EFFICIENT ABSORPTION-LINE SHAPE CALCULATIONS FOR AN ELECTRON COUPLED TO MANY QUANTUM DEGREES OF FREEDOM - APPLICATIONS TO AN ELECTRON SOLVATED IN DRY SODALITE AND HALO-SODALITES

We present quantum mechanical calculations of the absorption line shap e of an electron ''solvated'' in several sodalites. Photon absorption by the electron modifies the forces acting on the nuclei, setting the counterions in motion. This nuclear motion causes broadening and gives vibrational structure to the absorption spectrum of the electron. The major effort in the computation of the absorption spectrum is directe d toward the evaluation of an overlap integral that evolves in time be cause of nuclear motion. The systems considered here have a very large number of nuclear degrees of freedom, and this makes a brute-force qu antum mechanical calculation of the overlap impossible. Good results c an be obtained with a method that exploits the fact that in a system w ith many degrees of freedom the overlap integral decays rapidly to zer o, and can therefore be evaluated accurately and efficiently by short- time methods. The short-time method that seems most advantageous is th e Gaussian wave packet (GWP) procedure proposed some time ago by Helle r. This simplifies the nuclear dynamics and also substantially diminis hes the number of electron energy calculations needed for determining the forces acting on the nuclei. When the GWP method is used, the elec tronic wave function is calculated only for a small number of nuclear configurations along the classical trajectory on which the center of t he nuclear wave packet evolves. The present calculation is the first u se of this method to compute the absorption spectrum of a complex syst em. We study the absorption line shape for an electron solvated in a d ry sodalite, and in chloro-, bromo-, and iodo-sodalite. We find that t he homogeneous linewidth due to the nuclear motion is narrower than th at observed experimentally. This implies that the measured linewidth i s due to inhomogeneous broadening. For the dry sodalite the main inhom ogeneity is the disorder in the position of the counterions, and for h alo-sodalites, the presence of defects introduced during synthesis. Ou r results imply that a careful synthesis can improve the contrast in d isplays based on the cathodochromic effects in zeolites. (C) 1995 Amer ican Institute of Physics.