Bulletin of the American Physical Society
2005 36th Meeting of the Division of Atomic, Molecular and Optical Physics
Tuesday–Saturday, May 17–21, 2005; Lincoln, Nebraska
Session G5: Strong Field Molecular Processes |
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Chair: C. L. Cocke, Kansas State University Room: Burnham Yates Conference Center Arbor |
Thursday, May 19, 2005 1:30PM - 1:42PM |
G5.00001: Interaction of Coherent VUV Radiation with Xenon Clusters Zachary Walters, Chris H. Greene, Robin Santra The interaction of atomic clusters with short, intense pulses of laser light to form extremely hot, dense plasmas has attracted extensive experimental and theoretical interest. The high density of atoms within the cluster greatly enhances the atom-laser interaction, while the finite size of the cluster prevents energy from escaping the interaction region. A model of the laser-cluster interaction is presented which uses non-perturbative R-matrix techniques to calculate inverse bremsstrahlung and photoionization cross sections for Herman-Skillman atomic potentials. We describe the evolution of the cluster under the influence of the processes of inverse bremsstrahlung heating, photoionization, collisional ionization and recombination, and expansion of the cluster. Results are compared with the Hamburg experiment of Wabnitz et al [Nature {\bf 420}, 482 (2002)]. This work was supported by the Department of Energy, Office of Science. [Preview Abstract] |
Thursday, May 19, 2005 1:42PM - 1:54PM |
G5.00002: A semi-quantitative model for molecular dissociation in an intense laser pulse B.D. Esry, I. Ben-Itzhak, P.Q. Wang, A.M. Sayler, K.D. Carnes We have developed a simple model that describes the intensity and angular dependence of molecular dissociation and predicts the kinetic energy of the fragments. Moreover, the model yields relatively simple analytic expressions for each of these dependencies that makes possible semi-quantitative estimates for the mechanisms of bond-softening and bond-hardening. The predictions of the model are in general agreement with our measurements for net one, two, and three photon dissociation of H$_2^+$ in a 45~fs pulse. [Preview Abstract] |
Thursday, May 19, 2005 1:54PM - 2:06PM |
G5.00003: Controlling H$_2^+$ dissociation pathways in an intense laser pulse with the carrier-envelope phase Vladimir Roudnev, B.D. Esry The dissociation of H$_{2}^{+}$ and HD$^{+}$ in an intense ($>$5$\times $10$^{14}$W/cm$^{2}$) laser field has been found to be sensitive to the carrier-envelope phase difference (CEPD) for short ($\sim$10~fs) pulses when initially in their ground vibrational state. In this case, the ratio of $p$+H to H+$p$ can vary by as much as a factor of three. Experimentally, though, these molecules are typically in a distribution of vibrational states. We will show that the CEPD dependence of dissociation varies with the initial vibrational state, thus reducing the contrast between the two channels after averaging over this distribution. The velocity distribution of the fragments, however, could help to reveal CEPD effects in molecular dissociation. [Preview Abstract] |
Thursday, May 19, 2005 2:06PM - 2:18PM |
G5.00004: Parallel and perpendicular transitions in the dissociation of O$_{2}^{+}$ caused by an ultrashort intense laser pulse A.M. Sayler, P.Q. Wang, K.D. Carnes, B.D. Esry, I. Ben-Itzhak Laser-induced dissociation of O$_{2}^{+}$ has been experimentally studied using ultrashort ($\sim $50 fs) intense ($\sim $10$^{14}$ W/cm$^ {2})$ laser pulses at 790 nm. The O$^{+}$ and O fragments are measured in coincidence with a 3-dimensional momentum imaging system, which provides both angular and kinetic energy release (KER) distributions for dissociation. By analyzing the angular distribution of a specific range in the KER spectrum, the dissociation pathway may be deduced. In particular, a single photon parallel transition ($\Delta \Lambda $=0) is expected to have a cos$^{2}$\textit{$\theta $} distribution, while a perpendicular one ($\Delta \Lambda $=1) leads to a sin$^{2}$\textit{$\theta $} distribution. Therefore, a dissociation pathway requiring the exchange of $n \quad \Delta \Lambda $=0 photons and $m \quad \Delta \Lambda $=1 photons is expected to have a cos$^{2n}$\textit {$\theta $}sin$^{2m}$\textit{$\theta $} distribution. Thus, knowing $n$ and $m$ along with the KER range allows a plausible identification of the dissociation pathway. [Preview Abstract] |
Thursday, May 19, 2005 2:18PM - 2:30PM |
G5.00005: Probing Vibrational and Dissociative Wavepacket Dynamics in H$_2$ and D$_2$ Molecules with Ultra Short Laser Pulses Ali Alnaser, B. Ulrich, X.-M. Tong, C.M. Maharjan, P. Ranitovic, T. Osipov, R. Ali, B. Shan, Z. Chang, C.D. Lin, I.V. Litvinyuk, C.L. Cocke We have used ultra short laser pulses of ~ 8fs duration in pump- probe arrangement to probe fast wavepacket dynamics in H$_2$ (D$_2$) molecules. A vibrational wave packet is launched onto the $1s\sigma_g$ potential curve of the singly charged molecule by the (tunneling) removal of one electron from the neutral molecule. By choosing the proper laser intensities for the pump and the probe pulses, we were able to monitor the simultaneous propagation of this wave packet on the (coupled) $1s\sigma_g$ (bound) and $2p\sigma_u$(dissociative) potential curves of H$_2^+$ and D$_2^+$ over a time delay of 0-100 fs between the two pulses. We used the COLTRIMS technique to measure the kinetic energy release (KER) of the proton (deuteron) pairs produced in the Coulomb explosion of these molecules as function of time. A quantitative model has been constructed which accounts for the major features observed in the experiment. [Preview Abstract] |
