NONLINEAR-OPTICAL SPECTRA IN HALOGEN-BRIDGED MIXED-VALENT PLATINUM COMPLEXES

Nonlinear optical properties in halogen-bridged mixed-valent platinum complexes are studied theoretically for the first time. Two types of n onlinearities are treated here. One is the electric-field modulation o f the absorption spectrum, and the other is the third-harmonic generat ion. The spectral shapes of these nonlinearities are determined by a m ethod in which both effects of the electron-hole correlation and the l attice fluctuations are taken into account. As for the electric-field modulation, the difference spectrum observed in the visible region for [Pt(en)2][Pt(en)2Cl2)(ClO4)4 is well reproduced with features such as the rigid shift of the main charge-transfer band and the newly appear ing peak due to the parity-even exciton. On the other hand, in the spe ctrum of the frequency-dependent intensity of the third-harmonic gener ation, several enhancements are found: the three-photon resonance stru cture due to the parity-odd exciton and the free elctron-hole continuu m, and the two-photon resonance peak due to the parity-even exciton. I n particular, the last one seems to correspond to that observed in the experiment.