The fluorescence intensities for the D lines in non-resonant excitation of the alkali-helium pairs under single-collision conditions were obtained from calculations of the optical collision spectra by the quantum coupled-channels method. The close relation of the non-adiabatic transition probability from a molecular level to an atomic state during the final stage of an optical collision and the spectral dependence of the relative fluorescence intensities is investigated. An analysis of the impact region in terms of line-broadening and fine-structure cross sections calculated separately for all systems confirms the factorisation approximation.
The spectra of fluorescence depolarisation (orientation, alignment) applying to single collision conditions are analysed for the alkali-helium series. These are derived from a quantum calculation of the optical collision cross sections for non-resonant collision pair excitation and include non-adiabatic couplings and thermal averaging over the collision energy. The authors find strong spectral variations of the polarisation which reflect the potential characteristics of these systems. A numerical test is performed for the classical Lewis model using realistic trajectories and a reasonable choice of the decoupling radius and fair quantitative agreement with the quantum calculation is shown. Large residual alignments in the far wings demonstrate a lack of complete reorientation of the transition dipole in a strong collision.
Due to the large values of the fine structure interval for the first resonance lines of Rb and Cs, we may treat separately the 2P1/2 and 2P3/2 atomic levels when perturbed by Helium atoms. Within the framework of semi-classical and impact approximations, and assuming an exchange interaction, the diffusion matrices S1/2 and S3/2 are numerically computed ; firstly by neglecting all 1/2 - 3/2 coupling, and secondly, by a more accurate treatment which makes use of the molecular potentials and shows that the D1-lines depolarization is entirely due to the short range nonadiabatic coupling. The validity of analytical approximate resolutions for asymptotic regions is studied, and also the trajectory for the P1/2 levels, correlated with a single molecular curve; the rectilinear assumption appears to be insufficient for very short range phenomena. The agreement with experimental results confirms that the exchange interaction predominates for intermediate distances.
Excitation transfer between the
62P21 and
62P23 resonance states in cesium, induced by Cs-
N2 and Cs-
H2 collisions, was observed in the first experiment of its kind, in which techniques of sensitized fluorescence are used to investigate collisional mixing between resonance states of alkali atoms in mixtures with molecular gases. The cesium-vapor-gas system, in which the cesium vapor pressure was low enough to avoid radiation trapping, was irradiated with a single component of the resonance doublet. The fluorescent light, which contained both components of the doublet, was monitored at right angles to the direction of excitation. Measurements of relative intensities of the fluorescent components yielded the following cross sections for excitation transfer and for quenching of resonance radiation. For Cs-
N2 collisions: ${Q}_{12}(^{2}P_{\frac{1}{2}}\ensuremath{\rightarrow}^{2}P_{\frac{3}{2}})=3.6$
A˚2, ${Q}_{21}(^{2}P_{\frac{1}{2}}\ensuremath{\leftarrow}^{2}P_{\frac{3}{2}})=16.2$
A˚2, ${Q}_{10}(^{2}S_{\frac{1}{2}}\ensuremath{\leftarrow}^{2}P_{\frac{1}{2}})=55.2$
A˚2, ${Q}_{20}(^{2}S_{\frac{1}{2}}\ensuremath{\leftarrow}^{2}P_{\frac{3}{2}})=63.5$
A˚2. For Cs-
H2 collisions:
Q12=6.7 A˚2,
Q21=29.6 A˚2,
Q10=11.7 A˚2,
Q20=6.7 A˚2. The large sizes of the excitation-transfer cross sections, which are comparable to the corresponding Cs-Cs cross sections, are attributed to resonances with transitions in the molecules.
Fine-structure mixing cross-sections for the alkalis in collisions with the rare gases are reviewed. Included in the review are all the rare gases in collisions with all of the first excited state of the alkalis, the second excited state for K, Rb and Cs and the third excited state for Rb and Cs. The cross-sections are converted to probabilities for energy transfer using a quantum-defect calculated cross-section and are then presented as a function of adiabaticity. The data shows a clear decreasing trend with adiabaticity but secondary factors prevent the probabilities from decreasing as quickly as expected. Polarizability is introduced as a proxy for the secondary influences on the data as it increases with both rare gas partner and alkali excited state. The polarizability is shown to cause the probability of fine structure transition to be higher than expected. An empirical model is introduced and fit to the data. Future work will develop a model using time-independent perturbation theory in order to further develop a physical rational for the dependence of fine structure cross sections on adiabaticity and to further understand the secondary influences on the probability for fine structure transition.
Calculated potentials for the NaHe molecule are used to obtain collision-induced widths and shifts for the sodium
D lines together with cross sections for fine-structure transitions and the relaxation of the multipole polarizations of the resonance levels. The atomic collision is treated in an adiabatic approximation in which the colliding atoms form a molecular system which rotates during the collision. We find the resonance lines to be broadened equally and to be approximately 70% wider, and with associated shifts of an order of magnitude smaller, than expected on the basis of van der Waals forces. The dependence of the cross sections for relaxation of the multipole polarizations of the resonance levels on the molecular-coupling conditions and the rotation of the system during collision is discussed and indicates a limitation on the sum rule for these cross sections. Good over-all agreement with the existing experimental data is obtained.