Proposal for Observing XUV-Induced Rabi Oscillation Using Superfluorescent Emission.

Intense x-ray and extreme ultraviolet (XUV) light sources have been available for decades, however, due to weak nonlinear interaction in the XUV photon energy range, observation of Rabi oscillation induced by XUV pulse remains a very challenging experimental task. Here we suggest a scheme where photoionization of a He medium by an intense XUV pump pulse is followed by a strong population inversion and Rabi oscillation at the He^{+}(1s-3p) transition and is accompanied by superfluorescence (SF) of the 7.56 eV pulse at the He^{+}(3p-2s) transition. Our numerical simulations show that the Rabi oscillation at the He^{+}(1s-3p) transition induced by an XUV pulse with photon energy 48.36 eV results in significant signatures in the SF spectra, allowing us to identify and characterize the XUV induced Rabi-oscillatory regime. The proposed scheme provides a sensitive tool to monitor and control ultrafast nonlinear dynamics in atoms and molecules triggered by intense XUV.

Time-Resolved Resonant Auger Scattering Clocks Distortion of a Molecule.

Resonant Auger scattering (RAS) provides information on the core-valence electronic transition and impresses a rich fingerprint of the electronic structure and nuclear configuration at the time-initiating RAS process. Here, we suggest using a femtosecond X-ray pulse to trigger RAS in a distorted molecule, which is generated from the nuclear evolution on a valence excited state pumped by a femtosecond ultraviolet pulse. With the time delay varied, the amount of molecular distortion can be controlled and the RAS measurements imprint both their electronic structures and changing geometries. This strategy is showcased in H2O prepared in an O-H dissociative valence state, where molecular and fragment lines appear in RAS spectra as signatures of ultrafast dissociation. Given the generality of this approach for a broad class of molecules, this work opens a new alternative pump-probe technique for mapping the core and valence dynamics with ultrashort X-ray probe pulses.

Ultraviolet Pump-Probe Photodissociation Spectroscopy of Electron-Rotation Coupling in Diatomics.

The electronic angular momentum projected onto the diatomic axis couples with the angular momentum of the nuclei, significantly affecting the rotational motion of the system under electronic excitations by intense lasers. In this letter, we propose a pump-probe photodissociation scheme for an accurate determination of electron-rotation coupling effects induced by the strong fields. As a showcase we study the CH+ molecule excited by a short intense ultraviolet pump pulse to the A1Π state, which triggers coupled rovibrational dynamics. The dynamics is observed by measuring the kinetic energy release and angular resolved photofragmentation upon photodissociation induced by the time-delayed probe pulse populating the C1Σ+ state. Simulations of the rovibrational dynamics unravel clear fingerprints of the electron-rotation coupling effects that can be observed experimentally. The proposed pump-probe scheme opens new possibilities for the study of ultrafast dynamics following valence electronic transitions with current laser technology, and possible applications are also discussed.

Predissociation resonances and accurate ab initio calculations of dication HF2.

It is very interesting and challenging to investigate the electronic structures of diatomic dications, due to the nature of coulombic repulsive and bound attractive dissociation limits and their avoided diabatic interactions. Using the multi-reference configuration interaction approach, comprehensive ab initio calculations of the first 36 electronic states, corresponding to 15 dissociation limits, of dication HF2+ are reported. Good agreements for the vertical excitation energies and dissociation limits are achieved with the available references. Besides the common interesting quantities as adiabatic potential energy curves, dipole moments and spectral constants for the bound states, the nonadiabatic radial coupling matrix elements for the 1,3Π states are also presented. A showcase for the diabatic potentials of 3Π states are presented and discussed. Furthermore, predissociation states from the nonadiabatic couplings or avoided crossing of potential energy curves, known as shape resonances in collisions, are also investigated by using the WKB and scattering methods.

Theoretical study of (e, 2e) triple differential cross sections of tetrahydrofuran using multicenter distorted-wave method.

We present in this work a theoretical study of the triple differential cross sections for electron-impact single ionization of 12a' and 9b orbitals of a tetrahydrofuran molecule. The calculations are performed by the multicenter distorted-wave (MCDW) method in coplanar asymmetric kinematics with an incident electron energy of 250 eV, where previous (e, 2e) experiments and theories are available for comparison. The present calculations reproduce the experimental measurements satisfactorily, indicating that the MCDW method can be easily extended to study biomolecules with sufficiently high calculation efficiency.

