Bulletin of the American Physical Society
38th Annual Meeting of the Division of Atomic, Molecular, and Optical Physics
Volume 52, Number 7
Tuesday–Saturday, June 5–9, 2007; Calgary, Alberta, Canada
Session J6: Short Pulse Interactions with Molecules |
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Chair: B. Esry, Kansas State University Room: TELUS Convention Centre Olde Scotch Room |
Thursday, June 7, 2007 1:30PM - 1:42PM |
J6.00001: Intense field ionization of diatomic molecules in the tunneling region. Zi Jian Long, Wing-Ki Liu There are two theoretical approaches to study the strong field ionization of molecules.One is based on a static picture in which the electron is considered to tunnel out through a potential barrier (ADK Theory [1]). In this approach the time dependence of laser field is taken into account by averaging the static ionization rate over time. A simple ionization formula can be obtained and it well predicts the experimental result in the tunneling region. Another theoretical approach is the time-dependent picture based on an S-matrix formulism. An essential feature of this approach is Strong Field Approximation (SFA). We expect that in the tunneling region the SFA will give similar analytical expression as in the ADK theory. Much work has been done in the atomic case to demonstrate that the SFA asymptotically gives ADK tunneling rate [2]. Here we study the strong field ionization of simple diatomic molecules in which the electronic wave functions will be represented by linear combination of atomic orbitals (LCAO). We will consider the ionization rate in tunnelling regime for diatomic molecules and investigate the interference effects between different atomic orbitals. We will compare our asymptotic formula with numerical calculation for simple systems. \newline [1]Ammosov, Delone, Krainov Sov. Phys. Jept. 64 1191 \newline [2]L. V. Keldysh Sov. Phys.Jept. 20 1307 [Preview Abstract] |
Thursday, June 7, 2007 1:42PM - 1:54PM |
J6.00002: Direct time-resolved observation of molecular dynamics induced by extreme- ultraviolet photoionization Arvinder Sandhu, Etienne Gagnon, Ariel Paul, Margaret Murnane, Henry Kapteyn, Predrag Ranitovic, C. Lewis Cocke Laser-generated high-order harmonics provide a source of extreme-ultraviolet radiation with unique capabilities for probing atomic and molecular dynamics. Here we present the first such studies by employing high harmonics in conjunction with coincident electron-ion 3D momentum imaging (COLTRIMS) technique. We generate femtosecond EUV pulses at $\sim $ 42 eV photon energy by upconverting intense ($>$ 10$^{14}$ Wcm$^{-2})$ 25 fs laser pulses in an argon filled waveguide. Using this ultrashort EUV pulse as a pump, we launch D$_{2}$, N$_{2}$ and CO into highly excited states near the molecular double-ionization threshold. The dynamics of these highly excited states unfold along different channels, which are identified by electron-ion correlation. By employing moderate intensity infrared fields, we show that we can influence and probe these dynamics on femtosecond timescales. We observe that the double ionization yield is significantly enhanced by the presence of an IR field in conjunction with the EUV pump. The kinetic energy release evolves as a function of delay time (internuclear distance), allowing us to map excited state dynamics. [Preview Abstract] |
Thursday, June 7, 2007 1:54PM - 2:06PM |
J6.00003: Electron Momentum Distributions from Strong-Field Ionization of Aligned Molecules Daniel Comtois, Heidi Bandulet, Domagoj Pavicic, Moritz Meckel, Andr\'e Staudte, Dirk Zeidler, Reinhard D\"orner, Henri P\'epin, Jean-Claude Kieffer, David Villeneuve, Paul Corkum Using a velocity map imaging electron spectrometer, we measured the velocity distribution of electrons produced by strong laser field ionization of aligned molecules. By taking the ratio of velocity distributions obtained for different molecular orientation distributions, we are able to isolate the modulations due to molecular alignment. The low momentum part of the distributions is populated by electrons that are directly emitted from the molecule without subsequent rescattering with the ion core. For these electrons, the modulations of the electron velocity distribution are observed solely along the axis perpendicular to the laser field. They are caused by the projection of the ionized molecular orbital's wavefunction into the perpendicular momentum distribution of the ionized electron wavepacket. The high momentum part of the distributions is populated by electrons that have rescattered with the ion core. The modulations seen in the ratio of the velocity distributions for these electrons are believed to be due to diffraction of the electron wavepacket by its parent ion. [Preview Abstract] |
Thursday, June 7, 2007 2:06PM - 2:18PM |
