Momentum imaging experiments on dissociative electron attachment (DEA) to CO2 are combined with the results of ab initio calculations to provide a detailed and consistent picture of the dissociation dynamics through the 8.2 eV resonance, which is the major channel for DEA in CO2. The present study resolves several puzzling misconceptions about this system. PACS numbers: 34.80.HtNegative ion resonances are ubiquitous in low-energy electron-molecule collisions and provide an efficient vehicle for the transfer of electronic energy to nuclear motion either through vibrational excitation or dissociative electron attachment, the latter process resulting in the formation of both charged and neutral fragments. Recent dynamical studies [1] have shown that DEA to fundamental polyatomic systems can exhibit complex electronic and nuclear dynamics involving symmetry breaking target deformations [2] and, in some cases, conical intersections [3,4]. Mechanistic studies of the DEA process may give insight into their behavior in the condensed phase [5] and in biological environments [6].Carbon dioxide offers an interesting case in point. The inverse of DEA to CO 2 , i.e. associative detachment, is thought to be important in the catalytic oxidation of CO on a metal surface [7]. In light of its fundamental importance to the understanding of such processes, it is noteworthy that the electronic structure of CO 2 and its metastable anions has not been completely characterized. Most of the extant literature on low-energy electron-CO 2 scattering deals with the short-lived 2 Π u shape resonance near 4 eV, which provides the dominant mechanism for vibrational excitation, while experimental studies of DEA to CO 2 [8][9][10][11][12][13][14] have focused mainly on total cross sections and their dependence on electron energy and ion kinetic energy release. The 2 Π u resonance also feeds the CO( 1 Σ + ) + O − ( 2 P) DEA channel whose thermodynamic threshold lies at 3.99 eV. Scattering calculations [15] show that the 2 Π u resonance becomes sharper and finally electronically bound as the CO bonds are increased along the symmetric stretching coordinate. It is also known that the CO − 2 ion becomes stable upon bending. It was a long-held belief [16][17][18] [5,21], the 2 Π u resonance accounts for the other two states, then we are led to the puzzle whose resolution is a subject of this Letter. The dominant DEA channel in CO 2 is observed at 8.2 eV. Since this energy is less than the 10.0 eV required to produce electronically excited CO* + O − , the 8.2 eV resonance must necessarily result in electronic ground-state products. So how is this possible when, according to current thinking, all three states arising from this asymptote have already been accounted for? The early theoretical work of Claydon et al.[21] and England et al. [22] assigned the 8.2 eV peak to a 2 Σ + g shape resonance. This assignment has since been disputed. Srivastava and Orient [13], having found little or no dependence of the 8.2 eV DEA peak on vibrational excitation of the...

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Momentum imaging experiments on dissociative electron attachment to the water molecule are combined with ab initio theoretical calculations of the angular dependence of the quantum mechanical amplitude for electron attachment to provide a detailed picture of the molecular dynamics of dissociation attachment via the two lowest energy Feshbach resonances. The combination of momentum imaging experiments and theory can reveal dissociation dynamics for which the axial recoil approximation breaks down and thus provides a powerful reaction microscope for DEA to polyatomics.PACS numbers: 34.80.HtResonant collisions between low-energy electrons and molecules can provide an efficient pathway for channeling electronic energy into nuclear motion. In dissociative electron attachment (DEA), the transient negative ions so formed fragment to form neutral plus ionic fragments. The resurgence of interest in this process in recent years has been due in large part to the key role it plays in radiation damage in a number of different contexts and to the discovery that low-energy DEA can be responsible for double-strand breaks in DNA [1]. It is therefore not surprising that dissociative electron attachment to water has been the target of much recent experimental and theoretical work, since water is the principal constituent is living tissue and DEA can produce free radicals that affect that tissue. The application of modern imaging techniques, such as velocity slice imaging [2], can bring a new level of sophistication to the study of the angular dependence of fragment ions produced in DEA.Dissociative electronic attachment to the deceptively simple water molecule involves complex electronic and nuclear dynamics. 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...

The results of a theoretical study of HOCO − photodetachment are presented, with a view toward understanding the origin of two peaks observed by Lu and Continetti (Phys. Rev. Lett. 99, 113005 (2007)) in the photoelectron kinetic energy spectrum very close to threshold. It is shown that the peaks can be attributed to vibrational Feshbach resonances of dipole-bound trans-HOCO − , and not s-and p-wave shape resonances as previously assumed. Fixed-nuclei variational electron-HOCO scattering calculations are used to compute photodetachment cross sections and laboratory-frame photoelectron angular distributions. The calculations show a broad A ′′ (π*)-shape resonance several eV above threshold.

We present experimental results for dissociative electron attachment to acetylene near the 3 eV 2 Π g resonance. In particular, we use an ion-momentum imaging technique to investigate the dissociation channel leading to C 2 H − fragments. From our measured ion-momentum results we extract fragment kinetic energy and angular distributions. We directly observe a significant dissociation bending dynamic associated with the formation of the transitory negative ion. In modeling this bending dynamic with ab initio electronic structure and fixed-nuclei scattering calculations we obtain good agreement with the experiment.