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Femtosecond Raman spectroscopy has been developed to investigate ultrafast photoinduced structural changes of materials. Vibrational modes in the photogenerated transient species are measured by stimulated Raman scattering using a Raman pump pulse with narrow bandwidth and a femtosecond supercontinuum probe pulse. The Raman signal can be measured without slowing the temporal response and broadening the spectrum, because the temporal and spectral resolutions of the present method can be improved independently without the restriction of the transform limit. The transient Raman spectra of the trans-cis photoisomerization process in the DCM ͑4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-Pyran͒ dye solution were observed with the resolutions of 250 fs and 25 cm Ϫ1 .

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The ultrafast relaxation kinetics of all-trans-beta-carotene homologs with varying numbers of conjugated double bonds n(n=7-15) and lycopene (n=11) has been investigated using femtosecond time-resolved absorption and Kerr-gate fluorescence spectroscopies, both carried out under identical excitation conditions. The nonradiative relaxation rates of the optically allowed S(2)(1(1)B(u) (+)) state were precisely determined by the time-resolved fluorescence. The kinetics of the optically forbidden S(1)(2(1)A(g) (-)) state were observed by the time-resolved absorption measurements. The dependence of the S(1) relaxation rates upon the conjugation length is adequately described by application of the energy gap law. In contrast to this, the nonradiative relaxation rates of S(2) have a minimum at n=9 and show a reverse energy gap law dependence for values of n above 11. This anomalous behavior of the S(2) relaxation rates can be explained by the presence of an intermediate state (here called the S(x) state) located between the S(2) and S(1) states at large values of n (such as n=11). The presence of such an intermediate state would then result in the following sequential relaxation pathway S(2)-->S(x)-->S(1)-->S(0). A model based on conical intersections between the potential energy curves of these excited singlet states can readily explain the measured relationships between the decay rates and the energy gaps.

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