Laser ablation mass spectrometry of inorganic transition metal compounds. Additional knowledge for the understanding of ion formation

Frdric Aubriet aubriet@univ- 0 1 Jean-Franois Muller 0 1 0 Address reprint requests to Dr. F. Aubriet, Laboratoire de Spectromtrie de Masse et de Chimie Laser, Universit Paul Verlaine-Metz , 1 Boulevard Arago, F-57078 Metz Technople Cedex 03, France 1 Laboratoire de Spectromtrie de Masse et de Chimie Laser, Universit Paul Verlaine-Metz , Metz, France Laser ablation of transition-metal oxides have been investigated to better understand the formation processes of inorganic cluster ions. The study of binary oxide mixtures and the relative distribution of the ions produced suggest three salient mechanisms that occur after laser/matter interaction, that function to produce the observed ensemble of ionic species. Molecular recombination reactions, unimolecular dissociation processes, emission of small neutrals, including molecular oxygen from transition-metal oxide samples, or from species expelled in gas phase appear to be a significant mechanism, especially under high laser irradiance conditions. These processes are used to propose a set of pathways to rationalize the envelope of ionic clusters formed under photon bombardment. (J Am Soc Mass Spectrom 2008, 19, 488 -501) 2008 American Society for Mass Spectrometry - Tspectrometry during inorganic compound laser he study of ion formation processes by mass ablation is not yet fully understood. However, several models have been proposed to describe and explain the laser ablation of inorganic compounds [17]. The interaction of a laser beam with a non-metallic solid induces different processes, leading to ejection of neutral and ionized species into the gas phase. Haglund proposed a qualitative description of these processes [7], which begins with the assumption that laser sputtering could be decomposed into four phases: (1) the absorption of laser energy by one or multiple-photon processes; (2) the conversion of the incident energy through radiative and non-radiative relaxation processes; (3) the ejection of speciesatoms, molecules, neutrals, ions, excited speciesfrom the irradiated surface; and (4) the formation and the expansion of a more or less dense plume of neutrals and ions. Two laser matter interaction regimes have to be considered: the laser desorption (LD) and the laser ablation (LA). It is generally reported that LD results in emission of ions, atoms, and molecules without any substantial disturbance in the surrounding surface. LA implies a largescale disruption of surface and near-surface geometrical and electronic structure. These substantial qualitative differences allowed Haglung to identify desorption with a microscopic, and ablation with a mesoscopic view point. LD and LA have to be viewed as the extremes of a continuum, which ranges from the emission of isolated neutrals or ions in the case of LD to the massive removal of material resulting from the collective effects of multiple photons irradiating the same spatial locale in the case of LA. The phenomenology of LA is thought of in terms of formation of a plume of ejected species, which follows the laws of plasma and gas dynamics, and produces a large number of ionized species. Collisions of ions with neutrals occurring in the gas-phase plume after LA lead to ion-molecule condensation reactions. On the other hand, ions and neutrals possess a significant amount of energy, which can result in dissociation reactions. The length scale associated with laser ablation is on the order of d laser vs, where laser is the duration of the laser pulse and vs is the speed of the sound. Indeed, the excitation, thermalization and lattice instability, which drive ablation process, occurred during the time scale of the full laser pulse duration. Consequently, the scale length for ablation is up to a few tens of microns and the amount of species expelled in gas phase is important [7]. Molecular dynamics (MD) simulation have been used to investigate and differentiate ablation and desorption processes. Prior MD studies have principally emphasized laser organic matter interaction, motivated by interest in the field of matrix assixted laser desorption ionization (MALDI) experiments [8]. Recently, MD simulation of ultrafast laser ablation of silica has been reported [9]. During ultrafast laser irradiation (i.e., laser pulsewidths in the femtosecond time scale), the energy deposited at the surface of silica excites valence electrons in the material to the conduction band. The excited electrons absorb laser energy that is then transferred to the lattice, which induces an increase of the temperature and involves phase change (boiling and vaporization) that accompanies material decomposition. This process is commonly referred to as the thermal ablation component of the lasermatter interaction [9]. The liberation of free electrons may also have a significant influence on the ablation, leaving behind positively charged moieties subject to Coulombic repulsive forces, and contributing to ejection of the ablated material. Such electrostatic events are referred to as nonthermal ablation. Two MD simulations have been conducted by Cheng et al. at a laser energy higher than the reported laser ablation threshold for fused silica ( 4.5 J/cm2). The first one neglected Coulombic repulsive forces associated with free electrons whereas the second one took them into account. In the first MD simulation, no material was ejected into the gas phase, which is in disagreement with experiments. In contrast, the calculations including electron effects demonstrated the separation of material and the ejection of species in the gas phase [9]. These calculations are in accordance with those reported by Knochenmuss and Zhigilei on the importance of electrons in lasermatter interaction of organic compounds [8]. Both the desorption and ablation regimes lead to the emission of free electrons in the gas phase from the top of the laser irradiated material, inducing a significant surface charge. The first ionic species emitted in the gas phasemainly positive ionsare ejected by Coulomb repulsion. Later ions are entrained in the gas of neutral species that are in rough local equilibrium with surface (desorption) or cluster (ablation). Calculations also clearly indicated that ablation involves the disintegration of a relatively thick surface region and ejection of individual molecules as well as large clusters, producing a plume is dense enough to enable collisions. Collisions were also observed for MD simulations of desorption phenomenon 500 ps after the lasermatter interaction, when the gas phase became dense enough [8]. The descriptions of Haglung [7], Knochenmuss and Zhigilei [8] are consistent with the two mechanisms reported to explain cluster ion formation, namely recombination and unimolecular dissociation. The factors that govern each of the processes are different. The intact ejection of a lattice fragment occurs if the energy involved is low enough to avoid complete atomization, but still sufficient to prefe (...truncated)