Thermalization of Fast Cesium Atoms Produced Using Molecular Photodissociation

Oleg S. Vasyutinskii, K. O. Korovin, B. V. Picheyev

A. P. Hickman, Alan D. Streater, Aaron Marks

Aaron Marks, A. P. Hickman, Alan D. Streater +1 more

We have investigated collisions involving fast, excited Cs atoms produced by photodissociating ${\mathrm{Cs}}_{2}$ molecules with a pulsed dye laser. The velocities of the atoms in the $5D$ state formed by the process ${\mathrm{Cs}}_{2}(X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}_{g}^{+})+\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{pump}}\ensuremath{\rightarrow}{\mathrm{Cs}}_{2}^{*}\ensuremath{\rightarrow}\mathrm{Cs}(5D)+\mathrm{Cs}(6S)$ are much greater than typical thermal velocities associated with the cell temperature. Using a narrow-band cw probe laser to observe the increased Doppler broadening of the $5{D}_{3∕2}\ensuremath{\rightarrow}5{F}_{5∕2}$ excitation line shape, we are able to monitor the time evolution of the velocity distribution of these $5D$ atoms. We analyze the data using a model that predicts the time-dependent excitation line shape of the fast atoms. Because the photons used to dissociate the molecules have a well-defined energy, the velocity distribution of the excited atoms in the early time after they are produced can be fairly well determined. Over time, velocity-changing collisions with ground-state Cs atoms cause the velocity distribution of excited atoms to approach the thermal limit. An analysis based on the strong-collision model leads to a prediction that the observed line shape at intermediate times will be a linear combination of contributions from distinct ``fast'' and ``thermalized'' atomic populations. By fitting our data to this model, a rate coefficient for velocity-changing collisions of fast $\mathrm{Cs}(5{D}_{3∕2})$ atoms with ground-state Cs atoms has been determined. The result ${k}_{\mathrm{VCC}}=(6.1\ifmmode\pm\else\textpm\fi{}1.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ corresponds to an effective velocity-changing collision cross section of ${\ensuremath{\sigma}}_{\mathrm{VCC}}^{\mathrm{Cs},\mathrm{eff}}=(1.2\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}$.

K. J. Nygaard, R. E. Hebner

M. Defour, J. Keller, J.-L. Le Gouët

Stimulated photon echoes (three-pulse echoes) produced by broadband (incoherent) light pulses are discussed. A switching time corresponding to the reciprocal inhomogeneous width is predicted for the echo signal when the time interval between the first two time-overlapping, mutually coherent pulses is scanned across 0. The effect is demonstrated in an experiment performed with cesium vapor. A new method for fast sampling of atomic or molecular velocity distribution is proposed with potential applications to photodissociation and photofragmentation studies.

Alexander Z. Devdariani, Andrey N. Klyucharev