Bulletin of the American Physical Society
43rd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 57, Number 5
Monday–Friday, June 4–8, 2012; Orange County, California
Session M4: Focus Session: Control of Molecular Dynamics |
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Chair: Jonathan Wrubel, NIST Room: Garden 1-2 |
Thursday, June 7, 2012 8:00AM - 8:30AM |
M4.00001: Clocking Ultrafast Wave Packet Dynamics in Molecules through UV-induced Symmetry Breaking Invited Speaker: Alicia Palacios A UV pump - UV probe scheme is used to trace the evolution of nuclear wave packets in excited molecular states. A direct map is obtained by analyzing the asymmetry of the electron angular distributions resulting from dissociative ionization. The asymmetry results from the coherent superposition of gerade and ungerade states of the remaining molecular ion in the region where the pumped nuclear wave packet is located. The variation of this asymmetry with the time delay between the pump and the probe pulses thus parallels that of the moving wave packet and, consequently, can be used to clock its field-free evolution. The hydrogen molecule is used as benchmark target, which is represented within the Born-Oppenheimer approximation including all electronic, including correlation, and nuclear degrees of freedom. Two identical UV pulses of a few fs of duration constitute a typical UV pump - UV probe scheme. The photon energy is chosen such that the pump pulse excites the system in the lowest single excited states of the neutral, and the probe pulse will ionize system. Two-photon ionization is the major channel leaving the ion in both its ground (1s s g ) and its first excited state (2p s u ). As expected, the proton kinetic energy release (KER) distributions vary with the time-delay between the pulses, but any signature of the pumped wave packet is hardly visible. However, the superposition of the gerade (1s s g ) and ungerade (2p s u ) electronic states induces an asymmetry in the angular distributions, whose dependence with time delay leads to a sharp image of the time evolution of the wave packet. [Preview Abstract] |
Thursday, June 7, 2012 8:30AM - 9:00AM |
M4.00002: Ultralong-range energy transfer by interatomic Coulombic decay in the giant helium dimer Invited Speaker: Nicolas Sisourat Interatomic (molecular) Coulombic decay (ICD) is an ultrafast non-radiative electronic decay process for excited atoms or molecules embedded in a chemical environment. Via ICD, the excited system can get rid of the excess energy, which is transferred to one of the neighbors and ionize it. It should be stressed that whereas the same excited atom when isolated relaxes only by emitting a photon in a time range of picoseconds to nanoseconds, ICD takes place in the femtosecond range. Thus, ICD is generally the most favorable decay process. A key feature of ICD is that the energy transfer between the two involved atoms can take place over large distances, even beyond distances where the overlap of the involved wavefunctions becomes negligible. A question which arises is how far two atoms can exchange energy? The giant helium dimer is a perfect candidate to investigate this issue. It is the most weakly bound system in nature, with a binding energy of about 10$^{-7}$ eV ($\approx$ 1.1 mK) and a very large average bond length of around 52 {\AA}! Thanks to the extremely large interatomic distance distribution of the helium dimer, the latter allows to study ICD over such large distances. The presentation will focus on recent theoretical results on ICD in helium dimer. It be will shown that after simultaneous ionization and excitation of one helium atom in the dimer, the excited ion can relax through ICD. From the kinetic energy release (KER) spectra, it will be demonstrated that the two helium atoms can exchange energy via ICD over distances up to 14 {\AA} and that the observed oscillatory structures in the KER spectra reflect the nodal structures of vibrational wavefunctions involved in the decay process. Finally, computed time-resolved KER spectra will be presented and discussed. [Preview Abstract] |
Thursday, June 7, 2012 9:00AM - 9:12AM |
M4.00003: Attosecond coherent control of C2D4 dynamics Predrag Ranitovic, Craig Hogle, Margaret Murnane, Henry Kapteyn We employ ultrashort VUV pulses to initiate, and coherently control ultrafast dynamics and fragmentation of a C2D4 molecule by using an IR/VUV time-resolved attosecond COLTRIMS technique. A VUV frequency comb, containing the 5$^{th}$, 7$^{th}$, 9$^{th}$,11$^{th,}$ and 13$^{th}$ harmonic of the fundamental laser field (785 nm), was used to coherently populate several potential energy surfaces of a neutral C2D4 molecule, and a C2D4+ ion. By ionizing the neutral C2D4* molecule in a time-resolved fashion, using a strong (3x10$^{12}$ W/cm$^2$) laser field, we follow the fast relaxation of several excited states through conical intersections where the electronic excitation gets converted to vibrational motion. We investigate these dynamics on femtosecond and attosecond time scales. On the femtosecond time scale, we measure the decay constants of the fragments of interest (i.e. C2D4+, C2D3+, C2D2+, D+, and D2+), and find that the dynamics occur within the first 50 fs after the VUV pump pulse. On the attosecond time scale, we find that we can control the fragmentation through interference of electron wave packets and by changing the laser intensities. We find that the absolute phases of the fragmentation yields are sensitive to the VUV/IR delay, and change as the molecule relaxes through conical intersections. The relaxation through conical intersections is a complex and important mechanism that we studied using an electron/ion 3D momentum imaging COLTRIMS technique. The ability to demonstrate coherent control of this relaxation process on an ultrafast time scale, is an important step towards control of chemical reactions. [Preview Abstract] |
