DOI: 10.1007/s00340-006-2259-1 Appl. Phys. B 83, 651–657 (2006) Lasers and Optics Applied Physics B k. amal 1 s.h. elnaby 1 v. palleschi 2 a. salvetti 2 m.a. harith 1, ✉ Comparison between single- and double-pulse LIBS at different air pressures on silicon target 1 National Institute of Laser Enhanced Science (NILES), Cairo University, Giza, Egypt 2 IPCF-CNR, Applied Laser Spectroscopy Laboratory, Pisa, Italy Received: 16 May 2005/Final version: 27 March 2006 Published online: 5 May 2006 • © Springer-Verlag 2006 ABSTRACT A comparative study between single- and double- pulse laser-induced breakdown spectroscopy (LIBS) was per- formed on an n-type silicon(111) target. A new mobile double- pulse instrument for LIBS analysis was used for the measure- ments. The experiment was carried out at different air pressures of 0.7, 470 and 1000 hPa. It has been found that, in the case of double-pulse LIBS, the emission intensities of atomic and ionic lines are strongly enhanced at higher pressures. Using Stark broadening of the atomic lines of silicon, it was found that the electron number densities for single and double pulses are approximately the same ( N e ∼ 10 17 cm −3 ). Plasma excitation and ionization temperatures were determined from a Boltzmann plot. The double-pulse laser-induced plasma was studied at dif- ferent interpulse delay times of 1, 2, 5, 10, 15, 25 and 50 μ s. The results indicated that the interaction between the laser, plasma and target gives higher atomic and ionic intensities at shorter interpulse delay times. PACS 52.38.Mf; 79.20.Ds; 52.50.Jm 1 Introduction The interaction of laser radiation with solid tar- gets is involved in a large variety of applications, including plasma characterization, laser ablation [1], thin film study ap- plications and depth profiling [2]. In particular, laser-induced plasma spectroscopy (LIPS) is considered one of the most promising analytical techniques, since its first introduction in the second half of the twentieth century [3]. In analyti- cal atomic spectroscopy, LIPS has been frequently used and proposed for atomic emission spectrometry. In this case the technique is most often referred to as laser-induced break- down spectrometry (LIBS) [4]. In its original version, LIBS analysis used a single laser pulse, that is, the plasma on the target was produced by a single laser shot directed onto the sample surface. Even in this relatively simple configura- tion, the interaction of laser radiation with a solid target is a complex phenomenon, up to now not completely under- stood and still under intensive investigation. In addition, it is ✉ Fax: +202-5675335, E-mail: mharithm@egypt.com difficult to obtain suitable standards, because of interference effects (the matrix effect). Conventional (single-pulse) LIBS has consequently a poorer sensitivity than several competing atomic spectroscopic techniques such as inductively coupled plasma atomic emission spectrometry (ICP-AES) or induc- tively coupled plasma mass spectroscopy (ICP-MS) [4]. In recent years, one way to overcome these problems was pro- posed through the use of double-pulse LIBS, when two lasers or two pulses from one laser, separated by a delay time of the order of microseconds, are used. Double-pulse LIBS was first applied to the analysis of liquid samples, then solids immersed in liquids [5–7]. In recent years, double-pulse LIBS has also been extensively applied to the analysis of solid samples in a gaseous or ambient air environment [8–16]. Different geometrical configurations are used in double- pulse LIBS. In the so-called ‘collinear’ configuration, the two laser pulses have the same axis of propagation and are both directed orthogonally to the sample surface [8, 12–16]. Stud- ies of collinear double pulses were performed by Sattmann et al. [12–14] using a single Nd:YAG laser capable of emitting two sequential pulses. The authors compared the double-pulse results with the ones obtained using a single pulse with the same energy of the two combined laser pulses. St-Onge et al. [8] investigated the physical properties and emission yield of the plasma formed from solid Al alloys in air in a collinear double-pulse configuration, using a Nd:YAG laser operated in double-pulse mode at the fundamental wave- length (1064 nm). They found that the Al II line at 281.6 nm is enhanced when using two sequential laser pulses instead of a single pulse of equal energy. However, the electron density was found to be approximately the same for both the sin- gle and double pulses and the plasma temperature was less than 10% higher in the double-pulse configuration. The ef- fect of the interpulse interval on the plasma emission was also studied. The other common configuration of double- pulse LIBS is the ‘orthogonal’ configuration, where the two laser beams are orthogonal to each other. In this case, the laser parallel to the target comes first, producing preabla- tion plasma in air [9, 10], or after the perpendicular one, causing reheating and excitation of the already ablated ma- terial [11]. Angel et al. [10] proposed a perpendicular pre- ablation setup using two Nd:YAG lasers. The first pulse was directed parallel to the sample and produced a preablation