VIBRONIC STRUCTURE INDUCED IN SPIN-FORBIDDEN TRANSITIONS IN EMISSION AND ABSORPTION-SPECTRA BY EXCITED-STATE COUPLING

Spectroscopic effects of spin-orbit coupling of excited-state potential surfaces are calculated by using the numerical integration of the time-dependent Schrodinger equation and the time-dependent theory of electronic spectroscopy. Intensity borrowing by a spin-forbidden transition from a nearby spin-allowed transition is calculated in terms of amplitude transfer of the wave packet between states. The main emphasis of the calculations is to analyze the vibronic structure in emission and absorption spectra arising from coupled surfaces. The coupling causes dramatic changes in both the relative intensities of the vibronic bands and the spacings between members of a progression. These changes are quantitatively calculated, and the theory is applied to the spectra of transition-metal complexes. The intensity and spacing between vibronic peaks in the absorption spectrum of K2NiO2 are calculated and analyzed. A striking example of relative intensities in vibronic peaks induced by spin-orbit coupling is found in the emission spectra of d2 and d3 metal ions in octahedral environments where the lowest energy spin-forbidden transitions arise from a change in the spin state with no change in the orbital component. Short progressions in totally symmetric modes are frequently observed even though no changes in the orbital populations, bond properties, or force constants are expected. The vibronic structure in spectra of Ti2+, V3+, Cr3+, and Mn4+ ions in octahedral halide lattices is analyzed.