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Exploratory work on the precision wavelength measurement of the hydrogen Lyman spectra Dalby, Frederick William 1952

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EXPLORATORY WORK ON THE.PRECISION WAVELENGTH MEASUREMENT  OF THE HYDROGEN LYMAN.SPECTRA by FREDERICK WILLIAM DALBY A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ARTS In the Department of P h y s i c s .. We accept, t h i s t h e s i s as conforming t o the standard r e q u i r e d , from candidates f o r the degree of MASTER OF ARTS Members of the Department of P h y s i c s THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1952 ABSTRACT The L u b z i n s k i . s p e c t r o g r a p h has been a c c u r a t e l y f o c u s s e d and i t s performance as a h i g h d i s p e r s i o n vacuum u l t r a v i o l e t instrument has been s t u d i e d . L i g h t sources have been designed f o r the e x c i t a -t i o n of the hydrogen Lyman s e r i e s . The a l i n e of t h i s s e r i e s has been o b t a i n e d u s i n g exposure times r a n g i n g from twenty minutes t o seven hours. A spectrogram of the hydrogen-deuterium i s o t o p e s t r u c t u r e o b t a i n e d i n the f i f t h g r a t i n g order a t a d i s p e r s i o n of 1 A ° / W « has been o b t a i n e d p e r m i t t i n g p o s i t i v e i d e n t i f i c a t i o n of the hydrogen Lyman a l i n e • L i g h t sources f o r the e x c i t a t i o n of the f i r s t -spark s p e c t r a of copper have, been c o n s t r u c t e d and experiment-a l l y s t u d i e d . T h i s spectrum c o n t a i n s a l a r g e number:of standard wavelengths, i n the vacuum u l t r a v i o l e t ; however, because of low g r a t i n g i n t e n s i t y we were unable to observe these l i n e s . An e x i s t e n t d i s c r e p a n c y between c a l c u l a t e d and measured, v a l u e s f o r the i o n i z a t i o n p o t e n t i a l of H e l i u m - l i k e atoms has been r e s o l v e d by a g e n e r a l i z e d Lamb electromag-n e t i c s h i f t . iv ACKNOWLEDGMENTS I am p l e a s e d t o acknowlege the h e l p and ad v i c e g i v e n by Dr. A. M. Crooker who suggested the problem. To Dr. J . B. Warren, f o r the l o a n of platinum, f o i l and a g i f t of heavy water, t o Mr. J . Lees, f o r h i s s generous h e l p w i t h glass, blowing problems, and t o Mr. T. Reesor f o r h i s a i d i n d r a f t i n g , I extend my warmest thanks. I am inde b t e d to Dr. G. Herzberg f o r v a l u a b l e d i s c u s s i o n of many of the problems e n t a i l e d i n t h i s work. I t i s a p l e a s u r e to acknowledge that t h i s work was completed d u r i n g the tenure of a N a t i o n a l Research C o u n c i l B u r s a r y . i i i TABLE OF CONTENTS Pap;e Acknowledgements 1 A b s t r a c t i i Table of Contents i i i L i s t of I l l u s t r a t i o n s i v I n t r o d u c t i o n 1 I . Theory of Hydrogen-like Spectra.. 3 (a) E a r l y Work 3 (b) Welton's Theory of the E l e c t r o m a g n e t i c S h i f t 7 I I . P r e v i o u s Experimental Work on Lyman S e r i e s S p e c t r a 12 (a) On the Hydrogen Lyman S e r i e s 12 (b) On Other Hydrogenic Lyman S e r i e s 15 I I I . tamb S h i f t i n H e l i u m - l i k e Atoms 16 IV. Design of Experimental Programme 19 (a) Estimate of O p t i c a l Performance of the L u b z i n s k i Spectrograph 19 (b) Standard Wavelengths In the Vacuum U l t r a v i o l e t 25 (c) Widths of S p e c t r a l L i n e s . 31 V. Experimental - 33 (a) The L u b z i n s k i Spectrograph 33 (b) F o c u s s i n g the Spectrograph 37 (c) Lyman S e r i e s S p e c t r a 41 (d) F i r s t Spark Spectrum of Copper 45 V I . Conclusions and Recommendations 48 B i b l i o g r a p h y 50 i v LIST OF ILLUSTRATIONS A . Figures I . Energy Level Diagram of the Hydrogen Atom. to fol low page 5 I I . Optics of the L u b z i n s k i Spectrograph.to f o l l o w page 19 I I I . Vacuum Wavelength Standards from the R i t z Combination P r i n c i p l e to f o l l o w page 19 IV. Spectrograph Pressure Against Time from Cessation of Pumping page 34 V . C i r c u i t Diagrams of Power Supply and High Frequency O s c i l l a t o r to fol low page 4 l B. Tables —,,, I . Experimental Hydrogen Lyman Wavelengths According to Boyce and Rieke page 14 I I . I o n i z a t i o n Energies f o r the Ground States of H e l i u m - l i k e Atoms page 17, I I I . L i n e a r D i s p e r s i o n and Wavelength Against D i f f r a c t i o n Angle page 21 IV. Vacuum Wavelength Standards i n the F i r s t Spark Spectrum of Copper page 30 V . Doppler Half Widths- f o r Hydrogen and Copper page 32 C . P l a t e s I . Reproduction of T y p i c a l Spectragrams. to f o l l o w page 42 INTRODUCTION The. theory of D i r a e e ( l 8 ) p r e d i c t s t h a t the a_lSj_ energy l e v e l and the P± l e v e l of the hydrogen atom have the same.energy (he degenerate). However, Lamb and R e t h e r f o r d (26) have shown e x p e r i m e n t a l l y t h a t t h e r e i s an -1 energy d i f f e r e n c e of about 0.03 cm between these l e v e l s . Bethe. (14), has. deduced from the new. quantum el e c t r o d y n a m i c s , an e q u a t i o n which p r e d i c t s an upward s h i f t i n a l l S l e v e l s r e l a t i v e t o the P l e v e l s . A c c o r d i n g to the Bethe t h e o r y t h i s s h i f t amounts to 0.03 cm f o r the A Jx l e v e l , i n good:, agreement w i t h t h e . e x p e r i m e n t a l . v a l u e . F o r the ground s t a t e , / *" 5j. , a s h i f t of 0.264 cm"""*- i s p r e d i c t e d , which corresponds t o a wavelength s h i f t of 0.004 A 0 f o r the hydrogen Lyman a l i n e a t 1216 A ° . I t was our purpose to measure the wavelength of t h i s L a l i n e w i t h an accuracy of 0.001 A ° i n o r d e r to be a b l e to t e s t the Bethe p r e d i c t i o n . I t was soon d i s c o v e r e d t h a t the b l a z e of the o n l y a v a i l a b l e g r a t i n g . f o r the L u b z l n s k i vacuum spetrograph n e c e s s i t a t e d such e x c e s s i v e l y l o n g exposure times t h a t precieionviWorkoonttheLL^mansseries, would be I m p r a c t i c a l . Our experimental programme then became: (1) To focus the spectrograph a c c u r a t e l y and t o study I t s performance w i t h the e x i s t i n g g r a t i n g i n o r d e r to f a c i l i t a t e u t i l i z a t i o n when the improved g r a t i n g a r r i v e d . (2) To develop a s u i t a b l e source f o r the e x c i t a t i o n of the hydrogen Lyman s e r i e s s p e c t r a ^ (3) To attempt to solve the fundamental problem i n v o l v e d i n a l l p r e c i s i o n wavelength measurements i n the vacuum u l t r a - r v i o l e t s p e c t r a l r e g i o n i . e . the development of s u i t a b l e wavelength standards, A d e t a i l e d account of the r e s u l t s of t h i s programme i s p r e s e n t e d . An i n t e r e s t i n g d i s c r e p a n c y between measured and c a l c u l a t e d i o n i z a t i o n p o t e n t i a l s of h e l i u m - l i k e atoms-was encountered. I t i s shown th a t t h i s d i s c r e p a n c y can be r e a d i l y r e s o l v e d by c o n s i d e r i n g a m o d i f i e d Bethe equat i o n . . We commence w i t h an o u t l i n e , of the h i s t o r y of the theory of h y d r o g e n - l i k e atoms i n c l u d i n g Walton's elementary s e m i - q u a n t i t a t i v e treatment of the e l e c t r o -magnetic s h i f t , f o l l o w e d by a review of p r e v i o u s experiment-a l work on the Lyman s e r i e s s p e c t r a * - 3 -If".' THEORY OF HYDROGEN-LIKE. SPECTRA, (a) E a r l y - Work. The problem of the i n t e r p r e t a t i o n of the s p e c t r a of the hydrogen atom has p l a y e d an almost.unique r o l e i n the development of the new quantum.mechanics. From the time, of Balmer, c o n t i n u a l major, advances were made by e i t h e r i n v e n t i n g more r e f i n e d t h e o r i e s to account f o r new experimental data, on the. one hand, or d e v i s i n g more-e l a b o r a t e e x p e r i m e n tal techniques to observe p r e d i c t i o n s of new t h e o r i e s on the o t h e r . U n t i l r e c e n t l y i t . had been thought t h a t t h i s p r o c e s s had exhausted i t s e l f . . A g r e a t range of p r e c i s e experimental d a t a s e e m e d w e l l accounted f o r by the t h e o r y of D i r a c . However, r e c e n t e x p e r i m e n t a l work by Lamb and R e t h e r f o r d (26) e x p l o i t i n g wartime developments i n microwave technique, and subsequent t h e o r e t i c a l work, e s p e c i a l l y by. Bethe (14), have shown r e a l d e f i c i e n c i e s i n the D i r a c : t h e o r y . Balmer (13) i n 1885 showed, t h a t the wavelengths . ( >\ ) of a l l the then known l i n e s a t t r i b u t e d , t o hydrogen c o u l d be expressed by the simple e m p i r i c a l f o r m u l a : h * . where b i s . an e m p i r i c a l l y d e r i v e d constant and n = 3, 4, ... This, formula p r e d i c t e d , the. wavelengths of the observed l i n e s ? w i t h e r r o r s . o f l e s s than one p a r t i n one thousand. - 4 -M i c h e l s o n and Morley ( J I ) soon showed t h a t the f i r s t l i n e i n the s e r i e s . (H a) was i n r e a l i t y a c l o s e doublet ..and t h a t t h e r e f o r e the Balmer formula c o u l d not be s t r i c t l y c o r r e c t . I t was n e v e r t h e l e s s an important, stimulus f o r f u r t h e r work. The f i r s t s u c c e s s f u l t h e o r e t i c a l treatment of the hydrogen atom problem.was; r e p o r t e d by Bohr (15) i n 1913. By combining R u t h e r f o r d ' s i d e a (34) of a n u c l e a r atom w i t h the quantum c o n d i t i o n s of Planck and E i n s t e i n (19), Bohr was a b l e t o show that, o n l y .discrete, energy l e v e l s were p o s s i b l e and g i v e n by fhe formula:.-7 2 E(n) = - hcR . . . (1-2) n The number of " o r b i t s " p o s s i b l e f o r ea,ch energy was e q u a l t o n. Thus f o r n —. 