PHYSICAL REVIEW A 82, 042704 (2010) Orientation and alignment effects in electron-induced ionization of a single oriented water molecule C. Champion 1,* and R. D. Rivarola 2 1 Laboratoire de Physique Mol´ eculaire et des Collisions, ICPMB (FR CNRS 2843),Universit´ e Paul Verlaine-Metz, 1 boulevard Arago, F-57078 Metz Cedex 3, France 2 Instituto de F´ ısica Rosario, CONICET and Universidad Nacional de Rosario, Avenida Pellegrini 250, 2000 Rosario, Argentina (Received 18 May 2010; published 11 October 2010) We here report a theoretical study about the orientation effect on the total ionization cross sections for a single oriented water molecule. The theoretical description of the ionization process is performed within the first Born framework with a collisional system including an initial state composed of a projectile and a water target molecule described by a plane wave and an accurate one-center molecular wave function, respectively, and a final state constituted by a slow ejected electron represented by a Coulomb wave and a scattered (fast) electron projectile described by a plane wave. Secondary electron energetic distributions as well as total cross sections are then compared for particular target configurations pointing out strong alignment and orientation effects on the description of the ionization process. DOI: 10.1103/PhysRevA.82.042704 PACS number(s): 34.50.Gb, 34.10.+x I. INTRODUCTION Target orientation effects on electron-induced ionization and fragmentation of molecules have been the subject of scarce experimental as well as theoretical investigations. In a general way, for high-enough projectile velocities, the reaction time τ ( ∼ =10 −16 s) remains considerably shorter than the rotational and vibrational periods of the molecules which permits us to assume that the impacted target molecule remains frozen during the collision [1,2]. Under these conditions, for the case of diatomic targets, the initial orientation of the molecular axis can be experimentally derived from the measured fragment velocity vectors and the ionization cross section studied as a function of the alignment of the molecular axis with respect to the electron beam. On this matter, this progress is essentially due to the use of the coincidence multihit cold-target recoil-ion-momentum spectroscopy (COLTRIMS) imaging technique, which made possible the study of the different dissociation channels with, in particular, the mo- mentum detection of the target fragments following molecular dissociation [3]. The existing studies remain, up to now, essentially focused at the multidifferential scale for simple molecules like H 2 impacted by electrons [4] or positrons [5] and to the best of our knowledge only a few studies have reported total ionization cross sections for oriented molecules since the pioneering works of Kasai et al. [6] dedicated to indirect ionization induced by a 700 eV electron beam. Furthermore, let us cite the more recent work of Aitken et al. [7] where the authors investigated the ionization of the prolate symmetric top molecule CH 3 Cl impacted by 200 eV electrons to determine the production of the molecular CH 3 Cl + and the fragmentation product CH 3 + . The ionization cross section for the CH 3 Cl + formation was then found higher at the positive end of the molecule whereas the observed cross section for the formation of the CH 3 + fragmentation product was relatively independent of the target orientation. On the theoretical side, let us mention the recent work provided by Kretinin * Corresponding author: champion@univ-metz.fr et al. [8] where total inelastic and integrated cross sections were reported within a Born-Bethe-type approximation and that performed by Gorfinkiel and Tennyson [9] where cross sections for the electron impact ionization of H 3 + and H 2 molecules were determined by using the molecular R-matrix method with pseudostates. Finally, the orientation effect on single-electron-induced ionization was also studied by Stia et al. [10] who pointed out that the interference structures coming from the two-center geometry of the target molecule, namely, the H 2 molecule, were markedly dependent on the molecular orientation as already observed by other authors for heavy ion impact [11]. For single oriented water molecules there are no investiga- tions except the very recent three-dimensional mapping of pho- toemission provided by Yamazaki et al. [12] in which the O 1s photoelectron angular distributions from a single oriented H 2 O molecule were studied in detail. The experimental results also obtained within the quadruple coincidence framework clearly revealed the anisotropic feature of the photoionization dynam- ics. The lack of experimental information makes reliable theo- retical predictions valuable. In this context, we have previously reported multidifferential cross sections for the (e,2e) process on a single oriented water molecule within the framework of the first Born approximation and clearly identified a molecular orientation effect on the angular distributions of the secondary ejected electron for three particular geometric configurations, namely, a “parallel,” an “antiparallel,” and a “perpendicular” configuration which correspond to three target positions differing by the position of the hydrogen atoms with respect to the incident electron beam. Secondary electron angular distributions were then investigated in detail for each ionized molecular subshell via eightfold differential cross-section calculations and have evidently demonstrated their relative predominance with respect to the target orientation [13,14]. In the present work, we aim to point out that the molecular orientation is still crucial when less differential ionization cross sections are investigated, namely, in terms of secondary electron energy distributions (and more particularly the mean kinetic energy transfer) and total cross sections. In the sequel, we deal with the theoretical model developed here for calculating the total ionization cross sections for 1050-2947/2010/82(4)/042704(8) 042704-1 ©2010 The American Physical Society