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The Journal of Physical Chemistry A

Ground State Polarized Photofragments Study by Using Resonance and Off-Resonance Probe Beam Techniques

B. V. Picheyev, Andrey G. Smolin, Oleg S. Vasyutinskii

Published Oct 1, 1997• 66 citations
Publisher

Abstract

The optical magnetic birefringence (Faraday) technique that can be used to study the anisotropy distributions of the angular momenta of photofragments produced in molecular photodissociation is compared theoretically and experimentally with the magnetic dichroism technique. The frame independent tensor form of the general expression describing the difference between an initial and a final polarization matrix of a probe light beam passing through the photofragment vapor is presented and analyzed. The procedure of detection of the photofragment electron orientation and alignment angular distributions by Doppler spectroscopy using each of the techniques is studied theoretically. The angular independent anisotropy orientation and alignment parameters describing photodissociation dynamics that can be determined from these studies are presented. It is shown that in principle each of the techniques can give the same information about the photodissociation dynamics; however, the birefringence technique has an undeniable advantage since it leads to an increase in accuracy of the experimental data. Both magnetic birefringence and magnetic dichroism techniques were used experimentally to study the spin orientation of ground state Rb atoms produced in photodissociation of RbI at 266 nm by circularly polarized laser light. The experiment was not of the Doppler spectroscopy type, but use of different 85 Rb and 87 Rb isotopes in the dissociation cell and in the probe light source allows detection of the oriented atomic photofragments separately by each of the techniques. Obtained experimental results are in agreement with the theoretical predictions. Contribution of the orientation of the Rb hyperfine structure energy states to the observed signals were also studied and treated theoretically. The initial degree of the electron spin orientation of the Rb atoms produced in the reaction was found to be P e = 0.11 ± 0.02.

DOI
10.1021/jp971287y

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The optical magnetic birefringence (Faraday) technique that can be used to study the anisotropy distributions of the angular momenta of photofragments produced in molecular photodissociation is compared theoretically and experimentally with the magnetic dichroism technique. The frame independent tensor form of the general expression describing the difference between an initial and a final polarization matrix of a probe light beam passing through the photofragment vapor is presented and analyzed. The procedure of detection of the photofragment electron orientation and alignment angular distributions by Doppler spectroscopy using each of the techniques is studied theoretically. The angular independent anisotropy orientation and alignment parameters describing photodissociation dynamics that can be determined from these studies are presented. It is shown that in principle each of the techniques can give the same information about the photodissociation dynamics; however, the birefringence technique has an undeniable advantage since it leads to an increase in accuracy of the experimental data. Both magnetic birefringence and magnetic dichroism techniques were used experimentally to study the spin orientation of ground state Rb atoms produced in photodissociation of RbI at 266 nm by circularly polarized laser light. The experiment was not of the Doppler spectroscopy type, but use of different 85 Rb and 87 Rb isotopes in the dissociation cell and in the probe light source allows detection of the oriented atomic photofragments separately by each of the techniques. Obtained experimental results are in agreement with the theoretical predictions. Contribution of the orientation of the Rb hyperfine structure energy states to the observed signals were also studied and treated theoretically. The initial degree of the electron spin orientation of the Rb atoms produced in the reaction was found to be P e = 0.11 ± 0.02.
The angular distribution of spin-oriented Rb photofragments produced by photodissociation of RbI molecules using circularly polarized UV light at 266 nm has been investigated for the first time applying the method of Doppler-resolved paramagnetic Faraday rotation. The observed signals could successfully be explained as the sum of two contributions being due to incoherent and coherent excitation of the RbI molecules. For the ratio of the two corresponding anisotropy parameters we obtain a value of −0.33 as a preliminary result.
The photodissociation of RbI molecules via the second excited state under the action of circularly polarized radiation with a wavelength of 266 nm is studied. By using sub-Doppler Faraday spectroscopy, the angular distributions of the orientation of spins of Rb(5 2 S 1/2 ) atoms formed are studied and the anisotropy parameters describing these angular distributions are determined. Based on the anisotropy parameters, the ratio between the amplitudes of the inelastic scattering matrix elements corresponding to the two possible photodissociation channels is determined to be r Ω=1 / r Ω=0 =0.71±0.15, while the phase difference for these matrix elements is found to amount to Δφ = 173° ± 29°.
A study has been made on the oriented ground state 5 2 S 1/2 Rb atoms produced in molecular photodissociation. Low-pressure RbI vapours have been photolysed by a beam of circularly polarized impulse laser radiation at 266 nm. The resulting Rb atoms were probed by a resonance atomic line at 794.8 nm from a discharge Rb lamp in two different experimental geometries. A free precession of the oriented atomic spins in an external magnetic field was detected by a dichroism of the atomic vapours. The initial degree of spin orientation was found to be P c = (20 +or- 2)%.
Abstract In this study we explored the possibilities for observing the angular momentum alignment-to-orientation conversion (AOC) in the ground state of various alkali metals: K, Rb, Cs. For theoretical analysis we used a model that is based on the Optical Bloch equations for the density matrix. In our model we took into account the interaction of the laser light with all hyperfine levels, which are mixed as the magnetic field increases. The radiation’s coherence properties were also included. Finally, the results were averaged over the Doppler profile. Additionally we simulated signals where the ground- or the excited-state coherent processes were numerically switched off in order to determine the origins of the features of the obtained signals. We also performed experiments on Cs atoms with two laser beams: a linearly polarized Cs D 1 pump and circularly polarized Cs D 2 probe. We used the pump beam to create angular momentum alignment in the ground state and observed the transmission signal of the probe beam as we changed the magnetic field. A detailed analysis of the experimentally obtained transmission signal from a single circularly polarized probe laser component is provided. Finally, prospects for observing AOC experimentally are discussed, as well as experiments were even a weak AOC signal could lead to systematic errors.
We investigated experimentally and theoretically angular momentum alignment-to-orientation conversion created by the joint interaction of laser radiation and an external magnetic field with atomic rubidium at room temperature. In particular, we were interested in alignment-to-orientation conversion in the atomic ground state. In the experiment the laser frequency was fixed to the hyperfine transitions of the D1{D}_{1}D1​ line of rubidium. To simulate the measured signals we used a theoretical model that takes into account all neighboring hyperfine levels, the mixing of magnetic sublevels in an external magnetic field, the coherence properties of the exciting laser radiation, and the Doppler effect. The experiments were carried out by exciting the atoms with linearly polarized laser radiation. Two orthogonal circularly polarized laser induced fluorescence (LIF) components were detected and afterward their difference was taken. The combined LIF signals originating from the hyperfine magnetic sublevel transitions of 85Rb^{85}\mathrm{Rb}85Rb and 87Rb^{87}\mathrm{Rb}87Rb rubidium isotopes were included. The alignment-to-orientation conversion can be incontrovertibly identified in the difference signals for various laser frequencies, and a change in signal shapes can be observed when the laser power density is increased. We studied the formation of and the underlying physical processes behind the observed signals of the LIF components and their difference by performing an analysis of the influence of incoherent and coherent effects. We performed simulations of theoretical signals that showed the influence of ground-state coherent effects on the LIF difference signal.