We report new results on the effect of collisions on radio-frequency (rf) -laser double resonances in an atomic vapor under conditions of velocity-selective optical pumping. Our experiments are performed on Zeeman resonances in the 4${f}^{6}
6{s}^{2}$
7F1--4${f}^{6}$6s6p
7F0 transition in samarium vapor in the presence of rare-gas perturbers (He, Ne, Ar, Xe). The collisional relaxation of the rf-generated sublevel coherence is studied using Raman heterodyne detection; this recently introduced spectroscopic technique is capable of separately yielding signal contributions originating from optically resonant and off-resonant atoms. The corresponding measurements clearly reveal that velocity-changing collisions (VCC) can completely determine the characteristics of the rf-laser double-resonance signals. Our experimental findings on the linewidths and line shapes of the rf resonances are in satisfactory quantitative agreement with theoretical predictions that are based on a novel four-level density-matrix calculation; here a Keilson-Storer collision kernel is used to describe the velocity diffusion processes. The comparison of calculated and measured results also permits us to derive cross sections for VCC and depolarizing collisions; moreover, information on the spectrum of the velocity changes connected with alignment-preserving collisions can be obtained.
We report detailed studies of Ramsey-type resonances due to collisional velocity diffusion of Sm atoms within the Doppler-broadened
7F1\ensuremath{\rightarrow}
7F0 154Sm transition at \ensuremath{\lambda}=570.68 nm in the presence of rare-gas perturbers (He,Ne,Ar,Kr,Xe). The experimental technique uses counterpropagating laser fields and relies on coherent resonance Raman processes to excite and detect sublevel coherence optically. A modulated pump laser field is used for velocity-selective resonance Raman excitation of oscillating Zeeman coherence in the J=1 ground state of Sm; the sublevel coherence is monitored by Raman heterodyne detection using a counterpropagating, unmodulated probe laser field. If the pump and probe fields interact with different atomic velocity subgroups, a signal only occurs in the presence of velocity-changing collisions which provide a transport of the Sm atoms with sublevel coherence in velocity space. The sublevel resonance curves turn out to be sensitive to both depolarizing and velocity-changing collisions; the measured line shapes show the characteristic features of Ramsey-type resonances. Our theoretical treatment is based on a novel four-level density matrix calculation; the analysis reveals that the temporal evolution of the collisional velocity-diffusion processes can simply be obtained by means of a Fourier transformation of the measured sublevel resonance curves. In the case of He perturbers the diffusion in velocity space can be described by a Fokker-Planck equation; here, the theoretical fits of the measured sublevel resonance curves directly yield the time constant for collisional thermalization and the cross section for depolarizing collisions. In the case of the heavier rare gases, however, the Fokker-Planck model fails to describe the velocity thermalization; here, the Keilson-Storer kernel yields a better agreement with the experimental data. We derive cross sections for depolarizing and velocity-changing collisions and values for the collisional velocity changes associated with orientation-preserving collisions.
We report on the observation of novel Ramsey-type resonances due to collisional velocity diffusion in atomic samarium vapor in the presence of rare-gas perturbers. The experimental technique uses counterpropagating laser fields and relies on coherent resonance Raman processes to optically excite and detect sublevel coherence within the Doppler-broadened
154Sm $\ensuremath{\lambda}=570.68$-nm $^{7}F_{1}\ensuremath{-}^{7}F_{0}$ transition. The experimental findings are in good agreement with theoretical predictions based on a Fokker-Planck equation; time constants for the collisional thermalization of Sm in He and Ne atmospheres are derived from our data.
A novel coherent Raman effect induced by a laser and a radio-frequency (rf) field is used to detect cw and pulsed nuclear magnetic resonance (NMR) in ground and excited electronic states. The effect is illustrated in the impurity-ion solid
Pr3+: La
F3 at 1.6 K utilizing the
Pr3+ optical transition $^{3}H_{4}({\ensuremath{\Gamma}}_{1})\ensuremath{\rightarrow}^{1}D_{2}({\ensuremath{\Gamma}}_{1})$. The laser field of frequency ${\ensuremath{\omega}}_{E}$ and the rf field (${\ensuremath{\omega}}_{H}$) induce a light wave at the sum ${\ensuremath{\omega}}_{E}+{\ensuremath{\omega}}_{H}$ (anti-Stokes) and difference ${\ensuremath{\omega}}_{E}\ensuremath{-}{\ensuremath{\omega}}_{H}$ (Stokes) frequencies, generating an absorptive or dispersive heterodyne beat signal (${\ensuremath{\omega}}_{H}$) with the laser field at a photodetector. The theory of this effect is characterized in a new three-level perturbation calculation which requires, unlike the usual stimulated Raman effect, that all three transitions be electric- or magnetic-dipole allowed. Detailed predictions are confirmed by cw measurements of the
Pr3+: La
F3 hyperfine splittings where the optical heterodyne signals are shot-noise limited. The
Pr3+ nuclear quadrupole parameters are obtained for the
3H4 and
1D2 states where the line centers are determined with kilohertz precision. The corresponding wave functions show significant hyperfine-state mixing, as required for all three transitions to be dipole allowed. The cw line shapes are narrow (30-160 kHz), inhomogeneously broadened by nuclear magnetic interactions, and reveal either a Gaussian or an anomalous second-derivative---like line shape. The spin-echo measurements for the
3H4 and
1D2 hyperfine transitions yield homogeneous line shapes which are Lorentzian, and rather surprisingly, linewidths in the narrow range 10-20 kHz, a result which tests current line-broadening theories.