Total and partial cross sections for breakup of ground rovibronic state of H$_2^+$by photon impact are calculated using the exact nonadiabatic nonrelativistic Hamiltonian without approximation. The converged results span six orders of magnitude. The
breakup cross section is divided into dissociative excitation and dissociative ionization. The dissociative excitation channels are divided into contributions from principal quantum numbers 1 through 4. For dissociative ionization the kinetic energy sharing is calculated using a formally exact expression. These results are compared with approximate expressions, and it is shown that the Born-Oppenheimer result is surprisingly accurate, whereas using Born-Oppenheimer final states to extract the cross sections from the full nonadiabatic wave function produces pathologies near threshold.
We estimate rates for the dissociative recombination (DR) of NO$_2^+$ + e$^-$. Although accurate excited state potential energy curves for the excited states of the neutral are not available, we estimate that the 1 $^2${Phi}$_g$ and the 1 $^2${Pi}$_g
$ states of the neutral may intersect the ground state cation potential energy surface near its equilibrium geometry. Using fixed nuclei scattering calculations we estimate the rate for direct DR via these states and find it to be significant. We also perform approximate calculations of DR triggered by the indirect mechanism, which suggest that the indirect DR rate for NO$_2^+$ is insignificant compared to the direct rate.
The multiconfiguration time-dependent Hartree-Fock (MCTDHF) method is formulated for treating the coupled electronic and nuclear dynamics of diatomic molecules without the Born- Oppenheimer approximation. The method treats the full dimensionality of
the electronic motion, uses no model interactions, and is in principle capable of an exact nonrelativistic description of diatomics in electromagnetic fields. An expansion of the wave function in terms of configurations of orbitals whose dependence on internuclear distance is only that provided by the underlying prolate spheroidal coordinate system is demonstrated to provide the key simplifications of the working equations that allow their practical solution. Photoionization cross sections are also computed from the MCTDHF wave function in calculations using short pulses.
We examine the quenching reaction Rb (doublet S) + NH (singlet Delta) goes to Rb (doublet P one-half) + NH (ground triplet Sigma minus). This reaction may be utilized to produce ground state NH molecules for studies of ultracold physics or for other
purposes, and is interesting in that it involves initial and final states that are nearly degenerate. This near degeneracy is expected to lead to a large reaction rate. We examine this system using ab initio quantum chemistry calculations and scattering calculations, which include spin-orbit effects, and find that the reaction rate is large and, in fact, approaches the quantum mechanical unitarity limit. We discuss the prospects for an experimental examination of this system.
The HeH$^+$ cation undergoes dissociative recombination with a free electron to produce neutral He and H fragments. We present calculations using ab initio quantum defects and Fanos rovibrational frame transformation technique, along with the methodo
logy of PRL 89, 263003 (2002), to obtain the recombination rate both in the low-energy (1-300 meV) and high-energy (ca. 0.6 hartree) regions. We obtain very good agreement with experimental results, demonstrating that this relatively simple method is able to reproduce observed rates for both indirect dissociative recombination, driven by rovibrationally autoionizing states in the low-energy region, and direct dissociative recombination, driven by electronically autoionizing Rydberg states attached to higher-energy excited cation channels.
We present the results of calculations determining the cross sections for indirect dissociative recombination of LiH$_2^+$ + $e^-$. These calculations employ multichannel quantum defect theory and Fanos rovibrational frame transformation technique to
obtain the indirect DR cross section in the manner described by Ref.cite{hamilton}. We use textit{ab initio} electron-molecule scattering codes to calculate quantum defects. In contrast to H$_3^+$, the LiH$_2^+$ molecule exhibits considerable mixing between rotation and vibration; however, by incorporating an exact treatment of the rovibrational dynamics of the LiH$_2^+$, we show that this mixing has only a small effect on the observed DR rate. We calculate a large DR rate for this cation, 4.0 $times$ 10$^{-7}$ cm$^{3}$ s$^{-1}$ at 1 meV incident electron energy.
We report the results of a first-principles study of dissociative electron attachment to H2O. The cross sections are obtained from nuclear dynamics calculations carried out in full dimensionality within the local complex potential model by using the
multi-configuration time-dependent Hartree method. The calculations employ our previously obtained global, complex-valued, potential-energy surfaces for the three (doublet B1, doublet A1, and doublet B2) electronic Feshbach resonances involved in this process. These three metastable states of H2O- undergo several degeneracies, and we incorporate both the Renner-Teller coupling between the B1 and A1 states as well as the conical intersection between the A1 and B2 states into our treatment. The nuclear dynamics are inherently multidimensional and involve branching between different final product arrangements as well as extensive excitation of the diatomic fragment. Our results successfully mirror the qualitative features of the major fragment channels observed, but are less successful in reproducing the available results for some of the minor channels. We comment on the applicability of the local complex potential model to such a complicated resonant system.
