Electronic spectroscopy and excited state dynamics of the Al–H2/D2 complex

Abstract

The electronic spectra of the Al–H₂ and Al–D₂ complexes are investigated in a collaborative experimental and theoretical study. The complexes were prepared in a pulsed supersonic beam and detected with laser fluorescence excitation spectroscopy. Transitions to bound vibrational levels in electronic states correlating with the excited-state Al(3d, 4p, 4d) + H₂/D₂ asymptotes were observed by monitoring emission from lower excited Al atomic levels, formed in the non-radiative decay of the excited complex. Fluorescence depletion has also been used to verify that the observed Al–H₂ bands all involve the same molecular carrier. The bands have been assigned to the more strongly bound Al–oH₂ and Al–pD₂ nuclear spin modifications. In contrast to our previous observations for Al(5s)–H₂ [X. Yang and P. J. Dagdigian, J. Chem. Phys., 1998, 109, 8920], for which only one potential energy surface (PES) emanates from the dissociation asymptote, the Lorentzian widths of the different vibrational bands in the 3d, 4p, 4d←3p transitions vary widely, in some cases allowing resolution of the rotational structure of the bands. With the help of the calculated Al(3p)–oH₂/pD₂ dissociation energies, binding energies of the observed excited vibronic levels are reported. The mechanism of predissociation is investigated theoretically through ab initio calculation of C_2v cuts of the excited PESs. It is concluded that predissociation occurs through coupling with the repulsive Al(4s)–H₂ PES. With these calculations, a qualitative interpretation of the observed bands could be made.