1 o n l y a c i r c u l a r o r b i t was p e r m i t t e d whereas f o r n = 2 one c i r c u l a r and,one e l l i p t i c a l o r b i t were p e r m i t t e d , e t c e t e r a . Sommerfeld (37) In 1916 showed t h a t the r e l a t i v i s t i c v a r i a t i o n of mass w i t h v e l o c i t y l e d to. s m a l l energy d i f f e r e n c e s between o r b i t s of the same quantum number, n,.and the . t h e o r y e x p l a i n e d a l l the known expe r i m e n t a l r e s u l t s - I n c l u d i n g the doublet s p l i t t i n g of Michelson-Morley•. However, the Bohr theory was not completely adequate. No s a t i s f a c t o r y method f o r t r e a t i n g atoms w i t h more than one ..electron c o u l d be d e v i s e d ; . f u r t h e r d i f f i c u l t -i e s were encountered i n deducing the r u l e s f o r l i n e - 5 -I n t e n s i t i e s - the s o - c a l l e d s e l e c t i o n . r u l e s . These d i f f i c u l t -i e s were surmounted i n 1926 by the new wave mechanical > theory of Schrttdinger (35) (or the equivalent Heisenberg matrix mechanics (22)) based upon speculations of d e B r o g l i e . The p o s s i b l e energy l e v e l s , E , and the state f u n c t i o n s , Y , which replace the Bohr o r b i t , could be deduced from the eigenvalue equation HY - EY . . . (1-3) where H Is. a suitable. Hamiltonian .operator. Rules were given f o r deducing the appropriate form of H from c l a s s i c a l analogues... A f t e r the new mechanics had been modified by the i n t r o d u c t i o n of r e l a t i v i t y mass v a r i a t i o n e f f e c t s , and the s p i n o r b i t i n t e r a c t i o n of Uhlenbeck and Goudsmit (39) r e s u l t s were obtained f o r the hydrogen energy l e v e l s equivalent to those of Sommerfeld. The e l e c t r o n wave f u n c t i o n , Y, was a f u n c t i o n of the p r i n c i p a l quantum number, n , the o r b i t a l quantum.number, JL , whose p o s s i b l e values are n , (n-1), (n-2) . . . 0 each corresponding to a p o s s i b l e Bohr orbit., a magnetic quantum number m=>e, (-e-1), U - 2 ) , . . . 0 , - 1 , . . . -JL, and f i n a l l y a s p i n quantum number S whose p o s s i b l e values are +1/2 and - l / 2 . The d i s c r e t e energy states which are deduced from the Schrtidinger equation are (7) S(n.,Q = _ RZ£ he n' + rP 4n j + 1/2 • • • (1-4) where a = 2 n e 2 / c h (or 1/137) i s the f i n e s t r u c t u r e constant, Z and R a r e : t h e appropriate charge and Rydberg ! B, Bethe 1 1 t D irac FIGURE I, ENERGY LEVEL DIAGRAM F O R T H E H Y D R O G E N ATOM. To f o l l o w page 5 - 6 -constant, r e s p e c t i v e l y , f o r the atom co n s i d e r e d , and J i s the t o t a l a ngular momentum whose p o s s i b l e v a l u e s are X + 1/2 and A - l / 2 except f o r S s t a t e s , ( s t a t e s w i t h i = 0 ) , when i t s v a l u e i s 1/2. The f i r s t term of e q u a t i o n (1-4) i s j u s t the f a m i l i a r n o n - r e l a t i v i s t i c ; B o h r e x p r e s s i o n . The second term g i v e s the combined s p i n - r e l a t i v i t y c o r r e c t i o n p which i s , f o r hydrogen, an o r d e r of a s m a l l e r than the . f i r s t . An important consequence of t h i s . e q u a t i o n . i s the p r e d i c t i o n t h a t s t a t e s of d i f f e r e n t ^ , but i d e n t i c a l n and j have the same energy. Thus the t h e o r y p r e d i c t s t h a t the 2*$x and 2 * ^ l e v e l s i n hy d r o g e n - l i k e atoms be degenerate. F i g u r e I g i v e s the energy l e v e l scheme f o r hydrogen on the Schrtidlnger t h e o r y . The l i n e s a r i s i n g from t r a n s i t i o n s t o the ground s t a t e a re c a l l e d the Lyman s e r i e s . The p r i n c i p a l l i n e shown i n the f i g u r e i s c a l l e d the Lyman a l i n e ( L Q ) and a c c o r d i n g to the wave mechanical theory should be a doublet whose t h e o r e t i c a l i n t e n s i t i e s are I n d i c a t e d . The l i n e s ending on the 2 1 S i -and 2 1 ?JL s t a t e s , c a l l e d the Balmer s e r i e s , are not shown i n the diagram. The r e l a t i v i s t i c quantum, t h e o r y of D i r a c ..(18) which a u t o m a t i c a l l y endowed.the e l e c t r o n w i t h s p i n a n g u l a r momentum, i s c o n s i d e r e d a more s a t i s f a c t o r y theory than the o l d SchrBdinger t h e o r y . However, f o r our purposes i t i s s u f f i c i e n t to note t h a t f o r the hydrogen atom problem D i r a c 1 s theory gives the same r e s u l t ^ as the SchrBdinger t h e o r y . For hydrogen-l ike atoms,.the energy l e v e l s are again given by equation (1-4), and i n p a r t i c u l a r the 2*"Si.and. 2 . P i l e v e l s are degenerate. From 1887 to 1940 soma.thirty--two c a r e f u l o p t i c a l studies (27) were made of the hyperfine s t r u c t u r e of the Balmer a l i n e . . The . s l i g h t discrepancy, which seemed to be i n d i c a t e d , between the Dirac. theory and most of t h i s work could be r e s o l v e d by r a i s i n g the 2 ' "S i l e v e l about 0.03 c m " 1 r e l a t i v e to the 2 x l e v e l . In 1947 Lamb..: and Retherford (26) by observing t r a n s i t i o n s between f i n e s t r u c t u r e l e v e l s using.new microwave techniques found.that indeed the S l e v e l was s h i f t e d upwards. Bethe (14) then deduced from the quantum electrodynamics the expected magnitude of such a s h i f t . A s i m p l i f i e d account, of t h i s Bethe s h i f t , due.to Welton (40), w i l l now be presented. (b) Welton*s Theory of the Electromagnetic S h i f t The quantum theory of r a d i a t i o n p r e d i c t s an i n f i n i t e e l e c t r o n mass and hence an i n f i n i t e energy a s s o c i a t e d w i t h t h i s mass. As such q u a n t i t i e s a r e . c l e a r l y not observable t h i s . r e s u l t must be I n c o r r e c t • Bethe # The. p r e c i s e r e s u l t of the Dirac theory leads _to equation (1-4) when expanded In.powers of a to a 2 . Successive terms are n e g l i g i b l y s m a l l . suggested t h a t the i n f i n i t e mass, be re n ormalized, or. made f i n i t e , by a s u b t r a c t i o n procedure. I f t h i s r e n o r m a l i z a t i o n be performed " c o r r e c t l y " the the o r y l e a d s to the f e l i c i t o u s r e s u l t of zero electromagnetic.energy f o r a f r e e e l e c t r o n and a small, displacement of energy l e v e l s f o r a bound e l e c t r o n . Welton's s e m i - c l a s s i c a l theory (40) i l l u s t r a t e s the p h y s i c a l o r i g i n of t h i s displacement, of energy l e v e l s f o r a bound e l e c t r o n . Consider a qu a n t i z e d e l e c t r i c f i e l d , i n vacua, as a F o u r i e r expansion i n terms of plane waves^ E -- l_ EK e ... (1-5) k —» , where i s the. amplitude of. the. wave w i t h p r o p a g a t i o n v e c t o r k and c i r c u l a r frequency 3 * c i The lowest energy i s not zero, f o r a quantized, r a d i a t i o n . f i e l d , f o r the r e are the s o - c a l l e d zero p o i n t e n e r g i e s s i m i l a r t o those f o r a.simple harmonic o s c i l l a t o r . The zero p o i n t energy a s s o c i a t e d . w i t h each F o u r i e r component i s T ^ ^ so t h a t the amplitude can be determined by the e q u a t i o n £T(r+l]>)-i-^t)~E<r = J*£ H-IVCSJ ... ( i _ 6 ) where M-M£j = ~jp^s i s the number.of r a d i a t i o n o s c i l l a t o r s whose wave v e c t o r s l i e i n ^ k . Hence 1£K\ C . T^v d - 7 ) # We employ the conv e n t i o n t h a t the . summation be c o n s i d e r e d an i n t e g r a l f o r no n - i n t e g e r k. - 9 -These f l u c t u a t i n g e l e c t r i c - f i e l d s tend to spread the p o s i t i o n of an e l e c t r o n over a s m a l l volume. Such an e l e c t r o n w i l l not be so s t r o n g l y a t t r a c t e d c l when c l o s e to the nucleus, thus r a i s i n g . i n energy the s t a t e s of zero a n g u l a r momentum.relative to those s t a t e s of h i g h e r a n g u l a r momentum i n which the e l e c t r o n has. a s m a l l proba-b i l i t y of b e i n g found near the n u c l e u s . Now l e t . u s t r y t o . e s t i m a t e the change i n energy from such f l u c t u a t i o n s . . Let r + dr be the p o s i t i o n of the e l e c t r o n , where the second term a r i s e s from zero p o i n t f l u c t u a t i o n s . The k i n e t i c energy of the o s c i l l a t i o n s c o n t r i b u t e s t o t h e - e l e c t r o m a g n e t i c mass of the e l e c t r o n and can t h e r e f o r e be i g n o r e d . F o r a bound s t a t e t h e r e e x i s t s an e f f e c t upon the . p o t e n t i a l energy. Since d r i s small we assume V(? + ^)= vc?j + C ^ * )V ( ? J +it^-*)V?; + - . . . ; ( i _ 8 ) The time average v a l u e of the second term i s zero and t h e r e f o r e makes no c o n t r i b u t i o n . The c o n t r i b u t i o n of the l a s t term i s * v - - tC^)U. v l v ... (1_9) To c a l c u l a t e c^Mi, r e c a l l —i ^ <Ly - €. E E f c .... (1-10) o r - - e and f o r the r e s u l t a n t Vf- »-/-w*n - 10 -e l [A V I 4 fe . - ^ * <- V ^ c / / fe ... (1-11) The l i m i t s k-^  and k 2 which are i n s e r t e d t o prevent the i n t e g r a l from d i v e r g i n g . w i l l , b e . discussed, la t e r . . The e f f e c t of the zero p o i n t f l u c t u a t i o n s i s . t h e n - ( 1- 1 2 ) When t h i s , i s averaged over a quantum s t a t e ¥ the r e s u l t i n g energy s h i f t i s i« l ( ^ ) / * W P M $ ) ••• ( 1-13) so.that o n l y the S s t a t e s are s h i f t e d , (a i s the f i n e s t r u c t u r e constant and Z the n u c l e a r charge.) The Bethe e q u a t i o n o b t a i n e d from n o n - r - r e l a t i v i s t i c quantum ..electrodynamics, was the same as e q u a t i o n (1-13) except t h a t k]_ c o u l d be c a l c u l a t e d and a p h y s i c a l l y reasonable value: f o r kjj c o u l d be guessed. Bethe found f o r hydrogen k x = 17.8R ' ... (1-14) The upper, l i m i t Is^ was taken equal t o mc/h because of the e x p e c t a t i o n t h a t a r e l a t i v i s t i c c a l c u l a t i o n would l e a d t o convergence f o r l i g h t quantum e n e r g i e s exceeding.the r e s t energy.of the e l e c t r o n . The l o g a r i t h m a p p e a r i n g above was a c c o r d i n g l y , f o r hydrogen l o g ~ o r l o g 1 7^Q h c R or 7.63 ... (1-15) 11 -R e l a t i v i s t i c c a l c u l a t i o n s ( 2 7 ) have been subse-quently c a r r i e d out to second order p e r t u r b a t i o n y i e l d i n g r e s u l t s equivalent to those obtained here, w i t h i n the accuracy r e q u i r e d f o r o p t i c a l , spectroscopy. Whe n. hydro geni c - wave- f u n o t i o n s . a r e sub at 1 t u t ed i n t o equation (1-13) the r e s u l t a n t s h i f t i s e x p l i c i t l y dE = ^ h c l o g ( ^ ) ^ . . . (1-16) f o r the S s t a t e s . Other states are u n a f f e c t e d . The c o n -stant k^ f o r helium turns out to be four times that f o r hydrogen (1-14) and v a r i e s slowly f o r successive hydrogenlc atoms• The p o s i t i o n s of the S l e v e l s for.hydrogen p r e d i c t e d by equation (1-16) are shown c r o s s h a t c h e d . i n F i g u r e . I . As i s indicated/. in. the f i g u r e , the l x ^ a n d 2 " " f x energy l e v e l s are s h i f t e d upward by 0.265 cm""'' and 0.03 c m " 1 r e s p e c t i v e l y . This s h i f t s the H a l i n e of the Balmer s e r i e s by about 0.01 A ° , and the L a l i n e of the each Lyman s e r i e s by about 0.004 A 0 / t o longer wavelengths* than those p r e d i c t e d by the Dirac t h e o r y . X T I , PREVIOUS EXPERIMENTAL WORK ON LYMAN SERIES SPECTRA (a) On the Hydrogen Lyman S e r l e s I t has been .seen,that, t h e o r e t i c a l c o n s i d e r a t i o n s p r e d i c t t h a t Lg f o r hydrogen be a doublet of s e p a r a t i o n 0 . 