Abstract. It is shown experimentally that the optical break- down in water produced by nanosecond pulses is initiated by a gas dissolved in it due to the formation of bubston clusters playing the role of heterogeneous seeds of the breakdown. By using the method proposed in the paper, a completely degassed water is obtained, which is optically stable to the seed mechanism of breakdown both away and near the boiling point. The existence of long-lived hydrates of a dissolved gas in water is established and a new effect of water puriécation from such hydrate traps by `washing' it with helium is discovered. Keywords: optical breakdown, interaction of radiation with matter, optical breakdown mechanism. 1. Introduction: Optical breakdown mechanisms It was pointed out in many experimental papers that the optical breakdown in liquids (at least by nanosecond laser pulses) occurs sporadically, appearing not at each laser pulse. In addition, the breakdown takes place not only in the focus of a lens but can appear both in front of the beam waist and behind it. When long-focus lenses are used, the breakdown acquires the so-called multiple character, each êash consisting of individual microêashes (Fig. 1). It is found that the breakdown threshold increases after careful puriécation of a liquid. The spectrum of the optical- breakdown êash is continuous (it has no characteristic lines), which is inherent in thermal radiation. Based on these data, it was assumed that the breakdown is initiated by absorbing solid microimpurities (see, for example, review [1]). The breakdown threshold also very weakly depends on the laser radiation wavelength. This led to a conclusion that the breakdown is initiated by particles absorbing light in a very broad spectral range, in particular, by soot particles [2]. However, this model is not devoid of contradictions. Thus, in many experiments a breakdown was observed even in very carefully puriéed liquids (for example, puriéed by using the Milli-pore technology removing particles of size no less than 100 nm). In addition, the presence of absorbing microparticles in the focus of a lens should result in the heating of caustic and efécient thermal defocusing, i.e., the conditions for breakdown initiation should get worse upon `contamination' of the liquid by absorbing impurities. Thus, we found in our experiments [3] that under the invariable focusing conditions and parameters of the laser pulse, the breakdown disappeared during a gradual addition of micron activated coal particles into water preliminary puriéed in a Milli-pore unit. We also determined the dependence of the breakdown threshold on the cell height and showed that the breakdown had the seed nature and the seed density decreased with increasing the depth of a liquid layer [3]. The alternate mechanism of breakdown in liquids assumes that the breakdown occurs as in amorphous semiconductors (with some reservations). According to this mechanism, an electron avalanche develops in a liquid irrespective of the presence of seeds in it. The liquid breakdown model based on these concepts has become quite popular (see review [4] and references therein). Review [4] is entirely devoted to the optical breakdown in water, assuming that water is a semiconductor with an energy gap of 6.5 eV. This value of the energy gap was taken from experimental papers [5, 6] in which water was treated as an amorphous semiconductor. It is obvious that for such an energy gap and wave- lengths in the optical spectral range, the electron avalanche N.F. Bunkin Scientiéc Center for Wave Studies, A.M. Prokhorov General Physics Institute, Russian Academy of Sciences, ul. Vavilova 38, 119991 Moscow, Russia; e-mail: nbunkin@kapella.gpi.ru S.I. Bakum N.S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninsky prosp. 31, 117907 Moscow, Russia Received 10 August 2005; revision received 5 December 2005 Kvantovaya Elektronika 36 (2) 117 ë 124 (2006) Translated by M.N. Sapozhnikov PACS numbers: 61.25.Em; 62.50.+p; 81.20.Ym INTERACTION OF LASER RADIATION WITH MATTER. LASER PLASMA DOI:10.1070/QE2006v036n02ABEH013113 Role of a dissolved gas in the optical breakdown of water N.F. Bunkin, S.I. Bakum 945/024 ëVOLO ë 16/iii-06 ë SVERKA ë 8 ÒÑÎÑÔ ÍÑÏÒ. å 1 Quantum Electronics 36 (2) 117 ë 124 (2006) ß2006 Kvantovaya Elektronika and Turpion Ltd Figure 1. Picture of the multiple breakdown of water irradiated by a 50- ns pulse obtained with an optical microscope.