Photo-bleaching and melting of a 2d-crystal of a fluorescent molecule

Introduction The work presented in this paper concerns an experimental study of the melting of two-dimensional crystals in a Langmuir monolayer. In two dimensions, thermal fluctuations destroy the long-range positional order. Also, the resulting quasi long-range order in two-dimensional crystals is characterised by a very slow algebraic decay of the postional correlations [1]. The solids we study are made of a fluorescent molecule and coexist with their liquid phase. The melting occurs through three different ways : by decreasing the surface pressure of the monolayer from the liquid-solid coexistence, by heating the film, and finally the key observation is that melting can occur through illuminating the solid with blue light, strongly absorbed by the molecule. From the observations defects play an important role in the nucleation of liquid [2]. The melting by illumination leads indistinguishable from the way the thermodynamical processes occur, indicating a common mechanism in the initiation of melting. Our purpose in the present paper is to analyse the light-solid interaction and finally to understand how illumination induces melting. In addition, the first effect of the illumination on the fluorescent solid is the photobleaching. This last phenomenon is a dynamic process in which fluorescent molecules undergo photo-induced chemical destruction upon exposure to excitation light and thus lose their ability to fluorescence. The photobleaching phenomenon has been the basis of many fluorescence measurements techniques developed and successfully applied since 1970s ; thus, Fluorescence Photobleaching Recovery (FPR) is one of the most applied [3,4]. Photobleaching of a fluorochrome molecule is known to be an irreversible process. In the following experiments we show a partial fluorescence recovery after photobleaching which can presumably be explained by a reversible photochemical reaction. 2 Experimental The two-dimensional crystals are obtained in a Langmuir monolayer of a pure insoluble amphiphilic molecule at the air/water interface [5,6]. Two different monolayers were studied : one of pure NBD-stearic acid [or