Disorientation Cross Sections Research Articles

Optical–optical double resonance circular polarization spectroscopy: Measurement of the disorientation cross section in 6p2P1/2 Cs atoms

We have measured the cross section for the disorientation of excited 6p2P1/2133Cs atoms by collisions with ground state Ar atoms by optical–optical double resonance circular polarization spectroscopy. Circular polarization spectra were measured for the 6s2S1/2→ 6p2P1/2→ 10s2S1/2 transition. Orientation was optically induced in the 2P1/2 state by circularly polarized light. A polarization degree was measured when two pulsed lasers were circularly polarized with the same helicities and opposite helicities, respectively. The intensity of the cascade fluorescence signal, monitored from the 9p2P1/2→ 6s2S1/2 transition at 361.73nm, was analyzed in the presence of Ar pressures ranging from 7mbar to 133mbar. This is a direct measure of the importance of the orientation to inelastic process in alkali rare-gas collisions. A disorientation cross section value of 9.25 (1.4)Å2 was extracted from the measurements. Our result is in good agreement with the theoretical and experimental predictions within the error limits.

Depolarization of Ne*(2p2) atoms induced by isotropic collisions with He(1s2) atoms at temperatures between 10 K and 1000 K

Analysis of the atomic depolarization induced by collisions offers an accurate test for anisotropic atom–atom potentials. Based on a model potential proposed by Hennecart and Masnou-Seeuws in 1985, we report quantum calculations for disalignment and disorientation of Ne*(2p2) atoms induced by collisions with helium in a gaseous mixture at 10 K ⩽ T ⩽ 1000 K using a close-coupling many-channels method within a molecular approach. The goal is to explain the laser-induced fluorescence spectroscopy experiments realized by Fujimoto and his co-workers in 2004. Our quantum cross sections for both disalignment and disorientation show a resonant structure below 10 meV which is attributed to the rotational coupling between collisional channels, contrary to the experimental results which suggest that the intra-multiplet cross sections vanish at zero-energy. Agreement between theory and experiment below 77 K is found when the intra-multiplet cross sections are linearly extrapolated below 10 meV. The analysis of the long-range potentials for the He–Ne*(2p2) system suggests a more repulsive interaction than proposed in the original model potential, and leads to a better agreement with the experiment for temperatures between 10 K and 650 K.

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

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.

Disorientation and disalignment of 42P3/2 potassium atoms, induced in resonant collisions

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.

Disorientation of 42P1/2 potassium atoms, induced in resonant collisions

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.

Metastability-exchange and depolarising collisions in xenon and krypton

Collisions between rare-gas atoms in the 3P2 metastable state and in the ground state were studied. The metastable atoms were created by a discharge and oriented by optical pumping, and pressure broadening of their magnetic-resonance curves was measured. For xenon, mixtures of even and odd isotopes were used, leading to a determination of the cross section for metastability-exchange collisions. For krypton the results are reported for the disorientation of the metastable state in the natural gas. In addition the disorientation cross section has been measured of the krypton metastable state through collisions with helium.

Depolarization of6P322cesium atoms, induced in collisions with noble gases

Cross sections for relaxation of circular and linear polarization of $6^{2}P_{\frac{3}{2}}$ 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 $Q(\mathrm{circ}.)$ and $Q(\mathrm{lin}.)$, 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.

Disorientation of Cs(62P1/2) atoms, induced in collisions with noble gases

The disorientation of 62P1/2 cesium atoms, induced in collisions with noble gas atoms in their ground states, was systematically investigated by monitoring the depolarization of cesium resonance fluorescence in relation to noble gas pressures. The Cs atoms, contained together with a buffer gas in a fluorescence cell and located in zero magnetic field, were excited and oriented by irradiation with circularly polarized 8943 Å resonance radiation, and the resonance fluorescence, emitted in an approximately backward direction, was analyzed with respect to circular polarization. The experiments yielded the following disorientation cross sections which have been corrected for the effects of nuclear spin: Cs–He: 4.9 ± 0.7 Å2; Cs–Ne: 2.1 ± 0.3 Å2; Cs–Ar: 5.6 ± 0.8 Å2; Cs–Kr: 5.8 ± 0.9 Å2; Cs–Xe: 6.3 ± 0.9 Å2. The results are in good agreement with most of the available zero-field and low-field data.

⇄ Sz↩ and ⇄ S+↩ relaxation in optically pumped cesium vapour

Two-exponential relaxation of ⇄ S z ↩ in the ground state of optically pumped cesium atoms has been studied. The modified weak pumping transients method has been applied. The following cross sections for the nuclear spin independent disorientation of the electronic spin have been calculated:σ R(⇄ S z ↩) (Cs-He) = 2.3 x 10 -23 cm 2, σ R(⇄ S z ↩) (Cs-Ne) = 5.6 x 10 -23 cm 2, σ R(⇄ S z ↩) (Cs-Ar) = 94 x 10 -23 cm 2. Theoretically predicted relations between longit udinal and transverse relaxation times have been experimentally confirmed.

