We have investigated the desorption by nsec laser pulses of NO from adsorbed (NO)2 layers and of Xe from physisorbed Xe on Ag nanoparticles (8 nm) on alumina films. The layers have been characterized by TPD (temperature-programmed desorption) before and after irradiation. Photon energies below, on, and above the Mie plasmon resonance of the particles (3.6 eV) were used with s- and p-polarization. Desorption cross sections (from signal decay under irradiation), and mean translational energies of the desorbing particles (from time-of-flight spectra) have been obtained. For (NO)2 layers, photochemical conversion to N2O (which is inactive in photodesorption) is observed in addition to desorption of NO. Excitation in the plasmon resonance leads to strongly enhanced yields and cross sections for all these processes, compared to off-resonance excitation as well as to desorption from Ag(111) surfaces. However, the (hyperthermal) translational energies are similar in all cases, suggesting that a common mechanism - the formation of a transient negative ion state leading partly to desorption, partly to conversion - is operative in all cases.
For xenon monolayers we find a fluence range in the plasmon resonance where chaotic desorption, characterized by erratic variations of the desorbing Xe flux, is observed. The translational energy of the desorbing Xe atoms is clearly hyperthermal. At higher fluences this behaviour is superseded by fast heating with lower translational energies and nonchaotic behaviour. We interpret the chaotic nonthermal desorption as due to plasmonic coupling which leads to areas of very strong field enhancement. As no transient negative ions are possible for Xe, we postulate a new mechanism, plasmon-induced desorption, in which the acceleration of the Xe is due to the repetitive push of the Pauli wall, due to the periodic movement of the decaying electron density outside the Ag surface, against the adatom during the plasmon lifetime. A molecular dynamics modelling of this process using realistic parameters shows its possibility.
We put these results into a more general context of the electronic properties and excitations of metallic nanoparticles. |