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

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 ͑LIAD͒ 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 ͑poly͒dimethylsiloxane have been determined. ͓S1050-2947͑99͒01512-7͔

Low Energy Atomic Photodesorption from Organic Coatings

“…LIAD is a non-thermal effect, in fact the number of desorbed atoms does not increase exponentially with the desorbing light intensity and exhibits saturation at higher intensity [3]. For the maximum relative atomic density increase ∆ max as a function of the desorbing light intensity I L , a square root dependence was reported for Rb from PDMS [3,6], for Rb and Cs simultaneously desorbed from PDMS [13], for Rb from OCT [18], Rb from OTS [19], and K from PDMS [10]. For Na from PDMS, a linear dependence of the desorbed atoms has been found at low desorbing light intensity [1].…”

“…We limit the discussion to models developed for organic coatings in sealed cells, although phenomenological treatments of low-energy non-resonant photodesorption have been proposed for porous materials [26] and for metal layers in the context of cold atoms experiments [56]. These models, developed originally for PDMS [6] and paraffin [20], focus their attention on the complete system formed by the vapor phase and the coating, in order to model the complex interplay among adsorption and desorption fluxes in various conditions.…”

“…In [6], LIAD is directly related to the diffusion in the coating bulk. The system dynamics are modeled as a 1D diffusive process from the bulk to the surface of the coating, hence perpendicular with respect to the vapor/substrate interface.…”

“…Atutov et al [6] take into consideration the current fluxes of the alkali atoms in and out of the coating (PDMS), the total flux being:…”

“…Besides PDMS [6][7][8][9][10][11][12][13][14][15][16][17], organic films investigated so far include octamethylcyclotetrasiloxane (OCT) [18], octadecyltrichlorosilane dry film (OTS) [19] and paraffin [20,21]. Details on the preparation of different coatings are reported in [1,4,19,20].…”