Strong-field Ionization Of Molecules Research Articles

Reducing the Cost of TD-CI Simulations of Strong Field Ionization.

Strong field ionization of molecules by intense laser pulses can be simulated by time-dependent configuration interaction (TD-CI) with a complex absorbing potential (CAP). Standard molecular basis sets need to be augmented with several sets of diffuse functions for effective interaction with the CAP. This dramatically increases the number of configurations and the cost of the TD-CI simulations as the size of the molecules increases. The cost can be reduced by making use of spin symmetry and by employing an orbital energy cutoff to limit the number of virtual orbitals used to construct the excited configurations. Greater reductions in the number of virtual orbitals can be obtained by examining their interaction with the absorbing potential during simulations and their contributions to the strong field ionization rate. This can be determined from the matrix elements of the absorbing potential and the TD-CI coefficients from test simulations. Compared to a simple 3 hartree cutoff in the orbital energies, these approaches reduce the number of virtual orbitals by 20-35% for neutral molecules and 5-10% for cations. As a result, the cost of simulations is reduced by 35-60% for neutral molecules. The number of virtual orbitals needed can also be estimated by second-order perturbation theory without the need for test simulations. The number of virtual orbitals can be reduced further by adapting orbitals to the laser field using natural orbitals derived from test simulations. This is particularly effective for cations, yielding reductions of more than 20%.

Tracking the Ionization Site in Neutral Molecules.

When a diatomic molecule is exposed to intense light, the valence electron may tunnel from a higher potential (corresponding to an upfield atom) due to the suppressed internuclear barrier. This process is known as ionization enhancement and is a key mechanism in strong field ionization of molecules. Alternatively, the bound electron wave function can evolve adiabatically in the laser field, resulting in ionization from the downfield atom. Here, we introduce a method to quantify the relative contribution of these two processes. Applying this method to experimentally measured electron momenta distributions following strong field ionization of N_{2} with infrared laser light, we find approximately a 2∶1 ratio of electrons ionized from a downfield atom, relative to upfield. This suggests that the bound state wave function largely adapts adiabatically to the changing laser field, although the nonadiabatic process of ionization enhancement still contributes even in neutral molecules. Our method can be applied to any diatomic neutral molecule to better understand the evolution of the initially bound electron wave packet and hence the nature of the molecular ionization process.

Molecular potential anisotropy probed by electron rescattering in strong-field ionization of molecules

One of the main differences between rescattering following laser-induced ionization of atoms and molecules and electron-ion collisions performed with electron beams is the quivering dynamics of the electron around the ionic site in laser experiments. This report is aimed at probing the effect of the anisotropic potential of linear and symmetric molecular ions on the departing electron following tunnel ionization and rescattering by an intense femtosecond laser field at $0.8\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ and ${10}^{14}{\phantom{\rule{3.33333pt}{0ex}}\mathrm{Wcm}}^{\ensuremath{-}2}$. The laser excitation conditions are chosen so that the maximum collision energy remains of the order of 20 eV. In the energy range where rescattering is significant, the angular widths of the photoelectron angle-resolved energy spectra are found to be significantly larger for ${\mathrm{N}}_{2}$ and ${\phantom{\rule{3.33333pt}{0ex}}\mathrm{C}}_{2}{\mathrm{H}}_{2}$ molecules aligned perpendicularly to the laser polarization than the angular widths of angle-resolved energy spectra recorded in parallel alignment, although ${\mathrm{N}}_{2}$ and ${\phantom{\rule{3.33333pt}{0ex}}\mathrm{C}}_{2}{\mathrm{H}}_{2}$ obey opposite tunnel ionization dynamics with respect to alignment. In the associated collision energy and angle ranges where this effect is observed, the lowest values of the impact parameter are less than one atomic unit. It is conjectured that the approach of the quivering departing electron may be sufficient for its trajectories to be dependent on the molecular ion core alignment, i.e., the anisotropic molecular potential.

