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Femtosecond time-resolved absorption spectroscopy

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Abstract

Femtosecond time-resolved absorption spectroscopy is discussed in terms of a simple model in which the collision complex is regarded as a time- dependent dipole oscillator. This model is applied to recent measurements of the absorption spectra molecules in the process of dissociating. When the dissociation process yields a narrow wave packet, the dominant feature in the absorption spectra is generally a sharp feature at the transition frequency for separated atoms. The spectrum in this impact region reflects an effective phase- shift for a partial collision which begins at the time of excitation by the probe pulse. By inspection of the data, one can directly see when the atoms have reached the Weisskopf radius, the point where the partial collision provides a phase- shift of 1 radian. Moreover, by measuring the phase-shift as a function of pump-probe delay, one can recover the molecular transition frequency along the path of dissociation. For broad wave packets, there can be a large phase-shift spread, and the spectra approach a statistical limit, in which the absorption profile directly reflects the distribution of molecular transition frequencies at the time of excitation by the probe pulse. Measurements of the spectral power density of the transmitted probe pulse, are compared and contrasted with measurements based on laser-induced fluorescence.

© 1991 Optical Society of America

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