Three-body breakup in dissociative electron attachment to the water molecule

Abstract

We report the results of ab initio calculations on dissociative electron attachment (DEA) to water that demonstrate the importance of including three-body breakup in the dissociation dynamics. While three-body breakup is ubiquitous in the analogous process of dissociative recombination, its importance in low-energy dissociative electron attachment to a polyatomic target has not previously been quantified. Our calculations, along with our earlier studies of DEA into two-body channels, indicate that three-body breakup is a major component of the observed O − cross section. The local complex potential model provides a generally accurate picture of the experimentally observed features in this system, reproducing some quantitatively, others qualitatively, and one not at all.PACS numbers: 03.65. Nk, 34.80.Ht Dissociative electron attachment (DEA) to the water molecule, [9] observed the opening of the lower three-body channel, as discerned through the kinetic energy distribution of the O − fragment. However, because of the large O atom/H 2 mass ratio, such a measurement is difficult and a quantitative experimental determination of the final-state branching ratios among the two-and three-body breakup channels has yet to be published.There is strong reason to believe that three-body breakup is important for describing DEA leading to O − production. In particular, examination of the shape of the 2 A 1 surface suggests that three-body breakup may be the key to describing O − production via this resonance. The 2 A 1 (1 2 A ′ ) electronic surrface slopes downward toward linear H-O-H geometry and is dissociative along the OH bonds. It provides a path for symmetric dissociation of the OH bonds to produce O − + H + H, but also slopes downward toward the H − + OH asymptotes. Similarly, the 2 B 2 diabatic surface, which equals the 2 B 2 adiabatic surface for C 2v geometries, provides a path toward symmetric dissociation through the conical intersection with the 2 A 1 state. Symmetric dissociation of the 2 B 2 state from the equilibrium geometry of the neutral leads to the O − + H + H three-body asymptote.Our methodology is very similar to that used in our previous calculations, so we provide only a brief summary

Imaging the Molecular Dynamics of Dissociative Electron Attachment to Water

“…These calculations indicate that the observed angular distribution is a product of multiple dynamic effects, including the variation of entrance amplitude with nuclear geometry and the correlation between the geometry at attachment and the angle and kinetic energy of H − recoil. O − production through the 2 A 1 state appears to proceed predominantly through a three-body breakup, H + H + O − , as we recently predicted [9]. This breakup channel was previously observed for D 2 O [17].…”

“…In the gas phase, it proceeds via three transient anion states of 2 B 1 , 2 A 1 and 2 B 2 symmetries which are responsible for three distinct broad peaks in the DEA cross section at electron energies of 6.5, 9 and 12 eV [4], while in the condensed phase, there is evidence that deep-valence states may be responsible for a broad DEA peak centered at 25 eV [3]. The negative ion states subsequently fragment to produce the anions H − , O − and possibly OH − , in various two-body as well as three-body breakup channels [5][6][7][8][9]. In this Letter we present momentum imaging measurements of the angular distribution of the ionic fragments relative to the direction of the incident electron that allow us to probe those dynamics.…”

“…However, since the measurements are necessarily made in the laboratory frame, these observations can yield detailed information about the nuclear dynamics following electron attachment only if a reliable connection between the lab frame and molecular frame can be made. The key to that connection is a knowledge of the angular dependence of the electron attachment probability in the molecular frame, and that attachment probability can be calculated by ab initio methods [9][10][11][12]. The attachment probability can be directly related to the laboratory frame distribution when the axial recoil condition is met, requiring in the present context that the recoil axis which connects the atom and the diatom center of mass does not rotate during the dissociation.…”

Dynamics of the dissociative electron attachment in H2O and D2O: The A1 resonance and axial recoil approximation#

“…A series of papers on DEA to water has recently appeared which give a fairly accurate picture of the dynamics of the process. [1][2][3][4][5][6][7][8] It may be noted that apart from being one of the simplest polyatomic molecules, water is important from its ubiquitous presence and its heightened importance in biology.…”

Observation of the dynamics leading to a conical intersection in dissociative electron attachment to water

“…Dissociative electron attachment (DEA) to the H 2 O molecule proceeds via these and the 2 B 1 state at incident electron energies of approximately 12, 8.5 and 6.5 eV respectively, and additionally in the condensed phase via a deep-valence state at approximately 25eV [27]. The negative ion states subsequently fragment to produce the anions H − , O − and OH − , in various arrangements [28][29][30][31][32][33][34][35][36][37]. Attachment to the 2 B 2 state leads to H − + OH ( 2 Σ), avoiding the conical intersection, or H − + OH ( 2 Π), passing through it.…”