3 6 7 cm" 1 whose c e n t r e of g r a v i t y l i e s a t 1216...664 A ° by the unmodified D i r a c theory or a t 1216.664 A 0 + 0.004 A 0 a c c o r d i n g t o the Bethe.theory of e l e c t r o m a g n e t i c s h i f t . The doublet s t r u c t u r e of t h i s Lyman.a l i n e has never been observed. F u r t h e r i t s wavelength has.never been measured s u f f i c i e n t l y a c c u r a t e l y to permit v e r i f i c a t i o n of .the. e l e c t r o m a g n e t i c s h i f t . Indeed, u n t i l r e c e n t l y o b s e r v o r s placed, such confidence i n the. wavelengths c a l c u l a t e d from the D i r a c e q u a t i o n t h a t v e r y commonly c a l c u l a t e d Lyman l i n e s were used as wavelength standards. Because, such.great confidence was p l a c e d i n the t h e o r e t i c a l l y c a l c u l a t e d wavelengths,rand a a c o n s i d e r a b l e experimental d i f f i c u l t y i s encountered i n precision,measurement of the hydrogen Lyman l i n e s , the p u b l i s h e d work on t h i s s u b j e c t Is meagre. I n the s p e c t r a l r e g i o n of the hydrogen.Lyman l i n e , indeed from about 1900 A ° to below 10 A 0, a i r , q u a r t z , and g e l a t i n are opaque. Schumann (4) by r e p l a c i n g the quartz o p t i c s by the more, t r a n s p a r e n t f l u o r i t e , u s i n g a s p e c i a l l y prepared photographic plate., and. e v a c u a t i n g h i s spec t r o g r a p h was abl e t o r e g i s t e r s p e c t r a t o about 1250 A ° and thus c r e a t e d vacuum u l t r a v i o l e t s p e c t r o s c o p y . As f l u o r i t e becomes almost opaque .in the hydrogen Lyman a - 13 r e g i o n , Schumann probably did. not observe t h i s . line,. F u r t h e r as t h e . r e f r a c t i v e index of f l u o r i t e i n t h i s s p e c t r a l r e g i o n was unknown no.wavelength measurements were p o s s i b l e . Lyman . (28), who.was the f i r s t t o r e p l a c e , the. f l u o r i t e prism, by a concave Rowland d i f f r a c t i o n g r a t i n g , by 1914 d i s c o v e r e d LQ a t 1216 A 0 and Lp a t 1026 A ° . H i s wavelength measurements were based.upon.the geometry of h i s instrument and were ac c u r a t e t o perhaps 1 A 0 . In 1922 J.. J . Hopf leld...(.24.) observed . the. hydrogen Lyman spectrum i n the f i r s t f o u r o r d e r s of a f i f t y c e n t -imeter, g r a t i n g s p e c t r o g r a p h w i t h two minute exposures.. By c o i n c i d e n c e of the. f o u r t h order L a w i t h the f i r s t o r der Hp he e s t a b l i s h e d L a = 1215.68 ± 0 . 0 3 A 0 Takamine and Suga ( 3 8 ) , and Rao and Budami ( 3 2 ) , who observed the Lyman s e r i e s out t o the t w e n t i e t h and f i f t e e n t h o r d e r s r e s p e c t i v e l y , a t t r i b u t e d d i s c o v e r e d i n t e n s i t y anomalies to c o l l i s i o n s o f the second., k i n d ( i n e l a s t i c ) w i t h f o r e i g n .gas molecules .( and atoms). By the. method...of o v e r l a p p i n g o r d e r s , wavelengths were measured to about 0 .02 A 0 . B a l l a r d and White (12) o b t a i n e d spectrograms of the hydrogen and deuterium Lyman .series l i n e s , i n the f i r s t o rder of a t h r e e metre, g r a t i n g . The L a l i n e s , were a l s o observed i n the second o r d e r . Values of A A , the - 14 -wavelength i n t e r v a l f o r the hydrogen i s o t o p e s , were d e t e r -mined u s i n g i r o n arc:: l i n e standards, i n second order s p e c t r a a n d . c a l c u l a t e d hydrogen wavelengths i n the f i r s t . There was n a t u r a l l y r a t h e r good agreement, w i t h the t h e o r y . Probably the, most ..accurate e x i s t i n g , measurements on-the hydrogen Lyman wavelengths-are those of Boyce and Rieke (17). U s i n g a ;two metre g r a t i n g , wavelengths of the hydrogen Lyman l i n e s were determined..by comparison i n h i g h e r g r a t i n g o r d e r s a g a i n s t f i r s t . o r d e r i r o n l i n e s . T h e i r r e s u l t s are,,summarized i n Table I . The d i f f e r e n c e s between the. measured wavelengths and the D i r a c t h e o r e t i c a l r e s u l t s are compared w i t h the s h i f t p r e d i c t e d by the Bethe,. t h e o r y . Some s h i f t towards the Bethe result.seems to be indicated.. However, as this.method of o v e r l a p p i n g o r d e r s can e a s i l y l e a d to c o n s i d e r a b l e e r r o r such evidence i s r a t h e r i n c o n c l u s i v e . . - TABLE. I Experimental. Hydrogen Lyman Wavelengths Accordlnp; to Boyce and Rieke Experimental A ° D i r a c Theory A ° Experimental - C a l c u l a t e d I O" 3 A 0 Bethe S h i f t 10~ 3~A° 1215.666 1215.664 +3.9 1025.725 1025.717 + 8.. +3.5 972.538 972.532 + 6 +3.1 949.740 949.739 + 1 +3.0 939.792 939.799 - 7 +10 +2.-.9 +16.4 - 15 -(b) On O t h e r H y d r o g e n ! c Lyman S e r i e s H y d r o g e n - l i k e Lyman s e r i e s s p e c t r a , h a v e b e e n o b s e r v e d f o r He III,, L l I I I , Be IV, B V, C VI, K V I I , a n d 0 V I I I ( 6 ) . However, b e c a u s e o f t h e s c a r c i t y o f s t a n d a r d l i n e s i n t h i s s p e c t r a l r e g i o n (306 A 0 t o 19 A ° ) , p r e c i s i o n measurements o f w a v e l e n g t h were u s u a l l y i m p o s s i b l e . C a l c u l a t e d w a v e l e n g t h s f o r t h e s e l i n e s were u s e d a s s t a n d a r d s . F o r L i I I I , E d l e n (6) h a s p o i n t e d o u t t h a t c a r e f u l measurement o f t h e w a v e l e n g t h o f Lyman a l i n e s i n o r d e r s up t o t h e t w e l f t h show i t ' t o t h e r e d o f t h e v a l u e c a l c u l a t e d f r o m t h e D i r a c t h e o r y b y a b o u t 20 cm" 1. T h i s a g r e e s w e l l w i t h t h e I s e l e c t r o m a g n e t i c s h i f t o f 19 cm" 1 c a l c u l a t e d by Mack ( 6 ) . - 16 -I I I . LAMB SHIFT IN HELlUM-LIKE ATOMS Accu r a t e work on the s p e c t r a of atoms i s o - e l e c t r o n i c . w i t h He I, mostly,by the Swedish s c h o o l , permit experimental d e t e r m i n a t i o n of the p o s i t i o n of the ground energy s t a t e s of these atoms... These experimental v a l u e s taken from Atomic Energy T a b l e s (6) are l i s t e d i n Column 3 Table I I . The c a l c u l a t e d . r e s u l t s are those of E r i k s s o n (20) who used a m o d i f i e d H y l l e r a s formula w i t h r e l a t i v i t y and mas-s-p o l a r i z a t i o n corrections.. Tyren (11) has noted - "the t h e o r e t i c a l r e s u l t s are throughout g r e a t e r than the exper-imental ones, and f o r the l a s t two elements^ the d i s c r e p a n c y i s c o n s i d e r a b l y g r e a t e r than the experimental l i m i t s of e r r o r g i v e n - a noteworthy circumstance from the t h e o r e t i c a l p o i n t of view." I t i s i n t e r e s t i n g to ask i f such a d i s c r e p a n c y may be r e s o l v e d by a Lamb s h i f t f o r the S l e v e l s i n H e l i u m - l i k e atoms. Although quantum electrodymamic c a l c u l a t i o n s , have not yet been performed f o r such atoms an estimate, of the expected s h i f t can be r e a d i l y made. We approximate the ground s t a t e h e l i u m - l i k e wavefunctions by hydrogenic wave-f u n c t i o n s .so t h a t e q u a t i o n (1-13) becomes when numerical f a c t o r s are made e x p l i c i t 4 EC?-) = ( O - A J T ) • ' . . . (3-1) The Lamb s h i f t s c a l c u l a t e d from th i s , formula: a re g i v e n i n Column 5 of Table I I . The r a t h e r good.agreement of data # 0 VII and F V I I I . We have added new r e s u l t s of Mg XI and A l X I I . - 17 -i n the l a s t two columns of Table I I , In view of the large e x p e r i m e n t a l . e r r o r s and the c r u d i t y of our t h e o r e t i c a l treatment, suggests s t r o n g l y that a. Lamb s h i f t i s o p e r a t i v e . A f t e r completion of the above c o n s i d e r a t i o n s i t was discovered that s u b s t a n t i a l l y the sameargument had been p r e v i o u s l y employed by E r i k s s o n (21) TABLE II I o n i z a t i o n Energies f o r the 1 ' S0 Ground States of Hel ium-l ike, Atoms. Atom . C a l c u l a t e d , Experimental],Calculated Lamb,Shift cms ' cms . -Experimental cmd"3- cms"" He I 198319 198305115 + 14 + 2 L i II 610092 610079+25 + 13 + 19 Be I I I 1241308 1241225.1100 + 83 + 60 B IV 2092151 20919601200 + 191 + 150 C V 3162759 31624501300 + 309 + 300 N VI 4453336 44528001500 + 536 + 570 0 VII 5964057 59630001600 +1057 + 960 F VIII 7695209 76934001800 +1809 +15^0 Ne IX not yet observed Na X not yet observed Mg XI 14213753 1420920012500 +4553 +4850 A l XII 16829563 1682500013000 +4563 +6700 - 18 -I t may be concluded t h a t the few.atoms f o r which s u f f i c i e n t l y accurate, experimental data have, been obtained, i . e . L l I I I and some H e l i u m - l i k e atoms, the observed S ground state, s h i f t s agree, q u i t e w e l l w i t h those c a l c u l a t e d from .the Bethe.. equation.