Chiral discrimination by recollision enhanced femtosecond laser mass spectrometry

Chiral molecules and their interactions are critical in a variety of chemical and biological processes. Circular dichroism (CD) is the most widely used optical technique to study chirality, often performed in a solution phase. However, CD has low-efficiency on the order of 0.01–1%. Therefore, there is a growing need to develop high-efficiency chiroptical techniques, especially in gas-phase, to gain background-free in-depth insight into chiral interactions. By using mass spectrometry and strong-field ionization of limonene with elliptically polarized light, we demonstrate an efficient chiral discrimination method that produces a chiral signal of one to two orders of magnitude higher than the conventional CD. The chiral response exhibits a strong dependence on wavelength in the range of 1,300–2,400 nm, where the relative abundance of the ion yields alternates between the two enantiomers. The origin of enhanced enantio-sensitivity in intense laser fields is attributed to two mechanisms that rely on the recollision dynamics in a chiral system: (1) the excited ionic state dynamics mediated either by the laser field or by the recollision process, and (2) non-dipole effects that alter the electron’s trajectories. Our results can serve as a benchmark for testing and developing theoretical tools involving non-dipole effects in strong-field ionization of molecules.

Torsional effects in strong-field ionization of molecules

This paper elucidates effects of torsional motion on tunneling ionization in intense laser fields. Recent advances in the formulation and implementation of an integral representation of the weak-field asymptotic theory make it possible to study the behavior of the tunneling ionization rate with the dihedral angle between the two planes in biphenyl and substituted biphenyl molecules. These results have implications for control of torsional motion and deracemization schemes for axial chiral molecules.

Enhanced ionization of vibrational hot carbon disulfide molecules in strong femtosecond laser fields**Project supported by the National Natural Science Foundation of China (Grant Nos. 91750104, 11704004, 11704149, and 11474130) and the Natural Science Foundation of Jilin Province, China (Grant No. 20180101289JC).

By using a heated molecular beam in combination with a time-of-flight mass spectrometer, we experimentally study the ionization of vibrational-hot carbon disulfide (CS2) molecules irradiated by a linearly polarized 800-nm 50-fs strong laser field. The ion yields are measured in a laser intensity range of 7.0 × 1012 W/cm2–1.5 × 1014 W/cm2 at different molecular temperatures of up to 1400 K. Enhanced ionization yield is observed for vibrationally excited CS2 molecules. The results show that the enhancement decreases as the laser intensity increases, and exhibits non-monotonical dependence on the molecular temperature. According to the calculated potential energy curves of the neutral and ionic electronic states of CS2, as well as the theoretical models of molecular strong-field ionization available in the literature, we discuss the mechanism of the enhanced ionization of vibrational-hot molecules. It is indicated that the enhanced ionization could be attributed to both the reduced ionization potential with vibrational excitation and the Frank–Condon factors between the neutral and ionic electronic states. Our study paves the way to understanding the effect of nuclear motion on the strong-field ionization of molecules, which would give a further insight into theoretical and experimental investigations on the interaction of polyatomic molecules with strong laser fields.

Phase Structure of Strong-Field Tunneling Wave Packets from Molecules.

We study the phase structure of the tunneling wave packets from strong-field ionization of molecules and present a molecular quantum-trajectory MonteCarlo model to describe the laser-driven dynamics of photoelectron momentum distributions of molecules. Using our model, we reproduce and explain the alignment-dependent molecular frame photoelectron spectra of strong-field tunneling ionization of N_{2} reported by M. Meckel etal. [Nat. Phys. 10, 594 (2014)]. In addition to modeling the low-energy photoelectron angular distributions quantitatively, we extract the phase structure of strong-field molecular tunneling wave packets, shedding light on its physical origin. The initial phase of the tunneling wave packets at the tunnel exit depends on both the initial transverse momentum distribution and the molecular internuclear distance. We further show that the ionizing molecular orbital has a critical effect on the initial phase of the tunneling wave packets. The phase structure of the photoelectron wave packet is a key ingredient for modeling strong-field molecular photoelectron holography, high-harmonic generation, and molecular orbital imaging.

Dynamic Exchange in the Strong Field Ionization of Molecules.

