Collision-Induced Anisotropic Relaxation in Gases

Abstract

An expression is derived for the cross sections, Vj\ for the collision-induced relaxation of the multipole moments of an ensemble of excited atoms in a state of total angular momentum j. Various general sum rules are obtained for the &/ x \ as well as for the Kf x ' =aA x^ -cy^, the cross sections for reorienting collisions.For a j=l state, one obtains A^1* / A t {2) = $•. This result implies that the cross section for collision induced I Am I =2 transitions is always twice that for \Am\ = l transitions, independent of the assumed interaction potential. Absolute cross sections are obtained using the van der Waals collision model. Translational motion is treated in a classical linear path approximation. It is shown that the cross sections can be written in the form oj(#)where v is the relative velocity between the colliding atoms, D depends on the particular atoms and states considered, and <j)Ax) is a geometric factor which is tabulated for j values up to j=8. The A-*(#) are related to the cross sections for transfers between magnetic sublevels. It is shown that in the present model a transfer from sublevel a to -a is forbidden for states with half-integer j values.

Collisional depolarization of light alkali atoms excited to their lowest 2P levels

“…It can be seen that the average cross sections deduced from the leading tensor components of eq. 1221 are in the ratio given by Elbel and Naumann (1967) and by Wang and Tomlinson (1969). This result is, however, not too meaningful because it does not include the contribution of the pseudovector components.…”

“…Since the range of experimental methods is steadily increasing and now includes such techniques as, for instance, optical pumping with D2-light (Franz and Franz 1966;Elbel and Naumann 1967;Fricke et al 1967;Ackermann and Weber 1968;Zhitnikov et al 1969) and Zeeman scanning techniques (Berdowski and Krause 1968), a further improvement of the theoretical methods would be desirable. Most of the theoretical treatments which have been proposed so far (Callaway and Bauer 1965;Byron and Foley 1964;Omont 1965 ;Mandelberg 1968 ;Wang and Tomlinson 1969) are based on timedependent perturbation theory. The disadvantage of this approach lies in its inability to account for the effect of the transitions between the atomic energy levels on the atomic trajectories.…”

The Line Width of the EPR Signal of Gaseous Nitric Oxide as Determined by Pressure and Temperature-Dependent X-band Continuous Wave Measurements

“…Ultee [39] reported for several other paramagnetic gases cross sections determined from EPR line widths which are also generally larger than those determined from transport properties. Here we describe qualitatively an explanation for the difference between cross sections determined from EPR or viscosity data without going into the details of the quite elaborate theory of collision-induced relaxation of angular momenta in gases [40][41][42][43]. We only note that within these theoretical frameworks the presence of nonspherical electronic intermolecular interactions like the electrostatic dipole-dipole interaction or van der Waals interactions can contribute essentially to the corresponding cross sections.…”

Theoretical approach for collisional depolarization of Rydberg atoms

“…Analysis of (i0), (14), and (19) shows that the influence of the phase t/c is negligible and as a result we have ~0=0, ~1=~-o;(3/J, ~2=~;(3L).…”

“…The general problem of collisional depolarization of excited atoms has been treated by Dyakonov and Perel [123, Omont [13], and Wang and Tomlinson [14]. Collisional depolarization (relaxation of orientation and of alignment) of Rydberg states is one of the relaxation processes and can be investigated on the basis of the irreducible tensors of the density matrix technique [15,16].…”