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Journal of Physics B Atomic and Molecular Physics

Simultaneous measurements of the longitudinal and transverse relaxation times of aligned mercury atoms in the metastable 63P2state

E Jacobson

Published Dec 1, 1977• 3 citations
Publisher

Abstract

Mercury atoms are excited in a weak gas discharge to the metastable 63P2 state, forming a longitudinal alignment, which is observed in the absorption of the linearly polarised green Hg line ( lambda =5461 AA). By a pulsed RF field, transverse components of the alignment are produced additionally, giving rise to modulated absorption. During the free precession the signals decay due to depolarising collisions with mercury atoms in the ground state. The cross sections for the destruction of the alignment have been determined to be sigma 0=(262.9+or-5.2) AA2 for the longitudinal component, sigma 1=(257.8+or-7.5) AA2 for the first transverse component (modulated with the Larmor frequency) and sigma 2=(273.1+or-9.0) AA2 for the second transverse component (modulated with twice the Larmor frequency). It is emphasised that these values refer to the alignment.

DOI
10.1088/0022-3700/10/17/015

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Mercury atoms are excited in a weak gas discharge to the metastable 63P2 state, forming a longitudinal alignment, which is observed in the absorption of the linearly polarised green Hg line ( lambda =5461 AA). By a pulsed RF field, transverse components of the alignment are produced additionally, giving rise to modulated absorption. During the free precession the signals decay due to depolarising collisions with mercury atoms in the ground state. The cross sections for the destruction of the alignment have been determined to be sigma 0=(262.9+or-5.2) AA2 for the longitudinal component, sigma 1=(257.8+or-7.5) AA2 for the first transverse component (modulated with the Larmor frequency) and sigma 2=(273.1+or-9.0) AA2 for the second transverse component (modulated with twice the Larmor frequency). It is emphasised that these values refer to the alignment.
In radio frequency (RF) experiments on aligned atoms (e.g. optical double resonance) new phenomena can be found in the case of longitudinal excitation if a strong oscillating RF field is used: both the positions of the RF resonances (RF shift) and the widths (power broadening) depend on the direction of the light observed and on the state of its (linear) polarisation. This applies to the main (single-quantum) transitions as well as the (odd) multi-quanta transitions. These effects have been observed in an experiment on aligned mercury atoms in the metastable 6 3 P 2 state and have been confirmed by numerical calculations based on the semiclassical approach.
Inclusion of collisional destruction mechanisms as sink terms in the theory of imprisonment of resonance radiation appears to have been neglected in the case of the 2537-Å mercury line. In this paper, the Holstein photon transport equation is employed to determine the effect of destructive or self-quenching collisions between resonance state and ground state atoms, processes in which the resonance state is converted to another excited state with the release of kinetic energy. These mechanisms seem to explain experimental work in which the decay constant of the 2537-Å mercury line reaches a minimum and then increases with density. Comparison of theory with experimental data yields a cross section between 3.0 and 10.0 × 10−18 for the conversion of Hg(3P1) to metastable Hg(3P0) by two-body collisions with Hg(1S0). The sharp increase at high density of the decay constant of the 2537-Å mercury line has not been predicted by previous theories but is a general consequence of the inclusion of collisional destruction mechanisms.
The alignment of He4{\mathrm{He}}^{4}He4 atoms in the (n=2n=2n=2, metastable) 3S1^{3}S_{1}3S1​ state is described. Metastable atoms are produced by an rf discharge in a glass tube containing a few mm of pure helium, and the one micron pumping light ($2^{3}P\ensuremath{-}2^{3}S$) is provided by a helium lamp. A resonance signal is obtained from radio frequency disorientation by monitoring the transmitted pumping light. The double maximum line shape of this signal for strong rf magnetic fields is discussed. Included also is a discussion of the angular dependence of the signal when unpolarized light is used and an explanation of the inversion of the resonance signal for certain densities of the metastable helium atoms.The measured relaxation time of the oriented metastable atoms in the discharge is about 2.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ second and the pumping time is about a millisecond. A method is proposed and initial measurements are given for the cross section for destruction of metastable helium atoms by collision with foreign gas atoms. The application of optical pumping in helium to the measurement of weak magnetic fields is also discussed.
The authors report an application of the pulsed magnetic field technique to the study of resonant inter-atomic collisions on the decay of alignment ( gamma 2 ) of the 6 3 P 1 state of mercury. Resonance fluorescence scattered at right angles from atoms near the front face of a cell is observed; the depolarizing effect of multiple scattering is thereby reduced, allowing studies of same-isotope collisions to proceed to higher density. For this geometry the effect of wall collisions on gamma 2 is not large. Magnetic scanning of the lamp is employed, firstly, to determine the isotopic abundance in the cell, and, secondly to enable study of both 'same-isotope' and 'different-isotope' collisions using the same lamp and cell. The results for different-isotope collisions confirm those obtained by others. For Hg 198 -Hg 198 collisions, the contribution to gamma 2 is found to be (4.7+or-0.5)*10 -9 N cm 2 s -1 , in satisfactory agreement with the theoretical value.