Collisional relaxation of atomic excited states, line broadening and interatomic interactions

P. R. Berman, Willis E. Lamb

In the limit of a dipole-dipole interaction between neutral atoms and the binary-collision approximation, we present a theory of pressure broadening in which the multiplet structure of the atomic levels is considered. The appropriate Schr\"odinger equations for the collision problem are numerically integrated to obtain solutions for both resonant and foreign gas broadening. The presence of radiation fields is neglected in the first half of the paper, where the collisional time rate of change of the density matrix is found. The pertinent collisional decay parameters are extracted from these equations. The ratios of the parameters of the average magnetic dipole to electric quadrupole moment obtained were 1.21\ifmmode\pm\else\textpm\fi{}0.02 for the relaxation of the entire ensemble in resonant broadening, 0.966\ifmmode\pm\else\textpm\fi{}0.012 for the relaxation of the initially excited atoms in resonant broadening, and 1.12\ifmmode\pm\else\textpm\fi{}0.02 for the relaxation of the initially excited atoms in foreign gas broadening. In the second half of the paper, the collisional broadening results are incorporated in a systematic evaluation of (i) spectral profiels (effects arising from the velocity and recoil of the emitting atom are neglected), (ii) Hanle-effect line shapes, and (iii) laser phenomena. For the case of resonant broadening, the radiation trapping process is reviewed and its influence on line shapes discussed. Finally, a detailed comparison of the theory with experimental findings as well as with previous theoretical results is made. We conclude that the dipole-dipole interaction is sufficient to explain most cases of resonant and some cases of foreign gas broadening.

A. P. Kazantsev

Roberto Buffa

This Brief Report reports a study of coherent phenomena occurring in the collisional excitation of an atomic continuum of ionization structured by intense laser radiation. Two interfering pathways to the continuum, induced by interatomic interaction processes (collision-induced ionization and radiative-collisional excitation followed by photoionization), give rise to Fano-type line shapes for the cross section of the process.

E. E. Nikitin, Anatoly I. Burshtein

Henry Margenau, Neil R. Kestner

K. W. Godfrey

The work described in this thesis is an experimental study of collisional relaxation of two-excitation-pulse optical echoes on the caesium 6S-7P transitions perturbed by noble gases using the echo polarisation rotation discrimination technique. Theoretical preliminaries include the development of a Schrddinger interaction representation theory of the formation of echoes resulting from two resonant excitation pulses applied to a two-state system. Rotation of the echo polarisation enables a polariser to be used to discriminate against the excitation pulses in favour of the echo. To illustrate the mechanisms behind this rotation, a simplified theory based on a nearly degenerate two level system is developed. The theory of collisional relaxation of echoes by phase interrupting and quantum diffractive velocity changing collisions is introduced. In traditional line profile spectroscopy the former type of collision cause line broadening whilst the effects of the latter are unobservable. The experimental work of this thesis involves the measurement of collisional relaxation of two-excitation-pulse echoes on the caesium 6Si/2-7Pi/2 (459nm) and 6S1/2-7P3/2 (455nm) transitions perturbed by low pressure (< 1 torr) of noble gas (He, A t or Xe). The experiments were carried out using a range of interpulse times from about 30 to 200ns. The broadening constants, determined to about 3% from the relaxation measurements, show significant discrepancies with traditional line profile measurements. For the longer interpulse times there is evidence for the quantum diffractive velocity changing aspects of the collisions. The associated cross-sections and average magnitudes of the mean velocity changes are estimated from the data.