Thursday, June 7, 2012 9:12AM - 9:24AM |
M4.00004: Control of Molecular Rotation with a Chiral Train of Ultrashort Pulses Sergey Zhdanovich, Alexander Milner, Casey Bloomquist, Johannes Floss, Ilya Averbukh, John Hepburn, Valery Milner Trains of ultrashort laser pulses separated by the time of rotational revival (typically, tens of picoseconds) have been exploited for creating ensembles of aligned molecules. We introduce a chiral pulse train - a sequence of linearly polarized pulses with the polarization direction rotating from pulse to pulse by a controllable angle. The chirality of such a train, expressed through the period and direction of its polarization rotation, is used as a new control parameter for achieving selectivity and directionality of laser-induced rotational excitation. The method employs chiral trains with a large number of pulses separated on the time scale much shorter than the rotational revival (a few hundred femtosecond), enabling the use of conventional pulse shapers. [Preview Abstract] |
Thursday, June 7, 2012 9:24AM - 9:36AM |
M4.00005: Coherent Ro-vibrational Revivals in a Thermal Molecular Ensemble Martin Bitter, Evgeny A. Shapiro, Valery Milner We report an experimental and theoretical study of the evolution of vibrational coherence in a thermal ensemble of nitrogen molecules. Rotational dephasing and rephasing of the molecular vibrations is detected by coherent anti-Stokes Raman scattering. The existence of weak ro-vibrational coupling and the discrete nature of the rotational bath lead to a whole new class of full and fractional ro-vibrational revivals. Following the rich dynamics on a ns time scale with sub-ps resolution enables us to determine the rotational constant $\gamma_e$, as well as the intensity ratio between the different Raman transitions. [Preview Abstract] |
Thursday, June 7, 2012 9:36AM - 9:48AM |
M4.00006: Phase control with many cycle pulses in the absence of CEP stabilization Hyounguk Jang, Guan-Yeu Chen, Wendell T. Hill III Stabilization of the carrier envelope phase (CEP) of few-cycle pulses enhances our ability to control dynamics. When coupled with fixing the relative phase between two few-cycle pulses, control of molecular dynamics can be dramatic even when the pulse separation greatly exceeds the pulse widths. Here we present what we believe is the first demonstration of molecular dynamics control by a pair of many-cycle (t=50 fs) pulses separated by 3t with fixed relative CEP but in the absence of CEP stabilization of either pulse. In our experiment each pulse was intense enough to induce a Coulomb explosion of CO$_2$ into doubly charged atomic ions. By monitoring the ions, which carry information about the molecular geometry at the time of the explosion, we were able to determine how the relative separation and phase of the two pulses influence how the second pulse interacts with the ensemble. Specifically, we modified the bond angle by about 33\% and the strength of the second explosion by about a factor of 2.5. What makes our result noteworthy are (1) interference between the pulses plays no role and (2) coherence established by a long pulse is robust. Details of our experiment along with our result's implication on evolutionary control mechanisms will be discussed. [Preview Abstract] |
Thursday, June 7, 2012 9:48AM - 10:00AM |
M4.00007: Understanding Intense Field Two-Color and Carrier-Envelope Phase Control D. Ursrey, B.D. Esry The use of light to manipulate molecular dynamics and chemical reactions has become an increasingly important area of study in the past few decades. As intense laser pulses become more readily available, the ability to take advantage of multiphoton processes to enhance this control has become possible. Using tailored laser pulses, intense field control has already been demonstrated on a large number of physical observables including branching ratios, isomerization, and the alignment and orientation of molecules. Although there has been much success in achieving intense field control, there is still some ambiguity in understanding the physical mechanisms responsible. We present a recently developed non-perturbative method for interpreting two-color and carrier-envelope phase control in terms of interfering photon pathways [J. Phys. B {\bf 42}, 085601]. We will explain our method and demonstrate its utility in understanding and predicting control by extracting the relevant multiphoton pathways from both theoretical and experimental results. [Preview Abstract] |
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