Ultraviolet Spectra of Alkali Halides in Inert Matrices

Ultraviolet absorption spectra have been observed for some of the alkali halides in several inert matrices at wavelengths between 2000 and 4500 Å. The matrix‐isolated molecules exhibit simpler spectra than those in the gas phase, permitting vibrational analysis. The LiI spectrum contains a strong absorption showing at least nine peaks with a mean spacing of 363 cm⁻¹ in argon and 367 cm⁻¹ in krypton, at wavelengths below 2900 Å. In addition, LiI exhibits one broad structureless peak near 3500 Å in both matrices. The NaI spectrum shows a weak absorption with as many as five peaks just below 4000 Å. The mean spacings are 166 cm⁻¹ in argon, 129 cm⁻¹ in kyrpton, and 141 cm⁻¹ in nitrogen. NaBr exhibits a strong absorption at wavelengths below 3100 Å showing as many as 16 peaks with mean spacings of 453 cm⁻¹ in argon, 529 cm⁻¹ in krypton, and 472 cm⁻¹ in nitrogen. LiBr, KI, NaCl, and RbBr show no absorption in this region at similar concentrations. The observed absorption peaks are interpreted as arising from transitions between the ground states ${(}^1{\Sigma }^{+})$ and different vibronic levels of the first excited 0⁺ bound states of these molecules. The structureless peak near 3500 Å in LiI probably represents a transition to a purely repulsive state. The appearance of absorption in LiI in a region where none existed in the gas phase is attributed to a change in Franck–Condon factors induced by the matrix. The results are interpreted in terms of a charge transfer model of molecular absorption and support the interpretation of adiabatic behavior when the ionic and atomic potential curves cross at a relatively short internuclear separation; the smaller the internuclear distance at the crossing, the more likely the noncrossing rule is obeyed. The unexpected appearance of discrete structure in the NaBr spectrum is discussed in terms of matrix‐induced charge transfer.