Eur. Phys. J. D 28, 157–162 (2004) DOI: 10.1140/epjd/e2003-00300-9 T HE EUROPEAN P HYSICAL JOURNAL D Light-induced potassium desorption from polydimethylsiloxane film S. Gozzini and A. Lucchesini a Istituto per i Processi Chimico-Fisici del Consiglio Nazionale delle Ricerche, Area della Ricerca, Via G. Moruzzi 1, 56124 Pisa, Italy Received 20 June 2003 / Received in final form 15 September 2003 Published online 2nd December 2003 – c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2003 Abstract. Potassium photoejection from polydimethylsiloxane surfaces has been observed and analyzed in detail. By diode laser absorption spectroscopy the K concentration in a sealed measurement cell without buffer gas has been monitored as a function of the environment variables, like temperature, flooding light wavelength and power. The dynamic evolution of the process has been measured too, showing differences compared to previous measurements with different alkali-metal atoms. Saturation spectroscopy on the K D1 line has been performed at room temperature by taking advantage of this phenomenon. PACS. 34.50.Dy Interactions of atoms and molecules with surfaces; photon and electron emission; neu- tralization of ions – 42.50.Ct Quantum description of interaction of light and matter; related experiments – 68.43.Tj Photon stimulated desorption 1 Introduction The photodesorption effect called LIAD, from Light In- duced Atom Desorption, has been observed for the first time ten years ago in measurement cells coated with poly- dimethylsiloxane (PDMS) [1], a polymer with a chemically intermediate behavior between organic and inorganic ma- terials and a good stability after dehydration. For this reason it has been used to coat the inner walls of mea- surement cells in order to reduce the spin-relaxation in the wall–atoms collisions. At that time a huge increase of the fluorescence of alkali atoms like Na, K and Rb was observed when illuminating the glass cell by nonresonant and nonmonochromatic radiation (room light). Later on this effect has been deeply studied with Rb [2] in PDMS and Cs [3] in octamethyl-cyclotetrasiloxane (OCT) and also in paraffin [4], taking into account the mobility of the atoms on the surface and inside the coating in order to ex- plain the time evolution of the desorption process. Closed cells containing inert buffer gas have been used in almost all these cases. In the experiments reported so far it is evident the nonthermal characteristic of the phenomenon and its dependence on the wavelength of the incident ra- diation. This effect has been also utilized to improve the loading of Rb atoms in a magnetooptical trap [5]. In this work we describe the observation of the LIAD effect in closed measurement cells coated by PDMS and containing potassium, without any buffer gas. A huge ef- fect is observed still caused by nonresonant visible light. a e-mail: lucchesini@ipcf.cnr.it This effect has been detected in two different class of measurements: (a) low incident radiation power (w ≤ 0.5 W/cm 2 ); (b) high incident radiation power (w > 0.5 W/cm 2 ) giving two different time responses, according to the diffusion theory inside the coating polymer [6]. 2 Experimental apparatus The experimental apparatus adopted is shown in Figure 1. Most of the reported data are obtained from a spe- cial capillary cell consisting of a Pyrex cylindrical tube of 300 mm length, 8 mm external and 5 mm internal di- ameters: in this way we obtained a large surface/volume ratio and really we observed an increased LIAD effect and a good absorption signal from the potassium vapor that let us monitor the atomic concentration with low error. The cell is closed by two Brewster angle glass windows to minimize reflection losses and to avoid any radiation feed- back. Other measurements have been carried on cells with “standard” dimensions in our laboratory, that is 5 cm long and 2 cm diameter. In this case the surface/volume ratio value was less favorable to the surface and this influenced the time constants of the process too. After a careful cleaning, rinsing and pre-backing, the cell was internally coated by PDMS and baked for 4 hours at 490–500 K. Then the cell was evacuated down to 10 −5 Torr by a clean vacuum system. The cell was con- nected to a potassium reservoir where the alkali was dis- tilled; after that the cell was pumped by a ionic pump down to 6−8 × 10 −8 Torr and finally sealed. A Scanning