Time-dependent quantum wave packet dynamics to study charge transfer in heavy particle collisions.

The method of time-dependent quantum wave packet dynamics has been successfully extended to study the charge transfer/exchange process in low energy two-body heavy particle collisions. The collision process is described by coupled-channel equations with diabatic potentials and (radial and rotational) couplings. The time-dependent coupled equations are propagated with the multiconfiguration time-dependent Hartree method and the modulo squares of S-matrix is extracted from the wave packet by the flux operator with complex absorbing potential (FCAP) method. The calculations of the charge transfer process 12Σ+ H-(1s2)+Li(1s22s)→22Σ+/32Σ+/12Π H(1s)+Li-(1s22s2l)(l=s,p) at the incident energy of about [0.3, 1.3] eV are illustrated as an example. It shows that the calculated reaction probabilities by the present FCAP reproduce that of quantum-mechanical molecular-orbital close-coupling very well, including the peak structures contributed by the resonances. Since time-dependent external interactions can be directly included in the present FCAP calculations, the successful implementation of FCAP provides us a powerful potential tool to study the quantum control of heavy particle collisions by lasers in the near future.

Complex multireference configuration interaction calculations for the K-vacancy Auger states of N(q+) (q = 2-5) ions.

K-vacancy Auger states of N(q+) (q = 2-5) ions are studied by using the complex multireference single- and double-excitation configuration interaction (CMRD-CI) method. The calculated resonance parameters are in good agreement with the available experimental and theoretical data. It shows that the resonance positions and widths converge quickly with the increase of the atomic basis sets in the CMRD-CI calculations; the standard atomic basis set can be employed to describe the atomic K-vacancy Auger states well. The strong correlations between the valence and core electrons play important roles in accurately determining those resonance parameters, Rydberg electrons contribute negligibly in the calculations. Note that it is the first time that the complex scaling method has been successfully applied for the B-like nitrogen. CMRD-CI is readily extended to treat the resonance states of molecules in the near future.

The sensitivities of high-harmonic generation and strong-field ionization to coupled electronic and nuclear dynamics.

The sensitivities of high-harmonic generation (HHG) and strong-field ionization (SFI) to coupled electronic and nuclear dynamics are studied, using the nitric oxide (NO) molecule as an example. A coherent superposition of electronic and rotational states of NO is prepared by impulsive stimulated Raman scattering and probed by simultaneous detection of HHG and SFI yields. We observe a fourfold higher sensitivity of high-harmonic generation to electronic dynamics and attribute it to the presence of inelastic quantum paths connecting coherently related electronic states [Kraus et al., Phys. Rev. Lett.111, 243005 (2013)]. Whereas different harmonic orders display very different sensitivities to rotational or electronic dynamics, strong-field ionization is found to be most sensitive to electronic motion. We introduce a general theoretical formalism for high-harmonic generation from coupled nuclear-electronic wave packets. We show that the unequal sensitivities of different harmonic orders to electronic or rotational dynamics result from the angle dependence of the photorecombination matrix elements which encode several autoionizing and shape resonances in the photoionization continuum of NO. We further study the dependence of rotational and electronic coherences on the intensity of the excitation pulse and support the observations with calculations.

Crossover of feshbach resonances to shape-type resonances in electron-hydrogen atom excitation with a screened Coulomb interaction.

The effects of Coulomb interaction screening on electron-hydrogen atom excitation in the n=2 threshold region are investigated by using the R-matrix method with pseudostates. The interaction screening lifts the l degeneracy of n=2 Coulomb energy level, producing two distinct thresholds for 2s and 2p states. The phenomenon of transformation of ;{1,3}P and 1D Feshbach resonances into shape-type resonances is observed when they pass across the 2s and 2p threshold, respectively, as the interaction screening increases. It is shown that this resonance transformation leads to dramatic effects in the 1s-->2s and 1s-->2p excitation collision strengths in the n=2 threshold collision energy region.