Modeling of thermal and chemical non-equilibrium in a laser-induced aluminum plasma by means of a Collisional-Radiative model V. Morel ⁎, A. Bultel ⁎, B.G. Chéron CORIA, UMR CNRS 6614, Université de Rouen, Site Universitaire du Madrillet, Avenue de l'Université, 76801 Saint-Etienne du Rouvray Cedex, France abstract article info Article history: Received 14 June 2010 Accepted 4 August 2010 Available online 12 August 2010 Keywords: Aluminum Collisional-Radiative model Laser-induced plasma Non-equilibrium plasma LIBS A 0D numerical approach including a Collisional-Radiative model is elaborated in the purpose of describing the behavior of the nascent plasma resulting from the interaction between a 4 ns/65 mJ/532 nm Q-switched Nd:YAG laser pulse and an aluminum sample in vacuum. The heavy species considered are Al, Al + , Al 2+ and Al 3+ on their different excited states and free electrons. The translation temperatures of free electrons and heavy species are assumed different (T e and T A respectively). Numerous elementary processes are accounted for as electron impact induced excitation and ionization, elastic collisions, multiphoton ionization and inverse Bremsstrahlung. Atoms passing from the sample to gas phase are described by using classical vaporization theory so that the surface temperature is arbitrarily limited to values less than the critical point one at 6700 K. The laser flux density considered in the study is therefore moderate with a fluence lower than 7 J cm -2 . This model puts forward the major influence of multiphoton ionization in the plasma formation, whereas inverse Bremsstrahlung turns out to be quasi negligible. The increase of electron temperature is mainly due to multiphoton ionization and T e does not exceed 10,000 K. The electron induced collisions play an important role during the subsequent phase which corresponds to the relaxation of the excited states toward Boltzmann equilibrium. The electron density reaches its maximum during the laser pulse with a value ≈10 22 , 10 23 m -3 depending highly on the sample temperature. The ionization degree is of some percents in our conditions. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Beyond its practical interest, laser-matter interaction in the case of nanosecond pulses is a full research area and is far from being completely understood. From this point of view, Aragon and Aguilera did recently a very interesting review [1] where they pointed out some outstanding works which concern particularly the early stage of the interaction. The early stage mentioned has to be generally accepted as related to time scales lower than some hundreds of nanoseconds. This definition results from typical delay times at which Laser-Induced Breakdown Spectroscopy (LIBS) measurements are usually performed [2]. This phase is characterized by the emission of both intense continuum and lines corresponding to deexcitation of highly excited states of multicharged ions. Electron density n e and temperature T e are therefore both high. For example in vacuum, Liu et al. [3] deduced that n e ≈ 10 24 m -3 and T e ≈ 15,000 K in the case of a 3 ns laser pulse with 2 × 10 13 Wm -2 on silicon at a 30 ns delay time. Experiments performed at so early delay times are very scarce. We can cite experiments performed on titanium in similar conditions by De Giacomo [4], or experiments due to Diwakar and Hahn [5] and performed in gases. In the latter case, higher orders of magnitude for n e and T e have been highlighted. The duration of laser-matter interaction depends on the pulse duration itself. In the case of a nanosecond pulse, the related time scale is largely shorter than the relaxation times. In addition, the energy released to the sample is high: all these conditions lead to an initial strong non-equilibrium [6] and the situation evolves subse- quently until equilibrium is obtained. The open question is then to know if the equilibrium is fulfilled long after the end of the pulse. This question is important because one assumes equilibrium in the general treatment of the LIBS signals [7]. Some works have been devoted to the criticism of criteria usually adopted to estimate if equilibrium is fulfilled [8]: these works suggest that such criteria are not appropriate. In these conditions, the best way to assess if the plasma is in equilibrium or not is to develop a complete time dependent modeling. Some numerical codes have been elaborated with this aim in view and offer illustrations of general plasma modelings and numerical simulations reviewed by van Dijk et al. [9]. The codes developed so far may be arbitrarily divided in three groups according to the degree of attention paid to the numerical treatment of the plasma. The first one concerns codes based on a hydrodynamic approach of the plasma Spectrochimica Acta Part B 65 (2010) 830–841 ⁎ Corresponding authors. E-mail addresses: morel@coria.fr (V. Morel), arnaud.bultel@coria.fr (A. Bultel). URL: http://www.coria.fr (V. Morel). 0584-8547/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2010.08.002 Contents lists available at ScienceDirect Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab