| Research Abstract |
By using a infrared short pulse with a duration of 55 fs and energy more than 1 μJ/pulse, which is the shortest IR pulse with a high energy, we have developed third order optical nonlinear spectroscopy and applied to vibrational dynamics in condensed phases. The examples are three-pulse photon echo method, wavelength-resolved three-pulse photon echo method, and wavelength-resolved transient grating method. In these methods we can plot a signal as a function of multi-dimensional coordinates such as delay time and frequency. Especially, wavelength-resolved three-pulse photon echo method is a vibrational analogue of two-dimensional NMR. We applied these methods to several vibrational modes in condensed phases to obtain, for example, the time-correlation function of the transition frequency fluctuation, lifetime of the vibrational excited state, reorientational time, and anharmonicity. The examples are the anti-symmetric stretching modes of OCN^-and SCN^-in methanol, the anti-symmetric CN
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stretching mode of Fe(CN)_6^<4->, the CO stretching mode of coordinated to Ruporphyryine, anti-symmetric -N=C=N-stretching mode, and the OH stretching mode of water in a reverse micelle. One of the problems to be overcome for practical use is to obtain very stable IR pulse. At first, we thought that IR short pulse should have an enough high energy, but we found that rather than that shot-to-shot stability of the pulse is more important that the high energy. The present system uses superfluorescence for optical parametric generation (OPG), and this method easily introduces un-stable nature in the pulse. Therefore, we have constructed a new system which uses white light for OPG, and difference frequency mixing set up to generate IR pulses. We have obtained idler pulses in the near IR region from 1.7 to 2.4 mm, signal pulses in the near IR region from 1.1 to 1.5 mm, and IR pulse in the mid-IR region from 3.0 to 5.0 mm. A shot-to-shot stability of the mid IR pulse is about 0.3%, which is almost the best performance ever recorded. Less
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