Collision of atoms with porous polymer surface: evidence of influence of bulk diffusion on anomalously long dwell time of Rb atoms

We present the results of a systematic study of Knudsen’s flow of Rb atoms in cylindrical capillary cells coated with a polydimethylsiloxane (PDMS) compound. The purpose of the investigation is to measure the adsorption energy and dwell time of Rb atom on the two types of coating of high and median viscosities. We report the measurement of the adsorption energy which yields E = 0.026 eV. This parameter is the same for the two kinds of PDMS used. We find that at room temperature the dwell times for high viscosity and median viscosity PDMS are 22 ± 3 μs and 49 ± 6 μs, respectively. These dwell times are many orders of magnitude longer than the dwell time derived from the measured adsorption energy of Rb atom and temperature of the coating. A tentative explanation of this surprising result, based on the bulk diffusion of the atoms that collide with the surface and penetrate inside of the coating, is proposed

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

Peculiar long-term fluorescence of Rb atoms in a coated vapor cell

We report on an experiment in which the decay time of an oscillating fluorescence of rubidium atoms on a 5P state in a coated vapor cell exceeds several of milliseconds; that is, many orders of magnitude longer than the normal decay time of the excited atoms. We found that the amplitude of this peculiar feature with long-term decay strongly depends to which particular atomic ground sublevels of the Rb atom and to which particular Rb isotopes are used and it could be clearly observed in a high-quality cell only. All these cases are characterized by a sufficiently long lifetime of optically pumped atoms on a sublevel of Rb atom ground state. A numerical simulation based on a complete density matrix developed for the case of two intense pumping and probing laser fields interacting with low density Rb vapor is presented. The model supports the coherent coupling process of a population on excited level to the correspondingly long-lived population on the ground-state sublevel by both probing and pumping radiations as the most plausible mechanism to explain the observed spectacular phenomenon