Calculations on recent high temperature experiments involving alkali-rare-gas collisions

Calculations of finite structure transition and disorientation cross sections, and of broadening and shift parameters for some alkali-rare-gas collisions are performed using the recent interatomic potentials of Pascale and Vandeplanque (1974). These are compared with recent experiments at high temperatures.

Disorientation of K(42P1/2) Atoms, Induced in Collisions with Noble Gases

The cross sections for disorientation of 42P1/2 potassium atoms, induced in collisions with noble gas atoms, have been determined in zero magnetic field by studying the depolarization of K(7699 Å) resonance fluorescence in relation to noble gas pressures. Potassium vapor at low density, mixed with a noble gas in a fluorescence vessel, was irradiated with circularly polarized 7699 Å potassium resonance radiation and the resulting resonance fluorescence, observed in an approximately backward direction, was analyzed with respect to circular polarization. The variation of the degree of polarization with gas pressure was interpreted on the basis of a 'J randomization' model for the collisions and yielded the following disorientation cross sections which are appropriately corrected for the effect of nuclear spin: K–He, 24 ± 4 Å2; K–Ne, 21 ± 3 Å2; K–Ar, 37 ± 5 Å2; K–Kr, 51 ± 7 Å2; K–Xe, 69 ± 9 Å2. The cross sections are significantly smaller than values obtained previously in kilogauss fields.

Disorientation and disalignment cross sections of excited Rb-atoms colliding with noble gases

The disorientation and disalignment cross sections for the depolarization of Rb 52P1/2 and 52P3/2 states due to collisions with the inert gases He, Ne, Ar, Kr, Xe were measured at a temperature of 317 K. The experiments were done in a static magnetic field H0 which served to decouple the nuclear spin. Unpolarized Rb-D2-radiation was made incident on the Rb-vapour parallel to H0 thus generating an alignment in the2P3/2-state. The linear polarization of the fluorescent D2-light emitted at right angles towards the incident light beam was measured as a function of the rare gas pressure (0... 3 Torr). Similarly, for the disorientation measurements the vapour was excited by circularly polarized D2- or D1-radiation parallel to H0 and circularly polarized fluorescent light was observed at small angles towards the backward direction. From the decrease of the polarizations observed at rising pressure the cross sections were determined. The comparison with theoretical results gives satisfactory agreement.

Disorientation of Na(32P1/2) Atoms, Induced in Collisions with Noble Gases

The cross sections for disorientation of 32P1/2 sodium atoms, induced in collisions with noble gas atoms, have been determined by following the depolarization of Na–D1 resonance fluorescence in relation to noble gas pressure. Sodium vapor at low density, mixed with a noble gas in a fluorescence cell, was irradiated with circularly polarized D1 resonance radiation and the resulting D1 resonance fluorescence, observed in an approximately backward direction, was analyzed with respect to circular polarization. The variation of the degree of polarization with gas pressure was interpreted on the basis of a 'J randomization' model for the collisions, and yielded the following disorientation cross sections which are appropriately corrected for effects due to nuclear spin. Na–He: 28.1 ± 4.0 Å2; Na–Ne: 27.8 ± 4.0 Å2; Na–Ar: 57.0 ± 8.0 Å2; Na–Kr: 78.0 ± 10 Å2; Na–Xe: 87.0 ± 13 Å2.

Relaxation des Natrium-Grundzustandes bei Stößen mit den stabilen Helium- und Wasserstoff-Isotopen

The spin-polarization of the optically pumped Na-groundstate is investigated in presence of the stable He- and H2-isotopes. The following disorientation cross sections are derived from the pressure dependence of the relaxation rate:σ=(16.2±2.0) · 10−26 cm2 for He3σ=(2.4±0.5) · 10−26 cm2 for He4σ=(3.9±1.5) · 10−26 cm2 for H2σ=(2.3±1.0) · 10−26 cm2 for D2.

MJMixing in Oriented6P122Cesium Atoms, Induced in Collisions with Noble Gases

The cross sections for disorientation of magnetically oriented $6^{2}P_{\frac{1}{2}}$ cesium atoms, induced in collisions with noble gases, have been determined using Zeeman-scanning techniques at kilogauss magnetic fields. The ${m}_{J}=\ensuremath{-}\frac{1}{2}$, ${m}_{I}=\ensuremath{-}\frac{7}{2}$ hyperfine Zeeman substate was populated selectively by irradiating the cesium-vapor-noble-gas mixture with cesium-resonance radiation, and collisional transitions to the ${m}_{J}=+\frac{1}{2}$, ${m}_{I}=\ensuremath{-}\frac{7}{2}$ substate were monitored by measurements of the relative intensities of the ${\ensuremath{\sigma}}^{\ensuremath{-}}$ and ${\ensuremath{\sigma}}^{+}$ components in resonance fluorescence observed in the direction parallel to the magnetic field. The following cross sections were obtained: Cs-He, 11.8 ${\mathrm{\AA{}}}^{2}$; Cs-Ne, 4.7 ${\mathrm{\AA{}}}^{2}$; Cs-Ar, 10.7 ${\mathrm{\AA{}}}^{2}$; Cs-Kr, 37.9 ${\mathrm{\AA{}}}^{2}$; Cs-Xe, 71.7 ${\mathrm{\AA{}}}^{2}$.

