Vibrational distributions in desorption induced by femtosecond laser pulses: coupling of adsorbate vibration to substrate electronic excitation

Quantum-state distributions are reported for nitric oxide (NO) molecules desorbed from a Pd(111) surface at a base temperature of 140 K by laser pulses of 400 fs duration. Significant vibrational populations are observed in the V; = 0, 1, and 2 levels. For a femtosecond laser fluence capable of producing substrate electronic temperatures of ∼ 4500 K, the vibrational distribution of the desorbed NO molecules is roughly thermal with an average degree of vibrational excitation corresponding to a temperature of 2900 K. These measurements can be regarded as the reverse of the usual vibrational relaxation measurement in that here energy flows from the substrate to the adsorbate vibration. The high degree of excitation of the NO intramolecular vibration can be attributed directly to efficient coupling to the electron-hole pairs generated in the Pd substrate by the femtosecond laser pulse. The process is modeled by calculating electronic and lattice temperatures for the Pd substrate and then incorporating the electron temperature profile into a master equation for the NO vibrational populations. The analysis permits an effective coupling constant of ∼ 15 cm-1 to be inferred for the interaction between the substrate electronic excitation and the intramolecular vibration. © 1993