Surface-plasmon-induced desorption by the attenuated-total-reflection method

Abstract

Surface plasmons excited by a Nd: YAG (YAG, yttrium aluminum garnet) laser in an attenuated-total-refiection (ATR) geometry were used to desorb Al atoms from smooth Al films.The ejected Al atoms were ionized by a pulse-excimer laser and measured by time-of-flight mass spectroscopy. The desorption rate exhibits a sharp maximum at the surface-plasmon resonance angle. The experiment demonstrates that surface-plasmon-induced desorption can be accomplished on smooth surfaces. Since ATR excitation allows the use of a wide range of wavelengths, this method will be useful in the study of electronic desorption processes.Photodesorption by laser light has been accomplished by rapid surface heating, '2 by coupling to phonon modes, and by coupling to electronic excitation processes." Electronic excitation has attracted much interest recently, since it is a nonthermal mechanism that may avoid the fragmentation of desorbed molecules. Hoheisel et al. have observed desorption of metal atoms when laser light is coupled to the surface plasmon modes of small metal particles. "We report here the desorption of Al atoms from smooth Al films by surface plasmons excited by a Nd:YAG (YAG, yttrium aluminum garnet) laser in an attenuated total reflection (ATR) geometry. In contrast to the earlier work where plasmon coupling occurs only at selected wavelengths determined by the particle size and the metal's electron density, ATR excitation allows the study of plasmon-induced desorption on smooth surfaces over a broad range of frequencies below the plasmon frequency of the metal film. The existence of a tunable plasmon excitation mechanism that generates strong fields and field gradients at the surface will provide a novel and powerful tool for the study of electronic desorption mechanisms on surfaces.In the ATR method, a thin metal film is deposited on the base of a prism (or semicylinder). Surface plasmons are generated at the metal-vacuum interface when ppolarized light is incident through the prism onto the film at angles just above the angle for total internal reAection. ' The plasmon coupling occurs over a very narrow range of angles that varies with the frequency of the light. The plasmon excitation is most easily observed as a strong minimum in reflectance measured as a function of incident angle, ' but has also been observed in photoemission' and in other processes such as surface-enhanced Raman spectroscopy (SERS) that are sensitive to the strong surface fields associated with the plasmon excitation. In the experiments reported here, the existence of the plasmon-induced desorption mechanism is confirmed by comparing the angular dependence of the plasmongenerated peak in optical absorption with that of the desorption rate. The samples in our experiment were prepared by evaporating 99.999% aluminum onto the base of a glass prism. The data reported here were taken on a 27-nm film, as measured by a quartz-crystal thickness monitor. The desorption experiments were performed in a cryopumIied vacuum system with a base pressure of less th...

Kinetics of photo-induced dissociation of Na clusters deposited on Mica

“…Both a linear dependence of the rate of evaporated atoms on number of incident photons as well as an apparent maximum in the wavelength dependence of the evaporation rate that corresponds to the spectral position of the theoretically proposed Mie-resonance suggested a non-thermal, direct desorption process following electronic excitation of the clusters. Similar conclusions have been drawn to interpret laser-induced desorption measurements of A1 atoms [15] or Au and Ag atoms [ 16] from metallic films of the same materials.…”

Pulsed laser ablation and deposition of thin films

“…That there should be electronic (or photochemical) contributions to the ablation of molecular materials, like polymers, which contain well-defined chemical bonds, is unsurprising. 6 Direct evidence for electronic sputtering contributions in the case of metals and other extended solids is generally harder to discern, but reported examples include the observation of atoms with markedly non-thermal velocity distributions arising in laser-induced desorption from nano-sized metal particles 9 and from thin metallic films 10 (both of which findings have been attributed to surface-plasmon interactions) and even of graphite targets. 11 Most detailed mechanistic considerations of target excitation and sputtering involve electronic initiation.…”

Photodesorption of Na atoms from rough Na surfaces

“…12 Their calculations assumed spherical particles on the substrate surface and were performed on the basis of classical Mie theory. 13 Measurements of desorption of neutrals 14,15 and ions 16 from adsorbed metallic films using excitation via evanescent waves demonstrated the efficiency of plasmon-enhanced desorption processes. The field enhancement idea was subsequently included into a qualitative model of the overall desorption process 11 which, however, did not allow quantitative predictions on any of the experimental observations.…”