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Physical Review A

Collisional relaxation of the 6s[3/2 ]10 and 6s’[1/2 ]10 mJ levels of xenon using resonantly enhanced multiphoton ionization

Dimitrios Charalambidis, A. Lyras, P. Lambropoulos, Xian Tang, Stanley J. Bajic, R. N. Compton

Published Feb 1, 1990• 7 citations
Publisher

Abstract

Collisional relaxation of the 6s${[3/2]}_{1}^{0}$ and 6s'${[1/2]}_{1}^{0}$ ${m}_{J}$ levels of Xe has been investigated both experimentally and theoretically. Collisional disalignment and dephasing of the ${m}_{J}$ levels have been studied by measuring the pressure and laser intensity dependence of the ratio of the ionization from selectively excited ${m}_{J}$=0 and \ifmmode\pm\else\textpm\fi{}1 sublevels in a three-photon resonant-ionization experiment via the 6s and 6s' states of Xe. The selective ionization is achieved by a novel use of two counterpropagating laser beams having parallel and orthogonal polarization, respectively. The pressure dependence of the ionization ratio has been calculated by solving a system of rate equations derived from a full set of density-matrix equations pertaining to the particular experimental conditions. Collisional-relaxation cross sections of the investigated levels have been estimated by fitting the calculated results to the experimental data. The ionization ratio is found to be sensitive to both disalignment and dephasing of the ${m}_{J}$ levels.

