Physics

The dynamics of an x-ray-ionized two-component core-shell nanosystem is probed using doped helium (He) nanodroplets. First, a soft x-ray pump pulse selectively inner-shell ionizes the core cluster formed of heavier rare-gas atoms, causing electron migration from the He shell to the highly charged core. This ignites a He nanoplasma which is then driven by an intense near-infrared probe pulse. The ultrafast charge redistribution, evidenced by the rise of He + and He 2+ ion yields from the nanoplasma within <70 fs, leads to strong damping of the core cluster expansion. Thus, He droplets act as efficient tampers that reduce the radiation damage of embedded nanostructures, a property that could be exploited for improving coherent diffraction images.

In molecular collisions, resonances occur at specific energies where the colliding particles temporarily form quasi-bound complexes, resulting in rapid variations in the energy dependence of scattering cross sections. Experimentally, it has proven challenging to observe such scattering resonances, especially in differential cross sections. We report the observation of resonance fingerprints in the state-to-state differential cross sections for inelastic NO-He collisions in the 13 to 19 cm −1 energy range with 0.3 cm −1 resolution. The observed structures were in excellent agreement with quantum scattering calculations. They were analyzed by separating the resonance contributions to the differential cross sections from the background through a partitioning of the multichannel scattering matrix. This revealed the partial wave composition of the resonances, and their evolution during the collision.

We demonstrate the breakdown of molecular-frame dynamics induced by the uncoupling of molecular rotation from electronic motion in molecular Rydberg states. We observe this non-Born-Oppenheimer regime in the time domain through photoelectron imaging of a coherent molecular Rydberg wave packet in N 2 . The photoelectron angular distribution shows a radically different time evolution than that of a typical molecular-frame-fixed electron orbital, revealing the uncoupled motion of the electron as it precesses around the averaged anisotropic potential of the rotating ion-core.

The interaction of a helium atom with intense short 800 nm laser pulse is studied theoretically beyond the single-active-electron approximation. For this purpose, the time-dependent Schrödinger equation for the two-electron wave packet driven by a linearly-polarized infrared pulse is solved by the time-dependent restricted-active-space configuration-interaction method (TD-RASCI) in the dipole velocity gauge. By systematically extending the space of active configurations, we investigate the role of the collective two-electron dynamics in the strong field ionization and high-order harmonic generation (HHG) processes. Our numerical results demonstrate that allowing both electrons in He to be dynamically active results in a considerable extension of the computed HHG spectrum.

The ground state of atoms from H to Ar was calculated using a self-interaction correction to local and gradient dependent density functionals. The correction can significantly improve the total energy and makes the orbital energies consistent with ionization energies. However, when the calculation is restricted to real orbitals, application of the self-interaction correction can give significantly higher total energy and worse results, as illustrated by the case of the Perdew-Burke-Ernzerhof gradient dependent functional. This illustrates the importance of using complex orbitals for systems described by orbital density dependent energy functionals.

The radiative lifetime of molecules or atoms can be increased by placing them within a tuned conductive cavity that inhibits spontaneous emission. This was examined as a possible means of enhancing three-body, singlet-based upconversion, known as energy pooling. Achieving efficient upconversion of light has potential applications in the fields of photovoltaics, biofuels, and medicine. The affect of the photonically constrained environment on pooling efficiency was quantified using a kinetic model populated with data from molecular quantum electrodynamics, perturbation theory, and ab initio calculations. This model was applied to a system with fluorescein donors and a hexabenzocoronene acceptor. Placing the molecules within a conducting cavity was found to increase the efficiency of energy pooling by increasing both the donor lifetime and the acceptor emission rate--i.e. a combination of inhibited spontaneous emission and the Purcell effect. A model system with a free-space pooling efficiency of 23% was found to have an efficiency of 47% in a rectangular cavity.

We report on electron removal calculations for 2.81 keV/amu Li 3+ and O 3+ ion collisions with neon dimers. The target is described as two independent neon atoms fixed at the dimer's equilibrium bond length, whose electrons are subjected to the time-dependent bare and screened Coulomb potentials of the classically moving Li 3+ and O 3+ projectile ions, respectively. Three mutually perpendicular orientations of the dimer with respect to the rectilinear projectile trajectories are considered and collision events for the two ion-atom subsystems are combined in an impact parameter by impact parameter fashion and are orientation-averaged to calculate probabilities and cross sections for the ion-dimer system. The coupled-channel two-center basis generator method is used to solve the ion-atom collision problems. We concentrate the ion-dimer analysis on one-electron and two-electron removal processes which can be associated with interatomic Coulomb decay, Coulomb explosion, and radiative charge transfer. We find that the calculated relative yields are in fair agreement with recent experimental data for O 3+ -Ne 2 collisions if we represent the projectile by a screened Coulomb potential, but disagree markedly for a bare Coulomb potential, i.e., for Li 3+ impact. In particular, our calculations suggest that interatomic Coulomb decay is a significant reaction channel in the former case only, since capture of a Ne( 2s ) electron to form hydrogenlike Li 2+ is unlikely.

We present a detailed study of inelastic energy-loss collisions of photoelectrons emitted from He nanodroplets by tunable extreme ultraviolet (XUV) radiation. Using coincidence imaging detection of electrons and ions, we probe the lowest He droplet excited states up to the electron impact ionization threshold. We find significant signal contributions from photoelectrons emitted from free He atoms accompanying the He nanodroplet beam. Furthermore, signal contributions from photoionization and electron impact excitation/ionization occurring in pairs of nearest-neighbor atoms in the He droplets are detected. This work highlights the importance of inelastic electron scattering in the interaction of nanoparticles with XUV radiation.

We report observation of new infrared bands of (CS2)2 and (CS2)3 in the region of the CS2 {\nu}1+ {\nu}3 combination band (at 4.5 cm-1) using a quantum cascade laser. The complexes are formed in a pulsed supersonic slit-jet expansion of a gas mixture of carbon disulfide in helium. We have previously shown that the most stable isomer of (CS2)2 is a cross-shaped structure with D2d symmetry and that for (CS2)3 is a barrel-shaped structure with D3 symmetry. The dimer has one doubly degenerate infrared-active band in the {\nu}1+ {\nu}3 region of the CS2 monomer. This band is observed to have a rather small vibrational shift of -0.846 cm-1. We expect one parallel and one perpendicular infrared-active band for the trimer but observe two parallel and one perpendicular bands. Much larger vibrational shifts of -8.953 cm-1 for the perpendicular band and -8.845 cm-1 and +16.681 cm-1 for the parallel bands are observed. Vibrational shifts and possible vibrational assignments, in the case of the parallel bands of the trimer, are discussed using group theoretical arguments.

The fundamental band for the OC-C2H2 dimer and two combination bands involving the intermolecular bending modes nu9 and nu8 in the carbon monoxide CO stretch region are re-examined. Spectra are obtained using a pulsed supersonic slit jet expansion probed with a mode-hop free tunable infrared quantum cascade laser. Analogous bands for OC-C2D2 and the fundamental for OC-DCCH as an impurity are also observed and analysed. A much weaker band in the same spectral region is assigned to a new mixed trimer, CO-(C2H2)2. The trimer band is composed uniquely of a-type transitions, establishing that the CO monomer is nearly aligned with the a-inertial axis. The observed rotational constants agree well with ab initio calculations and a small inertial defect value indicates that the trimer is planar. The structure is a compromise between the T-shaped structure of free acetylene dimer and the linear geometry of free OC-C2H2. A similar band for the fully deuterated isotopologue CO-(C2D2)2 confirms our assignment.