Bulletin of the American Physical Society
41st Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 55, Number 5
Tuesday–Saturday, May 25–29, 2010; Houston, Texas
Session B4: Focus Session: Strong Field Alignment and Orientation |
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Chair: Oliver Gessner, Lawrence Berkeley National Laboratory Room: Regency Ballroom |
Wednesday, May 26, 2010 10:30AM - 11:00AM |
B4.00001: Ultrafast strong field xray interactions with aligned molecules at LCLS Invited Speaker: The laser-molecule interaction with focused intense ($10^{20} W/cm^2$) ultrafast (5-100 fsec) coherent x-rays (800-2000 eV) is the subject of ongoing research at the LCLS x-ray free electron laser. This unprecedented combination of short wavelength, short pulse duration, and high intensity leads to several new experimental regimes. We have performed the first x-ray studies of transient molecular ion states, including coherent rotational alignment of diatomic molecules, as well as the excited states of the nitrogen dication. We have also studied the intense-field phenomenon of simultaneous multiple core hole formation, both within the same atom and on different atoms in the same molecule. The x-ray matter interaction is affected by competition between the laser ionization rate and Auger relaxation rates. Our initial results show how the ultrashort pulse duration affects ionization and dissociation dynamics in the molecule. These early results also establish new techniques for ultrafast time resolution in pump-probe experiments. [Preview Abstract] |
Wednesday, May 26, 2010 11:00AM - 11:12AM |
B4.00002: Angle- and internuclear separation- resolved strong field processes in molecules George Gibson, Li Fang Strong field ionization of molecules is considerably more complicated than atoms, depending on both angular orientation and internuclear separation R. Unfortunately, ground state molecules are localized in R and are randomly oriented, making measurements as a function of angle and R difficult. However, by simply exciting iodine to the B state of the neutral molecule with a pump pulse, we can study ionization processes from this state with a probe pulse as a function of both of these variables. Because the X to B transition dipole lies along the internuclear axis, the B state population is very well aligned, without the need for impulsive or adiabatic aligning pulses. By changing the delay of the probe, we control the internuclear separation, as the vibrational wavepacket evolves in the B state, and by changing the polarization of the probe, we can study the dependence on molecular orientation. With these techniques, we measure R-critical in a neutral molecule, the first time, as a function of angular orientation. [Preview Abstract] |
Wednesday, May 26, 2010 11:12AM - 11:24AM |
B4.00003: Ultrafast Electron Diffraction from Aligned Molecules Martin Centurion, Peter Reckenthaeler, Werner Fuss, Sergei Trushin, Ferenc Krausz, Ernst Fill Electron diffraction has been very successful for determining the structure of molecules in the gas phase, and also for investigating ultrafast conformational changes. However, due to the random orientation of the molecules in the gas phase only 1D information (the interatomic distances) can be extracted from the diffraction patterns, which limits the size of molecular structures that can be studied. Having a sample of aligned molecules would greatly increase the information encoded in the diffraction pattern and potentially allow for reconstructing the full 3D molecular structure. Here we show electron diffraction patterns recorded from a sample of transiently aligned molecules. In our experiments molecules were aligned selectively using photodissociation of C2F4I2 (1,2- diiodotetrafluoroethane). Molecules oriented parallel to the laser polarization have a higher probability for dissociation, which generates an anisotropic angular distribution. The diffraction pattern is captured by probing the sample with picosecond electron pulses shortly after dissociation---before molecular rotation causes the alignment to vanish. Our results clearly show that the diffraction pattern becomes anisotropic after dissociation. [Preview Abstract] |
Wednesday, May 26, 2010 11:24AM - 11:36AM |