(1-1.6) *j However, the experimental data on the. hydrogen atom ground s t a t e i s too u n c e r t a i n to permit t h e o r e t i c a l v e r i f i c a t i o n . IV. DESIGN. OF EXPERIMENTAL PROGRAMME To determine the p o s i t i o n , of: the 1 x5_<_ ground energy s t a t e of the hydrogen atom r e l a t i v e to the 2 x s t a t e , n e c e s s i t a t e s measurement.of the wavelengths of Lyman s e r i e s l i n e s . The elegant, microwave techniques of. Lamb and R e t h e r f o r d , which f i r s t e s t a b l i s h e d the s h i f t . i n the 2 l e v e l , i s not a p p l i c a b l e t o the ground s t a t e . To v e r i f y the Bethe p r e d i c t i o n t o w i t h i n even t h i r t y per..cent r e q u i r e s an a b s o l u t e wavelength measurement of accuracy of one p a r t i n a m i l l i o n (to 0.001 A°)> . Although the. a t t a i n -ment of such accuracy p r e s e n t s no d i f f i c u l t y i n the v i s i b l e and near u l t r a - v i o l e t r e g i o n s of the spectrum where powerful i n t e r f e r o m e t r i c ^ methods are a v a i l a b l e , i n the vacuum u l t r a - v i o l e t f o r m i d a b l e problems are encountered.. The two most major problems, -the need of a vacuum spectrograph of adequate r e s o l v i n g power and the development of a s u i t a b l e source of standard wavelengths, w i l l now be c o n s i d e r e d i n some d e t a i l . (a) Estimate of O p t i c a l Performance of the L u b z l n s k i  Spectrograph The spectroscopy l a b o r a t o r y at the U n i v e r s i t y of B r i t i s h Columbia i s fortunate, i n p o s s e s s i n g the L u b z i n s k i two metre s p e c t r o g r a p h . The mechanical c o n s t r u c t i o n and 5p& rx •, * "# Note, however, t h a t the Lamb.shift i n the 2 l e v e l , which corresponds to a wavelength s h i f t of 0.01 A ° i n a component of the Balmer a line,- two and a h a l f times as l a r g e as the s h i f t i n the L a , was never c l e a r l y e s t a b l i s h e d o p t i c a l l y . FIGURE II OPTICS OF THE LUBZINSKI SPECTROMETER FIGURE HI VACUUM WAVELENGTH STANDARDS FROM RITZ COMBINATION PRINCIPLE To f o l l o w page 19 - 20 -performance of t h i s instrument'have been p r e v i o u s l y d e s c r i b e d ( 5 ) . We s h a l l now d i s c u s s i t s expected optimum o p t i c a l performance when used w i t h our Slegbahn g r a t i n g i n o rder to answer the f o l l o w i n g q u e s t i o n s : (1) Can the 0.368 cm" 1 doublet s t r u c t u r e of L a be r e s o l v e d ? (2) Can the wavelength of the centr e of g r a v i t y of L Q a t 1216 A 0 be measured s u f f i c i e n t l y a c c u r a t e l y t o d e t e c t the expected Lamb s h i f t ? Our d i s c u s s i o n w i l l show t h a t the f i r s t q u e s t i o n must be answered.in the negative and. the second In the a f f i r m a t i v e . ( 1 ) " O p t i c s of the L u b z i n s k i Spectrograph The o p t i c a l mounting, i s i n d i c a t e d . i n F i g u r e I I . L i g h t from the s l i t S i n c i d e n t upon the two metre Slegbahn g r a t i n g G a t an angle of about .20° a f t e r d i f f r a c t i o n i s f o c u s s e d upon the p l a t e h o l d e r P to P 1 . The angle of d i f f r a c t i o n 0 ranges from 85° at P to 40° a t P 1 . T h i s g r a t i n g mounting Is . q u i t e unique. b e i n g ..the. o n l y e x i s t e n t mounting u t i l i z i n g n e g a t i v e o r d e r s . (8 2 0 ° ) . Mack, Stehn and E d l e n (30) have, shown, t h a t a l t h o u g h the ..disper-s i o n i n c r e a s e s without l i m i t as © i n c r e a s e s t o 90°, a t the same time the r e s o l v i n g power approaches z e r o . Hence r a t h e r severe l i m i t a t i o n s on the use of t h e r e g i o n approaching P may be expected. ..For a rough estimate of the performance of t h i s s p ectrograph mounting f i r s t o rder theory of p h y s i c a l o p t i c s g i v e s --21 -h \ z. d(*t^ & - <**~s i ) ... (4-1) where i s the wavelength d i f f r a c t e d through an angle e i n g r a t i n g o r d e r n; i i s the i n c i d e n t angle and d the g r a t i n g l i n e s p a c i n g . F o r the L u b z i n s k i s p e c t r o g r a p h w i t h the Siegbahn g r a t i n g d = (1/576) 1G T A 0 = 17360 A 0 i = 20° So t h a t KA v a r i e s from about 4800 A° a t P t o 11500 A 0 a t P 1 . Thus the hydrogen Lyman a l i n e at. 1216 A 0 can be observed o n l y i n grating- o r d e r s exceeding the f o u r t h . (2) L i n e a r ^ D i s p e r s i o n of the: Spectrograph The l i n e a r d i s p e r s i o n of the L u b z i n s k i s p e c t r o -graph, i s g i v e n by h TT'~ ••• ^ where r i s the Rowland c i r c l e r a d i u s and. S t h e . d i s t a n c e a l o n g the p l a t e h o l d e r . Table. III. g i v e s the r e s u l t a n t l i n e a r d i s p e r s i o n f o r v a r i o u s u s e f u l angles of d i f f r a c t i o n . TABLE .III L i n e a r D i s p e r s i o n and Wavelength A g a i n s t D i f f r a c t i o n Angle (A°) (A°/mm) 40° 5220 6 . 7 50° 7360 5 . 6 60° $100 4 . 3 70° 10400 2.9 80° 11200 1.5 - 22 -As the p o s i t i o n of a sharp s p e c t r a l l i n e can be determined to one micron, i f L a be observed i n the f i f t h order, (n = 5 x 1216 A 0 ) i t s wavelength may be measured to 0 . 0 0 1 A 0 . I f o r d e r s as high.as the seventh c o u l d be employed the wavelength of t h i s hydrogen Lyman l i n e c o u l d be determined to 0.0007 A ° . (3) R e s o l v i n g Power The t h e o r e t i c a l r e s o l v i n g power,R, of a d i f f r a c t i o n g r a t i n g of N l i n e s , employed i n the n*1*1 order i s g i v e n by R = n N ... ( 4 - 3 ) F o r the Slegbahn g r a t i n g R = n (81 mm) (576 lines/mm) = 46700 n The minimum wavelength s e p a r a t i o n t h a t may be r e s o l v e d Is then Thus t h i s minimum wavelength s e p a r a t i o n f o r the hydrogen Lyman a l i n e . w o u l d be 0 . 0 0 3 A ° f o r f i f t h o r d e r s p e c t r a and 0 .002 A ° f o r s i x t h order.. The p r a c t i c a l r e s o l v i n g power a c t u a l l y , a c h i e v e d i n such h i g h order vacuum u l t r a -v i o l e t s p e c t r a would be a p p r e c i a b l y s m a l l e r than these t h e o r e t i c a l v a l u e s . F u r t h e r , the above c o n s i d e r a t i o n s a p p l y o n l y to the r e s o l v i n g power of the g r a t i n g . The r e a l l i m i t a t i o n f o r the L u b z i n s k i s p e c t r o g r a p h i s the r e s o l v i n g power of the s p e c t r o g r a p h s p l a t e . The most f i n e g r a i n e d p l a t e s - 23 -s e n s i t i v e t o f a r u l t r a v i o l e t r a d i a t i o n c a n r e s o l v e o n l y 50 l i n e s p e r m i l l i m e t r e . As t h e d i s p e r s i o n o f t h e s p e c t r o -g r a p h i s about. 1 A ° / m m f o r t h e h y d r o g e n Lyman l i n e s , t h i s p l a t e r e s o l u t i o n c o r r e s p o n d s . t o a w a v e l e n g t h r e s o l u t i o n o f 0.02 A ° . S u c h r e s o l u t i o n i s i n a d e q u a t e f o r t h e o b s e r -v a t i o n o f t h e d o u b l e t s e p a r a t i o n o f 0.006 A 0 i n t h e . h y d r o g e n Lyman a l i n e . (4) Optimum G r a t i n g a n d S l i t W i d t h As t h e c o n c a v e g r a t i n g i s r u l e d o n a s p h e r i c a l s u r f a c e t h e o r d i n a r y o p t i c a l a b e r r a t i o n s f o r s u c h a. s u r f a c e are. e n c o u n t e r e d . T h e s e a b e r r a t i o n s Seoomee'e s p e c i a l l y i m p o r t a n t f o r g r a t i n g m o u n t i n g s w i t h r a t h e r l a r g e a n g l e s o f i n c i d e n c e o r d i f f r a c t i o n a n d l i m i t t h e u s e f u l g r a t i n g w i d t h . Mack, S t e h n , a n d E d l e r (30) have shown t h a t t h e g r a t i n g w i d t h f o r maximum r e s o l v i n g power w 0 p . j . i s g i v e n by * A R. 3 .... (4-5) where R i s t h e g r a t i n g r a d i u s . F o r t h e L u b z i n s k i s p e c t r o -g r a p h t h i s optimum, w i d t h v a r i e s from..8.3 cms. f o r a d i f f -r a c t i o n a n g l e o f 4 0 ° t o 4.9 cms. f o r a d i f f r a c t i o n a n g l e o f 8 0 ° . A3 a l l o u r work u t i l i z e d d i f f r a c t i o n a n g l e s n e a r 4 0 ° t h e f u l l S i e g b a h n g r a t i n g w i d t h o f 8.1 cms. was employed. Mack, Stehm a n d E d l e r f u r t h e r s t a t e " E x t r a l e n g t h i s n o t , l i k e e x t r a w i d t h , p o s i t i v e l y h a r m f u l " . - 24 -The s l i t w i d t h should he s u f f i c i e n t l y narrow to permit attainment of the f u l l g r a t i n g r e s o l v i n g power , without e x c e s s i v e l o s s i n l i g h t i n t e n s i t y . I t can be (30) shown/that the w i d t h S g i v e n by the f o l l o w i n g e q u a t i o n s a t i s f i e s t h i s c r i t e r i o n . a R A S = • ... (4-6) Hence the " i d e a l " s l i t width f o r 1200 A 0 i n the f i f t h o r d e r o f our spectrograph i s about f i v e microns. P r a c t i c a l l y i t i s found t h a t imperfections, i n the l i n e image, make the t o l e r a b l e s l i t w idth l a r g e r than, t h i s . t h e o r e t i c a l v a l u e and u s u a l l y the best width is. determined e x p e r i m e n t a l l y . We may conclude from t h i s d i s c u s s i o n of the optimum o p t i c a l performance of the L u b z i n s k i s p e c t r o g r a p h t h a t (1) the doublet s t r u c t u r e of the hydrogen Lyman a l i n e can not be r e s o l v e d w i t h the e x i s t i n g i nstrument. (2) and the most:accurate wavelength measurements t h e o r e t i c a l l y p o s s i b l e w i t h the spectrograph permit d e t e r m i n a t i o n of the hydrogen ground s t a t e Lamb s h i f t t o o n l y t h i r t y per c e n t . - 25 -•(b) Standard Wavelengths I n the Vacuum U l t r a v i o l e t The most d i f f i c u l t problem i n v o l v e d i n p r e c i s i o n a b s o l u t e wavelength measurements i n t h i s s p e c t r a l r e g i o n i s the choice of s u i t a b l e standards. Most ot h e r problems encountered i n vacuum.spectroscopy - such as. the develop-ment of l i g h t sources f o r the e x c i t a t i o n of s p e c t r a to be s t u d i e d , or the d e s i g n of i n s t r u m e n t s o f adequate r e s o l v i n g power and d i s p e r s i o n - can be r e a d i l y r e s o l v e d by an experienced s p e c t r o s c o p i s t of s u f f i c i e n t i n g