Electronic Dynamics Of Molecules Research Articles

Spatiotemporal imaging and shaping of electron wave functions using novel attoclock interferometry

Electrons detached from atoms by photoionization carry valuable information about light-atom interactions. Characterizing and shaping the electron wave function on its natural timescale is of paramount importance for understanding and controlling ultrafast electron dynamics in atoms, molecules and condensed matter. Here we propose a novel attoclock interferometry to shape and image the electron wave function in atomic photoionization. Using a combination of a strong circularly polarized second harmonic and a weak linearly polarized fundamental field, we spatiotemporally modulate the atomic potential barrier and shape the electron wave functions, which are mapped into a temporal interferometry. By analyzing the two-color phase-resolved and angle-resolved photoelectron interference, we are able to reconstruct the spatiotemporal evolution of the shaping on the amplitude and phase of electron wave function in momentum space within the optical cycle, from which we identify the quantum nature of strong-field ionization and reveal the effect of the spatiotemporal properties of atomic potential on the departing electron. This study provides a new approach for spatiotemporal shaping and imaging of electron wave function in intense light-matter interactions and holds great potential for resolving ultrafast electronic dynamics in molecules, solids, and liquids.

Two-dimensional photoelectron holography in strong-field tunneling ionization by counter rotating two-color circularly polarized laser pulses.

Strong-field photoelectron holography (SFPH), originating from the interference of the direct electron and the rescattering electron in tunneling ionization, is a significant tool for probing structure and electronic dynamics in molecules. We theoretically study SFPH by counter rotating two-color circularly (CRTC) polarized laser pulses. Different from the case of the linearly polarized laser field, where the holographic structure in the photoelectron momentum distribution (PEMD) is clustered around the laser polarization direction, in the CRTC laser fields, the tunneling ionized electrons could recollide with the parent ion from different angles and thus the photoelectron hologram appears in the whole plane of laser polarization. This property enables structural information delivered by the electrons scattering the molecule from different angles to be recorded in the two-dimensional photoelectron hologram. Moreover, the electrons tunneling at different laser cycles are streaked to different angles in the two-dimensional polarization plane. This property enables us to probe the sub-cycle electronic dynamics in molecules over a long time window with the multiple-cycle CRTC laser pulses.

Two methods for restricted configuration spaces within the multiconfiguration time-dependent Hartree-Fock method

The multiconfiguration time-dependent Hartree-Fock (MCTDHF) method has shown promise in calculating electronic dynamics in molecules driven by strong and high-energy lasers. It must incorporate restricted configuration spaces (meaning that a particular combination of Slater determinants is used, instead of full configuration interaction) to be applied to big systems. Two different Ans\atze are used to determine the essential term in the equations. The first Ansatz is the Lagrangian variational principle. The explicit, complete MCTDHF equations of motion, satisfying that principle, for arbitrary configuration spaces, are given. The property that a restricted configuration list must satisfy in order for the Lagrangian and McLachlan variational principles to give different results is identified. The second Ansatz keeps the density matrix block diagonal among equivalent orbitals, in a generalization of the method of Worth [J. Chem. Phys. 112, 8322 (2000)]. The methods perform well in calculating the dynamics of Be and ${\mathrm{BC}}^{2+}$ subject to ultrafast, ultrastrong lasers in severely truncated Hilbert spaces, although they exhibit differing degrees of numerical stability as implemented.

Ultrafast population transfer to excited valence levels of a molecule driven by x-ray pulses

First-principles quantum-mechanical calculations of an intense-field ultrafast two-color core-hole stimulated Raman process in nitric oxide are presented. They employ the multiconfiguration time-dependent Hartree-Fock (MCTDHF) method with all 15 electrons active. These calculations demonstrate a robust excitation localized on an atom through a core-electron stimulated Raman transition, the first step in proposed stimulated x-ray Raman spectroscopy experiments. A total population transfer of approximately 41% into valence excited states and 30% ionization is obtained via two concurrent 1.31-fs pulses with maximum intensities of 0.5 and $3\ifmmode\times\else\texttimes\fi{}{10}^{17}\text{W}{\mathrm{cm}}^{\ensuremath{-}2}$. It is found that both resonant and nonresonant (via the continuum) Raman transitions contribute. All aspects of these calculations except for ac Stark shifts are converged with a modest basis of 11 orbitals, demonstrating the efficiency of MCTDHF for the treatment of nonperturbative electronic dynamics in molecules.

Quantum path interferences of electron trajectories in two-center molecules

We report a new quantum path interference effect of electron trajectories in high-order harmonic generation (HHG) from two-center molecules, in which the interference minima are mainly located in the high-energy portion of HHG spectrum. The quantum calculations of the time-frequency analyses and the classical results of the electron trajectories demonstrate very good agreement and reveal that the positions of the interference minima are associated with the cutoff of various kinds of molecular electron trajectories. The interference fringes within a half optical cycle can be clearly seen in the time-frequency analysis spectrum. Moreover, the characteristics of both the HHG in frequency domain and the corresponding attosecond pulse generation in time domain permit tracing back the interference information of these electron trajectories. These interference phenomena offer new possibilities for getting insight into the attosecond electronic dynamics in molecules.

Quantum Control of Molecular Reaction Dynamics by Laser Pulses: Development of Theory and Its Application

Abstract In this paper, we review our recent progress in theoretical studies of quantum control of chemical reactions. Coherent interaction between the reaction system and applied laser fields is utilized to maximize the yield of a desired product (a target state). We have developed several kinds of effective theoretical methods for designing optimized ultrashort pulses for a desired reaction. One kind of control method is based on the control theory of a linearly time-invariant (LTI) system, by which the total population of the nontarget states and the total energies of the laser fields are minimized. This procedure is carried out at each successive short stage (local optimization theory), in which the time-dependent Schrödinger equation can be approximated to the equation of motion of the LTI system. Another control method is based on a tracking problem. A given trace in the tracking problem is introduced in an objective function of the quantum control. We have also developed a method using classical mechanics that provides a guiding principle for the control of a multidimensional molecular system. These methods based on local optimization theory can generate a wide variety of pulses ranging from weak field cases to strong field cases. In our treatment, π– and chirped pulses are obtained a priori as examples of optimized pulses. These methods have been applied to control of nuclear motions on adiabatic or diabatic potential surfaces. In addition to quantum control of typical unimolecular reactions such as isomerization, predissociation of NaI and enantiomer selective preparation, we present applications to orientation of molecules and population transfer to dark states (i.e., control of intramolecular vibrational energy redistribution). Control of orientation of molecules is a prerequisite for enantiomer selective preparation. Electronic dynamics of molecules in intense laser fields such as intramolecular electron transfer on an attosecond time scale is next discussed in order to extend our methods to novel control scenarios of chemical reactions by ultrafast manipulation of electronic motions. We have developed an efficient grid point method for accurately solving the time-dependent Schrödinger equation for the electronic degrees of freedom of a molecule. This fundamental numerical tool which extends the molecular orbital description enables us to clarify the mechanisms of intense-field-electronic dynamics such as tunnel ionization and Coulomb explosions of molecules. We present applications of this method to H2+ and H2 in high-intensity and electronically nonresonant low-frequency fields (I > 1013 W cm−2 and λ > 700 nm). For H2, it has been revealed for the first time that the two localized ionic states, H−H+ and H+H−, are alternately created according to the laser cycle. Such an intramolecular electronic motion induces nuclear motions (e.g., bond stretching). Possible new control schemes of electronic and nuclear dynamics utilizing electron-electron and electron-nucleus correlations are finally proposed.