The photoabsorption spectrum of Na•••FH van der Waals molecule:: Comparison of theory and experiment for a harpooning reaction studied by transition state spectroscopy

The photodissociation of Na ... FH van der Waals molecules in the 1.5-2.3 eV energy region is a very interesting system for transition state spectroscopy, because the potential energy surfaces for electronically excited states funnel the system down to the ground electronic state in a critical region where detailed features of the potential energy surfaces may be important in determining the branching probability between the harpooning reaction to form NaF or the dissociative E-->V energy transfer process to form vibrationally excited HF. We used an analytic representation. reported earlier, of the potential surfaces for the two lowest A' electronic states of NaFa I as well as separable fits for two higher excited potential surfaces to simulate the experimental photodepletion spectrum of the Na FI-I van der Waals molecule. Franck-Condon analysis was performed for the (X) over tilde (2)A' --> (2)A', (X) over tilde (2)A' --> (B) over tilde (2)A ", and (X) over tilde (2)A' --> (B) over tilde' (2)A' transitions to predissociative states of the exciplex by making a separable approximation in Jacobi coordinates. Theoretical simulation based on ab initio energies and transition dipole moments produced an excitation spectrum that is in good agreement with the experimental data. Including the dependence of the transition dipole moment on nuclear geometry had only a small quantitative effect on the calculated photoabsorption spectrum. The present calculation, in spite of the approximations involved, provides a semiquantitative description of the experimental spectrum of the resonance states in the funnel and allows us to explain all the main features of the spectrum. (C) 1998 American Institute of Physics. [S0021-9606(98)00913-1].