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We present experimental measurements and theoretical calculations for the photoionization of CH 4 at the carbon K-edge. Measurements performed using cold target recoil ion momentum spectroscopy (COLTRIMS) combined with complex Kohn variational calculations of the photoelectron in the molecular frame demonstrate the surprising result that the low energy photoelectrons effectively image the molecule by emerging along the bond axes. Furthermore, we observe a dynamic breakdown of axial recoil behaviour in one of the dissociation pathways of the intermediate dication, which we interpret using electronic structure calculations.

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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