The saturation coverage of N 2 on Pt(111) at surface temperatures below 40 K is θ 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 θ g 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 ∼ 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 2π* antibonding band. Energy accumulated on the excited state potential prior to quenching back to the ground state drives the photodesorption.

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