DOI
10.1103/physreva.41.1457

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Collisional relaxation of the 6s[3/2]10{[3/2]}_{1}^{0}[3/2]10​ and 6s'[1/2]10{[1/2]}_{1}^{0}[1/2]10​ mJ{m}_{J}mJ​ levels of Xe has been investigated both experimentally and theoretically. Collisional disalignment and dephasing of the mJ{m}_{J}mJ​ levels have been studied by measuring the pressure and laser intensity dependence of the ratio of the ionization from selectively excited mJ{m}_{J}mJ​=0 and \ifmmode\pm\else\textpm\fi{}1 sublevels in a three-photon resonant-ionization experiment via the 6s and 6s' states of Xe. The selective ionization is achieved by a novel use of two counterpropagating laser beams having parallel and orthogonal polarization, respectively. The pressure dependence of the ionization ratio has been calculated by solving a system of rate equations derived from a full set of density-matrix equations pertaining to the particular experimental conditions. Collisional-relaxation cross sections of the investigated levels have been estimated by fitting the calculated results to the experimental data. The ionization ratio is found to be sensitive to both disalignment and dephasing of the mJ{m}_{J}mJ​ levels.
Two independently tunable, linearly polarized dye lasers have been used to study multiphoton ionization of gaseous xenon. One laser is tuned near three-photon resonance with the allowed 6s[3/2]J=1° resonance, which is not observed with one laser at these pressures because of the destructive interference between the three-photon-excitation and third-harmonic-excitation pathways to the 6s[3/2]J=1° state. The second laser is tuned to resonance with higher allowed np and np′ states of xenon. The missing ionization signal is seen to be dramatically restored by the addition of the second laser beam; however, the signal again disappears as the pressure is increased. These results are discussed in relation to recent experimental and theoretical studies of cancellation effects between three-photon excitation and third-harmonic generation. Spectroscopic information obtained from tuning each dye laser, together with data on the angular distributions resulting from rotating the plane of polarization of one laser relative to that of the second laser, clearly shows that two different ionization mechanisms are operative. In addition, these experiments unambiguously illustrate the importance of dissociative ionization of quasi-dimers of xenon over a rather broad frequency range.
The destruction rates of the xenon $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0}$ metastable and $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{1}$ resonance atoms by collisions with ground-state xenon atoms have been measured by studying the temporal dependence of the number densities of the $6{s}^{\ensuremath{'}}$ and 6p6p6p atoms. The studies have been made both during the earlier afterglow of a xenon pulsed discharge and after the selective production of the $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0,1}$ atoms by a laser excitation in the late afterglow. An original optical pumping technique, using a tunable pulsed dye laser, was employed to populate the $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0,1}$ levels by excitation of the 6s[32]26s{[\frac{3}{2}]}_{2}6s[23​]2​ long-lived metastable atoms to $6{p}^{\ensuremath{'}}$ levels. The pressure dependence of the decay frequencies of these $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0,1}$ atoms was analyzed in the xenon pressure range of 0.04 to 1 torr and a kinetic model describing the evolution of the densities of these atoms after their production was established. The rate coefficients for the relaxation of the $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0}$ metastable and $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{1}$ resonance atoms by two-body collisions involving a ground-state xenon atom are (8.45\ifmmode\pm\else\textpm\fi{}0.85) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}12}$ cm3{\mathrm{cm}}^{3}cm3 ${\mathrm{sec}}^{\ensuremath{-}1}$ and (6.65 \ifmmode\pm\else\textpm\fi{} 1.00) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}11}$ cm3{\mathrm{cm}}^{3}cm3 ${\mathrm{sec}}^{\ensuremath{-}1}$, respectively. Results show that, in fact, at least 75% of this relaxation is a transfer of population to the 6p[12]16p{[\frac{1}{2}]}_{1}6p[21​]1​ level following ${\mathrm{Xe}}^{*}(6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0,1})+\mathrm{Xe}\ensuremath{\rightleftarrows}{\mathrm{Xe}}^{*}(6p{[\frac{1}{2}]}_{1})+\mathrm{Xe}\ensuremath{-}\ensuremath{\Delta}E$. These collisional transfer processes have been explained by means of Mulliken's xenon molecular potential curves.
Three-photon excitation and associated wave mixing near the 6s6s6s and $6{s}^{\ensuremath{'}}$ resonances of xenon have been studied utilizing resonance-enhanced multiphoton ionization in angled beams with different polarizations. It has been shown that a complete cancellation of three-photon resonant atomic excitation caused by the well-known destructive wave-mixing interference occurs in sss and ppp polarization of angled beams but distinct resonance ionization enhancement is observed when pump beams have orthogonal polarization planes. Pressure-induced evolution of the resonance ionization peak in cross-polarized beams is identical to that observed with counterpropagating beams. The reason for such resonance ionization enhancement is unknown and cannot be explained within the frame work of existing theory. The effect may result from some peculiarities of wave-mixing interference in a multilevel atomic system, where different degenerate magnetic sublevels of the upper atomic state and multiple interfering excitation processes are involved. Another possibility is that the resonance ionization enhancement results from a process where weak counterpropagating light is generated within the excitation region of cross-polarized angled beams.
A novel one-color Xe + -N laser induced collisional charge transfer system is proposed, and preparation of the initial state of the system, i.e., Xe + is experimentally implemented through resonance enhanced multi-photon ionization (REMPI) by ∼440 nm dye laser. The REMPI of Xe is experimentally investigated through time-of-flight (TOF) mass spectrometry and the intensity dependence of Xe + is obtained, aiming at the preparation of Xe + . The resonant ionization spectra of Xe at ∼440 nm under several different conditions are measured, showing the impacts of mode purification and source pressure on the resonant ionization spectrum. The results indicate the feasibility of preparing the initial state of the Xe + -N system by ∼440 nm multi-photon resonant ionization, which prepares for a further experiment of laser-induced collisional charge transfer.
Competitive inhibition of a resonance enhanced multiphoton ionization process by a resonant four-wave mixing has been observed in Xe atoms. When an intense IR (1064 nm) laser was applied to a sample of Xe which was also being irradiated by a UV laser tuned to the two-photon absorption line of Xe, the two-photon-resonant three-photon ionization signals decreased with increasing IR laser power. This phenomenon is dependent on the resonant states of Xe and the polarization of the two laser beams. Three 6sss excited states [5/2]2{}_{2}2​, [3/2]2{}_{2}2​, and [1/2]0{}_{0}0​ were examined. At the [1/2]0{}_{0}0​ resonant state, the ion signals were not decreased but slightly increased with the increase of the IR laser power. No suppression of the ion signal was observed at the [5/2]2{}_{2}2​ resonant state, when the polarization directions of the lasers were perpendicular to each other. The result of the polarization dependence reflects the selection rules of four-wave mixing. A simple rate equation analysis including the contribution of two-photon ionization from the [1/2]0{}_{0}0​ state by the IR laser well represents the IR laser-power dependence of the ion signal.
A pressure-broadening study of three-photon and one-photon resonances was performed using counterpropagating, linearly polarized laser beams. Self-broadening, as well as the collision-induced effects due to the presence of a second noble gas, was investigated for the 6s, 6s′, 5d and 7s of xenon and the 5s′ of krypton. A multiphoton ionization scheme in a calibrated proportional counter is the basis of a new technique by which collisional processes can be investigated for high pressure regimes which have previously been inaccessible to investigation, particularly for self-broadening where the mean free path of the photons is of the order of 10−3 cm for pressures as low as 1 Torr. Using this technique, we have recorded the collision-induced widths and shifts of the resonance for pressures up to 1000 Torr. In the hundreds of torr range, asymmetric, self-broadening line shapes were observed with full widths and shifts which were greater than five times the laser bandwidth (0.002-0.005 nm).