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Physical Review A

Collisional redistribution by laser-induced fluorescence in the Ba-Ar system

Michael Scott Belsley, J. Cooper

Published Feb 1, 1987• 8 citations
Publisher

Abstract

The effects due to the presence of a (possibly intense) probe laser, which is used to interrogate the asymptotic excited-${m}_{j}$-state distribution resulting from the evolution through a half-collision, are systematically studied. Experimental results are obtained which are in good qualitative agreement with predictions based on a recent theoretical treatment. Finally, a simple interpretation of the relevant physical processes involved is suggested by considering the problem within a dressed-state representation.

DOI
10.1103/physreva.35.1013

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The effects due to the presence of a (possibly intense) probe laser, which is used to interrogate the asymptotic excited-mj{m}_{j}mj​-state distribution resulting from the evolution through a half-collision, are systematically studied. Experimental results are obtained which are in good qualitative agreement with predictions based on a recent theoretical treatment. Finally, a simple interpretation of the relevant physical processes involved is suggested by considering the problem within a dressed-state representation.
Based on the existing four-level model of laser-induced collisional energy transfer, the collision process in Ba-Sr system in both weak and strong fields is calculated by immediate integration of the equation of motion, and profiles of laser-induced transition probability and collision cross section under both conditions are obtained. Numerical results for the weak field are in fair to good agreement with former analytical results, while the ones for the strong field show that the effect of laser-induced collision is enhanced when the transfer laser intensity increases. Moreover, the peak of the laser-induced collisional energy transfer spectrum shows an evident shift from resonant frequency to the violet side in strong field, and full width at half maximum (i.e. tuning range) of the spectrum becomes obviously narrower compared with that in weak field.
We study laser-intensity effects in two-photon collisional redistribution of radiation. Thereby we concentrate on a special case of recent experimental interest, where the transition from the excited to the final state is saturated by a strong laser field. Both exciting lasers are, however, assumed weak in the sense that the dynamics of a collision between the radiator and a perturber is not significantly influenced by the laser fields. In particular, our treatment explains the intensity independence of the polarization dependence of total fluorescence from the final state which has been measured previously even if the transition from the excited to the final state is saturated.
The authors present an investigation of the modification caused by an intense laser field to the rates of m J changing collisions for Hg perturbed by argon. They have studied experimentally the competition between radiative and collisional coupling between m J states with a strong laser field tuned to the Hg J=1 to J=1 transition at lambda 435.8 nm. Population is pumped into the strongly dressed manifold from the ground state with a weak UV 'probe' laser detuned slightly from the lambda 253.7 nm intercombination line. The fluorescence observed at lambda 404.7 nm depends critically on whether the excitation proceeds via collisional redistribution or resonance through Stark shifted levels. The authors present data that clearly exhibit the effects of this competition. The data show in particular how one can produce excitation to transient Stark shifted levels in the presence of collisions. This will be critical in future experiments on collisions in strong fields.
Energy-pooling collisions between low-lying excited states of Ba have been studied. We have used a CW dye laser to excite the level in Ba. Radiative decay and collisions with Ba or buffer-gas atoms (Ar, He) populate also the , and metastable levels. The spatial density distributions of these excited levels were measured by the absorption of lines from a Ba hollow-cathode lamp as a function of laser frequency. When the laser was on the exact atomic resonance we observed fluorescence from the , , , , , , and states populated by energy-pooling collisions of Ba( , ). By changing the buffer gas pressure the fluorescence intensity ratio between - and - transitions changes dramatically. This is found to be due to redistribution of population among metastable levels, both in magnitude and space. In addition, we have found that the and metastable levels are also populated when the laser is tuned to the blue from the 553.5 nm line under which conditions a pronounced red-shifted light cone (cone emission) is observed.
Recent experimental results of Havey, Copeland, and Wang on the collisional redistribution of light by Na in Ar showed a dependence of the relative intensities of the Na D1{D}_{1}D1​ and D2{D}_{2}D2​ resonance lines on laser detuning. Calculations of these intensity ratios are reported here. The agreement between theory and experiment obtained here shows the importance of fine-structure transitions occurring during the dissociation of the excited-state molecule.