The effect of orbital alignment on the forward and reverse electronic energy transfer Ca(4s5p 1P1)+M⇄Ca(4s5p 3P J)+M with rare gases

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Abstract
Effects of orbital alignment on the relative cross sections for electronic energy transfer are determined for the near resonant transfer between Ca(4s5p 1P1) and Ca(4s5p 3PJ) states with rare gas collision partners. The experiments are carried out by pulsed laser excitation in a crossed beam. The results for the forward direction, 1P to 3P, formulated in terms of the ratio of the maximum to minimum transfer probability are: 3He 1.61±0.05; He 1.60±0.03; Ne 1.55±0.10; Ar 1.52±0.21; for Kr, transfer occurs, but no preference is distinguishable within 1±0.2; Xe 1.44±0.06. The results for He, Ne, and Ar indicate a clear preference in the transfer for the initially prepared molecular Π state. For Xe the molecular Σ state is dominant. The energy transfer is also carried out in the reverse direction, 3P1 to 1P, for He and Xe, obtaining 1.65±0.10 and 1.94±0.22, respectively. Analysis of the state preparation suggests that the reverse direction favors the asymptotic molecular Σ state for He and the molecular Π state for Xe. These alignment results provide a first experimental determination of the dominant electronic states involved in a collisional energy transfer process.