Diffusion and photodesorption of molecular gases in a polymer organic film

We report the first study of photodesorption of various molecular gases from a polymer organic film. This study was carried out in a Pyrex cell whose inner surface was covered by a polydimethylsiloxane (PDMS) compound. The cell was illuminated by a flash or a CW halogen lamp. The molecular gas composition was analyzed with a mass spectrometer. We observed the variation of the molecular gas density due to photodesorption in a vapor cell as a function of illumination time, intensity, wavelength and temperature of the coating. We have observed that the desorption rate strongly depends on the light wavelength, with a threshold at about 500 nm. A linear dependence of the desorption rate on the incident light intensity has been found. This means that this effect is not caused by the direct heating of the surface and is non-thermal in nature. We have found that, under continuous illumination of the cell by a halogen lamp, the molecular photodesorption yield shows a fast decay curve, which is then followed by a long diffusion tail. The molecular photodesorption yield drops rapidly with decreasing temperature because the diffusion in the polymer in a glassy state decreases. These results are a clear indication that bulk diffusion plays an important role in the observed molecular photodesorption process. This study could be useful for constructing light-driven sources of molecules.