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Physical Review A

Non-Markovian correlation effects in time-delayed femtosecond absorption spectroscopy

W. Vogel, D.‐G. Welsch, B. Wilhelmi

Published May 1, 1988• 47 citations

Abstract

The dependence of the electronic absorption spectrum of a dye molecule on the time of delay between the excitation of the molecule by a pump-laser pulse and the probing of the absorption by a test-laser pulse is studied. The effect of dephasing of the molecular transition arising from the interaction with dissipative systems is described by a stochastic modulation of the electronic transition frequency. It is shown that in the case when the two laser pulses are short compared with the correlation time of the frequency modulation and when the spectral linewidth of the pump-laser light is smaller than the bandwidth of the equilibrium absorption spectrum, hole burning appears although the molecules have equal mean transition frequencies. This effect arises from non-Markovian correlations between the dephasings of the molecule during the actions of the pump- and the test-laser pulse. With increasing delay time the holes are smoothed out and disappear when the delay time exceeds the correlation time. Such effects were found in recent femtosecond absorption experiments by Shank et al. [in Ultrafast Phenomena V, dited by G. R. Fleming and A. E. Siegman (Springer, Berlin, 1986), p. 179].

DOI: 10.1103/physreva.37.3825

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Femtosecond photon-echo experiments on sodium resorufin in dimethylsulfoxide at room temperature show that optical dephasing in solution is of non-Markovian character. A single Gauss-Markov stochastic modulation process is used to interpret both the femtosecond light-scattering results and the steady-state absorption spectrum. The optical dynamics of this system appears to fall in the intermediate modulation regime. No evidence was found for contributions of slower dynamical processes to the absorption line breadth.

Electronic dephasing of solutions of all-trans-bis-(dimethylamino)-heptamethinium chloride, its photoisomer, and of 1,

{1}^{|\mathrm{I}\mathrm{H}}

,3,3,

{3}^{|\mathrm{I}\mathrm{H}}

,

{3}^{|\mathrm{I}\mathrm{H}}

-hexamethylindotricarbocyanine iodide (HITCI) is studied using forced light scattering by broad-bandwidth incoherent pump lasers in the wavelength region from 440 to 770 nm. The dephasing slows down as the pump laser wavelength is tuned over the 0-0 transition. Photoisomer and parent heptamethine molecule show comparable behavior in dephasing experiments. Absorption line-shape analysis including resonance Raman experiments indicates a considerable spectral cross relaxation in the absorption band of the parent molecule. The results for HITCI are close to those reported for two-pulse femtosecond photon echoes.