An infrared study of lithium-ammonia and potassium-ammonia complexes trapped in solid argon

Abstract

Condensation of ammonia and Li or K atoms in solid argon led to the spontaneous formation of numerous products classified in three groups. The first group is constituted by mononuclear species M(NH 3) n mainly characterized by the strongly intensity enhanced ν 1 (A 1 stretching) and ν 2 (A 1 bending) modes of ammonia. For M=Li and n=1, 6Li/ 7Li and H/D, isotopic substitution allows complete assisgnment of the spectrum and force field calculation to be made. For increasing values of n, the frequency decrease of the ν 1 mode suggests an electron transfer reverse as compared to the one in the 1 : 1 complex, namely from M towards ammonia molecules, which corresponds to the situation described for dilute solution of alkali metal atoms in liquid ammonia. The second group involves species with several metal atoms and one ammonia molecule, in which the ν 1 mode is also red shifted with respect to its position in the 1 : 1 complex. Since the same trend for electron transfer as in the 1 : 1 case (from ammonia towards metal cluster) is expected, one is led to the conclusion that both kinds of electron transfer, from M towards ammonia (1: n species case) or from ammonia towards M ( m: 1 case) give rise to the same vibrational perturbations. At last, at high concentration in both dopants, the spectral changes, mainly characterized by band broadening in the ν 1 and ν 2 spectral regions, are not easily interpreted.