PhysicsLetters A 167 (1992) 272-276 P IqYSICS LETTERS A North-Holland Time evolution of the light induced condensation of sodium clusters K. Kolwas, M. Kolwas and P. Zalicki Institute of Physics of the PolishAcademy of Sciences, AI. Lotnikbw 32/46, 02-668 Warsaw,Poland Received 13 June 1991;revised manuscript received 20 May 1992;acceptedfor publication 27 May 1992 Communicatedby B. Fricke We report the observationof cluster formation initiated by laser light in a cell containingsodium vapor and a buffer gas (He) under high pressure ( 550 Torr). Switching on the laser light induces the process of cluster condensation, which develops sponta- neously;changesin size and densityof transient dusters are observedlastingfromten to twentysecondsafter the lightis switched on. The study of clusters constitutes a very interesting field between atomic and solid state physics. In re- cent years considerable interest has been shown in formation and structure of clusters [ 1-7 ]. In some papers special attention has been paid to phase tran- sitions of clusters with the idea that the structure of the cluster is also important for its stability [ 8,9 ]. A number of experiments has been focused on the problem of cluster properties such as ionization po- tentials, crystallographic structure, binding energy, chemical reactivity and its dependence on the par- ticle size, known as the "quantum size" effect [ 10- 13 ]. It has been shown that the occurrence of"magic numbers" - stability of certain cluster sizes - cor- responds to spherical shell closings of the electronic energy levels [2,13,14]. The studies of cluster interaction with light lead- ing to excitation spectra, light scattering, absorption as well as nonlinear optical phenomena are very im- portant for understanding the cluster properties [ 15- 19 ]. It was demonstrated e.g. that in addition to sin- gle-electron excitation, collective electronic motion is quite important. A great effort has been made to produce and study ultrafine particles. There are few methods using e.g. inert gases to cool in a way the alkali vapor to form small particles - let us mention only the production by seeded beam expansion [ 20 ] or the production of particles in a flowing inert gas (e.g. ref. [21]). In ref. [22] the ultrafine particles of potassium were created by resonant laser excitation of potas- sium molecules contained in a heat-pipe oven. The idea of the experiment was similar to the ones with inert gases - the vapor expansion was effected by the local heating by the laser light. The heating caused pressure/temperature imbalance and expansion of the metal vapor inside the colder buffer gas. In this paper we report the observation of the phe- nomenon of cluster formation initiated by light and the developing of a cluster buildup process until sta- ble conditions are reached. The process of cluster formation was induced by laser light interacting with sodium vapor in thermal equilibrium. The sodium vapor was contained in a cell in the presence of He buffer gas under high pressure. The initiation of the cluster condensation was possible only under special conditions, created by the temperature of the oven containing the sodium deposit (in the range from 380 to 560°C), and the He pressure sufficiently high (ca. 500 Torr). Our experiment shows that the laser light produces an aerosol of clusters developing in time in a state far from thermal equilibrium. In the reported experiment the sodium cell was il- luminated with the blue 488 nm laser light of an Ar + laser, resonant with respect to the well-defined tran- sition in the rovibronic structure of sodium dimers. The laser light excited the well-known [23] dimer fluorescence, which could be observed usually in the 272 0375-9601/92/$ 05.00 © 1992 Elsevier SciencePublishers B.V. All fights reserved.