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

Abstract

Abstract. Na clusters bound to mica surfaces have been irradiated with pulsed and cw visible laser light. Kinetic energy and angular distributions of the Na atoms desorbing from the clusters have been determined using cw two-photon laser-induced fluorescence detection. In addition the dependence of the desorption rate on laser power, wavelength and polarization has been measured. The most probable kinetic energy Eki n of the photodesorbed atoms at the surface temperature T s = 300 K was found to be Eki n = 18 + 5 meV, independent of taser irradiance (3~tJ/cm2...20mJ/cm 2) and wavelength (450nm, 505 rim, 658 urn). With increasing orientation angle between detection axis and surface normal (0°< O_< 90 °) Ekin was observed to decrease slightly, while it was nearly independent of surface temperature between Ts= 30 K and T s = 300 K. Also, with increasing radius of the Na clusters the desorbing Na atoms slowed down. The angular distribution of the Na atoms was of cos2-type with respect to the surface normal. These observations suggest that laser-induced desorption of Na from Na clusters bound to mica surfaces involves an initial rate-limiting step of direct surface plasmon excitation followed by a final step of delayed thermal desorption.

Photodesorption of Na atoms from rough Na surfaces

“…In order to do so one has to realize that the clusters possess a size distribution on the surface, which is determined by the Vollmer-Weber growth mode, 50 and that they are flat spheroids with the ratio of semiaxes depending on their size; e.g., for Na clusters bound to mica the axial ratio is 2:1 for small clusters, but increases to 5:1 for average radius r 0 ϭ40 nm. 22 The size distribution of the large semiaxes, parallel to the surface, is usually 51 parametered in terms of two Gaussian functions:…”

“…Having the above discussed uncertainties in mind we therefore have recently applied a two-photon laser-induced fluorescence ͑TPLIF͒ technique 20,21 to this problem. [22][23][24] This method involves a pulsed laser desorption and cw laser detection within a spatially well defined detection spot as well as additional spectral information about the desorbing atoms and thus allows us to determine TOF distributions with well defined initial parameters and superior signal to noise ratio. First results for desorption from Na clusters on mica have been published, 24,25 showing that the kinetic energies of the desorbing atoms are surprisingly small and also that they cannot be fitted by a Maxwellian distribution at the surface temperature.…”

<title>Influence of surface roughness on frequency shift and third-order nonlinear susceptibility of adsorbed particles</title>

“…However, for the cluster radii we are investigating here the ratio of semi axes is expected to be close to that of a sphere. Clusters with a radius of at least 400 A are needed to change the ratio to a value of about 0.2 [46]. As seen from Fig.…”

“…As radiation source a cw dye laser with output power of less than 100 mW is used to avoid laser-induced desorption of the clusters. The cluster size distribution on the surface is generated via evaporation of Na atoms at mica surface temperatures of about 150 K and is characterized via extinction spectroscopy [46] or helium atom scattering.…”