J6.00004: Mapping the rapid time evolution of impulsively aligned deuterium molecules using intense few-cycle infrared laser pulses. Jarlath McKenna, Chris Calvert, Ian Williams, William Bryan, Elizabeth English, Joseph Wood, Roy Newell, Ricardo Torres, Edmond Turcu Short intense laser pulses have recently become a viable and efficient method of impulsively inducing alignment of molecules, ranging from simple diatomics to more exotic structures. Key to the widespread applicability of this technique is that the maxima and minima of alignment occur under field-free conditions at delayed periodic intervals corresponding to the quantum rotational revival of the system. In effect, the laser pulse creates a coherent superposition of the rotational states of the molecule which undergo a quantum dephasing and rephasing as the system evolves in time. We present here an experiment where we use few-cycle infrared laser pulses to induce and map the ultrafast field-free alignment of deuterium molecules starting from a randomly distributed ensemble at room temperature. The results, including angular distribution measurements, are compared to theory which describes the experimental data to a high degree. [Preview Abstract] |
Thursday, June 7, 2007 2:18PM - 2:30PM |
J6.00005: Energy and angular structure in momentum space images of electrons ionized from aligned O$_{2}$ and CO$_{2}$ molecules by short, intense laser pulses Chakra Maharjan, Predrag Ranitovic, Irina Bocharova, Dipanwita Ray, Ben Gramkow, Igor Litvinyuk, Charles Cocke We have used COLTRIMS to measure momentum spectra of electrons generated by ionizing dynamically aligned molecules of O$_{2}$ and CO$_{2}$ by short laser pulses. An ultra-short, weak, vertically polarized laser pulse produced an aligned ensemble of molecules through rotational revivals. These molecules were then ionized with a subsequent strong horizontally polarized pulse. The rotational revival structure was controlled by measuring the angular distribution of dissociation fragment ions from each target molecule as a function of the time delay between the pump and probe pulses. Full momentum-space images of the electrons were then measured as a function of this time delay. We observed pronounced energy and angular structures of the momentum images which show a dependence on the alignment of the molecule. [Preview Abstract] |
Thursday, June 7, 2007 2:30PM - 2:42PM |
J6.00006: Few cycle ($<$10fs) intense laser-induced ionization and dissociation of simple diatomic molecules probed by coincidence 3D momentum imaging A. Max Sayler, Pengqian Wang, Jarlath McKenna, Fatima Anis, Bishwanath Gaire, Nora G. Johnson, Eli Parke, Mat Leonard, Kevin D. Carnes, B.D. Esry, Itzik Ben-Itzhak Laser-induced ionization and dissociation of H$_{2}^{+}$, HD$^{+}$, and D$_{2}^{+}$ by sub-10 fs 790 nm pulses with intensities up to 10$^{15}$ W/cm$^{2}$ have been studied using coincidence 3D momentum imaging. Since the hydrogen molecular-ion (and its isotopes) is the simplest molecule, it provides an excellent testing ground for the understanding of molecular behavior in intense few-cycle laser pulses. The experimental findings are contrasted with both existing experimental studies of these molecular ions in longer laser pulses and theoretical calculations, which include all possible physical processes except ionization. [Preview Abstract] |
Thursday, June 7, 2007 2:42PM - 2:54PM |
J6.00007: Phase-Sensitive Coulomb Explosion of CO Prepared by a 2-Color Pump Field Daniel Pinkham, Robert R. Jones We use an intense 30 fsec, 800 nm probe laser pulse to explore the Coulomb explosion dynamics of CO molecules coherently prepared by a 2-color (400 nm+800nm), phase-locked, pump laser pulse. When an intense, ultrashort laser pulse interacts with a molecule, it can create a rotational wavepacket through a series of Raman transitions. As the wavepacket evolves the molecule undergoes periodic field-free alignment along the laser polarization axis. In principle, by using an asymmetric laser field, produced by combining an 800 nm pulse with its second harmonic, a rotational wavepacket which exhibits periodic field-free orientation can be created. We use a single-stage time-of-flight spectrometer to monitor the forward/backward asymmetry in the ion fragments ejected along the probe laser polarization as a function of the pump-probe delay and the relative phase between the 2-colors in the probe. At the pulse overlap, we observe several delay-dependent changes in the TOF spectrum as well as a strong phase-sensitive asymmetry in the directionality of the fragment distribution. [Preview Abstract] |
Thursday, June 7, 2007 2:54PM - 3:06PM |
J6.00008: Vibrational Quantum Beats and High Harmonic Generation in SF$_{6}$ Zachary B. Walters, Stefano Tonzani, Chris H. Greene Although HHG is commonly understood as an electronic process, vibrational degrees of freedom in molecules allow for phenomena which have no analogue in atomic systems. This was recently demonstrated in experiments performed with SF$_{6}$ (Wagner et al, PNAS {\bf 103} 13279, 2006). If a HHG laser pulse is preceded by a weaker pulse which stimulates Raman-active vibrations, the harmonic intensity oscillates with the interpulse delay time at the frequencies of the stimulated modes. We explain this modulation as quantum interference between adjacent vibrational states of the molecule, which are mixed during the high harmonic process. We present an improved version of the three-step model, which uses nonperturbative electron-ion scattering wavefunctions to find the recombination dipole, and which tracks the vibrational wavefunction of the molecule throughout the high harmonic process. [Preview Abstract] |
