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Physical Review A

Laser-induced collisional avalanche in atomic cesium

Alan Lenef, D. Kreysar, K. Obermyer, Stephen Colby Rand

Published Mar 1, 1995• 2 citations
Publisher

Abstract

Radiation tuned to an excited-state transition in dense cesium vapor results in the appearance of induced absorption and fluorescence after delays of up to 5 s, an observation ascribed to a chain reaction of resonant binary collisions initiated by the incident light. Our results occur in a nonlinear regime characterized by high collision rates, which are variable in a controlled fashion at constant vapor density.

DOI
10.1103/physreva.51.r1731

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AIP conference proceedings•Jan 1, 1993•3 citations
Radiation tuned to an excited-state transition in dense cesium vapor results in the appearance of induced absorption and fluorescence after delays of up to 5 s, an observation ascribed to a chain reaction of resonant binary collisions initiated by the incident light. Our results occur in a nonlinear regime characterized by high collision rates, which are variable in a controlled fashion at constant vapor density.
Light tuned to the 6p 2P½ → 8s 2S1/2 excited state transition wavelength 761.09 nm in a region of (ground state) spectral transparency of Cs causes abrupt appearance of population in the 6p state after delays of up to five seconds following the introduction of radiation into the vapor cell. These nonlinear dynamics occur at laser intensities well below that necessary for laser-induced plasma formation or saturation, and only at vapor densities exceeding 1016 cm-3. The temporal evolution of p state fluorescence is reproduced by calculations of a laser-induced critical reaction (avalanche) involving resonant energy exchange between excited 6d 2D3/2 and ground state atoms during collisions. Experimental threshold density and the variation of avalanche delay with incident intensity are shown to be in good agreement with theoretical expectations based on measured excited state collisional cross-sections. Implications of this effect, in which the rate of excited state collisions can be varied optically at constant vapor density, for unique studies of local fields in nonlinear optical interactions of dense systems and collisional contributions to excited state dephasing are suggested.
Using stepwise excitation of the cesium 8S atoms level in Cs vapor,several collisional and radiative processes have been identified from the intensities and spectra of the Cs fluorescence.The 5D state was populated by 8S→7P→5D transitions.In cesium densities 1016-1017?cm-3,the energy pooling collisions 5D+5D→nL+6S(nL=9D,11S,7F are studied.Since the 5D→6P(3.0-3.6 μm) fluorescence could not be detected with this experiment,the EP rate coefficients have been measured relative to the known rate coefficient of the collision,i.e.,6P+5D→6S+7D.The excited-atom density and spatial distribution were mapped by monitoring the absorption of a counterpropagating single-mode laser beam,tuned to 6P3/2→8S1/2 transition,which could be translated parallel to the pump beam.The excited atom densities are combined with measured fluorescene ratios to determine cross sections for the cesium energy pooling process.The average values(in units of 10-10?cm3s-1) for nL=9D,11S and 7F are 8.4±4.2,7.3±3.6,and 9.7±4.8,respectively.Influence on k11S and k7F of the energy transfer process 11S+6S7F+6S is also dicussed.
Enhancement and reduction of the fluorescence intensity in Rb atomic vapor are observed. The reduction is due to a depopulation of the 5P - {3/2 } state upon illumination of a Rb vapor cell with light corresponding to the 5P - {3/2 }→5D - {5/2 } transition. Collisional transfer of the 5D - {5/2 } atoms to the 5D - {3/2 } state, followed by radiative decay to the 5P - {1/2 } state, creates an increase in the population of the 5P - {1/2 } state. The increase in the excited state population results in an increase in the fluorescence intensity of the 5P - {1/2 }→5S - {1/2 } transition. The laser-induced population changes are monitored by the decrease or increase of the corresponding fluorescence signals. Using the steady-state rate equations, the rate constants for the stimulated emission(i.e., 5D - {5/2 }→5P - {3/2 }) is 2.0×10 +8s + {-1 }. The cross section for the fine-structure changing collision(i.e., 5D - {5/2 }→5D - {3/2 }) is 6.4×10 + {-14 }cm +2.
We report an application of laser spectroscopy techniques to a study of collision-induced interactions in atomic excited states. Due to pulsed dye laser excitation a considerable selective population of highly excited states of ccsium is obtained. Collision-induced transfer of excitation energy between the excited states results in modifications in time and spectral characteristics of observed atomic fluorescence. Quantitative information on the efficiency of collisional processes can be obtained from the measurements of time constants of the time-resolved fluorescence signals and/or from those of the integrated intensities of the fluorescence lines. Both possibilities are used in the present work. Perturbation of nD (n=8-14) and nS (n=1O-15) states of cesium in collisions with noble-gas atoms is investigated. The cross sections for the transfer of excitation between fine-structure substates of the nD states (J mixing) and for the nS-(n-4)F intermultiplet transfer are obtained.