Trapped NH2 Radicals at 4.2°K

Abstract

A study has been made of the electronic absorption spectrum of the NH₂ radical trapped in an argon matrix on a liquid helium cooled surface. The radical is produced in a microwave discharge of argon mixed with a small amount of parent molecule, ammonia or hydrazine. The long effective absorbing path is illustrated by the presence of new bands of NH₂, too weak to have yet been observed in the flash photolysis spectrum. That the radicals have also been found in moderate concentration from low‐pressure discharges containing hydrogen and nitrogen in many forms indicates that they are easily built up from smaller fragments, a fact which may be important in the explanation of the origin of these radicals in comets. The radical is positively identified by the excellent correlation between the low temperature spectrum and the gas‐phase spectrum recently analyzed by Ramsay, and in agreement with this analysis only alternate bands appear strongly at 4.2°K. The observed features, which represent rotational lines of the electronic transition, surprisingly enough, are less than 3 cm^—1 wide, the lack of broadening indicating that the radicals are situated in equivalent matrix sites, possibly in an ordered argon crystal lattice, and evidence is presented which shows that NH₂ experiences nearly free end‐over‐end rotation in the matrix. The experimentally observed small population of the lowest state of para‐NH₂ shows a breakdown of the usual s↮a selection rule for exchange of identical nuclei. A determination from relative intensity measurements of interaction constants for Fermi resonance between certain ${\text{0ν}}_2^{\prime }0$ and ${\text{1(ν}}_2^{\prime }\text{−4)0}$ levels shows that the intensity of the latter is caused primarily by the resonance and not by the Franck‐Condon effect.