Session N6: Electron Impact Ionization, Recombination, and Attachment

Dissociative recombination of H3+

The process of dissociative recombination (DR) of the $^{H3+}$ ion has been studied over the past years and it was found that the coupling of vibrational and electronic degrees of freedom plays a crucial role in the mechanism: when the Janh-Teller coupling effect was incorporated into the theoretical treatmnet it yielded DR rates in much better agreement with experiments. The previous work on H3+ was performed using hyperspherical coordinates and Siegert states for the vibrational wave functions. SVD technique employed in this study provides more accurate vibrational energies than the Siegert state approach for it takes into account the non-adiabatic coupling between different adiabatic channels. Another improvement towards theory-experiment agreement was to take into account the conditions and parameters of the experiments performed. The present approach uses SVD vibrational states in the calculatiion of H3+ DR rates and accounts for experimental conditions. Incorporating averaging procedures that describe better the experimental conditions improves the agreement between theory and experiment. Results for vibrationally-excited initial states of H3+ are also presented in this work.   

Formation of H$_{3}^{+}$ in methanol : an intramolecular bond rearrangement study

We report here results of TOF multi-hit coincidence experiment [1] that provide evidence for intramolecular reactions involving proton coagulation in methanol [2] after interaction with 1.2 MeV Ar$^{8+}$ projectiles produced from the ECR ion source in the LEIBF laboratory of Inter-University Accelerator Centre, India. Quite remarkably, we have observed the formation of H$_{3}^{+}$ due to movement of protons within the multiply charged parent molecular ion through two-body process (CH$_{3}$OH$^{2+}\to $H$_{3}^{+}$ + COH$^{+})$ and such bond formation occurs before the Coulomb repulsion makes the fragment ions to fly apart. Analysis of the fragmentation pattern of CH$_{3}$OH$^{2+}$ has been carried out using \textit{ab initio} quantum chemical techniques. Structural calculations indicate that the formation of H$_{3}^{+}$ is the preferred pathway in the overall fragmentation dynamics of the ground state of this alcohol. The field generated from highly charged ions induces the system to rearrange its structure following a minimum energy pathway and form hydrogen molecular ions. Repeating the experiment with CH$_{3}$OD confirm our bond rearrangement phenomenon and establish that H$_{3}^{+}$ formation occurs only within the methyl group of the alcohol. Ref: [1] S. De et. al. NIMB, 243, 435 (2006) [2] Sankar De et. al. PRL, 97, 213201 (2006)   

Yields of electronically excited product states in the dissociative recombination of N$_{2}$H$^{+}$, HCO$^{+}$, HOC$^{+}$, and HNC$^{+}$

We have determined branching fractions of radiating products of the dissociative electron-ion recombination (DR) of the astrophysically important ions N$_{2}$H$^{+}$ , HCO$^{+}$, HOC$^{+}$, and HNC$^{+}$, using the flowing-afterglow technique and absolute spectroscopy. State-specific yields were derived by fitting spatially resolved emission band intensities to model calculations. We find that DR of N$_{2}$H$^{+}$ results in N$_{2}$(B$^{3}\Pi _{g}$ , v'$\ge $1), with a yield of (19$\pm $8){\%}. DR of HCO$^{+}$ forms the long-lived CO($a ^{3}\Pi )$ state with a yield of (23 $\pm $ 12){\%}, but DR of its isomeric form, HOC$^{+}$,$^{ }$favors formation of the triplet states CO($a\prime \quad ^{3}\Sigma ^{+})$ and CO($d \quad ^{3}\Delta )$ with a combined yield of greater than 40{\%}. The yield of CN(B) from DR of HNC$^{+ }$ was found to be (22$\pm $8){\%}, while that of CN($A)$ is (14$\pm $5){\%}. The vibrational distributions of the product electronic states do not follow a simple pattern. In some cases, the distributions are close to those predicted by Bates' impulse model but we also find partially inverted distributions, and some that extend to very high vibrational quantum numbers.   

Complete theoretical treatment of Dissociative Recombination of LiH$^+$ and LiH$_2^+$

Studies of LiH$^+$ and LiH$_2^+$ ions are motivated by their role in the chemistry of the early universe. They are diatomic and triatomic prototypes of of the indirect dissociative recombination (DR) process, in which a colliding electron destroys the molecule through Rydberg capture pathways. We apply ab-initio multi-channel quantum defect theory in combination with ro-vibrational frame transformation techniques to calculate DR for these fundamental ions. Alternate versions of the vibrational frame transformation employing either Siegert states or Exterior Complex Scaling are employed. In both cases every rovibrational degree of freedom is included in the calculations, without approximation to the nuclear kinetic energy operator. In case of the LiH$^+$ ion we identify the underlying mechanism behind the suprisingly high DR rate recently measured in storige-ring experiments. Calculated DR rate coefficients are in a good agreement with the experimental data. This work is supported in part by the NSF.   

Magnetic field enhancement of dielectronic recombination

We report the results of the effects of combined electric and magnetic fields on dielectronic recombination (DR) from a continuum of finite bandwidth. Specifically, we have examined the process $\textrm{Ba}~6p_{3/2}8g \rightarrow \textrm{Ba}~6p_{1/2} nk \rightarrow \textrm{Ba}~6s_{1/2}nk + h\nu{\ }$ in the presence of electric fields from 0-7 V/cm and magnetic fields from 0-250 G. Our observations elucidate the requirements for magnetic field enhancement of the DR rate. In particular, they demonstrate that the magnetic coupling must not only be comparable to the electric field splitting of the intermediate autoionizing Rydberg states, but also to their decay rates.   

