**1. Introduction**

Surface diffusion of indigenous and/or foreign atoms plays a key role in a number of physical and chemical processes. To name a few, it is important in crystal growth and epitaxy, heterogeneous catalysis, nucleation and growth of supported nanoparticles, and so on. Finding a reliable tool to control the surface diffusion processes is an attractive goal for many modern technologies.

Optical photons being absorbed by the surface or by the species adsorbed onto it can alter the surface diffusion considerably. At lager intensities of illumination these alternations are mainly due to the temperature rise, while at the lower intensities non-thermal mechanisms of light-induced surface diffusion are operative. The latter are the subjects of this chapter.

The electronic excitation follows after the photon absorption and changes the forces exerted by the surface onto the adsorbed atoms. After a short period of time the energy of the photon is partitioned between the surface and the adsorbed atom. The excess energy obtained by the adsorbed atom results in the increased desorption rates from as well as diffusion rates over the surface.

An inhomogeneous illumination of the surface leads to the inhomogeneous steady state distribution of the adsorbed atoms over the surface. The situation is similar to the Soret effect but require a special theoretical consideration that is presented in this chapter. An unexpected result of the theoretical analysis is that the spatial distribution of the surface number density of the adsorbed atoms is non monotone. There is a pronounced maximum of the surface number density of the adsorbed atoms at the boundary between the illuminated and the dark regions.

The shapes of the supported metal nanoparticles obtained via Volmer-Weber growth mode are metastable. Heating is known to speed up the equilibration of the particles shapes. In our experiments with silver and sodium nanoparticles supported on dielectric surfaces we found evidences of the reversible changes of the particle shapes. Hence, the temperature of the substrate determines the equilibrium shape of the nanoparticles. In the case of sodium, illumination speeds up the particles reshaping. This process is rationalized in terms of the light-induced diffusion of the indigenous atoms over the metal nanoparticle surface, while the main step of the process is identified as the photo-induced detachment of an atom from the terraces. The latter is found to be the rate limiting step in the nanoparticle reshaping process.

Light-Induced Surface Diffusion 417

Among the important advantages of the chosen system is a relatively low quantum yield of the photodesorption process. According to (Bonch-Bruevich et al., 1985, 1997), it is as low as 10−5. This allowed us to hope that the main mechanism for light-induced changes in the surface density of the atoms would be the photoinduced diffusion rather than photodesorption. As shown below, these expectations were fulfilled. Since the excitation of an adsorption system by high-power optical radiation induces both the photodesorption of atoms from the surface and their photodiffusion over the surface and, in addition, is accompanied by dark desorption and diffusion, we performed experiments of two types. In the experiments of the first type, we studied the processes of photodesorption and dark desorption. For this purpose, the cesium atoms adsorbed on the sapphire surface were desorbed by single pulses of a ruby laser generating at a wavelength of 694 nm. The diameter of the irradiated spot varied from 1 to 4 mm. After the end of the strong desorbing pulse, the surface density of the adsorbed atoms gradually recovered. The kinetics of the recovery of the surface density was detected by measuring the intensity of a semiconductor laser beam transmitted through the irradiated area of the sapphire window of the cell. The intensity of the transmitted light at the wavelength 840 nm was measured by an FD-7K photodiode and was monitored by an S8-17 storage oscilloscope. At room temperature, the characteristic time of recovery of the surface density was 25 s. It was verified that this time did not depend on the radius of the irradiated spot within the limits indicated above. This allowed us to assign it exclusively to the process of deposition of atoms from the gas phase, rather than to their surface diffusion from dark regions, because in the latter case the recovery time would be dependent on the radius of the irradiated spot. From these

experimental data, we determined the rate of thermal desorption to be τ=0.04 s–1.

the setup was 0.5 s.

The photostimulated diffusion was studied by irradiating the surface with a cw beam of an argon laser at the wavelength 514.5 nm and a power of 1 W. The focused probe beam of a semiconductor laser (20×30 μm2 in size) was scanned in such a way that it could repeatedly pass both regions of the surface subjected and regions not subjected to the optical treatment. The scanning was implemented using a lightweight mirror glued to the diaphragm of a high-power loudspeaker. The loudspeaker was fed by a sine voltage from a low frequency oscillator. The scanning range of the probe beam exceeded by approximately an order of magnitude the diameter of the exciting beam, which varied within the range 30–130 μm. After passing through the cell, the probe beam was detected by a photodiode. The signal thus obtained was amplified by a selective amplifier at a scanning frequency of 30 Hz and was detected by a lock-in amplifier with an XY recorder at the output. The time resolution of

Because of different absorption in regions with higher and lower surface densities of adsorbed atoms, the intensity of the transmitted beam was modulated at the beam scanning frequency. Under the conditions described above, when the changes in the transmittance are concentrated within a region that is small compared to the whole scanning length, the amplitude of the first modulation harmonic in the transmitted beam, detected in the experiment, is proportional to the integral of variation of the surface density of the adsorbed atoms along the beam scanning path. In the experiment, we measured the kinetics of changes in the surface density of atoms after the argon laser beam was rapidly switched on and off. It was found that, as in the first experiments, for large spots of the exciting beam (~0.5 mm), this kinetics was exponential, with a rise and decay time of 25 s, and did not depend on the exciting beam power. It was verified that the amplitudes of the observed
