3-PHOTON EXCITATION OF THE LOWER O-U(-PAIR STATES OF CL-2 VIA A CONTINUUM INTERMEDIATE STATE() ION)

Three-photon (1 + 2) excitation via the continuum of the B state is us ed to access relatively low vibrational levels of the alpha and gamma ion-pair states of Cl-2 in the region 59 000-72 000 cm(-1) through bon d stretching. The vibrational progressions are sufficiently extended f or the gamma state levels, previously assigned to the (1) Sigma(u)(+)( D-1) state to bi reassigned to the (3) Pi(0+) (P-3(0)) state in the fi rst ion-pair cluster. The (1) Sigma(u)(+)(D-1) ion-pair state is not S een directly in three-photon excitation in this spectral region but in stead produces a characteristic pattern of perturbations, particularly in the gamma(3) Pi(0+) state. Both the very selective vibronic excita tion and the restricted number of ion-pair states that can be accessed with three photons suggest propensity rules based largely on single e lectronic configurations for valence and ion-pair states.