Bulletin of the American Physical Society
40th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 54, Number 7
Tuesday–Saturday, May 19–23, 2009; Charlottesville, Virginia
Session S5: Short Pulse Processes |
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Chair: Cornelis Uiterwaal, University of Nebraska-Lincoln Room: Clark Hall 107 |
Friday, May 22, 2009 2:00PM - 2:12PM |
S5.00001: Photoionization energy and angle differential probabilities of He under chirped attosecond x-ray pulses Teck-Ghee Lee, Mitch Pindzola, Francis Robicheaux The time-dependent close-coupling (TDCC) method is used to investigate the energy and angular differential probabilities for various ionization processes of He atoms under chirped attosecond soft x-ray pulses with a photon energy of 91.6 eV and a peak intensity of 10$^{15}$ W/cm$^2$. It is shown that the ejected electrons probability density for two-photon double ionization is very sensitive to the chirp. We find a weakly chirped pulse modifies the electron energy distribution from an elliptical to an oval shape, whereas a slightly stronger chirp causes the oval distribution not only to grow, but broaden and stretch along the ejected-electron equal energy sharing direction. As a result, the total two-photon double ionization probabilities are enhanced relative to the zero chirp case. We also analyze their corresponding energy and angular differential probabilities in order to better understand the chirp effects on correlated electron emission. [Preview Abstract] |
Friday, May 22, 2009 2:12PM - 2:24PM |
S5.00002: Two-color attosecond control of helium Jes\'us V. Hern\'andez, B.D. Esry We present the results of \textit{ab initio} 6-D calculations of atomic He in intense laser fields below the $N=2$ threshold. In particular, we investigate the possibility of coherent control over the atom by using two-color photoexcitation schemes. By manipulating the relative phase between the excitation pulses via attosecond delays, the probability of excitation and single ionization can be enhanced or suppressed. We study recent experiments which use attosecond pulse trains synchronized to infrared fields to ionize He. By casting the attosecond pulse train and the infrared laser into a multi-color picture, we also develop an analytic framework to interpret the numerical results. [Preview Abstract] |
Friday, May 22, 2009 2:24PM - 2:36PM |
S5.00003: Two-Photon Resonant-Single and Double Ionization of Helium by Ultrashort Laser Pulses Alicia Palacios, Thomas N. Rescigno, C. William McCurdy Two-photon single and double ionization induced by ultrashort laser pulses in He are explored in accurate time-dependent calculations including a full treatment of electron correlation. Ionization amplitudes and cross sections are extracted from the wave packet using exterior complex scaling. For photon energies above the first ionization threshold, two-photon single ionization is enhanced by core excited resonances, in processes visible with pulses as short as two femtoseconds when the photon frequency is equal to a transition energy in He$^+$. Subfemtosecond pulses are seen to suppress the two-peak signature of sequential ionization in the energy sharing cross section, mostly due to Fourier broadening. However, triple differential cross sections from subfemtosecond pulses show the evidence of nonsequential ionization competing more strongly with the sequential process. Peaks in the single differential cross section due to sequential ionization via excited intermediate states of the ion are observed to occur at energies displaced by about two eV from the expected values by novel interference effects between continuum channels. [Preview Abstract] |
Friday, May 22, 2009 2:36PM - 2:48PM |
S5.00004: Strong field Cooper minimum in High Harmonic Generation Joseph Farrell, Brian McFarland, Phil Bucksbaum, Markus G\"uhr The Cooper minimum is a prominent dip in the VUV photoionization spectrum in Ar caused by a cancellation of the radial integral in the transition matrix element between the atomic ground state and the continuum. The minimum is accompanied by a $\pi$ phase jump in the matrix element [1]. The recombination step of High Harmonic Generation (HHG) can be described by the inverse photoionization matrix element. We observe a similar minimum in the HHG spectrum of argon, which we associate with the Cooper minimum. We have measured the spectral phase of the harmonics and observe the expected spectral phase jump of $\pi$ around this spectral minimum. This HHG Cooper minimum is shifted from the photoionization Cooper minimum by about 5 eV. Calculations show that this shift is due to strong-field mixing of the continuum states during the recombination step in HHG. The results show the opportunity for controlled attosecond pulse shaping. [1] J. W. Cooper, Physical Review 128, 681 (1962) [Preview Abstract] |
Friday, May 22, 2009 2:48PM - 3:00PM |
S5.00005: Quantitative rescattering theory for high-order harmonic generation from aligned molecules Anh-Thu Le, Robert Lucchese, Mu-Tao Lee, Chii-Dong Lin By employing the recently proposed Quantitative Rescattering Theory (QRS) combined with accurate photoionization transition dipoles for aligned molecules, we show that most of the existing experimental results for high-order harmonic generation (HHG) from CO$_{2, }$N$_{2}$, and O$_{2}$ can be nicely reproduced. In our calculations, the returning electron wavepackets are obtained from the strong-field approximation or from solution of the time-dependent Schrodinger equation for a reference atom, whereas the transition dipoles are obtained from state-of-the-art molecular photoionization calculations. Our results show that quantitative description of the HHG from aligned molecules has become possible. [Preview Abstract] |
Friday, May 22, 2009 3:00PM - 3:12PM |
