HALF-COLLISION STUDIES - ACTION SPECTROSCOPY OF ELECTRONIC-ENERGY TRANSFER WITHIN THE CD-CH4 VANDERWAALS COMPLEX

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 (P-3(1) <-- 5s5s S-1(0)) transition while monitoring Cd(5s5p P-3(0)) produces an anharmonic series of blue-shaded bands which have been assigned to the Cd.CH4 (A (3)0+ <-- X (1)0+) transition. The spectra are similar to the A (3)0+ <-- X (1)0+ laser-induced fluorescence (LIF) spectra of the Cd-Ar and Cd-Kr complexes, except that there are weaker subbands approximately 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 (1)0+ ground state and the A (3)0+ 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 (3)0+(E,K' = 1) <-- X (1)0+(E,K" = 1) transitions and the subbands as A (3)0+(2F,K' = 1) <-- X 1(0)+(2F,K" = 1) transitions, It is suggested that the expected A (3)0+(F,K' = 0) <-- X 1(0)+(F,K" = 0) and A (3)0+(A,K' = 0) <-- X (1)0+(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 (3)0+ (A1) state with the a (3)0- (A2) state [which correlates with Cd(5s5p P-3(0))]. Action spectra of Cd.CH4 were also obtained to the red of the Cd(5s5p P-1(1) <-- 5s5s S-1(0)) transition when the probe laser was monitoring Cd(5s5p P-3(2)) products. The severely broadened spectra are consistent with extremely rapid predissociation of the C 1-PI-1 (C (1)1) van der Waals state by the repulsive c 3-SIGMA-1 (c (3)1) state, which correlates with Cd(5s5p P-3(2)) + CH4.