We have experimentally studied the nonradiative transport of excitation in a dense
[N=(0.3-2.5)\ifmmode\times\else\texttimes\fi{}${10}^{17}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}]$ potassium vapor. We show that in such a high-density vapor the diffusive nonradiative transport of excitation via ``hopping'' of excitation in dipole-dipole collisions between excited- and ground-state atoms is dominant over the radiative transport through photons. The nonradiative transport mechanism becomes visible in the fluorescence-excitation spectrum signal as a sharp dip close to the resonance frequency. The appearance of the dip, its shape, width, and density dependence can all be explained in terms of a simple diffusion model for the nonradiative transport. This yields values for the diffusion coefficient and the resonance-exchange rate coefficient.
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.
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.
Near-resonant four-wave light mixing in Na vapor with inert buffer gases reveals collision-induced coherences when the difference frequency between two incident light beams equals the hyperfine splitting or the Zeeman splitting between two states of the
32S ground-state manifold. The Doppler broadening of the corresponding resonances decreases with increasing buffer-gas pressure. The width of the resonances becomes significantly narrower than the spontaneous linewidth of the near-resonant intermediate state and is determined by spin-exchange collisions between ground-state atoms. The dependence of the intensity of these collision-induced resonances on buffer-gas pressure and detuning from one-photon resonances is measured. Selection rules for various polarizations of the incident light fields and directions of an externally applied magnetic field are established.
mJ mixing in
42P21 and
42P23 potassium atoms as well as atomic multipole relaxation induced in K-K and K-Ar collisions were investigated by methods of atomic fluorescence spectroscopy. K vapor, pure or mixed with Ar, was placed in a kG magnetic field and irradiated with a single Zeeman component of 7665- or 7699-\AA{} resonance radiation, causing the selective excitation of the $^{2}P_{\frac{1}{2},\ensuremath{-}\frac{1}{2}}$ or $^{2}P_{\frac{3}{2},\ensuremath{-}\frac{3}{2}}$ substate. Collisions of the excited atoms with ground-state K or Ar atoms caused
mJ mixing which was monitored by analysis of the resulting fluorescent spectrum with a scanning Fabry-Perot interferometer. Intensity measurements on the fluorescent Zeeman components in relation to ground-state atomic densities yielded the following multipole relaxation cross sections ${\ensuremath{\Lambda}}_{J}^{(L)}$: K-K (${10}^{\ensuremath{-}12}$
cm2): ${\ensuremath{\Lambda}}_{\frac{1}{2}}^{(1)}=5.7\ifmmode\pm\else\textpm\fi{}0.9$, ${\ensuremath{\Lambda}}_{\frac{3}{2}}^{(1)}=8.1\ifmmode\pm\else\textpm\fi{}1.4$, ${\ensuremath{\Lambda}}_{\frac{3}{2}}^{(2)}=11.4\ifmmode\pm\else\textpm\fi{}2.0$, ${\ensuremath{\Lambda}}_{\frac{3}{2}}^{(3)}=9.8\ifmmode\pm\else\textpm\fi{}1.5$; K-Ar (${10}^{\ensuremath{-}16}$
cm2): ${\ensuremath{\Lambda}}_{\frac{1}{2}}^{(1)}=65\ifmmode\pm\else\textpm\fi{}10$, ${\ensuremath{\Lambda}}_{\frac{3}{2}}^{(1)}=175\ifmmode\pm\else\textpm\fi{}25$, ${\ensuremath{\Lambda}}_{\frac{3}{2}}^{(2)}=230\ifmmode\pm\else\textpm\fi{}35$, ${\ensuremath{\Lambda}}_{\frac{3}{2}}^{(3)}=190\ifmmode\pm\else\textpm\fi{}30$. The ${Q}_{J}(m\ensuremath{\leftrightarrow}{m}^{\ensuremath{'}})$ cross sections for
mJ mixing were derived from the ${\ensuremath{\Lambda}}_{J}^{(L)}$ as well as the cross sections for multipole decay ${\ensuremath{\sigma}}_{J}^{(L)}={\ensuremath{\Lambda}}_{J}^{(L)}+{\ensuremath{\sigma}}_{J}^{(0)}$. The experimental results are in good agreement with theoretical calculations.
The effects of imprisonment of resonance radiation in potassium vapor and of collisions between excited potassium atoms and N2, H2, HD, and D2 molecules, on the measured lifetimes of the 42P states, have been investigated by the method of delayed coincidences. The potassium atoms were excited with short pulses of resonance radiation, and the delays between the exciting and fluorescent pulses were measured with a time-to-amplitude converter. At pure potassium densities below 1010 atoms/cm3, the measured lifetimes approach the constant values 27.8 ± 0.8 and 27.6 ± 0.8 ns, corresponding to the natural lifetimes of the 42P1/2 and 42P3/2 states, respectively. At higher densities the effective lifetimes are in satisfactory agreement with the predictions of Holstein's theory. The decrease in the measured decay rates observed upon addition of the diatomic gases yielded cross sections for quenching collisions with N2, H2, HD, and D2 which equal 34.0 ± 2.0 Å2, 9.4 ± 1.3 Å2, 11.9 ± 1.5 Å2, and 8.0 ± 2.0 Å2, respectively.