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Canadian Journal of Physics

Collisionally induced depolarization of 2P1/2 alkali atoms fluorescence: validity criteria of weak-pumping and broad-line approximations

P. Rudecki, Stanisław Chwirot, S. Łęgowski

Published Jan 1, 1986• 0 citations
Publisher

Abstract

The dependence of the circular polarization of the fluorescence of alkali atoms on the intensity and spectral composition of the σ+-polarized exciting D1 line has been investigated by considering the stationary solutions of rate equations for populations of the Zeeman sublevels. The calculations were performed for pumping-light intensities corresponding to those of classical light sources, e.g., radio frequency or microwave discharge lamps, and commonly accepted values of cross sections for collisional relaxation of ground-state and excited atoms. The systematic errors of cross sections for collisional disorientation of 2P1/2 alkali atoms due to the neglect of intensity and spectral composition of the exciting light are estimated.

DOI
10.1139/p86-010

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R. W. Berends, W. Kedzierski, W. E. Baylis +1 more

The determination of cross sections for the relaxation of atomic multipole moments from the intensities of the components of a resolved Zeeman fluorescence spectrum may be carried out using the density-matrix formalism. However, the spherical-tensor basis is not well adapted to state-selected measurements and can allow a significant propagation of error. We can reduce the error by determining the Q(j,m\ensuremath{\rightarrow}j',m') cross sections directly from the data taken at the lowest pressures. A comparison is given between recent experimental and theoretical relaxation cross sections for collisions of 5 $^{2}P$ potassium atoms with ground-state He, Ne, and Ar atoms.

