A new class of photo-induced phenomena in siloxane films

Abstract

The light induced atomic desorption effect, known as LIAD, is observed whenever Pyrex cells,\ud coated with siloxane films and containing alkali atoms, are illuminated. LIAD is a non-thermal phenomenon\ud and it can be observed even with very weak light intensities. We show that the simultaneous contribution\ud to the photo-emission of atoms adsorbed both at the film surface and within the film must be taken into\ud account in order to fit the experimental data. We demonstrate that both the desorption efficiency and\ud the dffusion coefficient of the alkali atoms embedded in the dielectric film depend on the desorbing light\ud intensity. These features characterize a new class of photo-induced phenomena whose analysis gives new\ud insights in the comprehension of the atom-surface interaction and of the atom-bulk dffusion and opens\ud interesting perspectives for applications

Low Energy Atomic Photodesorption from Organic Coatings

“…For Na from PDMS, a linear dependence of the desorbed atoms has been found at low desorbing light intensity [1]. For Rb from PDMS and OCT [18], for Rb and Cs from PDMS [13] and Rb from OTS [19], the desorption rate R = dn/dt at t = t on at the beginning of the illumination as a function of the desorbing light intensity has been reported. This parameter has a smaller dependence on the cell geometry compared to ∆ max .…”

“…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].…”

“…When the desorbing light is turned off at time t off , a faster decay of the vapor density is observed. Multiple illumination of the cell leads to the decrease of the maximum number of desorbed atoms and this difference becomes less pronounced by increasing the time delay between the light pulses [18]. This was explained by assuming that a fast depletion of the coating region close to the surface is induced when the cell is illuminated, while this same part is filled back in more slowly in the dark, and patterned with both the desorption efficiency and the diffusion coefficient of the alkali atoms embedded in the dielectric film.…”

“…The enhancement of diffusion induced by light is studied also in [18], in which a relative increase of the parameter d(λ)/D 0 of the order of 10 2 cm 2 /W is reported for Rb atoms photodesorbed from PDMS and OCT.…”

“…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].…”