Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session J26: Focus Session: Quantum Control I |
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Sponsoring Units: DCP Chair: Vlasta Bonacic-Koutecky, Humboldt-Universitaet zu Berlin Room: Morial Convention Center 218 |
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J26.00001: Quantum Control of Femtochemistry in the Gas Phase, Liquid Phase and on Surfaces Invited Speaker: By using coherent control techniques we control the behavior of quantum systems on their natural fs-time scale by applying ultrashort coherent light fields in the wavelength range from the IR to the UV. These laser pulses can be variably shaped in space and time using a laser pulse shaper consisting of a liquid-crystal display [1]. Laser-optimized femtochemistry in the gas phase and liquid phase is one field in which this new technique is successfully employed. Automated optimization of branching ratios and total product yields of gas phase photodissociation reactions as well as chemically selective molecular excitation in the liquid phase is performed~[2][3]. Structural changes of a molecule in the liquid phase have been controlled by laser-optimized photoisomerization of a cyanine dye molecule [4] and of retinal in bacteriorhodopsin [5]. So far, optimal control techniques have been restricted to gas phase and condensed phase optimization experiments. Recently we have demonstrated femtosecond laser-assisted catalytic reactions on a Pd(100) single crystal surface. By applying a closed-loop optimal control scheme, we manipulate these reactions and selectively optimize the ratio of different bond-forming reaction channels, in contrast to previous quantum control experiments aiming at bond-cleavage. The results represent a first step towards selective photocatalysis of molecules. \newline [1] T. Baumert et al, Appl. Phys. B 65, 779 (1997) \newline [2] A. Assion et al, Science 282, 919(1998); T. Brixner et al, J. Mod. Opt. 50, 539 (2003) \newline [3] T. Brixner et al, Nature, Vol. 414, 57 (2001) and J.~Chem. Phys. 118, 3692 (2003) \newline [4] G. Krampert et al, Phys. Rev. Lett. 94, 068305 (2005) \newline [5] G. Vogt et al, Chem. Phys. Lett. 433, 211 (2006) P. Nuernberger et al, Phys. Chem. Chem. Phys. 9, 2470 (2007) [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:27PM |
J26.00002: Quantum control spectroscopy with multipulses Invited Speaker: The manipulation of molecular vibrations by laser light has been always considered as a very promising means to control chemical reaction. The coherently controlled time-dependent superposition of vibrational states may represent motion along a reaction coordinate and therefore allows for a high degree of selectivity. Pulse shapes for manipulating vibrations can be predicted to be trains of pulses with temporal spacing between the sub-pulses equal to an integer of the vibrational phase. If the manipulation of molecular vibrations with pulse trains is expected to be one of the important mechanisms on the long standing aim of mode selective chemistry, it is necessary to understand its application limits. In this work, the interaction of pulse trains with matter is discussed under the light of time-resolved nonlinear experiments and density matrix simulations. Emphasis is given to the role of electronic coherence between excited and ground-state, to the excited state population relaxation time and to the electronic resonance. In particular the lifetime of the excited state poses a challenge for the coherent control with multipulses and, thus, for the mode filtering capability in the excited state. This is investigated by applying a shaped femtosecond excitation pulse to different molecules in solution and probing the response by transient absorption, nonlinear Raman and DFWM spectroscopy. Finally, the effect of the phase of sinusoidal modulation on the envelope of the multipulse sequence and its consequences on pump-probe spectroscopy is discussed, particularly near zero delay between pump and probe pulses. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 1:03PM |
J26.00003: Strategies for optimal control in complex systems Invited Speaker: We present strategies for the optimal control of the ground and excited state dynamics in complex systems, based on the combination of the quantum chemical molecular dynamics ``on the fly'' with the semiclassical Wigner distribution approach [1]. We first demonstrate our strategy for the optimal control of the ground state dynamics based on the MD ``on the fly'' with explicit treatment of the interaction with the laser field which is optimized using a genetic algorithm [2]. This approach will be illustrated on two prototype systems representing rigid symmetrical molecules and floppy biomolecules with low frequency modes. Our results show that the ground state isomerization process can be selectively driven by ultrashort laser pulses with different shapes which are characteristic for the prototype systems. Furthermore, for the optimal pump-dump control involving ground and excited electronic states we have developed a new ``field induced surface hopping'' method in which the nuclear dynamics is treated classically while the laser induced electronic transitions are treated fully quantum mechanically. We illustrate our approach on the optimal control of cis-trans isomerization in prototype Schiff base molecular switches. Our theoretical approach allows us to explore the controllability of dynamics in complex systems and to unravel the mechanisms underlying the control of molecular processes. Furthermore, the outlook for laser selective photochemistry of nanoparticles and nanoparticle-biomolecule hybrid systems will be given. \newline \newline [1] V. Bona\v{c}i\'c-Kouteck\'y, R. Mitri\'c , Chem. Rev. 105, 11 (2005). \newline [2] R. Mitri\'c, V. Bona\v{c}i\'c-Kouteck\'y, Phys. Rev. A, 76, 031405 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J26.00004: Phase control of molecular fragmentation with a pair of femtosecond-laser pulses Karl-Michael Weitzel, Georg Breunig, Gunter Urbasch We demonstrate the control of molecular fragmentation on a femtosecond-time scale in two-pulse measurements with a pair of femtosecond-laser pulses. The measurements were performed with o-xylene (C$_{8}$H$_{10})$. Parent and fragment-ion yields were recorded as a function of inter-pulse delays, i.e. different relative phases of the excitation pulses. The experiments revealed different fragmentation mechanisms in the temporal region of direct optical overlap and for separated pulses. For overlapping pulses all ion yields followed the excitation intensity which oscillated as a function of inter-pulse delay due to the change of constructive and destructive interference. For larger delays, in particular the oscillations of the C$^{+}$ and CH$_{3}^{+}$ fragment-ion yield showed a significant deviation from each other. This deviation vanished in measurements with chirped femtosecond-laser pulses where both parent and fragment-ion yields oscillated in phase for all investigated delays. The results are interpreted as a manifestation of optical phase-dependent electronic excitations mapped onto the nuclear fragmentation dynamics. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J26.00005: Coherent control on cold alkali systems Albrecht Lindinger Optimal control of photo-induced molecular processes has attained considerable success in recent years. An important issue in this regard is the information coded in the optimized laser pulse shape which supplies insight about the underlying processes. Small alkali systems are suitable since they exhibit bound states available for resonant transitions with weak fields which aids the theoretical description and thus the interpretation. New control methods are presented to extract the most relevant information from the optimized laser field. Moreover, novel pulse shaper schemes for simultaneous phase, amplitude, and polarization pulse control were designed and applied to alkali dimers, even in a parametric encoding. The results demonstrate the perspectives of adding the polarization and hence all properties of the light field in the pulse modulation. Currently, coherent control was applied to ultracold ensembles motivated by the perspective to perform photoassociation and photostabilization of alkali systems. First results are received regarding optimized multi-photonic excitation to molecular ions and pump-probe experiments exposing signal oscillations. They provide indications for photoassociation and open the perspective for transitions to lower vibrational levels in the electronic ground state, which would be a first step to an internally cold molecular Bose Einstein condensate. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J26.00006: Direct frequency comb measurement and control of vibrational dynamics in ultracold molecular samples Avi Pe'er, Evgeny Shapiro, Moshe Shapiro, Jun Ye We propose a new class of control schemes for robust transfer of population between quantum states via a wave packet that utilize trains of coherent pulses (optical frequency comb). Our approach draws from analogy to adiabatic passage techniques in three-level systems, but is more general. We show that breaking a slow adiabatic passage into a train of short, perturbative pulses, enables highly efficient population transfer between single states through an arbitrary wave packet. Alternatively, it is possible to directly deduce the intermediate multi-state structure by a simple scan of the pulse train parameters (repetition rate and envelope phase), in a method similar to two-dimensional Fourier spectroscopy. Viewed in the spectral domain, these techniques rely on quantum pathway interference in an adiabatic passage. The scheme is most suitable for applications in cold and ultracold molecular samples. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J26.00007: Formation of a gas of ultracold LiCs molecules J. Deiglmayr, J. Lange, S.D. Kraft, A. Grochola, R. Wester, M. Weidem\"uller, M. Aymar, O. Dulieu Ultracold polar molecules offer intriguing perspectives for the study of many-body effects in strongly interacting gases and the manipulation by external fields. A promising approach to the creation of a large ensemble of ultracold polar molecules in the absolute translational and electronic ground state is the direct formation of ultracold molecules through photoassociation of ultracold atoms. We recently observed the spontaneous formation of ultracold LiCs molecules in a double species magneto optical trap. After spontaneous decay into the electronic ground state, the molecules were ionized by one-color two-photon ionization and detected with a high-resolution time-of-flight mass spectrometer~[1]. Here we present the active photoassociation of ultracold LiCs molecules and discuss the state distribution of the produced ground state molecules. Precise knowledge of the molecular structure is required to find the most efficient route for the creation of molecules. We present ab-initio calculations of excited molecular states of LiCs including spin-orbit coupling and study the alignment and orientation of bialkali molecules in combinations of static electric fields and strong laser fields. The perspectives for the production of molecules in the absolute ground state are evaluated. [1] S. D. Kraft \textit{et al.}, J. Phys. B {\bf 39}, S993 [Preview Abstract] |
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