Physical Review A•Feb 1, 1989•3 citations
The dependence of the circular polarization of the fluorescence of alkali atoms on the intensity and spectral composition of the σ+-polarized exciting D1 line has been investigated by considering the stationary solutions of rate equations for populations of the Zeeman sublevels. The calculations were performed for pumping-light intensities corresponding to those of classical light sources, e.g., radio frequency or microwave discharge lamps, and commonly accepted values of cross sections for collisional relaxation of ground-state and excited atoms. The systematic errors of cross sections for collisional disorientation of 2P1/2 alkali atoms due to the neglect of intensity and spectral composition of the exciting light are estimated.
A (modified) Zeeman scanning method was used to determine the cross sections for disorientation of potassium atoms in the 42P1/2 resonance substate, induced in collisions with the ground-state atoms. Potassium vapor at densities of the order of 1011 cm−3 placed in a variable magnetic field was irradiated with σ+ polarized resonance radiation of wavelength 7699 Å emitted from a discharge lamp located in a constant field of 5.4 kG. The σ+ and σ− components of the resonance fluorescence emitted at right angles to the direction of excitation were monitored in relation to the vapor density as the variable field was scanned to make the σ+ resonance transition in the absorbing vapor coincide with the σ+ component present in the exciting light. As the vapor density increased, so did the admixture of the σ− component in the fluorescent light, which arises from collisional mixing between the mJ = 1/2 and mJ = −1/2 Zeeman substates. The resulting depolarization curve yielded the disorientation cross section σ1 = 4.0 × 10−11 cm2.
Cross sections for relaxation of circular and linear polarization of 62P326^{2}P_{\frac{3}{2}}62P23​​ cesium atoms, induced in collisions with ground-state He, Ne, Ar, and Xe, have been determined by Zeeman scanning techniques. A fluorescence cell containing low-pressure cesium vapor together with a buffer gas was irradiated with circularly or linearly polarized 8521-\AA{} Cs resonance radiation, producing excited and polarized Cs atoms which became depolarized in subsequent collisions with the buffer-gas atoms. The resulting depolarization of the cesium resonance fluorescence was monitored in relation to buffer-gas pressure (0-10 Torr) and to the strength of the ambient magnetic field (0-10 kG). At low fields at which the nuclear spin $\stackrel{\ensuremath{\rightarrow}}{\mathrm{I}}$ is coupled to the electronic angular momentum $\stackrel{\ensuremath{\rightarrow}}{\mathrm{J}}$, the measurements yielded depolarization cross sections Q(circ.)Q(\mathrm{circ}.)Q(circ.) and Q(lin.)Q(\mathrm{lin}.)Q(lin.), which agree within experimental error with theoretical values calculated for spin-coupled conditions. At kilogauss fields with $\stackrel{\ensuremath{\rightarrow}}{\mathrm{I}}$ and $\stackrel{\ensuremath{\rightarrow}}{\mathrm{J}}$ decoupled, the experiments produced cross sections for disorientation and disalignment, which are also in satisfactory agreement with the appropriate theoretical predictions. The results indicate that the nuclear spin has a more pronounced effect on the collisional relaxation of the atomic dipole than of the quadrupole.
The cross sections for the decay of circular and of linear polarization in oriented 42P324^{2}P_{\frac{3}{2}}42P23​​ potassium atoms, induced in collision with inert-gas atoms, were determined in a series of fluorescence experiments. A modified Zeeman scanning method was used to populate selectively the mJ{m}_{J}mJ​ sublevels of the 2P32^{2}P_{\frac{3}{2}}2P23​​ potassium atoms mixed with inert gases and placed in a strong magnetic field. Measurements of depolarization of the fluorescence, in relation to gas pressures, yielded the following depolarization cross sections: ${\ensuremath{\sigma}}_{1}$ (disorientation): K-He, 86 A˚2{\mathrm{\AA{}}}^{2}A˚2; K-Ne, 86 A˚2{\mathrm{\AA{}}}^{2}A˚2; K-Ar, 164 A˚2{\mathrm{\AA{}}}^{2}A˚2; K-Kr, 248 A˚2{\mathrm{\AA{}}}^{2}A˚2; K-Xe, 251 \AA{}. ${\ensuremath{\sigma}}_{2}$ (disalignment): K-He, 127 \AA{}; K-Ne, 120 A˚2{\mathrm{\AA{}}}^{2}A˚2; K-Ar, 240 A˚2{\mathrm{\AA{}}}^{2}A˚2, K-Kr, 301 A˚2{\mathrm{\AA{}}}^{2}A˚2, K-Xe, 336 A˚2{\mathrm{\AA{}}}^{2}A˚2. The experimental values are in satisfactory agreement with the calculations of Nikitin and Ovchinnikova.
An account of recent work is given by the author and co-workers on the experimental determination of cross sections for excitation transfer between the (2)P((1/2)) and (2)P(3/2)resonance levels in potassium, rubidium and cesium, induced by inelastic collisions with alkali atoms and inert gas atoms in their ground states. Forty values of cross sections were obtained in a series of sensitized fluorescence experiments, in which alkali vapors and vapor-gas mixtures were irradiated with one component of the appropriate alkali resonance doublet, and the relative intensities of both components appearing in the fluorescent light were measured. The experiments indicate that the collisional interaction between an alkali atom in a (2)P state and a ground state inert gas atom probably occurs between the inert gas atom and the (2)P alkali valence electron which behaves like a quasi-free particle.
The total cross sections for disorientation (σ1) and disalignment (σ2) of 42P3/2 potassium atoms, induced in collisions with the ground-state atoms, have been determined using a modified Zeeman scanning method. Potassium vapor at densities of the order of 1011 cm−3, contained in a fluorescence cell located in a kilogauss variable magnetic field, was irradiated with circularly polarized 7665 Å resonance radiation emitted from a discharge lamp located in a constant field of 5.4 kG. Scans of the variable field permitted selective excitation of single Zeeman components in the absorbing vapor. The σ+ and σ− components of the resulting resonance fluorescence emitted parallel to the scanning field were monitored in relation to the vapor density, as were the π and σ components emitted in the perpendicular direction. As the vapor density increased so did the frequency of the collisions which caused transfers among the Zeeman states in the vapor and thus disorientation and disalignment. The observed dependence of circular and linear depolarization of the fluorescence on the potassium density yielded the cross sections σ1 = 17 × 10−12 cm2 and σ2 = 21 × 10−2 cm2, corrected for imprisonment of radiation.
The determination of cross sections for the relaxation of atomic multipole moments from the intensities of the components of a resolved Zeeman fluorescence spectrum may be carried out using the density-matrix formalism. However, the spherical-tensor basis is not well adapted to state-selected measurements and can allow a significant propagation of error. We can reduce the error by determining the Q(j,m\ensuremath{\rightarrow}j',m') cross sections directly from the data taken at the lowest pressures. A comparison is given between recent experimental and theoretical relaxation cross sections for collisions of 5 2P^{2}P2P potassium atoms with ground-state He, Ne, and Ar atoms.