Nonthermal photodesorption of N2 from Ag(111)

Photoexcitation of adsorbates on metal surfaces: one-step or three-step

In this essay we discuss the light-matter interactions at molecule-covered metal surfaces that initiate surface photochemistry. The hot-electron mechanism for surface photochemistry, whereby the absorption of light by a metal surface creates an electron-hole pair, and the hot electron scatters through an unoccupied resonance of adsorbate to initiate nuclear dynamics leading to photochemistry, has become widely accepted. Yet, ultrafast spectroscopic measurements of molecule-surface electronic structure and photoexcitation dynamics provide scant support for the hot electron mechanism. Instead, in most cases the adsorbate resonances are excited through photoinduced substrate-to-adsorbate charge transfer. Based on recent studies of the role of coherence in adsorbate photoexcitation, as measured by the optical phase and momentum resolved two-photon photoemission measurements, we examine critically the hot electron mechanism, and propose an alternative description based on direct charge transfer of electrons from the substrate to adsorbate. The advantage of this more quantum mechanically rigorous description is that it informs how material properties of the substrate and adsorbate, as well as their interaction, influence the frequency dependent probability of photoexcitation and ultimately how light can be used to probe and control surface femtochemistry.

UV photo-dissociation and photodesorption of N2O on Ag(111)

Nanosecond laser induced photoreactions of N2O adsorbed on Ag(111) have been studied by temperature programmed desorption (TPD) and mass-selected, angle-dependent time-of-flight (MS-TOF) measurements of neutral desorbing particles. N2O molecules in the first monolayer are thermally inert but photo-dissociate into N2 + O, or photodesorb molecularly or dissociatively, at photon energies above 3.5 eV. We have found that TOF spectra of photodesorbed N2 as well as of N2O measured at hν = 4.7 eV consist of two velocity components. The desorption flux of the fastest component of N2O peaks ∼ 25° off the surface normal, whereas the others are directed in the surface normal. Origins and photo-excitation as well as photodesorption mechanisms of the N2O and N2 signals are discussed.

Temperature-dependent femtosecond photoinduced desorption in CO/Pd(111)

The desorption of CO from a Pd(111) surface following absorption of 120 fs pulses of 780 nm light occurs on two distinct and well-separated time scales. Two-pulse correlation measurements show a fast subpicosecond decay followed by a slower, approximately 40 ps decay. Simulations based on the two-temperature model of electron and phonon heat baths within the substrate, and an empirical friction model to treat coupling to the adsorbate, support the assignment of the desorption mechanism as an electron-mediated process. The photodesorption yield and overall width of the temporal response exhibit a marked dependence on the initial surface temperature in the 100-375 K range despite the much higher transient electronic temperatures (approximately 7000 K) achieved. The observed temperature dependences can be attributed directly to variations in the initial temperature within the frictional coupling picture. Simulations of this extended data set imply that the activation barrier to photoinduced desorption is equal in magnitude to that derived from thermal desorption experiments for this system within the limits of a one-dimensional Arrhenius desorption model. The simulations also imply that the slower decay is not the result of phonon-driven desorption. Though we cannot unambiguously determine the strength of the adsorbate-phonon coupling, our results suggest that its role is to moderate the degree of the adsorbate excitation.