Half-collision studies: Action spectroscopy of electronic energy transfer within the Cd⋅CH4 van der Waals complex

Abstract

The technique of action spectroscopy has been used to characterize the ground state and excited states of the Cd–CH4 van der Waals complex and to study ‘‘half-collision’’ electronic energy transfer processes within the complex. By tuning one laser pulse in the spectral region near an atomic transition of the free Cd atom and monitoring a predissociation product with a second, delayed laser pulse, it has been possible to obtain ‘‘action’’ spectra which provide information about the excited Cd–CH4 potential surfaces from which electronic energy transfer processes occur. Excitation of the Cd⋅CH4 complex to the red of the Cd(5s5p 3P1 ← 5s5s 1S0) transition while monitoring Cd(5s5p 3P0) produces an anharmonic series of blue-shaded bands which have been assigned to the Cd⋅CH4(A 30+ ← X 10+) transition. The spectra are similar to the A 30+ ← X 10+ laser-induced fluorescence (LIF) spectra of the Cd⋅Ar and Cd⋅Kr complexes, except that there are weaker subbands ∼9 cm−1 to the blue of each main band. Computer simulations of the rotational structure of several of the bands were successful, and spectroscopic parameters for both the X 10+ ground state and the A 30+ upper state are reported. It is postulated that Cd⋅CH4 is a hindered rotor with C3v (facial) geometry in both ground and upper states. Detailed considerations of the possible nuclear-spin isomers of symmetries A, F, and E, using the CH4 hindered-rotor calculations of Endo and Ohshima, lead to an assignment of the main bands as A 30+(E,K′ = 1) ← X 10+(E,K″ = 1) transitions and the subbands as A 30+(2F,K′ = 1) ← X 10+(2F,K″ = 1) transitions. It is suggested that the expected A 30+(F,K′ = 0) ← X 10+(F,K″ = 0) and A 30+(A,K′ = 0) ← X 10+(A,K″ = 0) transitions were not observed because the K′=0 states lack the rotational motion about the C3v axis which can couple the A 30+ (A1) state with the a 30− (A2) state [which correlates with Cd(5s5p 3P0)]. Action spectra of Cd⋅CH4 were also obtained to the red of the Cd(5s5p 1P1 ← 5s5s 1S0) transition when the probe laser was monitoring Cd(5s5p 3P2) products. The severely broadened spectra are consistent with extremely rapid predissociation of the C 1Π1 (C 11) van der Waals state by the repulsive c 3Σ1 (c 31) state, which correlates with Cd(5s5p 3P2) + CH4.