Evidence for Rb-Rare-Gas Molecules from the Relaxation of Polarized Rb Atoms in a Rare Gas. Experimental Results

Relaxation experiments have been performed between 0–200 G on optically polarized Rb atoms in argon, krypton or xenon, at pressures ranging from 0.1 to 20 torr. They clearly show the existence of Rb–Ar, Rb–Kr and Rb–Xe molecules of low binding energy (smaller than kT). Molecular parameters such as lifetime, spin-orbit coupling constant, formation rate, constant of mass action, etc., are measured, or evaluated when they cannot be directly measured. Experimental values of the diffusion coefficient of Rb in the gas (except xenon) and of the disorientation cross sections by usual binary collisions are also obtained.

Transverse Relaxation of Optically Pumped Cesium Atoms

Transverse relaxation time of optically pumped cesium atoms in the ground state has been measured by use of the technique of multiple spin echo and cross-beam. Longitudinal relaxation time has also been measured by use of the pulse technique. The variation of relaxation time with the buffer gas pressure and the cell temperature has been studied in an unlined glass cell. Transverse relaxation time ( T 2 ) F + depends strongly on the cell temperature but longitudinal relaxation time T n does not. From the temperature dependence of ( T 2 ) F + , the spin-exchange cross section has been evaluated as 1.4×10 -14 cm 2 . Disorientation cross sections have been evaluated as follows: σ(( T 2 ) F + )=1.8×10 -23 cm 2 and σ( T n )=3.2×10 -24 cm 2 for cesium-neon collisions; σ(( T 2 ) F + )=5.9×10 -22 cm 2 and σ( T n )=2.5×10 -23 cm 2 for cesium-argon collisions. Diffusion coefficients for cesium in neon and in argon have been evaluated as 0.19 cm 2 /sec and 0.14 cm 2 /sec, respectively. For the interrelation of transverse and longitudinal relaxation times, it is revealed that (1/ T e +1/ T n ) is equal to (1/( T 2 ) F + +1/( T 2 ) F - ) in the case of extreme motion narrowing.

Polarization of Pb Vapor. II. Disorientation of thePb207Ground State andμI(Pb207)μI(Hg199)

Optical pumping in the $^{3}P_{0}$ ground state of ${\mathrm{Pb}}^{207}$ is studied in detail. Pumping and nutation transient signals are used to investigate relaxation phenomena and to place upper limits on buffer-gas disorientation cross sections. ${\mathrm{Hg}}^{199}$ has been optically pumped in the same cell yielding $|\frac{{\ensuremath{\mu}}_{I}({\mathrm{Pb}}^{207})}{{\ensuremath{\mu}}_{I}({\mathrm{Hg}}^{199})}|=1.14960(4)$ and ${|{\ensuremath{\mu}}_{I}({\mathrm{Pb}}^{207})|}_{\mathrm{uncorrected}}=0.57235(2){\ensuremath{\mu}}_{N}$. The large (2%) difference between the accepted NMR value and the previously reported optical-pumping value is thus confirmed.

Calculation of cross sections for the depolarization of 2P states in alkali atoms

Optical pumping experiments have yielded very large disorientation cross sections (40–100 Å2) for collisions between oriented 2P1/2 atoms of potassium, rubidium, cesium and thalium, and helium, neon, argon, krypton, and xenon atoms in their ground states, in contrast to the extraordinarily small cross section (< 10−5 Å2) for the depolarization of 2S1/2 atoms, as found by Gallagher and by Anderson and Ramsey. Franz, Leutert, and Shuey pointed out that, within the adiabatic model, the transition ψ(j = 1/2, m = 1/2) → ψ(j = 1/2, m = −1/2) is strongly forbidden in conformity with Kramer's theorem according to which it is impossible to split the two components ψ(j = 1/2, m = ± 1/2) in an electric field. In the present investigation it has been shown that the inclusion of an angular nonadiabatic operator (i.e. the operator responsible for Coriolis mixing of adiabatic electronic molecular states) makes possible such transitions. Calculations are presented of cross sections for the resulting depolarization induced in collisions between potassium, rubidium, and cesium, and inert gas atoms.

Optical Orientation of theP03Ground State ofPb207

The method of optical pumping of diamagnetic atoms with zero orbital angular momentum ($^{1}S_{0}$) has been extended to an atom with nonzero orbital angular momentum ($^{3}P_{0}$), yielding ${|{\ensuremath{\mu}}_{I}|}_{\mathrm{uncorrected}}=0.5726(6){\ensuremath{\mu}}_{N}$ for the nuclear moment of ${\mathrm{Pb}}^{207}$. The cross sections for disorientation by collisions with the inert gases, hydrogen, and nitrogen are all less than ${10}^{\ensuremath{-}20}$ ${\mathrm{cm}}^{2}$.