Dynamics of Tunneling Ionization using Bohmian Mechanics

Recent attoclock experiments and theoretical studies regarding the strong-field ionization of atoms by few-cycle infrared pulses revealed new features that have attracted much attention. Here we investigate tunneling ionization and the dynamics of the electron probability using Bohmian Mechanics. We consider a one-dimensional problem to illustrate the underlying mechanisms of the ionization process. It is revealed that in the major part of the below-the-barrier ionization regime, in an intense and short infrared pulse, the electron does not tunnel \through" the entire barrier, but rather already starts from the classically forbidden region. Moreover, we highlight the correspondence between the probability of locating the electron at a particular initial position and its asymptotic momentum. Bohmian Mechanics also provides a natural definition of mean tunneling time and exit position, taking account of the time dependence of the barrier. Finally, we find that the electron can exit the barrier with significant kinetic energy, thereby corroborating the results of a recent study [Camus et al., Phys. Rev. Lett. 119 (2017) 023201]

Symmetry In The Dissociative Recombination Of Polyatomic Ions And In Ultra-cold Few Body Collisions

We discuss the role of symmetries in the dissociative recombinations (DR) of three polyatomic ions, namely the linear HCO+ (formyl) ion and the two highly symmetric H+3 and H3O+ (hydronium) molecular ions. Regarding the HCO+ ion, we apply a quantum mechanical treatment using the Multi-channel Quantum Defect Theory (MQDT) formalism to describe the ion-electron scattering process. Our study takes into account the Renner-Teller effect in order to model the non Born-Oppenheimer vibronic coupling in linear polyatomic ions. The coupling has shown to represent the main mechanism responsible for electronic capturing in highly excited Rydberg states associated with excited vibrational levels of the ionic core. We consider all internal degrees of freedom of HCO+ and obtain the dissociative cross section as a function of the incident electron kinetic energy. We have also improved the theoretical approach by including the large permanent dipole moment of HCO+ using a generalization of the MQDT formalism. To our knowledge, this is the rst time the permanent dipole moment of an ion is included in a DR study. The obtained results are in good agreement with experimental data. We also study the DR of H+3 and H3O+ symmetric ions using a simpli ed theoretical treatment, which focuses on the key ingredient of the DR process, the electron capture in the rst excited degenerate vibrational normal mode of the ions through non Born-Oppenheimer Jahn-Teller coupling. For both ions the obtained cross sections are in very good agreement with the available experimental data. Moreover, in the case of H+3 , the results reproduce previous calculations from two independent theoretical studies. Finally, we investigate the role of symmetries in few body ultra-cold collisions by considering both three and four identical atoms systems. We derive allowed rearrangements of different fragments of the system, satisfying the complete symmetry of the molecular Hamiltonian. For that purpose we establish a correspondence between constants of motion of the system in di erent large-distance con gurations and irreducible representations of the total symmetry group. Selection rules (forbidden transitions) and allowed states, which depend on the fermionic or bosonic nature of the atoms, can be derived from these results

Photodetachment cross sections of the C2nH- (n=1-3) hydrocarbon-chain anions

We report theoretical results of the low-energy photodetachment cross sections of the C2H-, C4H-, and C6H- hydrocarbon-chain anions. The complex Kohn variational technique is used to calculate molecular-frame transition dipole moments from the anion ground state to a photoelectron in the continuum of the neutral radical. We employ the Franck-Condon approximation and include interchannel electronic coupling to determine the low-energy photodetachment cross sections and asymmetry parameters. We discuss the behavior of the cross section, especially near thresholds, and describe the role of electronic resonances and excited channels. The theoretical results reproduce the main characteristics of recent measurements of absolute photodetachment cross sections

Theory of dissociative recombination of highly-symmetric polyatomic ions

A general first-principles theory of dissociative recombination is developed for highly-symmetric molecular ions and applied to H$_3$O$^{+}$ and CH$_3^+$, which play an important role in astrophysical, combustion, and laboratory plasma environments. The theoretical cross-sections obtained for the dissociative recombination of the two ions are in good agreement with existing experimental data from storage ring experiments

