Bulletin of the American Physical Society
2006 37th Meeting of the Division of Atomic, Molecular and Optical Physics
Tuesday–Saturday, May 16–20, 2006; Knoxville, TN
Session C4: Short Pulse Processes I |
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Chair: Chii-Dong Lin, Kansas State University Room: Knoxville Convention Center 301E |
Wednesday, May 17, 2006 10:30AM - 10:42AM |
C4.00001: Measuring Attosecond Electron Motion in Molecules Andre D. Bandrauk, Gennady Yudin, Paul B. Corkum We study analytically the possibility of monitoring electron motion in a molecule using two ultrashort pulses [1].The first pump pulse prepares a coherent superposition of two electronic molecular states whereas the second (attosecond) probe photoionizes the molecule. We show that direct information about electron dynamics on an attosecond time scale can be obtained from the photoelectron spectra as a function of time delay between the two pulses. In particular asymmetries in photoelectron angukar distributions provide a simple signature of the electron motion within the initial time-dependent coherently coupled two electron molecular states.It is further shown that ``chirped'' pulses can measure attosecond time scale electron dynamics just as effectively as ``transform-limited'' attosecond pulses of the same bandwith [2].Chirped pulses with frequency dependent phases create interferences in the photionization spectrum which are absent from transform-limited pulses. These interferences contain important information about the chirp rate and the attosecond electron dynamics. [1] G Yudin,S Chelkowski,A D Bandrauk, P B Corkum,Phys Rev A 72,051401(2005) [2] G Yudin,A D Bandrauk,P B Corkum, Phys Rev Lett (2006) [Preview Abstract] |
Wednesday, May 17, 2006 10:42AM - 10:54AM |
C4.00002: Attosecond pulse formation via harmonic generation driven by filamentation-compressed few-cycle laser pulses Himadri Chakraborty, Mette Gaarde, Arnaud Couairon It has recently been demonstrated that the self-compression during laser-driven filamentation can be an efficient and alternative source of intense few-cycle laser pulses. We explore the use of such pulses for attosecond pulse formation via high harmonic generation. We present a theoretical study of harmonic generation in argon driven by single or few cycle pulses with peak intensities of a few times $10^{14}$~W/cm$^2$. These pulses are the result of filamentation-driven self-compression of an 800 nm, 30 fs laser pulse in neon. Our results indicate that it is possible to generate isolated XUV pulses with durations of a few hundred attoseconds in this way. We explore the dependence of the attosecond pulse generation on the time-dependent phase of the driving pulse. [Preview Abstract] |
Wednesday, May 17, 2006 10:54AM - 11:06AM |
C4.00003: Attosecond pulse train modulation of non-sequential ionization B. Zimmermann, K.J. Schafer We examine the sub-cycle dynamics of non-sequential double ionization (NSDI) in a model helium atom using the combination of a strong infrared (IR) laser and an attosecond pulse train (APT). In the regime where the APT is the dominant source of single ionization we find a significant modulation of the NSDI signal versus APT-IR delay. In addition to controlling the time of ionization we also examine the effect of changing the ionized electron's initial energy by varying the harmonics used to synthesize the APT. [Preview Abstract] |
Wednesday, May 17, 2006 11:06AM - 11:18AM |
C4.00004: A simple model of laser-induced molecular ionization and dissociation B.D. Esry, P.Q. Wang, A.M. Sayler, K.D. Carnes, I. Ben-Itzhak We will present a simple qualitative picture with which both dissociation and ionization of a molecule in an intense laser can be understood. Moreover, the model has predictive power. One can use it, for instance, to deduce the main features of the angular distribution of the nuclear fragments and how it depends on the laser frequency and intensity. A comparison of this prediction with experiment will be presented. [Preview Abstract] |
Wednesday, May 17, 2006 11:18AM - 11:30AM |
C4.00005: Experimental study of structure in enhanced ionization of H$_{2}^{+}$ in intense ultrashort laser pulses I. Ben-Itzhak, P.Q. Wang, A.M. Sayler, K.D. Carnes, B.D. Esry, A.S. Alnaser, B. Ulrich, X.M. Tong, I.V. Litvinyuk, C.M. Maharjan, P. Ranitovic, T. Osipov, R. Ali, S. Ghimire, Z. Chang, C.D. Lin, C.L. Cocke Charge resonance enhanced ionization (CREI) of H$_{2}^{+}$ in intense ultrashort laser pulses was measured using molecular dissociation imaging of an H$_{2}^{+}$ beam and with a pump-probe technique using COLTRIMS on an H$_{2}$ target. The dependence of the ionization probability on the internuclear distance is determined independently from both measurements using a simple model for the initial dissociation step. Both data sets lead to a similar R-dependence which indicates that ionization is enhanced at large internuclear distances as predicted by theory [1]. However, in contrast to the predictions for a double peak structure, only one broad peak is observed. [1] T. Zuo and A.D. Bandrauk, Phys. Rev. A \textbf{52}, R2511 (1995) [Preview Abstract] |
