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Physical Review A

Light-induced diffusion and desorption of alkali metals in a siloxane film: Theory and experiment

Sergey Nikitich Atutov, V. Biancalana, Paola Bicchi, C. Marinelli, E. Mariotti, M. Meucci, Andrew M. Nagel, K. A. Nasyrov, S. Rachini, L. Moi

Published Dec 1, 1999• 76 citations
Publisher

Abstract

The light-induced desorption and diffusion of alkali-metal atoms in organic films are interesting fields of investigation. An impressive demonstration is given by the recently observed light-induced atomic desorption (LIAD) effect, where a huge alkali-metal atom desorption from siloxane films, previously exposed to atomic vapors, is induced by weak and nonresonant light. In this paper, experimental data and a one-dimensional theoretical model of the effect are presented. The model gives a good description of the vapor density dynamics by taking into account both the atomic diffusion inside the coating and the surface desorption. General equations are reported and discussed within the limits of experimental interest. The potential barrier at the vapor-surface interface and the activation energy for Rb in (poly)dimethylsiloxane have been determined.

DOI
10.1103/physreva.60.4693

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We review some recent studies on light-induced atomic desorption (LIAD) from dielectric surfaces. Alkali-metal atoms adsorbed either on organic films or on porous glass are released into the vapor phase under illumination. The measurements were performed in Pyrex resonance cells either coated with siloxane films or containing a porous glass sample. In both cases, the experimental results show that LIAD can be used to produce atomic densities suitable for most atomic physics experiments. Moreover, we find that photoinduced effects, correlated with LIAD, produce reversible formation and evaporation of alkali-metal clusters in porous glass. These processes depend on the light frequency, making the porous glass transmittance controllable by light.

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Mar 2, 2007•0 citations
The light-induced desorption and diffusion of alkali-metal atoms in organic films are interesting fields of investigation. An impressive demonstration is given by the recently observed light-induced atomic desorption (LIAD) effect, where a huge alkali-metal atom desorption from siloxane films, previously exposed to atomic vapors, is induced by weak and nonresonant light. In this paper, experimental data and a one-dimensional theoretical model of the effect are presented. The model gives a good description of the vapor density dynamics by taking into account both the atomic diffusion inside the coating and the surface desorption. General equations are reported and discussed within the limits of experimental interest. The potential barrier at the vapor-surface interface and the activation energy for Rb in (poly)dimethylsiloxane have been determined.
We review some recent studies on light-induced atomic desorption (LIAD) from dielectric surfaces. Alkali-metal atoms adsorbed either on organic films or on porous glass are released into the vapor phase under illumination. The measurements were performed in Pyrex resonance cells either coated with siloxane films or containing a porous glass sample. In both cases, the experimental results show that LIAD can be used to produce atomic densities suitable for most atomic physics experiments. Moreover, we find that photoinduced effects, correlated with LIAD, produce reversible formation and evaporation of alkali-metal clusters in porous glass. These processes depend on the light frequency, making the porous glass transmittance controllable by light.
The commonly accepted theory of light-induced atomic desorption (LIAD) [S. N. Atutov et al., Phys. Rev. A 60, 4693 (1999)] explains the dynamics of this effect by referring to a light-enhanced diffusion process proceeding in the absorbing medium. On the other hand, recently performed measurements show that in siloxane films which are used in LIAD experiments, the characteristic time of diffusion is extremely short (a fraction of 1 s) compared to the duration of the LIAD effect (hundreds of seconds). This is in contradiction to the conceptual basis of the theory of Atutov et al. that requires the characteristic diffusion time to be sufficiently long. What is more, the theory of Atutov et al. relies on an unjustified assumption about the dependence of the diffusion coefficient on the light intensity. In consequence, the theoretical results of Atutov et al. yield unsatisfactory predictions and do not fit well with experimental data. In this paper, we consider a partially illuminated siloxane-coated cell and propose an alternative theory of LIAD dynamics based on an assumption about lateral diffusion proceeding in the siloxane coating. Our theory provides a unique solution for the desorbed atoms' density regardless of the power of the laser light, explains the lengthy duration of LIAD, and accurately reproduces experimental results. A reasoning which explains how the boundary condition at the siloxane surface is established by the incident light gives us a proper relation between the desorbed atoms' density and the intensity of the desorbing light.
We observe Light-induced atomic desorption (LIAD) of Rb and Cs atoms from a PDMS-coated cell containing both atoms at the same time. The desorption rate and the relative vapor density variation have been simultaneously measured for both alkali atoms as a function of the desorbing light intensity and frequency. They show the same behaviour upon the different illumination conditions. This work demonstrates the possibility to provide light controlled atomic dispenser delivering different atomic species using the LIAD effect.
We present in this paper recent results on Light - Induced Atom Desorption (LIAD) in sealed and open coated cells. LIAD is defined via the description of an experiment on rubidium atoms stored in a dry - film coated cell, where a few milliwatts of even non coherent and non resonant light are able to increase the alkali atomic density for more than one order of magnitude. Modeling of the effect is given. New features become relevant in the case of LIAD in porous glasses: in fact, although the photodesorption efficiency per unit area in bare glass is much lower, photoatomic sources can be prepared, due to the huge inner surface of porous samples. We applied LIAD from organic coatings to the stabilization of alkali densities out of equilibrium: sodium case is here discussed. Finally, we report on fully original, preliminary measurements of rubidium Magneto - Optical Trap loading via LIAD from a dry - film coated cell.
We present a study of light-induced atom desorption (LIAD) of an alkali-metal atom (Rb) in porous alumina. We observe the variation due to LIAD of the rubidium density in a vapor cell as a function of illumination time, intensity, and wavelength. The simple and regular structure of the alumina pores allows a description of the atomic diffusion in the porous medium in which the diffusion constant only depends on the known pore geometry and the atomic sticking time to the pore wall. A simple one-dimensional theoretical model is presented which reproduces the essential features of the observed signals. Fitting of the model to the experimental data gives access to the diffusion constant and consequently the atom-wall sticking time and its dependence on light intensity and wavelength. The nonmonotonic dependence of the LIAD yield on the illumination light frequency is indicative of the existence of Rb clusters in the porous medium.
The study of light induced processes on atoms and nanoparticles confined in organic films or in dielectric structures is motivated both by fundamental interest and applications in optics and photonics. Depending on the light intensity and frequency and the kind of confinement, different processes can be activated. Among them photodesorption processes have a key role. Non thermal light induced atomic desorption has been observed from siloxane and paraffin films previously exposed to alkali vapors. This effect has been extensively investigated and used both to develop photo-atom sources and to load magneto-optical traps. Recently we observed huge photodesorption of alkali atoms embedded in nanoporous silica. In this case the atomic photodesorption causes, by properly tuning the light frequency, either formation or evaporation of clusters inside the silica matrix. Green-blue light desorbs isolated adatoms from the glass surface eventually producing clusters, whereas red-near infrared (NIR) light causes cluster evaporation due to direct excitation of surface plasmon oscillations. Green-blue light induces cluster formation taking advantage of the dense atomic vapor, which diffuses through the glass nano-cavities. Both processes are reversible and even visible to the naked eye. By alternatively illuminating the porous glass sample with blue-green and red-NIR light we demonstrate that the glass remembers the illumination sequences behaving as an effective rereadable and rewritable optical medium.