PHOTODESORPTION AND PHOTODISSOCIATION OF OCS ON GAAS(100)

Adsorption of OCS on GaAs(100) has been investigated using temperature -programmed desorption and photoinduced desorption/dissociation spectr oscopy. It was observed that OCS molecularly adsorbs on GaAs(100) and thermally desorbs at 160 K. Adsorbed OCS molecules either dissociate i nto gaseous CO, leaving S adsorbed on GaAs(100), or desorb as molecula r OCS under irradiation of lambda = 300 similar to 770 nm. The photodi ssociation cross section is about 5.0 x 10(-19) cm(2) at 300 nm, which is about 2 orders of magnitude higher than that for gaseous OCS at 25 0 nm and even 10 times greater compared with that for OCS on Ag(111) a t 254 nm. It drops rapidly as the wavelength increases and becomes und etectable at about 480 nm. In addition, the photodesorption cross sect ion was found to be about 8.4 x 10(-20) cm(2) at 300 nm with a thresho ld at about 770 nm. We also observed that the photodissociation cross section is strongly dependent on the coverage of OCS on GaAs(100), cha nging from 2.0 x 10(-18) cm(2) for 0.03 L OCS on GaAs(100) at 320 nm t o 3.6 x 10(-19) cm(2) for 3.3 L OCS. A 2.4 eV red shift of threshold p hoton energy for photodissociation of OCS on GaAs(100) was observed in our experiment relative to gaseous OCS. Our results support the inter action of photogenerated carriers with adsorbed OCS in the photochemic al processes. The observed difference in the threshold energies for ph otodissociation and photodesorption is possibly attributable to the di fferent final states involved.