The relaxation of atomic polarization in buffer-gas-free, paraffin-coated cesium vapor cells is studied using a variation on Franzen's technique of ``relaxation in the dark'' [Franzen, Phys. Rev. 115 850 (1959)]. In the present experiment, narrow-band, circularly polarized pump light, resonant with the Cs D2 transition, orients atoms along a longitudinal magnetic field, and time-dependent optical rotation of linearly polarized probe light is measured to determine the relaxation rates of the atomic orientation of a particular hyperfine level. The change in relaxation rates during light-induced atomic desorption (LIAD) is studied. No significant change in the spin relaxation rate during LIAD is found beyond that expected from the faster rate of spin-exchange collisions due to the increase in Cs density.
Observation of nonthermal, light-induced desorption of Cs and Rb atoms from paraffin coating is reported. Buffer-gas-free paraffin-coated cells containing Cs and Rb were exposed to laser light of various intensities and frequencies, and the change of the vapor densities as a function of time was investigated. Because atomic polarization relaxes very slowly in paraffin-coated cells (spin-relaxation times can be $\ensuremath{\gtrsim}1\mathrm{s}),$ they are applied in a variety of precision measurements (e.g., magnetometry, discrete symmetry tests, etc.). Light-induced desorption may be useful in such measurements as a method to control atomic density without changing the temperature of the cell.
Atomic-vapor density change due to light induced atomic desorption (LIAD) is studied in paraffin-coated rubidium, cesium, sodium, and potassium cells. In the present experiment, low-intensity probe light is used to obtain an absorption spectrum and measure the vapor density, while light from an argon-ion laser, array of light emitting diodes, or discharge lamp is used for desorption. Potassium is found to exhibit significantly weaker LIAD from paraffin compared to Rb and Cs, and we were unable to observe LIAD with sodium. A simple LIAD model is applied to describe the observed vapor-density dynamics, and the role of the cell's stem is explored through the use of cells with lockable stems. Stabilization of Cs vapor density above its equilibrium value over
25min is demonstrated. The results of this work could be used to assess the use of LIAD for vapor-density control in magnetometers, clocks, and gyroscopes utilizing coated cells.
The interaction of spin polarized cesium atoms on coated surfaces is examined. Coated surfaces find extensive applications in atomic devices-hydrogen masers and advanced rubidium frequency standards utilize wall coatings to obtain very narrow hyperfine resonances. A Cs atomic beam is hyperfine or spin polarized by optical pumping techniques using a single mode AlGaAs diode laser. The ground state distribution is probed before and after scattering from coated surfaces using laser induced fluorescence spectroscopy. The coatings under investigations are paraffin and organosilanes and these are known to be very weakly relaxing surfaces for polarized alkali atoms. The spin relaxation properties of paraffin and organosilane coatings have been studied in glass cell-vapor environment. In our experiments using polarized atomic beams, our goal is to examine in detail the hyperfine and spin relaxation properties of these coatings in a carefully controlled environment which is not possible in the gas cell environment. We measure the spin relaxation probability per wall collision as a function of temperature of the surface. Results of our studies will be presented.
Alkali-metal vapor cells with antirelaxation coating (especially paraffin-coated cells) have been a central tool in optical pumping and atomic spectroscopy experiments for 50 years. We have discovered a dramatic change of the alkali-metal vapor density in a paraffin-coated cell upon application of an electric field to the cell. A systematic experimental characterization of the phenomenon is carried out for electric fields ranging in strength from 0--8 kV/cm for paraffin-coated cells containing rubidium and cells containing cesium. The typical response of the vapor density to a rapid (duration $\ensuremath{\lesssim}100\text{ }\text{ms}$) change in electric field of sufficient magnitude includes (a) a rapid (duration of $\ensuremath{\lesssim}100\text{ }\text{ms}$) and significant increase in alkali-metal vapor density followed by (b) a less rapid (duration of $\ensuremath{\sim}1\text{ }\text{s}$) and significant decrease in vapor density (below the equilibrium vapor density), and then (c) a slow (duration of $\ensuremath{\sim}100\text{ }\text{s}$) recovery of the vapor density to its equilibrium value. Measurements conducted after the alkali-metal vapor density has returned to its equilibrium value indicate minimal change (at the level of $\ensuremath{\lesssim}10%$) in the relaxation rate of atomic polarization. Experiments suggest that the phenomenon is related to an electric-field-induced modification of the paraffin coating.
