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 preponder
ance 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 th
e basis of its 0.15 eV adsorption energy and 2266 cm(-1) intramolecular str
etching frequency. Thermal programmed desorption, reflection-absorption inf
rared spectroscopy (RAIRS), and photodesorption data indicate that a 2-D ga
s phase of chemisorbed N-2 and a condensed N-2 island phase coexist at cove
rages theta greater than or equal to 0.07 monolayer. Within the two-phase r
egime of coverage, the N-2 thermal desorption kinetics were zeroth order, w
ith a desorption energy of 0.13 eV. The data are broadly consistent with th
e speculation that the condensed island phase is composed of seven-membered
"pinwheels" in which vertically chemisorbed N-2 "pins" are decorated by "w
heels" of horizontally oriented physisorbed molecules. Only the weakly chem
isorbed N-2 on terrace sites exhibited photodesorption over the 308-193 nm
wavelength range. The more strongly chemisorbed N-2 molecules at Pt(111) st
ep 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 deso
rption mechanism involving transient attachment of a photoexcited substrate
electron into an affinity band of chemisorbed N-2 which lies between 3.5 a
nd 4.0 eV above the Fermi level; presumably the 2 pi* antibonding band. Ene
rgy accumulated on the excited state potential prior to quenching back to t
he ground state drives the photodesorption.