Multiphoton absorption by metal-metal long distance charge-transfer complexes in polar solvents

A theory of multiphoton absorption of mixed valence compounds in polar solvents is developed. When these systems interact with a strong laser field, there exist various absorption ''channels'' which correspond to one, two, three,..., absorbed photon quanta. The probability of each channel to be switched on or off is dependent on the laser intensity, which results in strong intensity dependence of the absorption spectrum. Bands can be eliminated from the spectrum simply by changing the laser intensity. The physical picture of channels is justified by a derivation based on a kinetic master equation for the flow of electronic population. Calculations are carried out for the case where the polar environment can be modeled by a single high frequency (''quantum'') vibrational mode plus a classical bath represented by a collection of low frequency harmonic oscillators. The spectrum is found to be only weakly sensitive to the quantum vibrational mode. Suggestions for experimental verification are made. (C) 1996 American Institute of Physics.