Application of the Complex Kohn Variational Method to Attosecond Spectroscopy

The complex Kohn variational method is extended to compute light-driven electronic transitions between continuum wavefunctions in atomic and molecular systems. This development enables the study of multiphoton processes in the perturbative regime for arbitrary light polarization. As a proof of principle, we apply the method to compute the photoelectron spectrum arising from the pump-probe two-photon ionization of helium induced by a sequence of extreme ultraviolet and infrared-light pulses. We compare several two-photon ionization pump-probe spectra, resonant with the (2s2p)1P1o Feshbach resonance, with independent simulations based on the atomic B-spline close- coupling STOCK code, and find good agreement between the two approaches. This new finite-pulse perturbative approach is a step towards the ab initio study of weak-field attosecond processes in poly-electronic molecules

Theory of dissociative recombination of a linear triatomic ion with permanent electric dipole moment: Study of HCO+

We present a theoretical description of dissociative recombination of triatomic molecular ions having large permanent dipole moments. The study has been partly motivated by a discrepancy between experimental and theoretical cross sections for dissociative recombination of the HCO+ ion. The HCO+ ion has a considerable permanent dipole moment (D approximate to 4 D), which has not been taken explicitly into account in previous theoretical studies. In the present study, we include explicitly the effect of the permanent electric dipole on the dynamics of the incident electron using the generalized quantum defect theory, and we present the resulting cross section obtained. This demonstrates the possibility of applying generalized quantum defect theory to the dissociative recombination of molecular ions

Theoretical rate of dissociative recombination of HCO(+) and DCO(+) ions

This Brief Report presents an improved theoretical description of dissociative recombination of HCO(+) and DCO(+) ions with a low-energy electron. In a previous theoretical study [Mikhailov et al., Phys. Rev. A 74, 032707 (2006)] on HCO(+), the vibrational motion along the CO coordinate was neglected. Here, all vibrational degrees of freedom, including the CO stretch coordinate, are taken into account. The theoretical dissociative recombination cross section obtained is similar to the previous theoretical result at low collision energies (0.1 eV) energies. Therefore, the present study suggests that motion along the CO coordinate does not play a significant role in the process at low collision energies. The theoretical cross section is still approximately 2-3 times lower than the data from a recent merged-beam experiment

Photoelectron angular distribution in two-pathway ionization of neon with femtosecond XUV pulses

We analyze the photoelectron angular distribution in two-pathway interference between non\-resonant one-photon and resonant two-photon ionization of neon. We consider a bichromatic femtosecond XUV pulse whose fundamental frequency is tuned near the $2p^5 3s$ atomic states of neon. The time-dependent Schr\"odinger equation is solved and the results are employed to compute the angular distribution and the associated anisotropy parameters at the main photoelectron line. We also employ a time-dependent perturbative approach, which allows obtaining information on the process for a large range of pulse parameters, including the steady-state case of continuous radiation, i.e., an infinitely long pulse. The results from the two methods are in relatively good agreement over the domain of applicability of perturbation theory

Theoretical study of radiative electron attachment to CN, C2H, and C4H radicals

A first-principle theoretical approach to study the process of radiative electron attachment is developed and applied to the negative molecular ions CN$^-$, C$_4$H$^-$, and C$_2$H$^-$. Among these anions, the first two have already been observed in the interstellar space. Cross sections and rate coefficients for formation of these ions by radiative electron attachment to the corresponding neutral radicals are calculated. For completeness of the theoretical approach, two pathways for the process have been considered: (i) A direct pathway, in which the electron in collision with the molecule spontaneously emits a photon and forms a negative ion in one of the lowest vibrational levels, and (ii) an indirect, or two-step pathway, in which the electron is initially captured through non-Born-Oppenheimer coupling into a vibrationally resonant excited state of the anion, which then stabilizes by radiative decay. We develop a general model to describe the second pathway and show that its contribution to the formation of cosmic anions is small in comparison to the direct mechanism. The obtained rate coefficients at 30~K are $7\times 10^{-16}$cm$^3$/s for CN$^-$, $7\times 10^{-17}$cm$^3$/s for C$_2$H$^-$, and $2\times 10^{-16}$cm$^3$/s for C$_4$H$^-$. These rates weakly depend on temperature between 10K and 100 K. The validity of our calculations is verified by comparing the present theoretical results with data from recent photodetachment experiments