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Physical Review A

Production of "hot" excited-state atoms in collisionally aided radiative transitions

E. Giacobino, M. Tawil, P. R. Berman, Olav Redi, H. Henry Stroke

Published Oct 1, 1983• 14 citations
Publisher

Abstract

Collisionally aided radiative excitation of atoms using a laser detuned to the blue is shown to lead to heating of the external degrees of freedom. The velocity distribution of the excited atoms is studied experimentally and compared with theoretical predictions.
DOI
10.1103/physreva.28.2555

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Populations of excited atoms that all have the same z component of velocity can be produced by pumping a vapor with a narrow-band laser. This velocity-selected population is then thermalized by velocity-changing collisions. However, in a pure vapor, even at low densities where velocity-changing collisions can be ignored, the mechanisms of radiation trapping and resonance exchange collisions can still lead to substantial thermalization of excited-atom velocity distributions. In this paper, we present data demonstrating these effects. We compare our data with results from a simple model of these processes. The model is in qualitative agreement with the experimental results.
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Collisional loss rates of ultracold atoms in a laser trap (formed by red-detuned laser beams) can be suppressed by using a sufficiently intense blue-detuned optical shielding laser to repel the atoms before they collide. We present calculations that reveal an additional mechanism for collisional loss in the presence of a blue-detuned laser that reduces optical shielding as the blue-detuned laser intensity increases, and even causes enhancement of collisional loss effects with increased blue-detuned intensity. This mechanism can strongly affect the saturation of the optical shielding.
We report the results of an experimental study of the effects of velocity-changing collisions on two-photon and stepwise-absorption line shapes. Excitation spectra for the $3{S}_{\frac{1}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}\ensuremath{\rightarrow}4{D}_{\frac{1}{2}}$ transitions of sodium atoms undergoing collisions with foreign gas perturbers are obtained. These spectra are obtained with two cw dye lasers. One laser, the pump laser, is tuned 1.6 GHz below the $3{S}_{\frac{1}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}$ transition frequency and excites a nonthermal longitudinal velocity distribution of excited 3P123{P}_{\frac{1}{2}}3P21​​ atoms in the vapor. Absorption of the second (probe) laser is used to monitor the steady-state excited-state distribution which is a result of collisions with rare gas atoms. The spectra are obtained for various pressures of He, Ne, and Kr gases and are fit to a theoretical model which utilizes either the phenomenological Keilson-St\"orer or the classical hardsphere collision kernel. The theoretical model includes the effects of collisionally aided excitation of the 3P123{P}_{\frac{1}{2}}3P21​​ state as well as effects due to fine-structure state-changing collisions. Although both kernels are found to predict line shapes which are in reasonable agreement with the experimental results, the hard-sphere kernel is found superior as it gives a better description of the effects of large-angle scattering for heavy perturbers. Neither kernel provides a fully adequate description over the entire line profile. The experimental data is used to extract effective hard-sphere collision cross sections for collisions between sodium 3P123{P}_{\frac{1}{2}}3P21​​ atoms and helium, neon, and krypton perturbers.