Cluster isolated chemical reaction (CICR) spectroscopy: Ba atoms and Ba(CH4)(n) complexes on large neon clusters

The cluster isolated chemical reaction (CICR) technique was applied to neon clusters (Ne-approximate to 7000) on which barium atoms and methane molecu les were deposited. Clusters carrying barium only were studied first. Quali tatively, the present results on neon clusters are in line with our previou s results on argon clusters. In particular, surface location of barium was observed. The central part of the present work concerns neon clusters carry ing both one barium atom and one to ten methane molecules. Several types of spectroscopy were performed in the region of the resonance transition (6s( 2))S-1 --> (6s6p)P-1 of barium (excitation spectrum of the total fluorescen ce, emission spectrum, action spectrum for forming (6s6p)P-3), and experime nts where the number of methane molecules per cluster, which was strictly c ontrolled, was varied systematically. The corresponding results were interp reted on the ground of a model, which transposes both chemical thermodynami cs of equilibria and reaction kinetics to CICR experiments. Such an approac h has a strong relationship, although it is more simple, with the thermodyn amical approach to reactions in micellar solutions. The present thermodynam ical model helped us to determine the origin of the action spectrum for for ming Ba(6s6p)P-3 in clusters carrying both one barium atom and an average o f 2.5 methane molecules. This action spectrum was assigned to direct excita tion of the Ba(CH4) and Ba(CH4)(2) complexes. The present thermodynamical m odel was also applied to our former results on argon clusters. This allowed us to derive a consistent picture of the association reaction of barium wi th methane and of the quenching of electronic excitation of barium by metha ne in both environments. (C) 2000 American Institute of Physics. [S0021-960 6(00)01402-1].