Surface-aligned photochemistry:: Photolysis of HCl adsorbed on LiF(001) studied by Rydberg-atom time-of-flight spectroscopy

Photolysis of HCl adsorbed on LiF(001) was studied by means of hydrogen Rydberg-atom time-of-flight (HRTOF) spectroscopy. Experiments were performed using 193.3 nm excimer laser radiation or 121.6 nm tripled dye-laser radiation for photolysis. The H-atom translational energy distributions using 193.3 nm (6.41 eV) radiation showed three main features: a high-energy channel corresponding to elastically scattered H atoms (EL) peaked at 1.85 and 1.65 eV (leaving behind Cl and Cl*, respectively); a second channel similar to 1.2 eV lower in energy resulting from surface-aligned inelastic collisions (INEL, peaking at 0.6 eV); and a thermalized channel thought to be the product of multiple collisions and trapping of the scattered H. For 121.6 nm (10.2 eV) photolysis, the H-atom translational energy distributions showed four features: EL, with a maximum at 5.5 eV (the Cl and Cl* channels could not be separated at this high recoil energy); INEL(1), peaking at 3.2 eV; a more complex inelastic pathway, INEL(2), with a maximum at 0.5 eV; and, finally, a thermalized channel. Angular distributions for the elastically scattered H atoms indicated that they scattered from F- in the underlying LiF(001) at 40 and 48 degrees off-normal for photodissociation of HCl(ad) at 193.3 and 121.6 nm, respectively. These two angles are far from 71 degrees, the angle that would be observed from specular scattering of the H atom from the surface plane; this is strong evidence for the localized atomic scattering (LAS) identified in earlier studies performed in this laboratory. It appears that the higher the energy of the H projectile, the deeper it penetrates into the impacted surface atom, giving rise to the larger scattering angle. As in previous work, an exchange reaction was invoked to explain the similar to 1.2 eV energy loss in the INEL(I) channel, together with observed retention of direction in H following this strongly inelastic encounter. (C) 1999 American Institute of Physics. [S0021-9606(99)70201-1].