Charge-transfer-to-solvent spectrum of potassium iodide and the configuration-coordinate model

Abstract

Accurate digital measurements of the u.-v. spectrum of iodide in water as a function of temperature and H₂O/D₂O composition are described and analyzed in terms of the configuration-coordinate model, which is commonly applied to absorption spectra of colour centres in alkali halide crystals. The low energy absorption band is almost but not exactly Gaussian in shape and the lognormal function is used to obtain accurate resolution of the two bands and to obtain estimates for the moments of the spectrum. The half-width (proportional to the standard deviation) increases with temperature and yields the information that the absorbing centre is coupled to a distribution of modes of the solvent shell. The Huang–Rhys phonon number was found to be equal to 10 and the effective wavenumber of the ground state is 0.49 kK.

The skewness decreases very slightly with increasing temperature. The shift in the position of the low-energy band with temperature is inconsistent with the low value of the skewness observed and must be due to some other mechanism than the difference in the ground and excited state fundamental frequencies, such as the expansion of the solvent with temperature. The polaron, diffuse and confined models all underestimate the fractional change in position if the thermal expansion of the solvent “cage” is equal to that of the bulk solvent.