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Electron spin resonance studies of NO₂ trapped in inert matrices at 4°K Hutchinson, Douglas Allen

Abstract

A 9 K Mc electron spin resonance spectrometer has been constructed and has been used in conjunction with a specially designed liquid helium dewar. Provision was made for condensing free radicals from the gas phase and for generating radicals by the irradiation of low temperature deposits. Experiments were performed in which monomeric NO₂ was trapped at 4°K in a series of different matrices. An attempt is made to explain the observed spectral lines in terms of magnetic interactions due to nuclear spins and electrostatic interactions due to neighbouring matrix particles. The equation which gives the electron spin resonance spectral lines as a function of magnetic field is [formula omitted] Thus an esr spectrum is characterized by two numbers, the hyperfine splitting constant A and the g value. When a free radical is trapped in an inert matrix, shifts occur in the hyperfine splitting constant and in the g value. Adrian developed a semiquantitative theory to explain these effects for hydrogen atoms trapped in inert matrices. This theory considers two important interactions between the free radical and matrix particle. The van der Waals interaction leads to a negative shift in A, while overlap effects lead to a positive shift in A and a negative shift in g value. Electron spin resonance spectra were obtained for nitrogen dioxide trapped in argon, methane and nitrogen at 4°K. Attempts were made to employ the ideas of Adrian's theory to explain the esr spectra obtained from these systems. Partial success was achieved in explaining the form of the spectra obtained. The divergences between the predictions of the theory and the experimental results leads to a questioning of the assumptions made and the approximations used in developing the theory.

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