e n u i t y . Yet the problem of standard wavelengths below 2000 A° remains l a r g e l y u n s o l v e d . Four methods of meeting t h i s problem of wavelength standards may be c o n s i d e r e d : (1) By employing c a l c u l a t e d wavelengths of hydrogenic or h e l i u m - l i k e atoms f o r which the theory i s r a t h e r w e l l e s t a b l i s h e d . (2) By the use of a . r e f l e c t i o n . . e c h e l o n i n t e r f e r o m e t e r . (3) By superimposing the unknown sp;:ectra upon known r e f e r e n c e l i n e s i n lower g r a t i n g o r d e r s . (4) F i n a l l y , by the use of wavelengths determined from the " R i t z combination p r i n c i p l e " . The f i r s t method which i s o f t e n the o n l y a v a i l a b l e method of e s t a b l i s h i n g standard wavelengths has been used e x t e n s i v e l y . (17, 11) The accuracy of the e x p e r i m e n t a l l y determined wavelengths, i s l i m i t e d o n l y by the range of - 26 -v a l i d i t y of the theory employed. C l e a r l y , however, t h i s method i s inadequate f o r our Lyman s e r i e s problem. For an i d e a l g r a t i n g i n perfect focus comparison # between spectra i n d i f f e r e n t g r a t i n g orders should be exceedingly a c c u r a t e . However, as.no g r a t i n g i s i d e a l and as small imperfections, i n focus are c e r t a i n l y , present t h i s method of overlapping orders i s subject to unknown e r r o r s , ( c f . Boyce (16)) For t h i s reason the proposed experiment of Wu (41) and the measurements,previously presented, by Boyce and Rieke (17) are suspect. This method i s not s u f f i c i e n t l y r e l i a b l e to permit determination of L a to the accuracy r e q u i r e d and was therefore d i s c a r d e d . The only work reported on the use of a r e f l e c t i o n echelon f o r i n t e r f e r o m e t r i c determination of wavelength standards i n the vacuum u l t r a v i o l e t i s that of MacAdam (29), who publ ished a p r e l i m i n a r y account of some experiments but no r e s u l t s . Asrthe r e f l e c t i o n echelon i s the only instrument of v e r y . h i g h r e s o l u t i o n which can be employed i n the vacuum u l t r a v i o l e t there would be considerable i n t e r e s t i n i t s f u r t h e r development f o r use i n t h i s s p e c t r a l region.. However, such work would be t e c h n i c a l l y very d i f f i c u l t . Tolansky (10) has discussed some of the problems which are encountered. The maximum e r r o r s permitted f o r the o p t i c a l surface are o n e - e i g h t i e t h of a wavelength. # I f the g r a t i n g be I l luminated exact ly the same way f o r both reference and unknown spectra-; This i s u s u a l l y achieved by simultaneous exposure of both s p e c t r a . - 27 -I f the instrument be b u i l t up u s i n g say green mercury l i g h t , these same e r r o r s amount t o o n e - f i f t e e n t h of a wavelength f o r M000 A ° . Thus the f r i n g e s become s t e a d i l y worse as the wavelength decreases. Tolansky f u r t h e r shows t h a t t o achieve an a n a l y s i s of the fr i n g e ; system produced r e q u i r e s t a k i n g a p l a t e w i t h a f o r e i g n gas i n the spec t r o g r a p h whose p r e s s u r e be maintained c o n s t a n t . t o b e t t e r than one t e n t h o f a m i l l i m e t r e of mercury. Such problems are much more d i f f i c u l t of s o l u t i o n t han those encountered i n more orthodox s p e c t r o s c o p y . A l s o . as. r e f l e c t i o n echelons ;?of s u f f i c i e n t l y h i g h q u a l i t y f o r vacuum spectroscopy are not y e t a v a i l a b l e t h i s method of e s t a b l i s h i n g standard wavelengths was d i s c a r d e d . The f i n a l . . p o s s i b i l i t y i s the one which was adopted h e r e . .If the energy l e v e l s A, C, and B (see F i g u r e I I I ) are so p l a c e d t h a t t r a n s i t i o n s A—*C and C—>B are observed i n the v i s i b l e where t h e i r a s s o c i a t e d wavelength can be very a c c u r a t e l y measured then the c o n s i d e r a b l y lower wavelength a s s o c i a t e d w i t h A—>B can be deduced from the " R l t z combination p r i n c i p l e " . The energy d i f f e r e n c e between A and B i s simply the sum of the other two d i f f e r -ences. F o r example, i f A—>C, and C—>-B each l i e a t about 2500 A ° (40000 cm - 1) and can be measured t o 0.001 A 0 (Oi016 cm" 1) then.the A-*B w i l l occur a t 1250 A G (80,000 cm" 1) and can be c a l c u l a t e d w i t h an accuracy of about 0.0005 A ° (O.O32 cm" 1). - 28 -At p r e s e n t , the s p e c t r a of v e r y few atoms have been s t u d i e d s u f f i c i e n t l y e x t e n s i v e l y f o permit wide use of such standards. Those few atoms, from which wavelength standards i n the r e g i o n M200 A 0 may be deduced, w i l l now be c o n s i d e r e d . The energy l e v e l diagrams ....of G r o t r i a n (2) were found very h e l p f u l i n the search f o r such atoms. He I I The wavelength a s s o c i a t e d w i t h the t r a n s i t i o n 4*"Dx—-v 2 P J - f o r once i o n i z e d Helium, which.should occur near 1215 A 0, can. be a c c u r a t e l y c a l c u l a t e d from known term v a l u e s . T h i s l i n e i s the analogue of Hp i n the hydrogen Balmer s e r i e s . As heliumccan be i n t r o d u c e d i n t o a d i s c h a r g e tube i n gaseous form, and as o n l y a v e r y short e x t r a p o l a t i o n Would be necessary f o r the c a l c u l a t i o n of the wavelength of L a , t h i s 1215 A 0 standard would be most convenient. However, t h i s l i n e i s v e r y d i f f i c u l t t o e x c i t e and has probably never been observed. Herzberg (25) has made an ext e n s i v e s e a r c h f o r t h i s , ^1215 A 0 l i n e without s u c c e s s . The wavelengths.of t h e second.and t h i r d memberss of the p r i n c i p a l s e r i e s i n the. f i r s t spectrum of mercury can a l s o be c a l c u l a t e d from known term v a l u e s . The second member, which l i e s at. X1402 A 0, has been..only weakly observed w i t h r a t h e r l o n g exposure times; The t h i r d member a t A1268 A 0'Would be very much l e s s i n t e n s e than the - 29 -second and has not yet been o b s e r v e d ^ CO The r o t a t i o n a l s t r u c t u r e of the e l e c t r o n i c b|nds of t h i s diatomic molecule might be used as.wavelength standards. The t r a n s i t i o n n B ' E + — , which has been observed i n emission and a b s o r p t i o n can be c a l c u l a t e d from known term values ( 3 ) . However, as other very expensive band.-systemsaof t h i s molecule would o v e r - l a p these known l i n e s causing severe experimental d i f f i c u l t y and as at present the accuracy of these standards i s r a t h e r low we have not yet studied t h i s method e x p e r i m e n t a l l y . Cu II Shenstone (36) as a r e s u l t of h i s exhaustive study of the apectra of once i o n i z e d Copper, has been able to. c a l c u l a t e the wavelengths of more than one hundred and f i f t y l i n e s between 2000 A° and 685 A ° . T h e i r accuracy i s b e l i e v e d to vary from about 0.003 A° at 1700 A 0 to less, than 0.001 A° at 800 A ° . Table IV presents the wavelengths of those l i n e s i n the r e g i o n of hydrogen L f l , t h e i r i n t e n s i t i e s on an a r b i t r a r y s c a l e , and t h e i r probable e r r o r s . Although these copper l i n e s are somewhat d i f f i c u l t to e x c i t e they have c e r t a i n l y been observed (25, 3 6 ) . # Herzberg (23) has not been able to e x c i t e this" l i n e i n emission. He i s now attempting observation i n a b s o r p t i o n . - R O -TABLE IV Vacuum Wavelength Standards i n the F i r s t Spark Specta of Copper.. . Wavelengths I n t e n s i t i e s Probable E r r o r (A°) _ _ _ _ _ (10-3 AQ) .1299.26? 10 2 1298.394 15 2 1297.549 2 2 1281.458 8 4 1275.570 30 2 1274.463 3 2 1266*308 10 1 1265.504 15 1 I25O.045 10 2 1248.790 • 5 2 1241.961 2 1 1219.332 1 2 1214.553 1 2 1185.899 2 2 1109.742 1 2 1106.446 3 2 1088.393 20 2 Careful-vacuum i n t e r f e r o n s try. .of the v i s i b l e parent l i n e s would reduce the above p r o b a b l e . e r r o r s . I t seems c l e a r that these Cu II l i n e s o f f e r the best hope of satisfactory-/ wavelength s t a n d a r d s . i n the r e g i o n of the hydrogen Lyman a l i n e . . Therefore, our programme Includes a c a r e f u l exper-imental study of l i g h t sources f o r the p r o d u c t i o n of these l i n e s . - 31 -(c) Widths of S p e c t r a l L i n e s No r a d i a t i o n i s p e r f e c t l y monochromatic* The f a c t o r s most, commonly pr o d u c i n g f i n i t e b r e a d t h of s p e c t r a l l i n e s . a r e - i n t r i n s i c r a d i a t i o n width,, p r e s s u r e broadening, S t a r k and Zeeman broadening, s e l f - r e v e r s a l width, and f i n a l l y Doppler w i d t h . For the c o n d i t i o n s of our experiments the Doppler width i s much the l a r g e s t and w i l l be the only e f f e c t d i s c u s s e d h e r e . ( c f . Tolansky (10)) atom at r e s t would emit s t r i c t l y monochromatic l i g h t . An atom moving w i t h a v e l o c i t y v towards the observor would emit r a d i a t i o n which i s d i s p l a c e d to h i g h e r f r e q u e n c i e s by ti v where T h i s i s simply a consequence of the f a m i l i a r . Doppler E f f e c t . Now the atoms of a gas move wit h a Maxwellian v e l o c i t y d i s t r i b u t i o n . Hence the r a d i a t i o n r e s u l t i n g from atoms, which i f a t r e s t would emit monochromatic.radiation of frequency v , w i l l .byyvirtue, of t h i s Maxwellian v e l o c i t y d i s t r i b u t i o n have f i n i t e width. The h a l f w i d t h of the r e s u l t a n t l i n e i s simply d e r i v e d from k i n e t i c theory and e q u a t i o n (4-7) as where T i s the temperature of the gas of atomic weight m and R i s the gas c o n s t a n t . I f V be measured i n wave numbers I f the i n t r i n s i c r a d i a t i o n width be n e g l e c t e d , an (4-8) - 32.-we nave Jiv = (p-fi) if" [ T t - H J . ....(4-9) The Doppler h a l f widths d e r i v e d from, e q u a t i o n (4-9) f o r s p e c t r a l l i n e s of hydrogen and copper i n the 1200 A 0 r e g i o n a t temperatures of l i q u i d n i t r o g e n (r210° C), room temperature.