Low-temperature and photon-induced chemistry of nitrogen on Pt(111)

The saturation coverage of N-2 on Pt(111) at surface temperatures below 40 K is theta(sat) = 0.41 monolayer +/- 10%. The N 1s binding energy of 403.8 eV measured by X-ray photoelectron spectroscopy suggests that the preponderance of the N-2 adlayer is physisorbed. Nevertheless, up to 0.07 monolayer of the N-2 was able to very weakly chemisorb on the Pt(111) terraces. This chemisorbed N-2 was assigned as vertically bound to Pt(111) top sites on the basis of its 0.15 eV adsorption energy and 2266 cm(-1) intramolecular stretching frequency. Thermal programmed desorption, reflection-absorption infrared spectroscopy (RAIRS), and photodesorption data indicate that a 2-D gas phase of chemisorbed N-2 and a condensed N-2 island phase coexist at coverages theta greater than or equal to 0.07 monolayer. Within the two-phase regime of coverage, the N-2 thermal desorption kinetics were zeroth order, with a desorption energy of 0.13 eV. The data are broadly consistent with the speculation that the condensed island phase is composed of seven-membered "pinwheels" in which vertically chemisorbed N-2 "pins" are decorated by "wheels" of horizontally oriented physisorbed molecules. Only the weakly chemisorbed N-2 on terrace sites exhibited photodesorption over the 308-193 nm wavelength range. The more strongly chemisorbed N-2 molecules at Pt(111) step defects (identified by a 2234 cm(-1) band in RAIRS) were photoinactive. The mean translational energy of the photodesorbing N-2 was < E-T> = 0.25 +/- 0.05 eV (< E-T>/2k(b) similar to 1450 K), independent of wavelength. The N-2 photodesorption dynamics and action spectra are consistent with a desorption mechanism involving transient attachment of a photoexcited substrate electron into an affinity band of chemisorbed N-2 which lies between 3.5 and 4.0 eV above the Fermi level; presumably the 2 pi* antibonding band. Energy accumulated on the excited state potential prior to quenching back to the ground state drives the photodesorption.