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We present the results of an ab initio study of elastic scattering and vibrational excitation of NO by electron impact in the low-energy (0-2 eV) region where the cross sections are dominated by resonance contributions. The 3 Σ − , 1 ∆ and 1 Σ + NO − resonance lifetimes are taken from our earlier study [Phys. Rev. A 69, 062711 (2004)], but the resonance energies used here are obtained from new configuration-interaction studies. Here we employ a more elaborate nonlocal treatment of the nuclear dynamics, which is found to remedy the principal deficiences of the local complex potential model we employed in our earlier study, and gives cross sections in better agreement with the most recent experiments. We also present cross sections for dissociative electron attachment to NO leading to ground state products, O − ( 2 P) + N( 4 S). The calculations show that, while the peak cross sections starting from NO in its ground vibrational state are very small (∼ 10 −20 cm 2 ), the cross sections are extremely sensitive to vibrational excitation of the target and should be readily observable for target NO molecules excited to ν = 10 and above.

We demonstrate a method for determining the full three-dimensional molecular-frame photoelectron angular distribution in polyatomic molecules using methane as a prototype. Simultaneous double Auger decay and subsequent dissociation allow measurement of the initial momentum vectors of the ionic fragments and the photoelectron in coincidence, allowing full orientation by observing a three-ion decay pathway, (H þ , H þ , CH þ 2 ). We find the striking result that at low photoelectron energies the molecule is effectively imaged by the focusing of photoelectrons along bond directions. DOI: 10.1103/PhysRevLett.108.233002 PACS numbers: 33.80.Eh, 33.60.+q Imaging molecular structure is a critical challenge in chemical physics recently highlighted by the emergence of techniques that, similar to ultrafast electron diffraction [1] or x-ray diffraction [2], have the potential to be taken to the time domain and thereby ultimately be used to make ''movies'' of chemical reactions on their natural time scale. Of particular interest is the development of such techniques that can be applied to the dynamics of isolated molecules. Here, the full three-dimensional orientation of a polyatomic molecule is measured simultaneously with the three components of the momentum of a photoelectron ejected from it with no underlying assumptions of symmetry or geometry. We present three-dimensional images of a polyatomic molecule measured with this technique, demonstrating an effect predicted [3] for polyatomic molecules with a heavy central atom bonded to hydrogens, namely that low-energy photoelectrons can directly image the molecular potential and bond structure.When a photoelectron is launched by photoabsorption of an inner shell, the outgoing photoelectron wave is then scattered by the aggregate potential of the molecule. The final angular distribution in the body-fixed frame of the molecule is an exquisitely sensitive probe of molecular structure and initial electronic state, which has been recently argued and demonstrated [4,5]. However, observing molecular-frame photoelectron angular distributions (MFPADs) at high resolution requires accurate orientation of the molecule in the gas phase. Three-dimensional laser alignment [6,7] can accomplish such orientation prior to photoionization but is limited to molecules with an asymmetric polarizability. In the case of simple diatomic molecules, orientation can also be accomplished by detecting the photoelectron in coincidence with positively charged fragments that emerge following prompt Auger decay and dissociation [8]. Progress has also been made toward threedimensional MFPAD measurement using coincidence detection and velocity map imaging [9]. Here we present photoelectron imaging of methane molecules, where both the photoelectron momentum and corresponding body frame of the polyatomic molecule are fully determined in all three dimensions.For many molecules, including CH 4 , core ionization opens a strong simultaneous double Auger decay channel that produces a trication that then can disso...

We present the results of ab initio calculations for elastic electron scattering by tetrahydrofuran (THF) using the complex Kohn variational method. We carried out fixed-nuclei calculations at the equilibrium geometry of the target molecule for incident electron energies up to 20 eV. The calculated momentum transfer cross sections clearly reveal the presence of broad shape resonance behaviour in the 8–10 eV energy range, in agreement with recent experiments. The calculated differential cross sections at 20 eV, which include the effects of the long-range electron–dipole interaction, are also found to be in agreement with the most recent experimental findings.

At temperatures below 15 000 K, the major pathway for the electron impact dissociation of H 2 is through excitation to the b 3 + u excited electronic state. Total cross sections and energy differential cross sections for threshold energies as a function of vibrational states (v) for H 2 v = 0-4, D 2 v = 0-6 and T 2 v = 0-7 are calculated. The rates of dissociation as a function of electron temperature for each state are parametrized. Near-threshold rates are shown to be so critically dependent on the vibrational level that dissociation from very high-lying vibrational levels must be included in calculations of the rate at local thermal equilibrium (LTE) even at low temperature. An adapted version of the extrapolation procedure of Stibbe and Tennyson (1999 Astrophys. J. 513 L147) is used to approximate the rates for all of the higher vibrational levels, which are then used to calculate the LTE rate. The LTE rate is an order of magnitude greater than the v = 0 rate. Calculations of energy differential cross sections suggest that impact dissociation of vibrationally excited molecules could be the source of low energy H atoms observed in tokamak plasmas.

Electron impact dissociation of molecular hydrogen is considered to be a major route to molecular break-up in cool hydrogen plasmas. Such plasmas are found in a number of astrophysical environments and in the divertor region of tokamak fusion plasma.At low energies, the major pathway for the electron impact dissociation of H is throughdissociative excited electronic state. The purpose of this work is to develop a theoretical formulation for this process for an arbitrary diatomic molecule, and to derive the formal expressions that describe the differential and total cross sections, which involve three fragments in the exit channel, and to study the electron impact dissociation of molecular hydrogen and its isotopomers at near-threshold energies.Ab initio calculations of total cross sections, angular differential cross sections, energy differential cross sections and double differential cross sections are made for the § © vibrational level of H . The effect of nuclear motion is then included in such calculations, with the dissociation taking place from the electronic ground state at different initial vibrational levels, dissociating into continuum states.Studies of mixed isotopes are performed using the R-matrix method, and total cross sections, energy differential cross sections and rate coefficients for electron impact dissociation of H , D , T , HD, HT and DT as a function of the vibrational states are presented.Finally, a scaling rule that describes integral cross sections as a function of the initial vibrational state, the reduced mass of the target molecule and the energy of the projectile electron is developed and compared against cross sections calculated accurately for the H molecule and its isotopomers.2

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.

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