B4.00004: Deflection of field-free aligned molecules Ilya Averbukh, Erez Gershnabel We suggest a new approach to the efficient control of molecular interaction with inhomogeneous optical or static fields by \textit{pre-shaping} molecular angular distribution with the help of short and strong femtosecond laser pulses. As an example, we consider laser deflection of $CS_2 $ molecules that are field-free pre-aligned before entering the interaction zone. Manipulating the polarization of the pre-aligning laser pulses, one may control the average deflection angle and its distribution, induce spectacular rainbow-like features in the distribution of the scattering angle, and reduce substantially the angular dispersion of the deflected molecules. This opens new ways for many applications involving molecular focusing, guiding and trapping by optical and static fields. We present both classical and fully quantum description of the new control scheme, and demonstrate its feasibility with the currently available laser technology. [Preview Abstract] |
Wednesday, May 26, 2010 11:36AM - 12:06PM |
B4.00005: Towards Control of 3D Alignment of Polyatomic Molecules Invited Speaker: 3D alignment of polyatomic molecules is the subject of a growing number of studies fueled by a host of potential applications. In general, to fully control the rotations of an asymmetric molecule, it is necessary to define two nonequivalent directions in space by means of two non-collinear fields. To this end, several strategies have been proposed; however, the question of the best approach in achieving 3D alignment remains open. One method to control the rotations of an asymmetric top molecule in 3D space is by using a combination of long and short (relative to the molecular rotational period) linearly polarized pulses. The molecular axis with the largest polarizability is sharply aligned along the polarization vector of the long pulse while the short pulse, orthogonally polarized and applied at the peak of the long pulse, spins the molecule about the ``held'' axis. By rapidly truncating the long pulse following the short pulse turn-off this method can be adopted to furnish field-free 3D alignment, a particularly attractive regime in terms of possible applications. Another approach to induce field-free 3D alignment is by using two short laser pulses linearly polarized in orthogonal directions. The temporal parameters of the pulses are tailored to a given molecule through optimization in order to produce the best overall 3D alignment or enhance the alignment in a particular angle. [Preview Abstract] |
Wednesday, May 26, 2010 12:06PM - 12:18PM |
B4.00006: Multi-dimensional quantum-beat spectroscopy of the rotational-vibrational dynamics in D$_2^+$ Uwe Thumm, Martin Winter, Ruediger Schmidt The ionization of D$_2$ in a short and intense laser pulse generates a rotational-vibrational (RV) nuclear wave packet in D$_2^+$. By solving the time-dependent Schr\"{o}dinger equation in full dimensionality, we simulate the coherent evolution of such wave packets and discuss their ro-vibrational dynamics. Within a harmonic time-series analysis of the evolving nuclear probability density [1], we characterize the RV dynamics in D$_2^+$ in an external intense linearly polarized infrared laser field in terms of quantum-beat (QB) spectra in which both, the internuclear distance and molecular orientation relative to the linearly polarized laser field are resolved. Based on numerical examples for the nuclear dynamics without and under the influence of pulsed and continuum-wave (cw) laser light, we discuss and quantify the signature of RV couplings in QB spectra [2] and to what extent the quantum-beat analysis of measured time-dependent fragment kinetic energy release spectra is expected to image the laser-dressed RV structure of D$_2^+$.\\[4pt] [1] U. Thumm {\it et al.}, Phys. Rev. A {\bf 77} 063401 (2008).\\[0pt] [2] M. Winter {\it et al.}, Phys. Rev. A {\bf 80} 063401(R) (2009). [Preview Abstract] |
Wednesday, May 26, 2010 12:18PM - 12:30PM |
B4.00007: Computational studies of x-ray scattering from threedimensionally- aligned asymmetric-top molecules Stefan Pabst, Phay Ho, Robin Santra We theoretically and numerically analyze x-ray scattering from asymmetric-top molecules three-dimensionally aligned using elliptically polarized laser light. The principal axes of the polarizability tensor are assumed to coincide with the principal axes of the moment of inertia tensor. Several symmetries in the Hamiltonian are identified and exploited to enhance the efficiency of solving the time-dependent Schr\"odinger equation for each rotational state initially populated in a thermal ensemble. Using a phase-retrieval algorithm, the feasibility of structure reconstruction from a quasi-adiabatically-aligned sample is illustrated for the organic molecule naphthalene. We demonstrate that for a laser peak intensity of 5~TW/cm$^2$, a laser pulse duration of 100~ps, a rotational temperature of 10~mK, and an x-ray pulse duration of 1~ps, the molecular structure may be probed at a resolution of 1 \AA. [Preview Abstract] |
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