Half-collision studies of singlet-to-triplet energy transfer: Action spectroscopy and predissociation dynamics of electronically excited Cd⋅H2 and Cd⋅D2 complexes

Abstract

The Cd⋅H2 and Cd⋅D2 van der Waals complexes were synthesized by expanding cadmium vapor and He/H2 mixtures into a supersonic free jet. The ‘‘half-collision’’ process, Cd(5s5p 1P1)⋅H2→Cd(5s5p 3PJ) +H2, was studied by fixing a probe dye laser pulse (delayed 10 ns) onto one of the Cd(5s6s 3S1←5s5p 3PJ) transitions while exciting the Cd⋅H2(Cd⋅D2) complex with a pump dye laser pulse tuned across frequencies near that of the free Cd(5s5p 1P1←5s5s 1S0) atomic transition. When the probe laser was tuned to detect Cd(5s5p 3P2), an action spectrum to the red of the atomic transition was obtained for Cd⋅H2 consisting of a broad continuum superimposed upon which was an anharmonic series of vibrational transitions with discernible, blue-shaded rotational structure. A similar spectrum was recorded for Cd⋅D2, except that only very broadened blue-shaded rotational structure was observed. From the isotopic band-head splittings, computer simulations of the rotational band structure, and recent ab initio calculations of Cd⋅H2 potential surfaces by Jack Simons and co-workers, it is tentatively concluded that the vibrational resonances are due to the (1B1;K′a=0) ←(1A1;K■a=1) perpendicular transition of ortho Cd⋅H2 (para Cd⋅D2 ). The continuum is assigned to the other perpendicular transitions of para and ortho Cd⋅H2(Cd⋅D2) which excite 1B2(K′a=0,1,2) and 1B1(Ka=1,2) states. The attractive 1B2 state predissociates rapidly due to potential surface crossings with both the 3A1 and 3B1 repulsive states of CdH2, and we postulate that the 1B1 energy levels with K′a≥1 are strongly coupled with the 1B2 levels with Ka≥1. The 1B1(K′a=0) level is predissociated via a less efficient surface crossing with the more strongly repulsive 3A1 state. The yield of Cd(5s5p 3P2) vs Cd(5s5p 3P1) for the ‘‘vibrational resonance’’ mechanism, 2.6±0.2, is quite different from that for the ‘‘continuum’’ mechanism, 1.17±0.05, consistent with this idea. The 3A1 state correlates only with Cd(5s5p 3P2), but the B2 spin–orbit component of the 3B1 state (which will be produced by predissociation of the 1B2 state) correlates with Cd(5s5p 3P1). We estimate that the CdH2(1B1) state has a value of De≊1700 cm−1 at R′e(Cd–H2)≊2.5 Å, qualitatively consistent with the ab initio calculations, which show that the CdH2(1B1) potential is quite flat, with no H–H bond stretching or appreciable repulsion until Cd–H2 distances as small as 2.0 Å are reached.