Wednesday, May 17, 2006 11:30AM - 11:42AM |
C4.00006: Carrier-envelope phase effects in molecular dissociation: practical considerations V. Roudnev, B. D. Esry We have performed three-dimensional calculations of the HD$^+$ dissociation in alinearly polarized laser field with the restriction that the nuclei are aligned with the field. Although dissociation from a single initial vibrational state can show substantial carrier-envelope phase (CEP) dependence, the CEP dependence of a mixed initial state is not as strong. A strong CEP effect can be recovered, though, by analyzing the kinetic energy of the fragments. In order to approach realistic experimental conditions, we perform focus volume averaging of the results and identify the kinetic energy ranges demonstrating the strongest CEP dependence. [Preview Abstract] |
Wednesday, May 17, 2006 11:42AM - 11:54AM |
C4.00007: Dynamical alignment of H$_{2}^+$ in an intense ultrashort laser pulse Fatima Anis, R. Cabrera-Trujillo, B. D. Esry We will present a study of ionization and dissociation of H$_{2}^+$ in an intense ultrashort laser pulse. Our results include all degrees of freedom $-$ classical for the nuclei and quantum mechanical for the electron. Similar previous calculations$^{1,2}$ have obtained the electronic wave function using a basis expansion. We, however, are solving the time-dependent Schr\"{o}dinger equation on a three dimensional grid in the lab frame. One of our goal is to study dynamical alignment of H$_{2}^+$ and its fragments. We consider a distribution of initial nuclear positions and momenta so that our results are suitable for direct comparison with experiment. \\ $^1$M. Ullmann, T. Kunert, F. Grossmann, R. Schmidt, Phys. Rev. A {\bf 67}, 013413 (2003). \\ $^2$E. Deumens, A. Diz, R. Longo, and Y. \"{O}hrn, Rev. Mod. Phys. {\bf 66}, 917 (1994) [Preview Abstract] |
Wednesday, May 17, 2006 11:54AM - 12:06PM |
C4.00008: Effect of Carrier Envelope Phase on Single Shot XUV Supercontinuum Measurements Mahendra Man Shakya, S. Gilbertson, C. Nakamura, C.Q. Li, E. Moon, Z. Duan, J. Tackett, S. Ghimire, Zenghu Chang High order harmonic generation in argon gas was made with polarization gated $\sim $6 fs laser pulses. Phase matching of the harmonic field was then investigated by changing the gas pressure in a gas cell which had two holes on its wall drilled by the laser beam itself. The highest number of photons of a single XUV pulse was on the order of 10$^{4}$. A glancing incident grating spectrometer with a MCP imager was integrated into the high harmonic generation setup to measure the XUV spectrum. By having the half divergence angle of the XUV beam (3.5 mrad) matched with the acceptance angle of the grating, the throughput of the spectrometer has been maximized. The XUV flux is sufficient for measuring the single shot XUV spectrum in the 35-60 eV photon energy range. The spectral profiles showed a supercontinuum for some shots and discrete harmonic peaks for the other shots. This shot to shot variation in the spectral profiles is attributed to a result of the changes of carrier envelope phase of the few-cycle laser pulses used for the polarization gating. [Preview Abstract] |
Wednesday, May 17, 2006 12:06PM - 12:18PM |
C4.00009: Three-active-electron mechanism of intense-field multiple ionization: recollisions, Coulomb focusing and excitation Phay Ho, Joseph Eberly We present the mechanism of intense-field multiple ionization obtained from 2-D classical three-electron ensemble calculations. The three electrons are dynamically active and are subjected to the nuclear attraction, e-e repulsion and laser interaction simultaneously throughout the whole duration of the laser pulse. The calculations are done at different laser intensities in the range of 10$^{14}$ W/cm$^{2}$ to 10$^{16}$ W/cm$^{2}$, but using the same laser wavelength of 780 nm. We found that the paths to multiple ionization require the combination of one or more recollisions, excitation and Coulomb focusing. These processes are dynamically intertwined to promote the removal of multiple electrons. [Preview Abstract] |
Wednesday, May 17, 2006 12:18PM - 12:30PM |
C4.00010: Momentum distribution of multiply charged ions ionized by intense lasers K. I. Dimitriou, S. Yoshida, J. Burgd\"orfer, H. Shimada, Y. Yamazaki The analysis of the momentum distribution of multiply charged ions ionized by intense laser field ($\sim$ 100PW/cm$^2$) is presented. Due to a spatial variation of the laser intensity, ions with different charge states are produced during a single laser shot. The measurements show a remarkably simple linear relation between the width of the momentum distributions and the ionization potential of the ions. This scaling appears to be universal, i.e. independent of the target atoms used. We show that the linear scaling can be explained by the semiclassical theory and discuss the effect of laser envelope in the ionization process. [Preview Abstract] |
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