A critical look at electron-photon coincidence experiments with heavy noble gases in the regime of large impact parameters

The electron-photon coincidence technique has been employed in experimental investigations aimed at elucidating the finer details of the excitation of the first excited states of the heavy noble gases by electron impact since the mid 1970s. The agreement between measured data and the predictions of first-order perturbative theories such as the distorted wave Bom approximation (dwba) and the first-order many-body theory (fombt) ranges from good in some cases to rather poor in other cases. In particular, early experimental evidence of the presence of significant spin effects in the regime of large impact parameters has not been supported by theoretical calculations. Recent developments showing how instrumental and physical effects can often obscure a meaningful comparison between measured and calculated data have largely removed the previous ambiguities. This article will review the current status of electron-photon coincidence experiments with the heavy noble gases, critically assess the level of agreement and disagreement between experiment and theory, discuss apparent trends in the data, make recommendations for the direction of future experiments and suggest ways to ensure that measured coherence parameters are free from instrumental and physical effects to the maximum extent possible.