James Williams

A Monte Carlo study of radiation trapping effects

A Monte Carlo simulation of radiative transfer in an atomic beam is carried out to investigate the effects of radiation trapping on electron–atom collision experiments. The collisionally excited atom is represented by a simple electric dipole, for which the emission intensity distribution is well known. The spatial distribution, frequency and free path of this and the sequential dipoles were determined by a computer random generator according to the probabilities given by quantum theory. By altering the atomic number density at the target site, the pressure dependence of the observed atomic lifetime, the angular intensity distribution and polarisation of the radiation field is studied.

Angular and polarisation correlations between cascade photons

The density matrix theory is applied to establish a connection between the configuration of the excited atomic states and the intensity and polarisation of the cascade photons emitted during the decay process. Explicit expressions are obtained for the P, D and F states of hydrogen and helium atoms. These formulae can be used to assist experimental design for the most efficient and revealing measurements. In addition, formulae for the integrated polarisation of the emitted photons from the P, D, F and G states of hydrogen and helium are presented in terms of partial excitation cross sections.

Resonance trapping effects in stepwise electron/laser excitation

Abstract This paper considers the relaxation processes in stepwise electron/laser excitations that are affected by radiation trapping. The pressure dependence of the observed linear and circular polarisations is examined and quantified. An extrapolation to zero pressure is found to be significantly different from a linear fitting procedure. This enables the extraction of true values of the state multipoles, which describe the alignment and orientation of the mercury 6 1 P 1 state immediately after electron-impact excitation.

Polarised electron study of angular momentum coupling in the neon 3p states

The excitation of the 2p5 3p[]3 and 2p5 3p′[h]1 states in the neon 3p manifold by transversally polarised electrons is studied in the electron energy region near threshold and beyond. The spin-orbit and exchange interactions during the excitation are explored by the measurement of integrated Stokes parameters P2 and P3 for the decay radiation from these states. Experimental evidence is given for the breakdown of LS coupling for the neon 3p (J = 1) state and the important role of spin–orbit and exchange interactions of the atomic electrons for the excitation of the state. Negative ion resonances strongly influence the polarisations of the decay radiations.