S5.00006: Polarization Resolved Characterization of High Harmonic Emission from Aligned Molecules Xibin Zhou, Robynne Lock, Henry Kapteyn, Margaret Murnane High-order harmonic emission from molecules that have been impulsively aligned can be used to obtain information on molecular structure and ultrafast dynamics. Complementary to the intensity measurement, the polarization properties of this emission can offer considerably more insight and can provide for critical tests of the theory of molecular high harmonic generation (HHG). In this work, we present the results of two experiments. First, we perform accurate polarimetry measurements of the HHG emission of aligned N$_{2}$ and CO$_{2}$ [1]. We find that in N$_{2}$, the HHG emission can be strongly elliptically polarized even when the laser used to drive the process is linearly polarized. In contrast, the harmonic emission from aligned CO$_{2}$ molecules shows a polarization rotation but no significant ellipticity. In the second experiment, we vary the ellipticity of the driving laser used to generate the high harmonic emission. We find that the HHG intensity from aligned molecules does not peak for linear polarization of the driving laser (as in the atomic case), but at a small positive or negative ellipticity. The sign and degree of this ellipticity depend on the molecular orientation. [1] X. Zhou \textit{et. al}., PRL \textit{accepted} (2009). [Preview Abstract] |
Friday, May 22, 2009 3:12PM - 3:24PM |
S5.00007: Effects of streaking laser intensity on the characterization of isolated attosecond pulses He Wang, Sabih Khan, Michael Chini, Shouyuan Chen, Zenghu Chang Single isolated attosecond extreme ultraviolet (XUV) pulses can be characterized by streaking photoelectrons using a near infrared (NIR) laser field. Classically, the streaking resolution is determined by the Rayleigh criterion, which requires the minimum NIR intensity of 5.5$\times $10$^{13}$ W/cm$^{2}$ to resolve 90 as XUV pulses. Under such high NIR intensity, the electrons generated from multi-photon processes overlap with the streaked electrons in the spectrogram, which unavoidably introduces errors in the final XUV reconstruction. When the FROG-CRAB (Frequency-Resolved Optical Gating for Complete Reconstruction of Attosecond Bursts) technique is used to reconstruct the XUV pulses from the spectrogram, it was found that the minimum streaking intensity needed to resolve single attosecond pulses is dependent on the maximum count of the spectrogram. With a peak count of 100 in the spectrogram, chirped attosecond pulses with spectral bandwidth supporting 90-as transform limited pulse durations can be retrieved from the spectrogram with streaking intensity two orders of magnitude smaller than that derived from the Rayleigh criterion. Such low streaking field intensity is desirable to suppress the ATI background, which is important for the characterization of even shorter XUV attosecond pulses because it significantly reduces the intensity constraints on the experiments. [Preview Abstract] |
Friday, May 22, 2009 3:24PM - 3:36PM |
S5.00008: Control of electron localization in a molecule using XUV and IR pulses Kamal P. Singh, P. Ranitovic, W. Cao, S. De, D. Ray, S. Chen, I. Bocharova, M. Magrakvelidze, H. Mashiko, F. He, U. Thumm, A. Becker, I. Litvinyuk, C.L. Cocke We demonstrate experimental control of electron localization in the deuterium molecular ion created and dissociated by the combined action of an attosecond pulse train (APT) and a femtosecond IR pulse. The APT is synthesized by a two-color method and consists of one attosecond pulse per optical cycle of the fundamental driving field. A left-right asymmetric ejection of deuterium ions is observed using COLTRIMS. This asymmetry undergoes oscillations with a full optical cycle period when the time delay between the APT and IR pulses is scanned in 0.3 fs steps. An analysis of the kinetic energies of the released ionic fragments suggests that the underlying mechanism of the localization control takes place primarily as the dissociating molecule passes through the bond-softening region of internuclear distance. Our results agree well with a theory based on the numerical solution of the corresponding Schrodinger's equation. [Preview Abstract] |
Friday, May 22, 2009 3:36PM - 3:48PM |
S5.00009: The Role of Mass in the Carrier-Envelope Phase Effect for $\rm H_2^+$ Dissociation Jianjun Hua, Brett Esry The carrier-envelope phase (CEP) dependence of the dissociation of H$_2^+$ is studied with special emphasis on the role of the nuclear mass. We find that the total dissociation probability displays a CEP effect that grows with increasing mass while the difference between dissociating to $p$+H and H+$p$ displays an effect that shrinks. Insight into the physical processes involved is given by an analytic description that casts CEP effects as interferences between pathways requiring different numbers of photons. [Preview Abstract] |
Friday, May 22, 2009 3:48PM - 4:00PM |
S5.00010: Strongly Polarized Plasma Emission Produced by Laser Ablation of Aluminum Robert Gordon, Yaoming Liu, John Penczak, Youbo Zhao We have found that continuum emission produced in the laser ablation of a material may be strongly polarized, whereas as discrete atomic or ionic emission lines appear as minima in the plasma polarization spectrum [1]. This effect is indicative of strong directionality of electrons recombining in the plasma. By placing a polarizer before the detector, it is possible to suppress the continuum background, thereby greatly increasing the resolution and detection sensitivity. Previously this technique used double pulses of femtosecond Ti:Sapphire radiation to achieve maximum polarization [2]. Here we show that single pulses on both the fs and ns time scales produce strongly polarized spectra of Al. The effects of laser intensity, focal position, angle of incidence, and polarization state of the laser are explored. \\[4pt] [1] Y. Liu, S. Singha, T. E. Witt, and R. J. Gordon, Appl. Phys. Lett. 93, 161502 (2008).\\[0pt] [2] Y. Zhao, S. Singha, Y. Liu, and R. J. Gordon, Opt. Lett. 34 (in press). [Preview Abstract] |
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