(30° C), and the m e l t i n g p o i n t of copper (1083° C) are pre s e n t e d i n Table V. TABLE V Doppler H a l f Widths, f o r Hydrogen, and Copper Temperature.,, Half. Width i n cm""1 (°C) Cu H -210 0.06 0.47 30 0.13 1.0 1083 0.28 2.2 Even a t the temperature of l i q u i d n i t r o g e n the.. Doppler h a l f w i d t h of the Lyman a hydrogen l i n e (0.47 cm" 1) would p r o b a b l y mask the expected doublet s t r u c t u r e (0.36 cm""1) • - 33 -V. EXPERIMENTAL A l l the common o p t i c a l m a t e r i a l s , ( g l a s s , quartz, e t c . ) , and most, gases,*; (n o t a b l y a i r ) , are opaque to r a d i a t i o n i n the s p e c t r a l r e g i o n below A 2000 A ° . F u r t h e r , because, of. the s t r o n g a b s o r p t i o n of t h e i r emulsion, a l l c o n v e n t i o n a l photographic, .materials aree a l s o i n s e n s i t i v e t o such r a d i a t i o n . . The spec t r o s c o p y of t h i s r e g i o n below A 2000 A 0, c a l l e d vacuum u l t r a v i o l e t , spectroscopy, r e q u i r e s the e l i m i n a t i o n of a l l such opaque m a t e r i a l from t h e s p e c t r o g r a p h . l i g h t p a t h . However, i n a l l o t h e r r e s p e c t s the o r d i n a r y techniques of spectroscopy i n the more-access-i b l e s p e c t r a l regions, are applicable,. The review o u t l i n e d by Boyce (16)., t r e a t i s e s by Bomke (1) and Lyman (4), and the r e l e v a n t chapter i n Sawyer's t e x t (6) a l l c o n t a i n v e r y h e l p f u l i n f o r m a t i o n on the techniques of spectroscopy i n the vacuum u l t r a v i o l e t i (a) The L u b z i n s k i . S p e c t r o g r a p h A d e t a i l e d account of the mechanical c o n s t r u c t i o n of t h i s s p e c t r o g r a p h has been g i v e n by L u b z i n s k i ( 5 ) • We hg,ve a l r e a d y examined, the main f e a t u r e s of the o p t i c a l system of t h i s , instrument and have, especially... noted the unique mounting, which u t i l i z e s n e g ative o r d e r s . The performance of. the remaining c r i t i c a l components w i l l now be d e s c r i b e d . Vacuum A l a r g e o i l . d i f f u s i o n pump backed by a mechanical Kenney pump serves t o evacuate the spectrograph i n about f o r t y minutes.. The. p r e s s u r e i s reco r d e d on a B i r a n i . guage. The u l t i m a t e p r e s s u r e , which i s . l i m i t e d by the. n a t u r a l leak of the spectrograph, d e s o r p t i o n of gases, and the water c o o l i n g employed, w i t h .the. d i f f u s i o n pump, i s about.one micron.of mercury. The. Importance of the second f a c t o r i s i l l u s t r a t e d , i n F i g u r e IV. FIGURE.IV ^ Spectrograph. P r e s s u r e .Against Time. From C e s s a t i o n of Pumping o U o > w ft; Iv l o J o % 9 I t can be seen t h a t the r a t e of pre s s u r e r i s e upon c e s s a t i o n of pumping decreases w i t h t o t a l e l a p s e d pumping time. T h i s phenomenon was a t t r i b u t e d to water vapour a d s o r p t i o n I n the porous, i r o n and i r o n oxide spectrograph h o u s i n g . " R i n s i n g " the spectr o g r a p h w i t h hydrogen or helium g r e a t l y reduced the time r e q u i r e d f o r degassing. I n a l l exposures a continuous flow.of c i r c u l a t i n g - 35 -gas from the. discharge through the spectrograph s l i t was maintained.. The r e s u l t a n t spectrograph pressure under such c o n d i t i o n s was about f i v e microns, r e s u l t i n g i n n e g l i g i b l e a b s o r p t i o n f o r the s p e c t r a l r e g i o n of the hydrogen Lyman s e r i e s ( c f . H o p f i e l d ( 2 4 ) ) . The G r a t i n g The g r a t i n g employed (#257655) was r u l e d at the P h y s i c a l I n s t i t u t e i n Uppsala on a r u l i n g engine designed by Slegbahn. 576 l i n e s per m i l l i m e t r e were r u l e d over an 81 m i l l i m e t r e width of the two metre g r a t i n g aluminlzed blank. The l e n g t h of the r u l i n g s was 48 m i l l i m e t r e s . T h i s g r a t i n g was appraised by Mr. David. Richardson (33) of Bausch.and Lomb and we r e p o r t his. f i n d i n g s . He s t a t e s : "(1) Weak Lyman ghosts, from t o o l bounce. (2) Rowland ghosts ••approximately 0.2$ i n f i r s t o r d e r . (3) E r r o r of run not s e r i o u s . (4) Target p a t t e r n .quite s t r o n g . (5) S a t t e l i t e s seen near parent l i n e i n f i r s t and second o r d e r s . (6) Resolving power good (estimate 15% of t h e o r e t i c a l value i n t h i r d o r d e r ) . (7) Energy d i s t r i b u t i o n very good - there being f i v e times:; as much l i g h t i n the f i r s t order on one side as on the o t h e r . (8) Diamond set down very hard and bounced. L i t t l e , evidence of diamond wear. , (9) We b e l i e v e the g r a t i n g to be quite a c c e p t a b l e . " - 36 -Most of the l i g h t from t h i s g r a t i n g tends to be d i f f r a c t e d i n the general, . d i r e c t i o n of the c e n t r a l image. ( c . f . items (4) .and (7) above) - 20° from the g r a t i n g normal f o r the L u b z i n s k i s p e c t r o g r a p h . , However, with the mounting employed i n thi s , spectrograph, (c . f . F i g u r e .II) o n l y l i g h t d i f f r a c t e d through angles g r e a t e r than 40° can be observed. T h i s r e s u l t s i n a g r e a t waste of l i g h t and consequently r a t h e r l o n g exposure times. Mechanlcal-The g r a t i n g mounting, p l a t e h o l d e r , and s l i t mechanism which L u b z i n s k i has d e s c r i b e d i n d e t a i l (5) a l l f u n c t i o n q u i t e adequately. Many of the e x c e l l e n t f e a t u r e s of t h e i r d e s i g n proved a g r e a t convenience i n . the f o c u s s i n g of the g r a t i n g . No d i f f u i c u l t y . was encount-ered i n any of the exposures w i t h v^bratlionsiiofi the o p t i c a l components. A s l i t w i d t h of 30 microns, which was found t o g i v e best d e f i n i t i o n w i t h reasonable exposure times, was employed f o r most of the spectrograms. (The optimum t h e o r e t i c a l s l i t w i d t h was p r e v i o u s l y shown to be 5 m i c r o n s ) . R e g i s t r a t i o n of the S p e c t r a A l l the vacuum s p e c t r a were r e c o r d e d on commercial I l f o r d Q-2, a mexllum g r a i n , r a t h e r h i g h s e n s i t i v i t y p l a t e . T h e . p l a t e s were developed i n Kodak D-19 f o r f o u r minutes a t • about 68° F. and t h e n f i x e d i n Kodak.F -5 s o l u t i o n and - 3 7 -washed. Fo r these p l a t e s of very t h i n , emulsion the l a s t two processes take l e s s than a minute each. When so t r e a t e d , these p l a t e s were found to g i v e s p e c t r a of good contrast, and low f o g . F o r r e c o r d i n g s p e c t r a i n the v i s i b l e r e g i o n e i t h e r Kodak 103a-0 or Kodak. IX F - 3 p l a t e s were employed. They were processed i n the same manner as- the Q,-2 p l a t e s . O c c a s i o n a l l y , p a r t i c u l a r l y w i t h the 103ar-0 p l a t e s , p l a t e s would break a f t e r b e i n g f o r c e d to the one metre r a d i u s curve of the p l a t e h o l d e r . (b) F o c u s s i n g the Spectrograph In a l l mountings of the concave d i f f r a c t i o n g r a t i n g , the s l i t and the diffracted...spectrum should b o t h l i e on a c i r c l e tangent to the g r a t i n g , c e n t r e which has as., i t s diameter the r a d i u s of c u r v a t u r e of the g r a t i n g b l a n k . F u r t h e r the s l i t should..be a c c u r a t e l y p a r a l l e l t o the g r a t i n g i r u l i n g s . The system i s then s a i d , to be f o c u s s e d . The. method of a c h i e v i n g t h i s f o c u s f o r the L u b z i n s k i spectrograph w i l l now be d e s c r i b e d . 1. The r a d i u s of c u r v a t u r e R of t h e . S l e g b a h n . g r a t i n g was determined by a F o u c a u l t > k n i f e edge method (9) t o be 199*55 cms ± 0.1 cms., as the average of e l e v e n independent o b s e r v a t i o n s shared by two o b s e r v o r s . The accuracy was l i m i t e d only by the u n c e r t a i n t y i n measurements.of d i s t a n c e w i t h a.good s t e e l r u l e . - 38 -2. A Z e i s s p r e c i s e l e v e l (#5672) k i n d l y l e n t us by:the C i v i l . Engineering.Department of the U n i v e r s i t y of B r i t i s h Columbia was,employed to set the centre of t h e . s l i t , the c e n t r e of the g r a t i n g , and the midpoint of t h e . p l a t e h o l d e r from P to P 1 (see F i g u r e I I ) i n the. same plane.. I t was, d i s c o v e r e d t h a t the mechanical c o n s t r u c t i o n n e c e s s a r i l y p l a c e d the g r a t i n g . c e n t r e about. 0.34 cms.... above the h o r i z o n t a l .plane through the s l i t . The plane of the Rowland, c i r c l e was t h e r e f o r e not h o r i z o n t a l , but r a t h e r . s l o p i n g upward from the s l i t to the p l a t e h o l d e r . 3» The g r a t i n g o r i e n t a t i o n was, f i x e d by v i s u a l l y f ocussing^ a . d i f f r a c t e d i r o n , a r c .spectrum upon the p l a t e ..holder. T h i s ..defines .the .Rowland c i r c l e . - a c i r c l e p e r p e n d i c u l a r t o the midpoint of the g r a t i n g and of r a d i u s 199.55/2 cms. 4 . To p o s i t i o n . t h e slit..upon..this R o w l a n d . c i r c l e the - s l i t - g r a t i n g d i s t a n c e was. a d j u s t e d u n t i l e q u a l t o the g r a t i n g - c e n t r a l , image.distance.. The s l i t - g r a t i n g d i s t a n c e was,.determined w i t h a good s t e e l tape and. a depth guage. The g r a t i n g - c e n t r a l image, distance, was deduced by measure-ment w i t h the s t e e l tape t o the focus of the c e n t r a l image, as determined u s i n g a F o u c a u l t k n i f e , edge. T h i s method p l a c e d the. s l i t upon the Rowland c i r c l e w i t h i n one m i l l i m e t r e a c c u r a c y . 