VIBRONIC INTENSITIES IN ELECTRONIC-SPECTRA INVOLVING EXCITED-STATES WITH DOUBLE-MINIMUM POTENTIAL SURFACES

Electronic spectra involving excited states with double-minimum potential surfaces are calculated by using the split-operator technique for numerical integration of the time-dependent Schrodinger equation, and the time-dependent theory of electronic spectroscopy. Absorption spectra to an excited state with a double-minimum potential surface contain unusual intensity distributions in the vibronic structure. Detailed spectra are calculated and trends are analyzed in terms of the width of the barrier and the width of the initial wave packet. Emission spectra from an excited state with the double-minimum surface to a harmonic ground state are calculated. Wave functions of the double-minimum surface are calculated and propagated on the ground-state surface. The doubling of the eigenvalues and the temperature effects on the spectra are calculated. The theory is applied to the spectroscopy of K2[PtCl4]. In this molecule the active asymmetric b1g mode is represented by the double-minimum potential. A two-dimensional surface consisting of the b1g Pt-Cl stretch and the totally symmetric Pt-Cl stretch is constructed. Both the absorption and the emission spectra including the unusual spectroscopic features of the MIME (missing mode effect) and the apparent "energy gap" between the emission and the absorption spectra are calculated. The molecule has D2h symmetry in its lowest excited electronic state with one pair of Pt-Cl bonds enlongated by 0.14 angstrom and the other pair contracted by 0.02 angstrom relative to the ground-state distances.