We have performed quantitative measurements of the variation of the on-resonance absorption coefficients ${\ensuremath{\kappa}}_{0}$ of the four hyperfine components of the Cs
D1 transition as a function of laser power
P, for pumping with linearly and with circularly polarized light. Sublevel populations derived from rate equations assuming isotropic population relaxation (at a rate ${\ensuremath{\gamma}}_{1}$) yield algebraic ${\ensuremath{\kappa}}_{0}(P)$ dependences that do not reproduce the experimental findings from Cs vapor in a paraffin-coated cell. However, numerical results that consider spin-exchange relaxation (at a rate ${\ensuremath{\gamma}}_{\mathrm{se}}$) and isotropic relaxation fit the experimental data perfectly well. The fit parameters, viz., the absolute value of ${\ensuremath{\kappa}}_{0}$, the optical pumping saturation power
Psat, and the ratio ${\ensuremath{\gamma}}_{\mathrm{se}}/{\ensuremath{\gamma}}_{1}$, are well described by the experimental conditions and yield absolute values for ${\ensuremath{\gamma}}_{1}$ and ${\ensuremath{\gamma}}_{\mathrm{se}}$. The latter is consistent with the previously published Cs-Cs spin-exchange relaxation cross section.
Relaxation of optically pumped Rb atoms on paraffin-coated walls has been studied using Franzen's sequence (relaxation in the dark) for observables
〈Sz〉 (mean electronic longitudinal polarization), and <S\ifmmode\cdot\else\textperiodcentered\fi{}I> (population difference between the two hfs levels). The physical meaning of optical signals under different conditions is analyzed in some detail, and it is pointed out that different observables relax differently. In the interpretation of our results, extensive use has been made of the theoretical analysis of the relaxation of a spin S (coupled to a spin I in the Rb atom) caused by a weak interaction of the type $_{\ensuremath{\gamma}s}H(t)\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{S}$,
H(t) being a random magnetic field acting on S during the dwell time of a Rb atom on the wall. Experimental results are in excellent agreement with this analysis, if one assumes the existence of two uncorrelated interactions of the above type. The first is the dipole-dipole interaction between S and the nuclear spins K of the protons (or deuterons) of the coating. This interaction has a long correlation time ${\ensuremath{\tau}}_{c1}\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ sec and dominates the relaxation of
〈Sz〉 in low fields. The second interaction which is independent of spins K, has a short correlation time ${\ensuremath{\tau}}_{c2}\ensuremath{\approx}{10}^{\ensuremath{-}12}$ sec, and dominates the relaxation of <S\ifmmode\cdot\else\textperiodcentered\fi{}I> in low fields, and of
〈Sz〉 in large fields. The values of these correlation times and of the strengths of the two interactions have been measured, first in low fields by combining results on
〈Sz〉 and <S\ifmmode\cdot\else\textperiodcentered\fi{}I> for the two Rb isotopes and for two types of coatings [
(CH2)n and
(CD2)n], and second, by studying the variations of the "pseudo" relaxation time
T1 of
〈Sz〉 for magnetic-field values up to 5000 G. We arrive at a detailed picture of physical adsorption of Rb on paraffin-coated walls, and find 0.1 eV for the adsorption energy. These results are compatible with those obtained for the relaxation of oriented Rb atoms in collisions with buffer-gas molecules.