Alkali-helium exciplex formation on the surface of helium nanodroplets. II. A time-resolved study

We have monitored the time evolution of the fluorescence of K*He exciplexes formed on the surface of helium nanodroplets using reversed time-correlated single photon counting. In modeling the present data and that from our previous work on Na*He, we find that partial spin-orbit coupling as well as the extraction energy of helium atoms from the droplet contribute to the observed dynamics of both K*He and Na*He formation, which differ considerably after either D-1(n P-2(1/2) <-- n S-2(1/2)) or D-2(n P-2(3/2)<--n S-2(1/2)) excitation for both K(n = 4) and Na(n = 3). Our quantitative prediction of the Na*He formation dynamics coupled with preliminary data on and modeling of the formation dynamics of K*He allow for extrapolation to the case of Rb*He. Spin-orbit considerations combined with a simple model of helium atom extraction from the matrix reveal the following predicted trend: as the choice of the alkali guest atom is moved down the periodic table, alkali atom-He exciplex formation along the 1 (II3/2)-I-2 surface occurs faster while formation along the 1 (II1/2)-I-2 surface occurs more slowly, ceasing to occur at all in the case of Rb. (C) 2000 American Institute of Physics. [S0021-9606(00)01245-9].