Selective laser desorption of ionic surfaces: Resonant surface excitation of KBr

We demonstrate evidence of selective laser-induced desorption of ground sta te Br(P-2(3/2)) and spin-orbit excited state Br(P-2(1/2)) atoms from KBr si ngle crystals following 6.4 eV irradiation. Laser excitation tuned selectiv ely to a surface resonance below the first bulk absorption band excites sur face states preferentially leading to surface specific reactions while indu cing relatively insignificant bulk reaction. The experimental results are s upported by embedded cluster ab initio calculations that indicate a reduced surface exciton energy compared to that of the bulk exciton with a slight further reduction for steps and kink sites. Low fluence irradiation of clea ved KBr crystals, near the calculated surface exciton energy of 6.2 eV, pro duces hyperthermal Br(P-2(3/2)) emission without a significant thermal or B r(P-2(1/2)) component. The hyperthermal emission is shown theoretically to be characteristic of surface induced reaction of exciton decomposition whil e thermal emission is attributed to bulk photoreaction. (C) 2001 American I nstitute of Physics.