[en] The method of far wing collision pair spectroscopy, providing dynamical information under conditions of a half collision with control of the potential curve, the collision energy and the impact parameter range is outlined and applied to fine structure changing and depolarization mechanisms of Na3P atoms interacting with rare gas targets at thermal energies. For this purpose experimental excitation spectra of the Na3P_J find structure population ratio and J=3/2 alignment obtained under single collision conditions are analyzed on the basis of a fully quantummechanical, nonadiabatic theory of optical line shapes and semiclassical model calculations. Discussed in detail are the effects on the alignment formation of molecular coherences, spin decoupling from the molecular axis, realistic trajectories and the type of transformation of molecular to atomic states during the half collision following excitation. (orig.)

No abstract available

[en] We have investigated experimentally and theoretically the spectral distribution and collisional depolarization of laser light near resonant to the Na transition 3 ²Ssub(1/2) reversible 3²Psub(3/2) scattered from a Na/Ar mixture in a cell (nsub(Na) approx. 4.0 x 10¹⁰ cm^-3, nsub(Ar) approx. 1.0 x 10¹⁷ cm^-3). In particular, at excitation intensity of approx. 3 mW/mm² and bandwidth of < 10 MHz intensity Isub(F) and linear polarization degree Psub(F) of the D₂-fluorescence component of the scattered spectrum have been measured as functions of the laser detuning Δlambdasub(L) from the D₂-resonance within two regions: 1) Δlambdasub(L) < approx. Doppler-width; 2) 20 >= Δlambdasub(L)/Δlambdasub(D) >= 1. Within the first region the influence of hyperfine splitting as well as pumping of the hyperfine and Zeeman sublevels of 3 ²Ssub(1/2) was studied in detail. Using an overall, total collision cross section for depolarization of 3 ²Psub(3/2) state atoms, calculations on the basis of the theory of redistribution reproduce qualitatively, but in a consistent manner, the experimental functions Isub(F)(lambdasub(L)) and Psub(F)(Δlambdasub(L)). The possibility to derive differential cross sections from Psub(F)(Δlambdasub(L)) is discussed. (orig.) 891 WL/orig. 892 MB

[en] By means of a N₂-laser pumped dye-laser (Psub(L) approx. 20 kW, Δνsub(L) < 10 GHz) different two-photon resonant four-wave coupling processes in Ba vapour (nsub(Ba) approx. 10¹⁶ cm^-3) using the Ba states 6s8s¹S₀ and 7s5d¹D₂ were investigated. Coherent line radiation with conversion rates up to 10^-3 was generated within the range of lambda = 190-200 nm by sum-frequency mixing νsub(UV) = 2ν₁ + ν₂₂ of 3 laser photons and within the range of lambda = 250-380 nm by coupling νsub(UV) = 2ν₁ +- νsub(IR) of 2 laser photons with one photon vsub(IR) stimulated emitted in laser-induced Ba transitions. For the second coupling type the various nonlinear processes contributing to the formation of the coupling components are discussed. The power of the UV-component as function of intensity and resonance detuning of the laser as well as on the phase-mismatch was calculated on the basis of the small signal theory and compared to the experimental data. (orig.)

[en] A new method for determination of relative dipole matrix elements for transitions between excited states is presented, using sum frequency generation spectroscopy. The method is based on measuring the frequency spacing of related extrema in the dispersion like shapes within the SFG-spectrum, caused by interference between resonant and nonresonant parts in the third order polarizability. It is applied to CdI with the generated frequency in the range of the principal series 5s¹S₀->np¹P₁, n=12-28. Values of the products μsub(6snp)μsub(np5s) are presented obeying the nsup(*-3) law. (orig.)

[en] We have investigated experimentally and theoretically the change in the polarization state of polarized laser light scattered near a ²P₃/₂-²S₁/₂ atomic resonance as function of the laser frequency. For a collision free Na vapor at number densities <10¹⁰ cm^-3 it is found experimentally, that the polarization degree changes from a small level at frequencies between hyperfine components of the 3 ²P₃/₂ - 3 ²S₁/₂ transition towards high values at separations large compared to the hyperfine splitting. By comparison with scattering theory it is shown, that this change is typical for the transition from resonant to nonresonant scattering in the presence of atomic hyperfine interaction. (orig.)

No abstract available

[en] By selective optical excitation of collision pairs and observation of the reemitted fluorescence information is obtained on the role of the molecular channels involved in inelastic collisions. As an example case we have studied experimentally the Li(3D→3P) excitation transfer in Li(3D)X systems with X=Ne, Ar by means of the optical collision process Li(2P)+X+hν →LiX(3DΛ)→Li(3P,3D)+X where LiX(3DΛ) collision molecules dissociate into Li(3P, 3D) atoms following laser excitation hν of Li(2P)+X pairs. For this purpose we measured the Li 3P/3D population ratio by the fluorescence from these levels as function of the laser detuning Δν from the Li(2P-3D) transition and the rare gas pressure, and determined from this the 3P/3D excitation ratio B(Δν) for single collision conditions. The experiments were performed using two step cw laser excitation of gaseous mixtures Li+X at temperatures around 600 K in the detuning range vertical stroke Δν vertical stroke ≤100 cm^-1. The B(Δν) profiles obtained display strong blue-red wing asymmetries both for Li^*Ne and Li^*Ar. This reflects different dissociation probabilities from the 3DΣ or 3D(Π,Δ) states that are initially prepared by blue wing or red wing excitation, respectively. The results are qualitatively discussed in terms of new ab initio potentials for the two systems. (orig.)

[en] A comparison is made of experimental far-wing profiles for the system Li(2P → 3D)He with quantum calculations of thermally averaged free-free continua. Using input adiabatic potentials and transition moment functions from both ab initio and semi-empirical approaches, the comparison shows: (i) new ab initio potentials for Li*(2P,3P,3D)He reproduce the spectral positions of the observed rainbow satellites well; the height of the 3DΣ barrier predicted agrees with experiment to within ±15 cm^-1, whereas its position is too large by 0.3 a₀; (ii) potentials obtained with various semi-empirical methods reproduce the satellite structure qualitatively, but are generally too repulsive in the 3DΣ state at intermediate and large internuclear separations; (iii) transition moment functions for the asymptotically forbidden 2P → 3P transitions reproduce, with different degrees of accuracy, the intensity of the red-wing satellite related to the 3PΣ potential extremum around 12 a₀. The vibrational energies in the 2PΠ and 3DΔ states calculated with the ab initio potentials reproduce to within a few cm^-1 those obtained from rotationally resolved band spectra reported in the literature. (Author)

[en] The depolarization mechanism of excited alkali atoms interacting with ground state rare gas atoms are investigated using the method of far wing spectroscopy of collision pairs under single collision conditions. From a semiclassical theory explicite expressions for the spectra of alkali multipole components σ_k^(J)(ω_L) are derived assuming quasistatic excitation at a localized internuclear separation R_L(ω_L) related to the laserfrequency ω_L as well as realistic models for the half collision following excitation. The collision models are characterized by different Hund's coupling regions, where excitation takes place and which are traversed on the collision path. Due to selection rules for excitation of populations and coherences and for support of multipoles the σ_k^(J)(ω_L) are shown to depend strongly on the collision model. From the spectra thus a labelling of the initial molecular states and mapping of the change in coupling case is possible. Estimations of the contributions of the various angular momentum recoupling effects are given. (orig.)