5. P l a c i n g the p l a t e h o l d e r i n the p o s i t i o n of best f o c u s ensures t h a t i t a l s o w i l l be on the Rowland c i r c l e . - 39 -T h i s adjustment was f i r s t made, v i s u a l l y u s i n g a m i c ro-scope whose f o c a l plane, c o u l d be made c o i n c i d e n t w i t h t h a t of the. p l a t e h o l d e r . The f i n a l , adjustment, was made, photo-g r a p h i c a l l y u s i n g an i r o n a r c spectrum. Exposures of about one minute g i v e w e l l developed s p e c t r a upon I I F - 3 p l a t e s . P l a t e 1(a) shows a r e p r o d u c t i o n of a t y p i c a l t r i a l exposure (H-73)•• The p l a t e h o l d e r was moved about 0.01 inches i n a d i r e c t i o n p a r a l l e l t o the Rowland r a d i u s between exposures. The p o s i t i o n of b e s t f o c u s corresponded to t h a t of the c e n t r e spectrum. T h i s c o n c l u s i o n f o l l o w s more e a s i l y from the o r i g i n a l p l a t e than from the r e p r o d u c t i o n . I r o n arc s p e c t r a are not.commonly employed i n the f o c u s s i n g of l a r g e g r a t i n g s . Wandering, of the a r c , by changing the alignment .of the o p t i c s may. w e l l change the. g r a t i n g i l l u m i n a t i o n and hence . the l i n e p r o f i l e on the p l a t e . T h i s causes, the s p e c t r a l l i n e s to appear badly out of f o c u s . Frequent c a r e f u l , alignment, of the a r c p o s i t i o n and d u p l i c a t i o n of t r i a l exposures served to minimize the d i f f i c u l t y . A c o n v e n t i o n a l type G e i s s l e r tube w i t h aluminum e l e c t r o d e s was c o n s t r u c t e d to g i v e f i n e diatomic band s p e c t r a which are v e r y convenient i n f o c u s s i n g . When run i n a i r a t a few m i l l i m e t e r s p r e s s u r e , c u r r e n t s of one ampere produced very b r i g h t s p e c t r a of NO, N 2, and Ng +. However, w i t h reasonable s l i t widths the l o n g exposure times r e q u i r e d p r e c l u d e d the use of t h i s , source i n f o c u s s i n g the g r a t i n g s - 40 -6. F i n a l l y the . s l i t , was rotated; u n t i l . . v e r y a c c u r a t e l y " p a r a l l e l to the g r a t i n g m i l i n g a . T h i s o p e r a t i o n was accomplished by v i e w i n g through a microscope an i r o n a r c spectrum p l a c e d . b e h i n d the s l i t which was s l o w l y r o t a t e d . P a r a l l e l i s m of the s l i t a n d . g r a t i n g r u l i n g s , ensures f i n e sharp l i n e s , while s m a l l e r r o r s o f f p a r a l l e l i s m are e a s i l y d e t e c t e d by the a s t i g m a t i c broadening. ... This...visual, adjustment was. found to. be more s e n s i t i v e than photographic methods. This, .completes, the f o c u s s i n g f o r the v i s i b l e r e g i o n . As this--ensures r a t h e r a c c u r a t e f o c u s s i n g f o r the vacuum u l t r a v i o l e t r e g i o n and a l s o as exposure times f o r s p e c t r a i n the. vacuum. r e g i o n were r a t h e r long, f u r t h e r f o c u s s i n g u s i n g vacuum u l t r a v i o l e t l i n e s was.not performed. T h i s f o c u s s i n g problem, can.be very time consuming. For example, a f t e r , f i x i n g , the. g r a t i n g o r i e n t a t i o n and the s l i t - g r a t i n g d i s t a n c e 'by.-«steps. (3) and. (4) one might d i s c o v e r i n s u f f i c i e n t . m e c h a n i c a l . f r e e d o m to e f f e c t f o c u s s i n g the p l a t e h o l d e r by step ( 5 ) . T h i s n e c e s s i t a t e s a s l i g h t change, i n g r a t i n g o r i e n t a t i o n and then a r e p e t i t i o n of steps (4) and.(5). I t i s e a s i l y p o s s i b l e to repeat t h i s process s e v e r a l times. - 41 -(c) Lyman S e r i e s S p e c t r a Two methods of o b t a i n i n g s p e c t r a i n the r e g i o n of Hydrogen L a were c o n s i d e r e d . E i t h e r the d i s c h a r g e t u b e . c o u l d be"coupled d i r e c t l y t o the spectr o g r a p h w i t h no o p t i c a l m a t e r i a l , i n the l i g h t p a t h o r f l u o r i t e , o r l i t h i u m f l u o r i d e , windows and condensing l e n s c o u l d be used t o i s o l a t e the spectrograph from the. d i s c h a r g e . As even the v e r y best fluorite.commences to absorb r a t h e r s t r o n g l y i n the o p t i c a l . r e g i o n near \ 1200 A 0 and l i t h i u m f l u o r i d e , the only o t h e r t r a n s p a r e n t m a t e r i a l i n t h i s r e g i o n , d i s -c o l o u r s v e r y badly under the. e l e c t r o n i c bombardment i n a gaseous d i s c h a r g e , ( c f . Boyce (16)) the f i r s t method u s i n g d i r e c t c o u p l i n g was employed. The hydrogen (or helium) gas was i n t r o d u c e d i n t o the d i s c h a r g e through a smalllMathew's needle type l e a k v a l v e . Continuous e v a c u a t i o n by a Hyvac pump served t o ma i n t a i n low p r e s s u r e s . The gas which escaped through the s l i t i n t o .the spectrograph was e l i m i n a t e d by the l a r g e o i l d i f f u s i o n pump. I n t h i s way a great range of p r e s s u r e s c o u l d be r e a l i z e d i n the di s c h a r g e tube, while the. s p e c t r o -graph p r e s s u r e was always v e r y low (about f i v e microns of mercury). The s p e c t r a were e x c i t e d i n a pyrex tube f o r t y cms., i n l e n g t h and about three cms.in diameter by e x t e r n a l h i g h frequency energy. The c i r c u i t . d i a g r a m of the-power supply and the o s c i l l a t o r which were employed are g i v e n i n 2 0 0 O V A . 2 0 0 0 VOLT POWER SUPPLY E T RADIO FREQUENCY OSCILLATOR FIGURE IL To f o l l o w page 41 ( - 42 -F i g u r e V. V o l t a g e s up t o two thousand v o l t s with, c u r r e n t s up to one ampere were used. The output frequency of the o s c i l l a t o r was found to he f i f t e e n megacycles,, w i t h a s t r o n g harmonic a t t h i r t y megacycles. In a l l exposures the e l e c t r o d e s were p l a c e d a t the e x t r e m i t i e s of the d i s -charge tube. The d i s c h a r g e was always c o n c e n t r a t e d between these e l e c t r o d e s . A l l the p l a t e s were exposed In the f i r s t s e c t i o n of the p l a t e h o l d e r where h\ v a r i e s from 4800 A ° t o 6200.A°. Thus, near L , spectra, i n the r e g i o n s X 1200 A ° , A 1500 A ° , A 2000 A ° , and A 3200 A ° were a l l overlapped. 1. Excitation...of Tank Hydrogen By e x c i t i n g tank hydrogen w i t h the h i g h frequency energy, a b r i g h t pink discharge was o b t a i n e d . The v i s i b l e spectrum c o n s i s t e d of s t r o n g Balmer l i n e s and v e r y much weaker hydrogen band s p e c t r a . Although the c h a r a c t e r i s t i c s of t h e . d i s c h a r g e v a r i e d very slowly w i t h p r e s s u r e some tendency f o r b e t t e r r e l a t i v e development of the atomic to the mol e c u l a r s p e c t r a was observed a t lower p r e s s u r e s . Low p r e s s u r e s were t h e r e f o r e employed (about 100 m i c r o n s ) . I n c r e a s i n g the v o l t a g e on the o s c i l l a t o r i n c r e a s e s the b r i g h t n e s s of the source. No other e f f e c t upon the c h a r a c t e r i s t i c s of the d i s c h a r g e was observed i n the v o l t a g e range 500 t o 2000 v o l t s . P l a t e 1 ( a ) I r o n A r c S p e c t r a u s e d i n F o c u s s i n g . . (H-73) E a c h e x p o s e d f o r 40 s e c . P l a t e h o l d e r moved a b o u t 0.01 i n c h e s between e a c h e x p o s u r e . *< I. III I I P l a t e 1 ( b ) H y d r o g e n E x c i t e d i n H i g h F r e q u e n c y D i s c h a r g e . (H-78) Most o f t h e l i n e s a r i s e f r o m t h e ba n d s p e c t r u m o f t h e h y d r o g e n m o l e c u l e . P l a t e 1 ( c ) T r a c e o f H y d r o g e n E x c i t e d I n E x c e s s o f H e l i u m i n H i g h F r e q u e n c y D i s c h a r g e . (H-88) P l a t e 1 ( d ) T r a c e o f H y d r o g e n a n d Heavy Water E x c i t e d i n E x c e s s o f H e l i u m i n H i g h F r e q u e n c y D i s c h a r g e . The o b s e r v e d i s o t o p e s h i f t p e r m i t s p o s i t i v e i d e n t i f i c a t i o n o f t h e h y d r o g e n Lyman a l i n e . To f o l l o w page 42 - 43 -P l a t e 1(b) i s a r e p r o d u c t i o n of p l a t e H-78 which was exposed f o r f o u r hours a t an o s c i l l a t o r v o l t a g e of 1600 v o l t s . Most o f the l i n e s on the p l a t e are hydrogen r o t a t i o n a l m olecular specjjra. near A.1506 A ° . The f e e b l e broad l i n e marked on the r e p r o d u c t i o n was a t t r i b u t e d t o hydrogen L a i n the f i f t h o r d e r . Because of the.very h i g h i n t e n s i t y of the m o l e c u l a r l i n e s r e l a t i v e to t h i s . s u p p o s e d atomic l i n e p o s i t i v e i d e n t i f i c a t i o n . w a s . i m p o s s i b l e . The v e r y l o n g exposures which were r e q u i r e d , and the s t r o n g o v e r l a p p i n g molecular l i n e s made a c c u r a t e work on the Lyman, s e r i e s impossible.. C l e a r l y a. more e f f i c i e n t method of e x c i t i n g the Lyman s e r i e s l i n e s was n e c e s s a r y . A condensed discharge, of the tank hydrogen i n the G e i s s l e r type d i s c h a r g e tube was examined w i t h a v i s u a l ..spectroscope. Over the complete range of hydrogen p r e s s u r e s a v a i l a b l e . o u t s i d e e x t i n c t i o n , and w i t h conden-sers from 0 to 1/2 F j no improvement i n b r i g h t n e s s i n the atomic l i n e s over t h a t of the h i g h frequency d i s c h a r g e was observed. • 2. E x c i t a t i o n of Tank Helium-When helium was e x c i t e d i n the h i g h frequency d i s c h a r g e a very marked improvement was observed. The # We have been unable t o f i n d a p u b l i s h e d a n a l y s i s of these . l i n e s . . A c a r e f u l study of t h i s spectrum might w e l l prove rewarding. - 44 -hydrogen band, s p e c t r a were v i r t u a l l y absent. fEurther, the l i n e a t t r i b u t e d to L a c o u l d be e a s i l y observed with-..exposure times of l e s s than an hour. No hydrogen was.introduced i n t o the d i s c h a r g e - traces, of e i t h e r water vapour or hydrogen i m p u r i t y i n the helium were s u f f i c i e n t t o produce v e r y b r i g h t s p e c t r a . P l a t e 1(c) i s a r e p r o d u c t i o n of p l a t e H-88 which was exposed f o r about f i f t y minutes. The l i n e , a t t r i b u t e d . t o L a i s marked. T h i s p l a t e should.be compared w i t h P l a t e . 