We show that dynamic exchange is a dominant effect in strong field ionization of molecules. In CO(2) it fixes the peak ionization yield at the experimentally observed angle of 45° between polarization direction and the molecular axis. For O(2) it changes the angle of peak emission and for N(2) the alignment dependence of yields is modified by up to a factor of 2. The effect appears on the Hartree-Fock level as well as in full ab initio solutions of the Schrödinger equation.

Theoretical approach to the study of vibrational effects on strong field ionization of molecules with alignment-dependent tunneling ionization rates**Project supported by the National Basic Research Program of China (Grant No. 2013CB922200) and the National Natural Science Foundation of China (Grant Nos. 11034003 and 11127403).

The tunneling ionization rates of vibrationally excited N2 molecules at the ground electronic state are calculated using molecular orbital Ammosov–Delone–Krainov theory considering R-dependence. The results show that molecular alignment significantly affects the ionization rate, as the rate is mainly determined by the electron density distribution of the highest occupied molecular orbital. The present work indicates that the ratios of alignment-dependent rates of different vibrational levels to that of the vibrational ground level increase for the aligned N2 at the angle θ = 0°, and suggests that the alignment-dependent tunneling ionization rates can be used as a diagnostics for the influence of vibrational excitation on the strong field ionization of molecules.

Signature of superradiance from a nitrogen-gas plasma channel produced by strong-field ionization

Recently, Yao et al. demonstrated the creation of coherent emissions in nitrogen gas with two-color (800 nm + 400 nm) ultrafast laser pulses [J. Yao, G. Li, C. Jing, B. Zeng, W. Chu, J. Ni, H. Zhang, H. Xie, C. Zhang, H. Li, H. Xu, S. L. Chin, Y. Cheng, and Z. Xu, New J. Phys. 15, 023046 (2013)]. Based on this two-color scheme, here we report on systematic investigation of temporal characteristics of the radiation emitted at 391 nm [N${}_{2}{}^{+}$: $B{\phantom{\rule{0.16em}{0ex}}}^{2}{\ensuremath{\Sigma}}_{u}^{+}\left(\ensuremath{\nu}=0\right)\ensuremath{-}X{\phantom{\rule{0.16em}{0ex}}}^{2}{\ensuremath{\Sigma}}_{g}^{+}\left(\ensuremath{\nu}=0\right)$] by experimentally examining its temporal profiles with the increase of the plasma channel induced by the intense 800-nm femtosecond laser pulses at a nitrogen-gas pressure of \ensuremath{\sim}25 mbar. We reveal unexpected temporal profiles of the coherent emissions, which show significant superradiance signatures owing to the cooperation of an ensemble of excited N${}_{2}$${}^{+}$ molecules that are coherently radiating in phase. Our findings shed more light on the mechanisms behind the coherent laserlike emissions induced by strong-field ionization of molecules.

Alignment-Dependent Fluorescence Emission Induced by Tunnel Ionization of Carbon Dioxide from Lower-Lying Orbitals

The study of the ionization process of molecules in an intense infrared laser field is of paramount interest in strong-field physics and constitutes the foundation of imaging of molecular valence orbitals and attosecond science. We show measurement of alignment-dependent ionization probabilities of the lower-lying orbitals of the molecules by experimentally detecting the alignment dependence of fluorescence emission from tunnel ionized carbon dioxide molecules. The experimental measurements are compared with the theoretical calculations of the strong field approximation and molecular Ammosov-Delone-Krainov models. Our results demonstrate the feasibility of an all-optical approach for probing the ionization dynamics of lower-lying orbitals of molecules, which is until now still difficult to achieve by other techniques. Moreover, the deviation between the experimental and theoretical results indicates the incompleteness of current theoretical models for describing strong field ionization of molecules.

Multielectron effects in charge asymmetric molecules induced by asymmetric laser fields.

Using a 45 fs pump pulse at 800 nm, a wave packet is created in a charge asymmetric dissociation channel of iodine, I(2)(2+)→I(2+)+I(0+) (2,0). As the molecule dissociates, a two-color (1ω2ω) probe pulse is used to study the dynamics as a function of internuclear separation R. We find a critical region of R in which there is spatially asymmetric enhanced ionization of the (2,0) channel to a counterintuitive (1,2) channel. In this region the I(0+) is ionized such that one electron is released to the continuum and another is transferred to the I(2+) resulting in I(0+)→I(2+) and I(2+)→I(1+). At larger R, the ionization is consistent with simple one-electron ionization in a double well where I(0+)→I(1+). We find qualitative agreement between simulations and experiment further highlighting the importance of multielectron effects in the strong-field ionization of molecules.

