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Physical Review A

Time-resolved luminescence from coherently excited molecules as a probe of molecular wave-packet dynamics

Pawel Kowalczyk, Czesław Radzewicz, Jan Mostowski, Ian A. Walmsley

Published Nov 1, 1990• 67 citations
Publisher

Abstract

We calculate the time-dependent spectrum of spontaneously emitted light from a coherently excited diatomic molecule. We show that this spectrum provides a tool for direct observation of the atomic vibrations in the excited electronic state of the molecule, and that the finite time and frequency resolution inherent in any experiment does not destroy all information about the time-dependent shape of the molecular wave packet.

DOI
10.1103/physreva.42.5622

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We calculate the time-dependent spectrum of spontaneously emitted light from a coherently excited diatomic molecule. We show that this spectrum provides a tool for direct observation of the atomic vibrations in the excited electronic state of the molecule, and that the finite time and frequency resolution inherent in any experiment does not destroy all information about the time-dependent shape of the molecular wave packet.
A method is presented for reconstructing the quantum state of vibrational wave packets in diatomic molecules by a heterodyne experiment. After the preparation of a wave packet, a femtosecond laser pulse interferes with the induced polarization and the resulting signal is frequency filtered and measured by a photodetector. The conditions are analyzed under which the resulting time- and frequency-resolved photocurrent contains the information needed to recover the quantum state of the prepared wave packet. It is shown that the vibrational wave function in the excited electronic state can be measured directly without the need of involved numerical reconstruction. Simulations of the proposed experiment are performed for sodium dimers.
It is well known that the application of a pulse to a molecular electronic transition leaves the molecules vibrational modes in different electronic states in well-localized states, or wave packets[1]. One method of detecting the wave packets is via the time-dependent spectra of spontaneous emission from the excited molecule[2]. The emission consists in general of two parts, an incoherent part that depends only on the wave packet in the excited state, and a coherent part, that depends on the wave packets in both excited and ground electronic states (of a two-state molecule). This can be seen from the following simple calculation.
A consistent quantum theory of a two-pulse pump-probe experiment in the femto-and picosecond spectroscopy of diatomic molecules is developed. The theory aims at the use of weak quantum femtosecond light pulses to excite molecular coherence. It is suggested that the experiment be realized using a double-frequency excitation scheme that is feasible for Na 2 molecules. On the basis of the theory developed, the luminescence signals are calculated at different temperatures and different optical densities of the medium for the time range from 100 fs to ~ 100 ps. The results obtained show the presence of a strong dependence of the time and frequency spectra of a luminescence signal on the optical density of the medium at temperatures from 50 to 300 K. The proposed approach makes it possible to use the quantum coherence properties of optically dense media for more detailed study of the vibronic dynamics of diatomic molecules, in particular, detection of weak optical transitions.
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