Thursday, May 19, 2005 2:30PM - 2:42PM |
G5.00006: Dissociation of H$_{2}^{+}$ in intense femtosecond laser fields probed at different pulse durations Pengqian Wang, A.M. Sayler, K.D. Carnes, J.F. Xia, M.A. Smith, B.D. Esry, I. Ben-Itzhak Laser-induced dissociation of H$_{2}^{+}$ has been experimentally studied using long (135 fs) and short (45 fs) laser pulses at 790 nm in the intensity range of 10$^{13}$-10$^{15}$ W/cm$^{2}$. The H$^{+}$ and H fragments from a vibrationally excited H$_{2}^{+}$ beam are measured in coincidence by a 3-dimensional momentum imaging system. The results are dramatically different for the long and short pulses, in contrast to the reported trend for longer pulses. At similar peak intensities, bond-softening is found to be the main feature in long pulses, while in short pulses it is a minor process having a low kinetic energy release and a very narrow angular distribution. Above threshold dissociation is dominant in short pulses whose durations are approaching the vibrational period of the molecule. The comparisons are made at specific intensity ranges using an intensity-difference spectrum method. [Preview Abstract] |
Thursday, May 19, 2005 2:42PM - 2:54PM |
G5.00007: Spectrally Resolved Transient Alignment of Molecular Iodine Doug Broege, Emily Peterson, Philip Bucksbaum The visible absorption spectrum of molecular iodine vapor develops transient anisotropic features following excitation by an intense ultrafast infrared laser pulse. We have studied the time evolution of this aniostotropic absorption for the first time. The transmission of polarized ultrafast continuum radiation through a thin (3mm) column of iodine vapor was measured following excitation of the vapor by an intense 800 nm sub-picosecond laser pulse. The results show the interplay between several multiphoton processes such as impulsive rotational and vibrational excitation, multiphoton absorption, dissociation, and ionization. [Preview Abstract] |
Thursday, May 19, 2005 2:54PM - 3:06PM |
G5.00008: Orientation-dependent strong field ionization of diatomic molecules Xi Chu, Shih-I Chu We present an all-electron 3D first principle study of strong field ionization of diatomic molecules in intense laser fields using time-dependent density functional theory [1]. Dependence of the ionization probability on the angle between the molecular axis and the field polarization is analyzed for both the highest and the inner-valence electrons. Our results are consistent with the experimental data [2] of the total ionization rate as a function of this angle. We demonstrate that the molecular orientation-dependent inner-valence-electron ionization is appreciable at strong laser intensities, and it is determined by both the molecular orbital type and the dynamic electron- electron interaction. [1] X. Chu and S. I. Chu, Phys. Rev. A {\bf 70}, 061402(R) (2004). [2] Litvinyuk IV, Lee KF, Dooley PW, {\it et. al.}, Phys. Rev. Lett. {\bf 90} 233003 (2003). [Preview Abstract] |
Thursday, May 19, 2005 3:06PM - 3:18PM |
G5.00009: Observation of selective charge separation following strong-field single ionization Chunlei Guo We report on an apparently anomalous observation in the dissociation channels of singly ionized nitric oxide (NO) molecules. We observe that the NO molecules, when oriented perpendicular to the laser polarization, appears to preferentially dissociate into a channel that is energetically unfavorable. A careful examination of the unique electronic structure of NO molecule suggests that this observation is due to the influence of the detailed electronic structure of NO on its strong field ionization dynamics, which leads singly ionized NO created in strong fields to dissociate differently from that in weak fields. This study reinforces our understanding of the fundamental differences between ionization dynamics in strong fields versus weak fields. [Preview Abstract] |
Thursday, May 19, 2005 3:18PM - 3:30PM |
G5.00010: Measurement of ionization of an H$_{2}^{+}$ beam by ultra-short intense laser pulses I. Ben-Itzhak, P.Q. Wang, A.M. Sayler, K.D. Carnes, V. Roudnev, B.D. Esry The ionization of H$_{2}^{+}$ by 45 fs, 10$^{14}$-10$^{15}$ W/cm$^{2}$ laser pulses was measured using coincidence 3D momentum imaging. The measured kinetic energy release distribution is broad and its peak shifts toward higher values as the laser intensity is increased indicating that ionization shifts to smaller internuclear distances. The angular distributions are strongly peaked along the laser polarization, resulting in much narrower distribution than the dissociation into H$^{+}$ + H. Our results are compared with theoretical calculations in which the molecule is aligned along the laser polarization by taking only those molecules which dissociated in a narrow cone around the polarization. The calculated and measured ionization rates exhibit a fast increase with laser intensity at this intensity range. [Preview Abstract] |
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