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The Journal of Chemical Physics

The use of weak magnetic fields in collisions of polarized atoms

Harro A. J. Meijer, Th. Zeegers, T. J. C. Pelgrim, H G M Heideman, Reinhard Morgenstern

Published Jan 15, 1989• 11 citations
Publisher

Abstract

We have experimentally and theoretically investigated the behavior of excited Na atoms under the combined influence of resonant laser light and a weak magnetic field. The use of weak magnetic fields gives another possibility to vary the polarization of the atoms, additional to variation of the polarization of the exciting laser. We calculated in detail the influence of weak magnetic fields on the density matrix describing the excited atoms, using generalized rate equations. We checked the results of our model experimentally, at the Na(3 2 S1/2, Fl=2) →(3 2 P3/2, Fu =3) transition, excited with circularly polarized light. We determined the polarization of the excited atoms, by measuring the fluorescence light at various angles, as a function of both magnetic field strength and laser irradiance. Model and experiment agreed very well. We applied our model to the associative ionization of the excited Na atoms. We measured the ion signal as a function of magnetic field. Analyzing the results, we could determine a specific associative ionization cross section, which is not accessible in our experimental geometry without the use of magnetic fields. Thereby we showed that the application of magnetic fields in polarization dependence studies can provide us with additional information about the process.

DOI
10.1063/1.456151

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Physical Review Letters•Mar 1, 1982•74 citations
We have experimentally and theoretically investigated the behavior of excited Na atoms under the combined influence of resonant laser light and a weak magnetic field. The use of weak magnetic fields gives another possibility to vary the polarization of the atoms, additional to variation of the polarization of the exciting laser. We calculated in detail the influence of weak magnetic fields on the density matrix describing the excited atoms, using generalized rate equations. We checked the results of our model experimentally, at the Na(3 2 S1/2, Fl=2) →(3 2 P3/2, Fu =3) transition, excited with circularly polarized light. We determined the polarization of the excited atoms, by measuring the fluorescence light at various angles, as a function of both magnetic field strength and laser irradiance. Model and experiment agreed very well. We applied our model to the associative ionization of the excited Na atoms. We measured the ion signal as a function of magnetic field. Analyzing the results, we could determine a specific associative ionization cross section, which is not accessible in our experimental geometry without the use of magnetic fields. Thereby we showed that the application of magnetic fields in polarization dependence studies can provide us with additional information about the process.
We have experimentally and theoretically investigated the behavior of Na atoms under the combined influence of resonant laser light and a magnetic field. Linearly polarized light was tuned to the (3s) 2{\mathrm{}}^{2}2S1/2{S}_{1/2}S1/2​ (F=2)\ensuremath{\rightarrow}(3p) 2{\mathrm{}}^{2}2P1/2{P}_{1/2}P1/2​ (F'=1) component of the D1{D}_{1}D1​ line. The intensity of fluorescent light was observed as a function of the laser intensity for various strengths of the magnetic field B. Trapping in the MF{M}_{F}MF​=\ifmmode\pm\else\textpm\fi{}2 levels of the ground state could be observed to be of decreasing importance for an increasing component of B perpendicular to the laser polarization \ensuremath{\epsilon}^. For laser intensities increasing beyond certain critical values, however, the fluorescence decreased and trapping seemed to become again important. For a quantitative explanation of the observed behavior we describe the system by generalized rate equations for the density matrix. We find that stimulated absorption and emission processes effectively inhibit the Larmor precession, thereby enforcing the trapping effect. Precession can only take place at sufficiently low light intensities.
We analyze the dependence of the rate of associative ionization of two radiatively excited atoms on the polarization properties of the exciting light. This provides a direct relation between the observed ion-production rate as a function of the polarization and the ionization rates for various combinations of the magnetic substates. We consider an atomic beam irradiated by a single laser beam, as well as an atomic vapor. The description applies to a recent experimental observation of a strong polarization dependence of the ionization rate in an atomic beam, and at the same time we suggest alternate experimental methods of analyzing the total cross sections of any inelastic or reactive binary collision of excited atoms as a function of the magnetic quantum numbers.
For the first time, associative ionization has been observed in an atomic beam of excited sodium atoms in both the and the state. Relative values for the partial cross sections were obtained specified according to the different magnetic substate combinations of the two fine-structure states. These were found to be comparable to those for associative ionization between two atoms. However, the effect of the excitation on the ion production rate was found to be considerable and to depend strongly on the relative polarizations of the laser beams producing the two populations due to the redistribution of the involved magnetic sublevels. The results indicate that the initial orientation of the electron spin S does not significantly influence the associative ionization reaction at the mean collision energy of 25 meV.
We report a detailed investigation of excited-state alignment-to-orientation conversion in the presence of an external magnetic field. This counterintuitive phenomenon occurs under intermediate-coupling conditions. A weak, linearly polarized, cw laser beam was used to excite and align the Na 3P3/2{\mathit{P}}_{3/2}P3/2​ state in an atomic beam along the z direction. The degree of circular polarization of the resulting fluorescence was detected along the z direction as a function of magnetic-field strength. The spectrally integrated transitions originating from individual F levels of the 3S1/2{\mathit{S}}_{1/2}S1/2​ state yield a maximum circular-polarization fraction of \ensuremath{\sim}40%; integrating the circular polarization over all the allowed 3S1/2{\mathit{S}}_{1/2}S1/2​-3P3/2{\mathit{P}}_{3/2}P3/2​ transitions gives rise to an \ensuremath{\sim}8% effect. The results are predicted by the Breit formula, which is in excellent agreement with our observations.
We consider collisions between (quasi-) one- or two-electron atoms, each of which has been prepared in a fixed distribution of hyperfine magnetic substates. The resulting ``polarization-dependent'' reaction rates are functions of (1) the (experimentally adjustable) orientation of the quantization axis associated with the hyperfine substates, (2) the hyperfine terms of the colliding atoms, and (3) the detailed reaction dynamics as characterized by preparation- and fine-structure-independent cross sections. These effects are deconvoluted and analyzed, and their use in elucidating reaction mechanisms is discussed. Particular emphasis is placed on the associative ionization of two Na(3p) atoms, each prepared by adsorption of resonant laser photons. The quasimolecular states of Na...Na that might react in this case are limited by selection rules. More information on the reaction mechanism is then obtained by using experimental data, in combination with the formal analysis, to draw further conclusions about the active quasimolecular states.
The associative ionization ${\mathrm{Na}}^{*}+{\mathrm{Na}}^{*}\ensuremath{\rightarrow}{{\mathrm{Na}}_{2}}^{+}+e$, when Na atoms in a single thermal beam are excited by linearly polarized light, was observed. The ion yield increases by a factor of 1.7 when the light polarization vector is rotated from a perpendicular to a parallel direction with respect to the beam. A theoretical analysis shows that the ionization rates KKK obey the relations K(12,32)=0K(\frac{1}{2}, \frac{3}{2})=0K(21​,23​)=0 and K(12,12)=1.4K(32,32)K(\frac{1}{2}, \frac{1}{2})=1.4K(\frac{3}{2}, \frac{3}{2})K(21​,21​)=1.4K(23​,23​), where the arguments indicate the initial magnetic substates ∣MJ∣|{M}_{J}|∣MJ​∣ of the two colliding atoms.