1 ( b ) d i r e c t l y above which was e x c i t e d w i t h hydrogen i n . a f o u r hour, exposure.. How t h a t a d i s t i n c t s p e c t r a l l i n e c o u l d be a t t r i b u t e d t o LQ w i t h reasonable, exposure time a p o s i t i v e i d e n t i f i c a t i o n of the e m i t t i n g atom became p o s s i b l e By a d d i t i o n of a few drops of heavy water to the helium supply, l i n e a p l a t e was obtained; of b o t h the hydrogen and deuterium spectra.... The...observed Isotope s h i f t p e r m i t t e d p o s i t i v e i d e n t i f i c a t i o n . o f the L a l i n e . P l a t e 1(d) reproduces p l a t e which was. exposed, f o r f o r t y minutes. The doublet, i s o t o p e s t r u c t u r e i s marked. I t i s . i n t e r e s t i n g t o note t h a t the, deuterium L Q i s much sharper than the c o r r e s p o n d i n g hydrogen, l i n e . ' A c c o r d i n g t o our d i s c u s s i o n of the Doppler width, the hydrogen.line B h o u l d be 1.41 < time's as broad as the deuterium l i n e . -The low l i g h t i n t e n s i t y from, the g r a t i n g d i d not permit o b s e r v a t i o n of h i g h e r members of the hydrogen Lyman - 45 -s e r i e s . No e f f o r t was made t o c o o l . t h e d i s c h a r g e tube and hence decrease., the hydrogen:Doppler teoudlening.. When measurements of wavelengthsoof these l i n e s can be made w i t h a c c u r a t e . s t a n d a r d s l i q u i d a i r c o o l i n g would much reduce the l i n e w i d t h s . (d) F i r s t Spark Spectrum of Copper T h i s spectrum has.been v e r y thoroughly s t u d i e d by Shenstone (36). A c c o r d i n g to Shenstone ..it. It..-futile to endeavor to o b t a i n the complete spectrum of a o n c e . i o n i z e d metal atom by means of a spark.alone. The source which was. f i n a l l y chosen by. t h i s worker was. a S c h u l e r tube whose c h a r a c t e r i s t i c s were d e s c r i b e d as f o l l o w s : " I n order to get r a p i d e v a p o r a t i o n of the metal i n t o the discharge, the hollow cathode. Was made of t h i n t u n g sten w i r e s . I t was about 1 cm. i n diameter and. 5. cm. l o n g and was u s u a l l y h a l f f i l l e d w i t h copper f o i l . Some o b s e r v a t i o n s were made w i t h neon as the conducting gas, but f o r the complete e x c i t a t i o n h e l i u m . i s n e c e s s a r y . The c u r r e n t was s u p p l i e d u s u a l l y w i t h a 700 v o l t D.C. generator and was c o n t r o l l e d . b y r e s i s t a n c e s . The tube when c o l d and f i l l e d w i t h c i r c u l a t i n g helium a t about 3 mm. p r e s s u r e , operates on about one ampere and 200 volts., and the spectrum i s mainly helium. As the temperature of the cathode reaches, the m e l t i n g p o i n t of copper, the d i s c h a r g e changes completely and becomes, a b r i l l i a n t green, the helium l i n e s f a d i n g and hundreds of copper a r c and spark l i n e s a p p e a r i n g . i n the v i s i b l e . At the same time the p o t e n t i a l r i s e s to 500 or - 46 -600 v o l t s , the current dropping to perhaps 0.4 amperes. When the tube, i s ..in t h i s condition the external resistance. . can be varied over a wide range without any great, e f f e c t on the current. A s t i l l . f u r t h e r i n t e n s i f i c a t i o n of the dopper l i n e s can.be produced by lowering the helium pressure." As no tungsten or molybdenum was availa b l e when t h i s work was begun an.attempt.was made.to excite t h i s copper spark spectrum i n other type sources. The f i r s t source studied was.a more conventional hollow cathode type, discharge ( c f . Tolansky (10)0'» Into the aluminum^, water..cooled cathode ^werepplaced t h i n f o i l s of copper. Because of the large mass of aluminum employed, and because of the water cooling and. f i n a l l y ..because the melting point of aluminum was. much lower than that of copper, i t was found impossible to reach the condition which Shenstone describes as. occurring upon melting of the copper. The spectrum of helium predominated. The next source was i d e n t i c a l with the Schuler tube of Shenstone except that fatherofthan tungsten^, iron$ was used as a.cathode. When the Iron cathode was half f i l l e d with copper f o i l and.excited i n helium the bright green discharge i n d i c a t i v e of copper spark spectra. # The melting points.of tungsten, platinum, iron, copper and aluminum are 3387oC, 1773°C, 1527°C, 1083°C, and 660°C respectively. 47 -c o u l d be obtained.. However, a t the same time, the v e r y s t r o n g i r o n a r c and spark spectra, e x c i t e d made o b s e r v a t i o n of the copper l i n e s , v e r y d i f f i c u l t . The i r o n s p e c t r a l l i n e s g a i n t h e i r i n t e n s i t y a t the expense of the copper.. T h i s source was n o t . c o n s i d e r e d adequate f o r . the e x c i t a t i o n of vacuum u l t r a v i o l e t copper spark s p e c t r a . As some p l a t i n u m ^ f o i l was a v a i l a b l e , a Shenstone type source u s i n g such a cathode was c o n s t r u c t e d . P l a t i n u m has, a m e l t i n g p o i n t about 700°C h i g h e r than t h a t of copper so we expected, t h a t , the spectra, of pl a t i n u m would be much l e s s i n t e n s e than t h a t of copper. I t was. d i s c o v e r e d , however, t h a t the pl a t i n u m s p u t t e r e d very b a d l y g i v i n g a s t r o n g spectrum. The e x c i t a t i o n of the copper spark spectrum was n e g l i g i b l e . No change i n the c o n d i t i o n s of t h e . d i s c h a r g e e f f e c t e d b e t t e r r e l a t i v e e x c i t a t i o n of the copper atom. # See f o o t n o t e on p r e v i o u s page - 48 -V I . CONCLUSIONS AND RECOMMENDATIONS The problem undertaken, was to measure the wave-l e n g t h of the hydrogen Lyman a l i n e w i t h an.absolute accur a c y of 0.001 A 0 . Some progress has been made towards s o l u t i o n of t h i s problem... The. hydrogen and deuterium Lyman a l i n e s have been o b t a i n e d i n the f i f t h g r a t i n g o r d e r w i t h d i s -p e r s i o n s of 1 A°/mm. T h i s i s s u f f i c i e n t d i s p e r s i o n to enable measurement ,of wavelength t o the r e q u i r e d a c c u r a c y (0.001 A G ) - i f r e l i a b l e standards were a v a i l a b l e . We have shown, t h a t the o n l y method of o b t a i n i n g standards, which.are of s u f f i c i e n t accuracy and r e l i a b i l i t y , i s to use wavelengths c a l c u l a t e d from the " R i t z Combination P r i n c i p l e " . The copper spark spectrum should supply such standards.. However we were unable to produce these l i n e s e x p e r i m e n t a l l y . When the tungsten f o i l a r r i v e s , and i f the new g r a t i n g has.the p r o p e r t y of good u t i l i s a t i o n of l i g h t near M 2 0 0 A ° , the Shenstone hollow cathode wMfeh has been d e s c r i b e d should g i v e the d e s i r e d s p e c t r a w i t h exposures under two hours. The major e x p e r i m e n t a l . d i f f i c u l t y encountered i n t h i s work arose from the long exposures which were;-found n e c e s s a r y . Exposures of one to two hours were r e q u i r e d f o r good p l a t e s of the hydrogen L Q l i n e . Commonly, w i t h ot h e r vacuum spectrographs, exposures of as many minutes were adequate (17, 24, 3 6 ) . . A new g r a t i n g has been ordered which w i l l have a m o d i f i e d groove form to concen-t r a t e r a d i a t i o n i n the d i r e c t i o n of the p l a t e . h o l d e r where I t can be u t i l i z e d . Such a g r a t i n g should reduce exposure times b e t t e r than t e n f o l d . When the copper spark standard l i n e s . h a v e been o b t a i n e d w i t h t h i s new g r a t i n g , . a c c u r a t e measurements, on the Lyman l i n e s e m i t t e d by a l i q u i d a i r . c o o l e d deuterium d i s c h a r g e should prove worthwhile. However, the ac c u r a c y r e q u i r e d w i l l not be e a s i l y a c h i e v e d and the experiment w i l l remain v e r y c h a l l e n g i n g . --50 -B i b l i o g r a p h y A. Handbooks, Monographs..' Textbooks. Tables 1. H. Bomke, Vakuumspektroskopie, ( J . W. Edwards,. Ann Arbor, 1944). 2. W. G r o t r i a n , Graphische D a r s t e l l u n g Der Spektren Von Atomen und Ionen. mit E i n , Zwei und D r e i V a l e n z e l e k t r o n e n , ( J . W. Edwards, Ann Arbor, 1946). 3. G. Herzberg, S p e c t r a of Diatomic Molecules, (Van Nostrand, New York, 1951). 4. T. Lyman, The Spectroscopy of the Extreme U l t r a v i o l e t , (Longmans, Green and Co., London, 1914). 5. J . L u b z i n s k i , M.A. T h e s i s ( B r i t i s h Columbia, 1950). 6. C. E. Moore, Atomic Energy L e v e l s , V o l . 1, ( N a t i o n a l Bureau of Standards, 1949). 7. F. K. Richtmyer, and E. H. Kennard, I n t r o d u c t i o n to Modem P h y s i c s , (Mc.Graw rHill, New-York, 1947). 8. R. A. Sawyer, Experimental Spectroscopy, ( P r e n t i c e - H a l l , New York, 1944). g..John Strong, Procedures i n Experimental P h y s i c s , ( P r e n t i c e - H a l l , New York, 1949). 10. S. Tolansky, High R e s o l u t i o n Spectroscopy, (Methuen, London, 1947). 11. F. Tyren, Nova. A c t a Reg. Soc. S c i . Ups.,Ser.4, Vol.12, No. 1, (1940). - 51 -B. References to I n d i v i d u a l Papers 12. S. S. Ballard, and H. E. White, Phys. Rev., 4J3_, 941, ( 1 9 3 3 ) . 13. Balmer, Ann.d. Physik, 25, 80, (1885). 14. H. A. Bethe, Phys. Rev., J_2, 339, (1947). 15. N. Bohr, Phil.. Mag., 26, 1, (1913). 16. J . C. Boyce, Rev. Mod. Phys., 13., 1, (1941). 17. J . C. Boyce, and C. A. Rieke, Phys.. 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Mag., 21, 6 6 9 , (1911). - 52 -35. E. Schrodinger, Ann. d. Physik, 22» 36:1, 389, 734, (1926). 36. A. G. Shenstone, P h i l . T r ans. R o y . S o c , A751. 195, (1936). 37. A. Sommerfeld, Ann. Phys.,Lpz., _ 1 , 1, (1916). 38. T. Takamine, and T. Suga., Nature, 13_7, 827, (1936). 39. G. E. Uhlenbeck, and S. Goudsmit, Nature, 117. 264, (1926). 40. T. A. Welton, Phys.. Rev., _4 , 1153, (1948). 41 . T. Y. Wu, Phys. Rev., J2, 977, (1947). 

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