Photodesorption of molecular nitrogen from Pt(111)

Ultraviolet photodesorption was observed from very weakly chemisorbed N2 on Pt(111) at surface temperatures less than 40 K. Photochemical data indicate that N2 chemisorbs on the surface at low coverage and that physisorbed N2 coexists on the surface at coverages greater than 0.07 ML. The low temperature saturation coverage of N2 on Pt(111) was 0.41 ML±10%. Laser induced photodesorption of N2 was observed at ultraviolet wavelengths from 193 to 308 nm, but not at 355 nm, nor at 532 nm. Laser polarization experiments were consistent with a substrate mediated excitation mechanism. The mean translational energy of the photodesorbing N2 was 〈ET〉=0.25±0.05 eV, independent of excitation wavelength. The 193 nm photodesorption cross-section of ∼5×10−20 cm−2 in the two-phase regime of coverage increased as the coverage was reduced. The N2 photodesorption dynamics and action spectrum are consistent with a desorption mechanism involving transient attachment of a photoexcited substrate electron into an affinity band of chemisorbed N2 located roughly 4.0 eV above the Fermi level; presumably the antibonding 2π∗ affinity band.