Bulletin of the American Physical Society
40th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 54, Number 7
Tuesday–Saturday, May 19–23, 2009; Charlottesville, Virginia
Session B6: Focus Session: Vibrational and Rotational Coherence |
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Chair: Albert Stolow, National Research Council Canada Room: Minor Hall 125 |
Wednesday, May 20, 2009 10:30AM - 11:00AM |
B6.00001: Quantum state selection of polar molecules: Alignment, orientation and conformational control Invited Speaker: Building on ideas that go back to Stern in the 1920s we use an inhomogeneous static electric field to deflect a cold beam of polar molecules. The deflection spatially disperses the rotational quantum states of the molecules. We show that the molecules residing in the lowest-lying rotational states can be selected and used as targets for further experiments. In particular, the quantum-state-selected molecules enable unprecedented strong alignment, induced by a moderately intense laser pulse, as well as strong orientation induced by a mixed laser and static electric field. Here, alignment refers to confinement of one or more molecule-fixed axes along laboratory-fixed axes, and orientation refers to the molecular dipole moments pointing in a specific direction. Also, it is shown that the deflection enables separation of the different conformers of a single molecule. We discuss new opportunities offered by the enhanced degree of orientational control, made possibly by quantum state selection, including time resolved studies of torsion, and eventually enantiomeric conversion, of axially chiral molecules. Collaborators: J. K\"{u}pper, G. Meijer, L. B. Madsen. \\[4pt] [1] \textit{Laser-Induced Alignment and Orientation of Quantum-State-Selected Large Molecules}, L. Holmegaard, J. H. Nielsen, I. Nevo, H. Stapelfeldt, F. Filsinger, J. K\"{u}pper, and G. Meijer, Phys. Rev. Lett. \textbf{102}. 023001 (2009). [Preview Abstract] |
Wednesday, May 20, 2009 11:00AM - 11:12AM |
B6.00002: Rotational decoherence in a dense gas of multiply kicked N$_2$ Ryan Coffee, James Cryan, Phillip Bucksbaum, Adam Pelzer, S. Ramakrishna, Tamar Seideman \newcommand{\expect}[1]{\left\langle #1 \right\rangle} We use a recently developed multiple-kick scheme to study relaxation and dephasing in rotational wavepackets. When properly timed, multiple pulses can produce substantial coherent alignment as well as significant Raman redistribution. Transient peaks in $\expect{\cos^2\theta}$ reflect coherent alignment while a time-average of $\expect{\cos^2\theta} > 1/3$ reflects Raman redistribution. We observe both features in nitrogen gas at 300K and 1 ATM following impulsive Raman excitation by a train of up to eight, 50 fs, 800 nm laser pulses. Using a quantum calculation to help disentangle population relaxation from phase decoherence [1], we experimentally investigate rotational decoherence in the context of $J$-changing and $M$-changing collisions as a function of both rotational energy and gas density. [1] S. Ramakrishna and T. Seideman, Phys.~Rev.~Lett.~{\bf 95}, 113001 (2005). [Preview Abstract] |
Wednesday, May 20, 2009 11:12AM - 11:24AM |
B6.00003: Dynamic field-free orientation of polar molecules by intense two-color femtosecond laser pulses S. De, D. Ray, F. Anis, N.G. Johnson, I. Bocharova, M. Magrakvelidze, B.D. Esry, C.L. Cocke, I.V. Litvinyuk, I. Znakovskaya, M.F. Kling Combining a fundamental frequency of a laser and its second harmonic with a definite relative phase results in an asymmetric electric field and broken inversion symmetry. Such fields can generate orientation in polar molecules. Here we present the first experimental observation of dynamic field-free orientation of a heteronuclear molecule (CO) induced by intense two color (800 and 400 nm) femtosecond laser pulses. We have used the two color pulse as pump to orient the molecules and a more intense 800 nm pulse as probe to measure the angular distributions of ionic fragments. In addition to dynamic alignment seen in the time dependence of $<$cos$^{2}\theta >$, we observe clear orientation in the $<$cos$\theta >$ traces, which revives with the rotational period and can be reversed by changing the relative phase of the two colors. We studied the dependence of degree of orientation on pump pulse intensity, and compared the results with theoretical calculations. [Preview Abstract] |
Wednesday, May 20, 2009 11:24AM - 11:36AM |