Thursday, June 7, 2007 3:06PM - 3:18PM |
J6.00009: Rotation of H$_{2}^+$ driven by 10fs laser pulse Fatima Anis, Brett Esry We have performed full-dimensional calculations for H$_{2}^+$ in ultrashort intense laser pulse including physical processes of dissociation, electronic excitation as well as nuclear vibration and rotation. The post-pulse time evolution of the bound wave function shows revivals due to impulsive alignment. Revival structure is more pronounced than the revivals observed experimentally for D$_2$[1], which makes H$_{2}^+$ more favorable to observe this process. Moreover, alignment depends strongly on the initial vibrational state, making it possible to control the alignment and use it in a pump-probe scheme to study dissociation and ionization. This work also shows that including rotation is important even for very short pulses. \newline [1] K. F. Lee, F. L\'{e}gar\'{e}, D. M. Villeneuve and P. B. Corkum, J. Phys. B, 39, 4081(2006) [Preview Abstract] |
Thursday, June 7, 2007 3:18PM - 3:30PM |
J6.00010: Rotational wave packets probed by high harmonic generation Brian K. McFarland, Markus Guehr, Joe P. Farrell, Philip H. Bucksbaum We prepare an aligned distribution of cold N$_{2}$ molecules by the interaction with an intense nonresonant fs laser pulse (pump pulse). The distribution is probed by high harmonic generation (HHG) using a time delayed probe pulse. The high harmonics show an enhancement if the molecules are aligned with the probe pulse polarization and a suppression if the molecules are aligned orthogonal to the polarization. We observe a first alignment 300 fs after the pump pulse. For longer time delays, we observe fractional and full revivals (the later at about 8 ps) of the rotational wave packet. We measure the relative phase of high harmonics from N$_{2}$ and Ar at the half revival for N$_{2}$, with a phase jump at the 23$^{rd}$ harmonic. The alignment contrast and phase relation among the high harmonics is discussed in the context of the two center model. [Preview Abstract] |
Thursday, June 7, 2007 3:30PM - 3:42PM |
J6.00011: High Harmonic Imaging of Conical Intersections Markus Guehr, Brian K. McFarland, Joe P. Farrell, Philip H. Bucksbaum Conical intersections (CI) are crucially involved in light harvesting, primary visual processes, DNA UV stabilization and atmospheric chemistry. A wave packet typically moves through the intersection on a femtosecond time scale, demonstrating the need for ultrafast tools that are sensitive to the electronic state change occurring in passing the CI. We propose a novel femtosecond pump-probe scheme based on high harmonic generation (HHG). A first pulse (pump) creates a molecular wave packet on excited electronic surfaces, and the time delayed, high intensity probe pulse produces HHG on the excited molecule as it moves through the CI region. We use the symmetry of the electronic wave functions [1] to detect the electronic state change in the CI via HHG. Furthermore, we use two center interference effects in the HHG [1] to determine the nuclear dynamics that is accompanied by the CI passage. To demonstrate our scheme, we perform simple model calculations on the triatomic molecule SO$_{2}$, which will be ideally suited for experiments because of its high UV excitation cross sections for pumping the wave packet to the CI region. [1] J. Itatani et al, Phys. Rev. Lett., \textbf{94}, 123902 (2005) [2] T. Kanai et al, Nature, \textbf{435}, 470 (2005) [Preview Abstract] |
Thursday, June 7, 2007 3:42PM - 3:54PM |
J6.00012: Investigating Excited Electronic States of I$_{2}^{+}$ and I$_{2}^{2+}$ Produced by Strong Field Ionization Using Vibrational Wave Packets Li Fang, George Gibson In pump-probe experiments with a femtosecond near-UV laser, a vibrational wave packet is produced in a potential well of an excited electronic state of I$_{2}^{+}$ through strong field ionization. The wave packet is further ionized by a probe pulse to an excited electronic state of I$_{2}^{2+}$ that dissociates into I$^{2+}$+I$^{0+}$. Projecting the vibrational wave packet onto the dissociation channel reveals characteristics of both potential curves via the periodic variations in kinetic energy distribution of the dissociating fragments. The experimental results are consistent with and interpreted by theoretical simulations. We find that the intermediate state in I$_{2}^{+}$ is $A^{2}\Pi_{u,3/2}$ and ionization to this state has an anomalous angular dependence. Moreover, we extract the I$^{2+}$+I$^{0+}$ potential energy curve and find evidence for a truly bound potential well, in contrast to the meta-stable ground state potential energy curves. [Preview Abstract] |
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