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IEEE Lasers and Electro-Optics Society Annual Meeting (LEOS)

Ultrafast spectroscopic tool for studying molecular dissociation

Andrey A. Senin, Zhongling Lu, J.R. Allen, James Gary Eden

Published Jun 26, 2003• 1 citation
Publisher

Abstract

In summary, the results of the time-frequency analysis of the output radiation suggest that the dissociation of electronically-excited molecules can be detected with an atomic wavepacket and four-wave mixing. The arrival of the dissociation transient is evidenced by changes in the temporal history of frequency components of the wavepacket. Thus, the technique presented in this work appears to be a novel, powerful spectroscopic tool for studying the dissociation of molecules. The potential of this technique for other molecules and optoelectronic materials (such as rare earth-doped crystals) will be discussed.

DOI
10.1109/leos.2002.1134030

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In summary, the results of the time-frequency analysis of the output radiation suggest that the dissociation of electronically-excited molecules can be detected with an atomic wavepacket and four-wave mixing. The arrival of the dissociation transient is evidenced by changes in the temporal history of frequency components of the wavepacket. Thus, the technique presented in this work appears to be a novel, powerful spectroscopic tool for studying the dissociation of molecules. The potential of this technique for other molecules and optoelectronic materials (such as rare earth-doped crystals) will be discussed.
We demonstrate that atomic wavepackets can serve as sensitive detectors for investigating atomic and molecular dynamics. In concert with parametric four-wave mixing, the interference between coherent superpositions of atomic excited states produced by ultrafast (/spl sim/150 fs) pump and probe pulses provides a new and powerful tool with which fundamental processes, such as molecular dissociation and Rydberg-Rydberg atomic collisions, can be observed with the extraordinary sensitivity afforded by a coherent nonlinear optical process. Experiments are described in which the dissociation of an electronically excited molecule (Rb/sub 2/) and the distribution of atomic fragments into excited states spanning >10 000 cm/sup -1/ are observed. Also, resonant collisions between Rb atoms in the 7s and 5d states are detected by monitoring the shift in the frequency of an atomic wavepacket induced by the dipole-dipole interaction.
Wavepackets are excited in rubidium vapor by intense ultrashort (∼100 fs) pulses centered at a wavelength of 620 nm or 770 nm. The generated atomic wavepackets are optically detected via a pump-probe experiment based on axially phase-matched parametric four-wave mixing (FWM). This nonlinear process results in coherent ultraviolet (UV) or violet emission that carries information about the relative number densities of the excited states. The frequency spectrum of the UV/violet signal intensity, recorded while varying the pump-probe time delay, shows the presence of quantum beats between energy levels involved in the FWM process. The time-frequency analysis of that temporal UV/violet data is done by means of the short-time Fourier transform that leads to retrieval of the temporal evolution of amplitude and phase of the quantum beats. The observed dynamics suggest that not only atomic but also molecular ( Rb*2 ) wavepackets are generated, and that the temporal history of the amplitude of the quantum beats reflects the production of the excited atomic fragments of Rb*2 dissociation. More precisely, changes in the Rb excited state number densities alter the third-order nonlinear susceptibility chi(3) of the Rb vapor, which affects generation of the output signal wave by FWM. Dissociation into several product states is detected, and their branching ratios are estimated. Thus, atomic wavepackets, combined with FWM, appear to be a sensitive new multichannel spectroscopic means to study the fundamental process of molecular dissociation.