B6.00004: Impulsive longitudinal molecular alignment Douglas Broege, Ryan Coffee, Philip Bucksbaum We report on measurements of a rotational wavepacket in diatomic nitrogen created impulsively with a circularly polarized 800 nm ultrfast pulse. This wavepacket differs from those typically produced in impulsive alignment experiments with linearly polarized light in that it exhibits transient alignment along the direction of laser propagation. In this experiment, The initial anisotropy in the angular distribution is torroidal within the plane of polarization, wheras the time averaged anisotropy lies perpendicular to this plane. This is in stark contrast to the linearly polarized case where both the initial and time averaged anisotropies lie along the electric field vector. In order to probe the wavepacket evolution in a co-propagating geometry, we use an interferometric measurement of the time varying index of refraction. [Preview Abstract] |
Wednesday, May 20, 2009 11:36AM - 12:06PM |
B6.00005: Strong field physics revealed through time-domain spectroscopy Invited Speaker: Pump-probe spectroscopy is generally used to study molecular dynamics. Over the past few years, we have turned this around and used molecular dynamics to study strong field interactions with molecules, using time-domain pump-probe spectroscopy. Many strong field experiments are insensitive to the final state of the atom or molecule under study, although the final state contains important information about the strong field interaction. By studying the vibrational dynamics of the final state we can learn about the interaction of the strong laser field with a molecule. In this talk, I will review a number of our results, including the measurement of a previously inaccessible potential energy curve in iodine, the demonstration of inner-orbital ionization, the observation of a new form of coherent control which works better as the temperature increases, and the demonstration of vibrational cooling of molecules through strong-field ionization. [Preview Abstract] |
Wednesday, May 20, 2009 12:06PM - 12:18PM |
B6.00006: Quantum-beat analysis of the rotational-vibrational dynamics in D$_2^+$ Uwe Thumm, Martin Winter, Ruediger Schmidt The rapid ionization of D$_2$ in a short and intense laser pulse generates a rotational-vibrational nuclear wave packet in D$_2^+$. By solving the time-dependent Schr\"odinger 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 ro-vibrational dynamics in D$_2^+$ in an external intense linearly polarized infrared laser field in terms of quantum-beat spectra in which both, the internuclear distance and molecular orientation relative to the laser field are resolved. In particular, we discuss quantum-beat spectra that reveal 1) the transition frequencies between stationary vibrational and rotational states, 2) the nodal structure of stationary vibrational rotational states, 3) ro-vibrational couplings, and 4) the imaging of field-dressed electronic potential curves of the molecular ion. [1] U. Thumm {\it et al.}, Phys. Rev. A {\bf 77}, 063401 (2008). [Preview Abstract] |
Wednesday, May 20, 2009 12:18PM - 12:30PM |
B6.00007: Internuclear-separation dependent ionization induced vibrational cooling in I$_2$ molecules Li Fang, George Gibson We present simulations and experimental results on the vibrational coherence induced by ``$R$-dependent ionization,'' also known as Lochfrass. The results are consistent and show that ``$R$-dependent ionization'' can lead to vibrational cooling. We attribute this cooling effect to the dissipative nature of the scheme for preparing the vibrational wave packets. In contrast, we show calculations for a non-dissipative scheme, bond softening, which does not lead to cooling. We compare the pulse-width and intensity dependence of the vibrational amplitude and average internuclear-separation. Finally, we compare trajectories of vibrational coherence from a thermal ensemble and from a single state as a function of intensity in a phase diagram for the two schemes. [Preview Abstract] |
Wednesday, May 20, 2009 12:30PM - 12:42PM |
B6.00008: Characterization of transient molecular vibrations on the way towards coherently controlled vibrational target state Stanislav Konorov, Xiaoji Xu, John Hepburn, Valery Milner We study molecular vibrational dynamics under the excitation by spectrally shaped broadband laser pulses. After performing a single measurement based on cross-correlation frequency resolved optical gating of molecular vibration, complete evolution of the complex-valued quantum coherence between the vibrational states is reconstructed with variable time and frequency resolution. The ability to change the resolution in the analysis of the transient molecular dynamics without repeating the experiment or changing experimental parameters is useful in designing and understanding various schemes of controlling quantum states of molecules. [Preview Abstract] |
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