PHOTODISSOCIATION DYNAMICS OF JET-COOLED H2O AND D2O IN THE NON-FRANCK-CONDON REGIME - RELATIVE ABSORPTION CROSS-SECTIONS AND PRODUCT STATEDISTRIBUTIONS AT 193 NM

Quantum state distributions for nascent OH and OD fragments generated by Franck-Condon ''forbidden'' 193 nm photodissociation of H2O and D2O are reported, with the two isotopomers initially prepared in their ze ro-point vibrational and lowest ortho/para nuclear spin allowed rotati onal states (i.e., J(KaKc)=1(01) and 0(00) in a 3:1 ratio for H2O and 1:2 ratio for D2O) by cooling in a slit supersonic expansion. Product state distributions are probed via OH/OD laser-induced fluorescence (L IF) with cylindrical mirror collection optics optimized for the slit e xpansion geometry, which makes photodissociation studies feasible with cross sections as low as approximate to 10(-26) cm(2). The OH and OD fragments are formed exclusively in v=0, but with highly structured qu antum state distributions in rotational, Lambda-doublet, and fine stru cture levels ((II3/2+)-I-2, (II1/2+)-I-2, and (II3/2-)-I-2) that exhib it qualitatively different trends than observed in previous jet photol ysis studies at 157 nm in the Franck-Condon ''allowed'' regime. The re lative OH/OD fragment yields at 193 nm indicate a 64+/-10 times greate r propensity for OH vs OD bond cleavage in H2O than D2O, which is more than three-fold smaller than predicted from full three-dimensional qu antum scattering calculations on ground ((X) over tilde(1)A(1)) and fi rst excited state ((A) over tilde(1)B(1)) potential surfaces. One-dime nsional semiclassical calculations of the Franck-Condon overlap matrix elements confirm these discrepancies to be considerably outside uncer tainties associated with the ground and excited state potential surfac es. These results indicate that the photodissociation dynamics for thi s benchmark system are not yet fully understood and suggest either non -Born-Oppenheimer effects or contributions from other electronic surfa ces may be important in the extreme non-Franck-Condon photolysis regim e. (C) 1997 American Institute of Physics.