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The microwave rotational spectrum of methane in the ground vibronic state Holt, Craig Ward

Abstract

The microwave rotational spectrum of ground state methane has been observed for the first time. Seven ΔJ=0 Q-branch transitions have been observed between 7.8 and 20 GHz with peak absorption coefficients < 6 x 10⁻¹¹ cm⁻¹: these transitions are the weakest ever observed with a Stark-modulated waveguide absorption microwave spectrometer. The spectrometer employed bolometer detectors in a source-noise cancellation scheme. The spectrometer signal was integrated by a signal averager with integration times ranging up to one week. Special experimental techniques are discussed in detail. The bolometer time constant is calculated from hot-wire anemometer heat loss data. The theory required to perform the experiment is presented using the octahedral group 0[sub h] and its rotational subgroup 0 along with the tetrahedral group T[sub d]. An understanding of the groups employed by various workers in the field removes many of the contradictions between them. The symmetric top rotational wavefunction parity introduced by Wang is used to obtain the correct total wavefunction parity from the electronic, nuclear spin, vibrational and rotational wavefunctions. The one quartic tensor distortion constant D[sub T] = 132943.41 ± .71 Hz, the two sextic constants H[sub 4T] =-16.9839 ±.0076 Hz and H[sub 6T] = 11.0342 ±.0086 Hz, and for the first time the three octic distortion constants L[sub 4T] = (20.07 ±.24) X 10⁻⁴ Hz, L[sub 6T] = (-26.77 ±.35) x 10⁻⁴ Hz and L[sub 8T] = (-30.0 ±1.8) x 10⁻⁴ Hz have been determined using the seven transition frequencies measured here, the J=2 ortho-para splitting known for methane and the two J=7 Q-branch transitions measured earlier in an infra-red laser - microwave double resonance experiment. The above errors are standard deviations given by weighted linear least squares analysis. The estimated absolute errors are also given. Term values are presented which allow the accurate calculation of all ground state splittings of methane up to J=21.

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