We have experimentally and theoretically investigated the mixed-field orientation of rotational-state-selected OCS molecules and achieved strong degrees of alignment and orientation. The applied moderately intense nanosecond laser pulses are long enough to adiabatically align molecules. However, in combination with a weak dc electric field, the same laser pulses result in nonadiabatic dynamics of the mixed-field orientation. These observations are fully explained by calculations employing both adiabatic and nonadiabatic (time-dependent) models.
We present a theoretical study of recent laser-alignment and mixed-field-orientation experiments of asymmetric top molecules. In these experiments, pendular states were created using linearly polarized strong ac electric fields from pulsed lasers in combination with weak electrostatic fields. We compare the outcome of our calculations with experimental results obtained for the prototypical large molecule benzonitrile (C(7)H(5)N) [J. L. Hansen et al., Phys. Rev. A, 2011, 83, 023406.] and explore the directional properties of the molecular ensemble for several field configurations, i.e., for various field strengths and angles between ac and dc fields. For perpendicular fields one obtains pure alignment, which is well reproduced by the simulations. For tilted fields, we show that a fully adiabatic description of the process does not reproduce the experimentally observed orientation, and it is mandatory to use a diabatic model for population transfer between rotational states. We develop such a model and compare its outcome to the experimental data confirming the importance of non-adiabatic processes in the field-dressed molecular dynamics.
We apply a three-dimensional (3D) implementation of the time-dependent restricted-active-space self-consistent-field (TD-RASSCF) method to investigate effects of electron correlation in the ground state of Be as well as in its photoionization dynamics by short XUV pulses, including time-delay in photoionization. First, we obtain the ground state by propagation in imaginary time. We show that the flexibility of the TD-RASSCF on the choice of the active orbital space makes it possible to consider only relevant active space orbitals, facilitating the convergence to the ground state compared to the multiconfigurational time-dependent Hartree-Fock method, used as a benchmark to show the accuracy and efficiency of TD-RASSCF. Second, we solve the equations of motion to compute photoelectron spectra of Be after interacting with a short linearly polarized XUV laser pulse. We compare the spectra for different RAS schemes, and in this way we identify the orbital spaces that are relevant for an accurate description of the photoelectron spectra. Finally, we investigate the effects of electron correlation on the magnitude of the relative Eisenbud-Wigner-Smith (EWS) timedelay in the photoionization process into two different ionic channels. One channel, the ground state channel in the ion, is accessible without electron correlation. The other channel is only accessible when including electron correlation. For theory beyond the mean-field time-dependent HartreeFock, the EWS time-delay for the photon energy analyzed is quite insensitive to the considered active orbital spaces.
We demonstrate and analyze a strongly driven quantum pendulum in the angular motion of state-selected and laser-aligned carbonyl sulfide molecules. Raman couplings during the rising edge of a 50-ps laser pulse create a wave packet of pendular states, which propagates in the confining potential formed by the polarizability interaction between the molecule and the laser field. This wave-packet dynamics manifests itself as pronounced oscillations in the degree of alignment with a laser-intensity-dependent period. Pendular states, directional superpositions of field-free rotational states, are created by the anisotropic interaction between an isolated molecule and a strong electric field [1][2][3][4]. In a classical sense, this corresponds to the free rotation of the molecule changing into a restricted angular motion, where a molecular axis librates about the field direction. In the case of a strong static electric field the pendular states result from the interaction with the permanent dipole moment. This was exploited, for instance, for the simplification of spectroscopic signatures of large molecular clusters [5]. In the case of a nonresonant laser field the pendular states are formed due to the interaction with the molecular polarizability. This interaction constitutes the basis for laser-induced alignment [3,4], the confinement of molecular axes to laboratory-fixed axes defined by the polarization of the alignment field. Notably, in the limit where the laser field is turned on slowly compared to the inherent rotational period(s) of the molecule, the initial field-free rotational states are converted into the corresponding pendular states. This process is called adiabatic alignment [4] and it has found widespread use in molecular sciences [6][7][8][9][10][11]. The pendular states persist for as long as the laser field is turned on and the molecules return to their initial field-free rotational states upon turning off the laser field, provided this occurs slowly compared to the rotational period(s), τ rot .Pendular states were investigated through frequencyresolved spectroscopy [12,13] and by photodissociation or Coulomb explosion imaging [4,14]. The former approach probes the field-induced changes of the rotational energy levels, thus the pendular state energies, while the latter approach probes the way the molecules are confined in space, i. e., the orientational character of the pendular states. So far these studies were all performed in the adiabatic limit where the classical signature of the pendular states, i. e., the librational motion of a molecular axis about the field direction, cannot be observed directly. To observe this motion it would * jochen.kuepper@cfel.de; http://desy.cfel.de/cid/cmi be necessary to create a coherent superposition of pendular states.Here, we demonstrate that such pendular motion can be induced through the use of a laser pulse with a duration τ laser ∼ τ rot in between the common limits of adiabatic (τ laser τ rot ) and impulsive (τ laser τ rot ) alignment. The intermediate regim...
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