Photodesorption of molecular nitrogen from Pt(111)

Ultraviolet photodesorption was observed from very weakly chemisorbed N-2 o n Pt(111) at surface temperatures less than 40 K. Photochemical data indica te that N-2 chemisorbs on the surface at low coverage and that physisorbed N-2 coexists on the surface at coverages greater than 0.07 ML. The low temp erature saturation coverage of N-2 on Pt(111) was 0.41 ML +/- 10%. Laser in duced photodesorption of N-2 was observed at ultraviolet wavelengths from 1 93 to 308 nm, but not at 355 nm, nor at 532 nm. Laser polarization experime nts were consistent with a substrate mediated excitation mechanism. The mea n translational energy of the photodesorbing N-2 was [E-T] = 0.25 +/- 0.05 eV, independent of excitation wavelength. The 193 nm photodesorption cross- section of similar to 5 x 10(-20) cm(-2) in the two-phase regime of coverag e increased as the coverage was reduced. The N-2 photodesorption dynamics a nd action spectrum are consistent with a desorption mechanism involving tra nsient attachment of a photoexcited substrate electron into an affinity ban d of chemisorbed N-2 located roughly 4.0 eV above the Fermi level; presumab ly the antibonding 2 pi* affinity band. (C) 2000 Elsevier Science B.V. All rights reserved.