Resonances are metastable states that decay after a finite period of time. These states play a role in many physical processes. For example, in recent cold collision experiments, autoionization from a resonance state was observed. Complementing such observations with theory provides insight into the reaction dynamics under study. Theoretical investigation of autoionization processes is enabled via complex potential energy surfaces (CPESs), where the real and imaginary parts, respectively, provide the energy and decay rate of the system. Unfortunately, calculation of ab initio polyatomic CPESs are cumbersome; hence, they are not in abundance. Here, we present an ab initio polyatomic CPES utilizing a recently developed approach, which makes such calculations feasible. This CPES helps interpret the autoionization process observed in the He(2S) + H collision. From the behavior of the calculated CPES we can conclusively determine the nature of the autoionization process. Moreover, this CPES was used to generate reaction rates for the collision of He with ortho- and para-H. These reaction rates are obtained from first principles. The results show a remarkable agreement with the cold collision experimental measurements, which demonstrates the robustness of our method. Hereby, we provide a computational tool for designing and interpreting new types of experiments that involve resonance states, e.g., in nucleobase damages (DNA or RNA) or in interatomic (intermolecular) Coulombic decay.
The
quantum phenomena of electronic and nuclear resonances are
associated with structures in measured cross sections. Such structures
were recently reported in a cold chemistry experiment of ground-state
hydrogen isotopologues (H2/HD) colliding with helium atoms
in the excited triplet P-state (He(23P)) [Shagam et al. Nature Chem.
2015, 7, 921], but a theoretical
explanation of their appearance was not given. This work presents
a quantum explanation and simulation of this experiment, which are
strictly based on ab initio calculations. We incorporate complex potential
energy surfaces into adiabatic variational theory, thereby reducing
the multidimensional scattering process to a series of uncoupled 1D
scattering “gedanken experiments”. Our theoretical result,
which is in remarkable agreement with the experimental data, manifests
that the structures in the observed reaction rate coefficient are
due to the spatial arrangement of the excited He p-orbitals with respect to the interaction axis, consequently changing
the system from a normal two-rotor model to a three-rotor one. This
theoretical scheme can be applied to explain and predict cross sections
or reaction rate coefficients for any resonance-related phenomenon.
Theoretical
description of potential energy curves (PECs) of molecular
ions is essential for interpretation and prediction of coupled electron-nuclear
dynamics following ionization of parent molecule. However, an accurate
representation of these PECs for core or inner valence ionized state
is nontrivial, especially at stretched geometries for double- or triple-bonded
systems. In this work, we report PECs of singly and doubly ionized
states of molecular nitrogen using state-of-the-art quantum chemical
methods. The valence, inner valence, and core ionized states have
been computed. A double-loop optimization scheme that separates the
treatment of the core and the valence orbitals during the orbital
optimization step of the multiconfiguration self-consistent field
method has been implemented. This technique allows the energy to be
converged to any desired ionized state with any number of core or
inner-shell holes. The present work also compares the PECs obtained
using both delocalized and localized sets of orbitals for the core
hole states. The PECs of a number of singly and doubly ionized valence
states have also been computed and compared with previous studies.
The computed PECs reported here are expected to be of importance for
future studies to understand the interplay between photoionization
and Auger spectra during the breakup of molecular nitrogen when interacting
with intense free electron lasers.
Within the Fock-space multi-reference coupled cluster framework, we have evaluated the electronic transition dipole moments, which determine absorption intensities. These depend on matrix elements between two different wave functions (e.g., ground state to the excited state). We present two different ways, to calculate these transition moments. In the first method, we construct the ground and excited state wave functions with the normal exponential ansatz of Fock-space coupled cluster method and then calculate the relevant off-diagonal matrix elements. In the second approach, we linearize the exponential form of the wave operator which will generate the left vector, by use of Lagrangian formulation. The right vector is obtained from the exponential ansatz. In order to relate the transition moments to oscillator strengths, excitation energies need to be evaluated. The excitation energies are obtained from the Fock-space multi-reference framework. The transition dipole moments of the ground to a few excited states, together with the oscillator strengths of a few molecules, are presented.
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