Molecular Dynamics in Strong Laser Fields

One of the goals for strong field physics is imaging the dynamics of the quantum systems, and in case of molecules, there is strong interest in imaging of the electron rearrangement during chemical reactions. Strong field ionization of molecules has many attractive features that make this process a candidate tool for strong field imaging of transient states and chemical reactions. In this paper, we present analysis of ionization dependence on orientation of molecular axis with respect to polarization of the electric field of the laser. By considering several examples of molecules at their equilibrium internuclear distances and an example of the simplest chemical reaction, namely, the dissociation of diatomic molecule, we present how symmetries of the molecular orbitals influence the alignment-dependent ionization and how this dependence can be used to follow molecular dynamics using strong field multiphoton ionization.

Strong-field ionization of molecules by few-cycle pulses

Few-cycle pulses of 800 nm light cause ionization and dissociation of triatomic molecules like H2O and CS2 in the temporal regime where only electron rescattering dominates the laser-molecule interaction. In case of ionization of H2O, significantly higher amounts of kinetic energy are released upon dissociation into atomic fragments. However, the case of CS2 is somewhat special in that the wave packet of the rescattered electron destructively interferes with the antibonding π orbital of CS2+ such that rescattering is also essentially turned off. Under such circumstances the dissociation channel is absent and long-lived singly, doubly, and triply charged molecular ions dominate the mass spectrum in the few-cycle regime. This brings to the fore the importance of molecular symmetry in strong-field ionization. Ultrafast atomic rearrangements are also observed: H atoms in H2O are rearranged by strong optical fields generated by intense 9.3 fs laser pulses to form a new bond leading to H2+; experiments show that this bond formation occurs within a single laser pulse.

Strong-field ionization of molecules in circularly polarized few-cycle pulses

We have numerically investigated ionization of molecules exposed to circularly polarized (CP) intense few-cycle pulses (FCPs) by solving the three-dimensional time-dependent Schroedinger equation. The resulting photoelectron spectra exhibit an interesting ''plateau'' feature that does not appear for atoms exposed to circularly-polarized laser fields. The origin comes from the intense CP field ionizing a portion of the electron wave packet about one nuclear center and driving it through the other. Moreover, the angular distribution of photoelectrons gives an indication of the carrier-envelope phase of the applied FCP, which implies that steering of photoelectrons in space can be achieved by controlling the FCP phase.

Resonant excitation during strong-field dissociative ionization

We studied dissociative ionization of oxygen by intense femtosecond laser pulses with central wavelengths between 550 and 1800 nm. We measured kinetic energy release spectra and angular distributions of fragments resulting from symmetric dissociation of doubly charged molecular ions (${\mathrm{O}}_{2}^{2+}\ensuremath{\rightarrow}{\mathrm{O}}^{+}+{\mathrm{O}}^{+}$ channel). In the kinetic energy release spectra we identified a number of distinct excited states of the molecular ion. Angular distributions for all but one of those states were consistent with recollision excitation following single ionization of ${\mathrm{O}}_{2}$. For the remaining $(B\phantom{\rule{0.2em}{0ex}}^{3}\ensuremath{\Pi}_{\mathbf{g}})$ excited state we observed a characteristic resonant dependence of its relative yield on the central optical frequency of the pulse, with the yield peaking at around 890 nm. This presents unambiguous evidence in support of importance of resonant excitation channels in strong field ionization of molecules.

Role of symmetry in strong-field ionization of molecules

Angular distributions of photoelectrons, produced by single ionization of ethylene or benzene molecules with an intense linearly polarized laser field, are calculated. The anisotropy and the yield of the photoelectrons depend strongly on the orientation between the molecular geometry and the field. In particular, electron emission in nodal planes is suppressed. The orientational dependence, also found in the total ionization rate, is a consequence of the symmetry of the highest occupied molecular orbital.