Optical Excitation Functions of N$_{2}^{+}$

Using a new apparatus designed to minimize systematic sources of error, we have measured the electron-impact optical excitation functions for the B$^{2}\Sigma _{u}^{+}$ -- X$^{2}\Sigma _{g}^{+}$ 391.4nm transition in N$_{2}^{+}$. Incident-electron energies ranged from 20-500 eV. By taking data at pressures less than 0.5 mTorr the possible effects of radiation trapping are significantly reduced. We compare our results with previously published data for this transition and find good agreement with them [1]. We discuss steps we have taken to ensure accurate normalization of photon count rates to both pressure and beam current, and discuss potential error in our results due to radiation trapping. [1] B.N. Srivastava and I.M. Mirza. Phys. Rev. \textbf{176}, 137 (1968).   

Single and Double Ionization of Helium by Charged Ions

We present total cross sections for single and double ionization of helium by various charged ion impact. A non-perturbative time-dependent close-coupling method (TDCC) has been developed to treat the correlated dynamics of ionized electrons by bare-ion impact [Journal of Physics B (accepted)]. The two-electron helium wavefunction is subject to a time-dependent projectile interaction. The projectile-atom interaction is constructed as a multipole expansion that includes monopole, dipole, quadrupole, and octopole terms. For proton, antiproton, and alpha particle impact, good agreement is obtained between our calculations and experimental measurements of total single and double ionization cross sections. We will also report on our progress in using the TDCC method to extract differential cross sections for double ionization by fast protons [Physical Review Letters 90, 243201 (2003)].   

Dissociative Electron Attachment to Acetylene

Experimental studies of electron impact on acetylene show the presence of a $\pi$*-shape resonance at 2.6 eV which leads to dissociation into C$_{2}$H$^{-}$ and H. In their ground state, these fragments have $^1\Sigma$ and $^1S$ symmetries respectively; therefore, the DEA process involves a break of the linear symmetry of acetylene and predissociation of the $^2\Pi_g$ resonance in bent geometries. We performed electron scattering calculations using the complex Kohn variational method to determine the resonance parameters of this system. We discuss the dynamics of dissociation into the product channels and report the computed DEA cross sections. The results are then compared to available experimental findings. Work supported by NSF PHY-05-55401.   

Dissociative Electron Attachment to chloroacetylene

The production of two fragments with $\Sigma$ symmetry from electron-impact dissociation of C$_2$H$_2$, which has only a low lying $\pi^*$ resonance at equilibrium geometry, has been explained by the existence of interactions with $\sigma^*$ resonances at bent geometries. We are investigating the presence of such multidimensional effects in the dissociative attachment of chloroacetylene (C$_2$HCl). We have performed electron scattering calculations using the Complex Kohn variational method to determine the resonance energies and widths of the chloroacetylene resonances as a function of both the Cl--C$_2$H bond distance as well as the variation with C-C stretch and bend. We will discuss our results and our prediction of the dissociation dynamics in comparison to the findings for for C$_2$H$_2$. This work was supported by the U.S. DOE Office of Basic Energy Sciences, Division of Chemical Sciences and the National Science Foundation, PHY-05-55401.   

A Universal model for electron impact ionization of K, L and M-shells.

A modified version of the original Kolbenstvedt model, which has its roots in quantum electrodynamical description of electron-electron scattering, will be presented. This modified model describes reasonably well cross sections of K-shell ionization of H, He, Li, C, N, O, Mg, Si, P and S, L-shell ionization of Ag, Sn, Ba, Ho, Ta, Au, Pb, Bi and U, M-shell ionization of Pb, Bi and U, from threshold to a few GeV incident energy. The same model with slight modification to account for the ionic charge is also applicable to ionic targets such as Ne$^{8+}$, Mo$^{41+}$, U$^{82+}$ from threshold to a few MeV incident energy. Experimentally observed increase of the cross section at high energies seems to be a consequence of the M{\o}ller interaction between two interacting electrons.   

Electron impact ionization of magnesium.

We have applied a hybrid method, combining a second-order distorted-wave method for a fast ionizing projectile with an $R$-matrix (close-coupling) approach for the initial bound state and the ejected electron~[1], to calculate electron impact ionization of magnesium. Our results will be compared with recent experimental data~[2] and predictions from other theoretical approaches~[3]. For various kinematical situations, in-plane and out-of-plane, we analyze the sensitivity of the theoretical results to the details of the computational model. We will suggest additional benchmark experiments to test the various theoretical approaches. \par\vspace{0.1truecm}\noindent [1] Y. Fang and K. Bartschat, J. Phys. B {\bf 34} (2001) L19. \par\noindent [2] R.W. van Boyen {\it et al.}, Phys. Rev. A {\bf 73} (2006) 032703. \par\noindent [3] M. Foster {\it et al.}, Phys. Rev. Lett. {\bf 97} (2006) 093202.   

The Role of the Projectile in Simultaneous Excitation-Ionization

The importance of projectile interactions in fully differential cross sections (FDCS) is explored for the problem of simultaneous excitation-ionization by electron impact. We will compare the results of two theories- the first Born approximation-Hartree Fock (FBA-HF) and the four-body distorted wave model (4DW). In the first Born approximation-Hartree Fock (FBA-HF), the projectile electron is treated as a plane wave, the ejected electron is treated as a Hartree Fock distorted wave, and the final state Coulomb interaction between the two continuum electrons is ignored. In the 4DW model, all continuum electrons are treated as Hartree Fock distorted waves, and a Coulomb distortion factor is included in the final state to account for the interaction between the two outgoing electrons. Results are presented for an incident electron energy of 500 eV and will be compared to experimental data.