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Journal of Physics B Atomic Molecular and Optical Physics

Laser-assisted collisions: Investigation of the optical collision process Na(3s)+Kr+h nu to Na(3p)+Kr in a crossed beam experiment

Joachim Großer, D Gundelfinger, A. Maetzing, Wolfgang Behmenburg

Published Jul 28, 1994• 10 citations
Publisher

Abstract

We measured the angular distribution of the excited Na atoms from the process Na(Ss)+Kr+h nu to Na(3p)+Kr. The photon energy is detuned by 50 cm -1 from the Na resonance energy and the optical transition can occur only when the atoms are close to each other. The angular resolution allows the observation only of the general structure of the cross section, but not of finer details. We describe the experimental method and the experimental difficulties, especially the origin of a number of competing signals. We discuss the process as a new way of observing the collision Na+Kr, using the photon as a probe.

DOI
10.1088/0953-4075/27/14/011

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We measured the angular distribution of the excited Na atoms from the process Na(Ss)+Kr+h nu to Na(3p)+Kr. The photon energy is detuned by 50 cm -1 from the Na resonance energy and the optical transition can occur only when the atoms are close to each other. The angular resolution allows the observation only of the general structure of the cross section, but not of finer details. We describe the experimental method and the experimental difficulties, especially the origin of a number of competing signals. We discuss the process as a new way of observing the collision Na+Kr, using the photon as a probe.
The optical collision Na(3s)+Kr+hν→Na(3p)+Kr has been investigated in an experiment with crossed atomic beams and with a differential detection of the excited collision products. Three groups of experimental results have been obtained, which all demonstrate unprecedented possibilities for the observation of atomic collisions. 1. The oscillatory structure of the differential cross sections allows a sensitive test of the molecular potentials. 2. Polarization experiments give direct information about the geometric characteristics of the collision complex. 3. State analysis of the collision products provides a tool for the observation of elementary nonadiabatic processes. A number of possible future experiments, which make use of the new ideas, is discussed, in addition.
An experimental investigation of electron - impact induced transitions from the laser - excited 3P state of sodium atom to the 4S and 3D excited states is presented, the main objective being measurement of the atomic collision parameters ( ACPs ) that are related to individual scattering amplitudes. These parameters are complementary to the differential excitation cross - sections and provide finer details on the collisional excitation process. The experiment involved the preparation of the 3P - state with coherent laser radiation of a particular polarisation and measuring the rate of electrons scattered from the laser - excited state as function of laser polarisations. The ACPs were then deduced from these measurements and optical pumping parameters, which account for the optical preparation of the 3P - state. To formulate different measurement schemes, a measurement theory based on the general description of transitions between states of different angular momentum using density matrix formalism is derived. The investigation of the 3P - 4S transition is very similar to the studies of the 3S - 3P excitation performed by superelastic method involving the 3P - 3S scattering in the 'time - inverse' geometry. 'Standard' ACPs are used and, similarly to the superelastic method, the measurements for the 3P - 4S excitation performed in the 'time - inverse' geometry also reveal ACPs for the 4S - 3P de -excitation process. For the more complex 3P - 3D transition, however, the 'standard' ACPs may be less informative. It is shown that certain sums of individual magnetic sublevel excitation probabilities can be measured which may be considered as atomic collision parameters for transitions between P and D - states.
We measured the cross section for the collision transfer process Na(4D)+Na (3S)Na(4F)+Na(3S). The 4D state was excited by two photon absorption using a tunable dye laser. Since the direct 4F→3D fluorescence can not be detected with our apparatus, we monitored instead the cascade 3D→3P and 4D→3P fluorescence. Relative intensity of the fluorescence, combined with the measurement of sodium density, yielded cross section σ_(4D)→(4F)=1.3×10~(-14)±28％(cm~2). Our result is compared to other experiments.
We report our measurements of the cross sections ${\ensuremath{\sigma}}_{5S}$ and ${\ensuremath{\sigma}}_{4D}$ for the electronic-energy-transfer process $\mathrm{Na}(3P)+\mathrm{Na}(3P)\ensuremath{\rightarrow}\mathrm{Na}(5S,4D)+\mathrm{Na}(3S)$. To obtain these cross sections we have measured the fluorescence intensity for the transitions $5S\ensuremath{\rightarrow}3P$, $4D\ensuremath{\rightarrow}3P$, and $3P\ensuremath{\rightarrow}3S$ in sodium vapor excited with a cw dye laser. The study of this electronic energy transfer is complicated and there is a large disagreement (\ensuremath{\gtrsim}104{10}^{4}104) among reported measurements. We believe we have avoided most of the difficulties of previous experiments. Our results are ${\ensuremath{\sigma}}_{5S}=(2.0\ifmmode\pm\else\textpm\fi{}0.7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ cm2{\mathrm{cm}}^{2}cm2 and ${\ensuremath{\sigma}}_{4D}=(3.2\ifmmode\pm\else\textpm\fi{}1.1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{\ensuremath{-}2}$, and they are compared with the results of other experiments.
We observe the production of Na2+ and Na+ arising from single collisions between crossed beams of sodium atoms when a laser field is tuned near the Na(3p 2P3/2) and Na(3p 2P1/2) transitions. Measurements of ion intensity vs laser intensity show that at moderately high power true laser-induced processes dominate over purely collisional effects. Relative intensity of mass-selected ions produced at either member of the Na resonance doublet shows conclusively that Na+ does not arise simply from photodissociation of Na2+ but must result from a direct, laser-induced collisional ionization.