Appl Phys A (2010) 101: 695–699 DOI 10.1007/s00339-010-5951-2 Material ablation and plasma plume expansion study from Fe and graphite targets in Ar gas atmosphere S. Mahmood · R.S. Rawat · S.V. Springham · T.L. Tan · P. Lee Received: 22 November 2009 / Accepted: 15 June 2010 / Published online: 21 July 2010 © Springer-Verlag 2010 Abstract Study of expansion dynamics of pulsed-laser ab- lation plasmas from Fe and graphite targets is presented. A 532 nm Q-switched Nd:YAG laser with fluence of 30 J cm −2 is used to ablate the Fe and graphite targets in various Ar ambient gas pressures. Plasma ablation parame- ters for the two target materials are estimated using snow- plow and shock-wave models, which show that the laser beam energy deposited to ablated species remains at 70% for both targets at all ambient pressures. The plume split- ting was observed, more prominently, for Fe plasma as it moves faster compared to graphite plasma. The difference in plasma plume fronts’ speeds for different targets was at- tributed to the significant difference in mass of the ablated plasma for two targets, as estimated from simulation results. 1 Introduction The expansion dynamics of laser ablation plasma through ambient gas is a complex mechanism [1, 2]. The fundamen- tal processes involved in plasma expansion like; decelera- tion, shock-wave formation, thermal conduction, diffusion, recombination, and clustering through ambient gas, are not S. Mahmood · R.S. Rawat ( ) · S.V. Springham · T.L. Tan · P. Lee NSSE, NIE, Nanyang Technological University, Singapore 637616, Republic of Singapore e-mail: rajdeep.rawat@nie.edu.sg S. Mahmood e-mail: shamahmood65@yahoo.com S. Mahmood Department of Physics, University of Karachi, Karachi 75270, Pakistan yet fully understood [1–5]. The initial expansion of the ab- lated plasma in a background gas atmosphere is almost in- dependent of the filling gas pressure due to the very high plasma expansion pressure. As the plasma plume moves far- ther from the target, the interaction processes between the ablated debris and the background gas control its expansion dynamics. In the last few decades, with the development of the high power pulsed lasers, extensive work has been done on the dynamics of plasma plumes in vacuum and in ambient gas pressure, during pulsed-laser deposition (PLD) [1–9]. When a pulse of energetic laser strikes the target with fluence in excess of the ablation threshold of the material, chemical bonds are broken and a shower of electrons, neu- tral atoms, molecules, and ions is generated [1, 2]. This very hot cloud of energetic fragments of the target material prop- agates towards the substrate, through the background gas, with a high velocity resulting in the formation of strong shock waves. Accordingly, the role of the background gas is crucial to the properties of the deposited thin films and nanostructures. During pulsed-laser ablation of solid materials, the hot plasma of material species is ejected from the target surface with very high kinetic energy. The temperature of these plas- mas can easily reach several hundreds of eV. During the ex- pansion, these species undergo various chemical reactions by themselves and with the ambient gas. The type of the tar- gets also plays an important role in controlling the dynamics of the plume due to the varying absorption coefficient of ma- terials for a particular wavelength of laser pulse. This paper presents the dynamics of the Fe and graphite plasma plume during their propagation through different Ar ambient gas pressures. The snow-plow and shock-wave models [10, 11] were used to fit the experimental data ob- tained from fast-gated imaging and to discuss the plume dy- namics. The snow-plow model provides a reasonably good