We have developed a femtosecond time-resolved luminescence spectroscopy by the optical Kerr-gate ͑OKG͒ method to investigate ultrafast carrier dynamics and relaxation processes of materials. Solid glasses with a high nonlinear refractive index were used as the Kerr media to obtain a subpicosecond time resolution. When a quartz plate was used as the Kerr medium, the Kerr efficiency and the instrumental response time of our spectroscopic system were 5-10 % and ϳ250 fs, respectively. By employing the OKG method, we revealed the internal conversion from S 2 to S 1 state of ␤-carotene with a low fluorescence quantum yield and an ultrafast fluorescence decay time, and the lifetime of the S 2 state was determined to be 210 fs. An advantage of the OKG method relative to the conventional up-conversion technique is its ability to directly obtain time-resolved luminescence spectra, and thus the OKG method might be superior to the up-conversion technique to investigate ultrafast carrier dynamics and relaxation processes of materials.

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The temporal evolution of the photoexcited state in quasi-one-dimensional ͑1D͒ halogen-bridged platinum complexes ͓Pt(en) 2 ͔͓Pt(en) 2 X 2 ͔(ClO 4 ) 4 ͑abbreviated as Pt-X, XϭCl, Br or I͒, has been comprehensively studied by femtosecond time-resolved luminescence spectroscopy. In Pt-Cl, new short-lived hot luminescence is found in the low-energy side of a self-trapped exciton ͑STE͒ luminescence band. The overall behavior of the STE luminescence band within 2 ps is well explained by the vibrational relaxation of the STE. The behavior is reproduced by a model calculation based on wave packet propagation on an interaction mode composed of frequency-dispersed bulk phonons. This model is also applied to the previous results in Pt-Br. For both Pt-Cl and Pt-Br, the frequency spectra of phonons which compose the interaction mode have been estimated. In Pt-I, the STE luminescence decays much faster than those in Pt-Cl and Pt-Br, showing existence of more effective nonradiative decay channel.

The photoinduced effect on the orbital-ordered phase in perovskite-type LaVO 3 has been investigated by femtosecond time-resolved reflection spectroscopy. Large reflectivity change has been observed immediately after photoirradiation. The spectral shape and polarization dependence of the reflectivity change indicate the melting of the orbital order induced by photoexcitation. The photocarriers annihilate in about 200 fs, and the orbital order partially recovers owing to thermalization with the lattice in 2-4 ps.

Using femtosecond time-resolved luminescence spectroscopy, we have observed a wave-packet oscillation in an adiabatic potential well associated with a self-trapped exciton ͑STE͒. We have investigated the STE luminescence band of a quasi-one-dimensional halogen-bridged platinum complex ͓Pt͑en͒ 2 ͔͓Pt͑en͒ 2 Br 2 ͔(ClO 4 ) 4 . In the time evolution of the luminescence, we have observed an oscillation with a period of about 300 fs. This result shows the formation of a wave packet and its oscillation, which is damped with the time constant of about 430 fs. The STE's have a lifetime of about 5.5 ps at room temperature.

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