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 L1: Interaction and Strong Lasers with Molecules and Clusters |
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Chair: Carlos Reinhold, Oak Ridge National Laboratory Room: Knoxville Convention Center Lecture Hall |
Thursday, May 18, 2006 10:30AM - 11:06AM |
L1.00001: Inverse Bremsstrahlung and High Harmonic Generation in Clusters and Molecules Invited Speaker: Clusters exposed to radiation from an intense VUV free-electron laser have been observed to absorb copious amounts of energy, which has been somewhat of a surprise. Different models by competing theory groups have proposed more than one mechanism for the energy absorption. We have studied the effect of incorporating a realistic atomic screening potential on the free-free or inverse bremsstrahlung absorption rate, and found important differences that emerge, compared to simpler hydrogenic models that are frequently adopted. In a separate project to be discussed at the meeting, some of the physical issues involved in high-harmonic generation from diatomic and polyatomic molecules will be assessed. These include the importance of treating the electron scattering from the molecular ion at a nonperturbative level of approximation, as well as the possible role of Jahn-Teller physics when the molecule possesses a high degree of symmetry. This theoretical project involves collaborative contributions from Zachary Walters, Stefano Tonzani, and Robin Santra. [Preview Abstract] |
Thursday, May 18, 2006 11:06AM - 11:42AM |
L1.00002: Interaction of clusters with intense soft x-ray radiation from the VUV-FEL Invited Speaker: The interaction of rare gas clusters with intense vacuum ultraviolett radiation from the DESY TTF1 FEL operating at 100 nm wavelength has yielded many surprising results. For rare gas clusters unexpected high energy absorption was measured and thermionic electron emission was observed. Already at 10$^{13}$ W/cm$^2$ the clusters completely disintegrated in a coulomb explosion. These results indicate that for cluster in intense laser fields down to 100 nm very efficient energy absorption mechanisms exist. From a theoretical point of view new explanations for the observed energy absorption were suggested, including atomic corrections to the inverse Bremsstrahlung potentials or high intermediate charge states in the cluster. In fall 2005 the DESY VUV-FEL became operational, currently producing intense soft x-ray radiation with 32 nm wavelength and power densities of up to 10$^{14}$ W/cm$^2$. In the talk first results of the laser cluster interaction at this reduced wavelength will be presented. The data show less efficient energy absorption from the laser field and no thermionic electrons. The results will be discussed and compared to the 100 nm experiments. [Preview Abstract] |
Thursday, May 18, 2006 11:42AM - 12:18PM |
L1.00003: Applications of Attosecond Lasers to Atoms and Molecules in Strong Laser Fields Invited Speaker: In the past two decades femtosecond time-resolved experiments have allowed the observation of molecular rotations and vibrations, and of photo-induced chemical processes. However, these experiments often tell only half the story: they show the motion of atoms moving under the influence of potential energy curves that result from a time-average over the motion of all electrons in the system. The natural time-unit for this electronic motion itself is the atomic unit of time (1 a.u. = 0.024 fsec = 24 attoseconds). \textit{Real-time }observation of this motion therefore requires recently developed attosecond laser techniques. When considering motions of electrons we may distinguish between motion that results from driving the electrons with a strong laser field and motion that results from photo-absorption in a weak laser field. In strong laser fields the electron motion can be quite intuitive. Eventually, studies of photo-absorption in weak laser fields are important, since all photo-absorption processes in nature (i.e. outside a laser laboratory) occur in this regime. At the meeting I will discuss experiments aimed at observing the motion of electrons on attosecond timescales in strong laser fields. An interesting example is the dissociative ionization of the hydrogen molecule (into a proton and a neutral atom), where we have recently observed that the dissociation process can be controlled by the carrier envelope phase of a few-cycle laser pulse. [Preview Abstract] |
Thursday, May 18, 2006 12:18PM - 12:54PM |
L1.00004: Rare Gas Clusters under an Intense Laser Pulse and Analytical Ion Kinetic Energy Spectra Invited Speaker: We will give an overview over the different mechanisms which emerge for the coupling of short light pulses into rare gas clusters when the laser frequency is increased from roughly 1 eV ($\sim$800 nm) over 12 eV ($\sim$100 nm) to 350 eV ($\sim$3 nm). In the second part of the talk I will present a simple analytic approach to quantitatively compare ion spectra of Coulomb exploding clusters under experimental conditions (spatial laser intensity distribution, cluster size distribution) to ideal single cluster results from theory. We will explain, how the kinetic energy distribution of ions emerging from a cluster irradiated by an intense laser pulse differs qualitatively from the distribution of a single cluster illuminated by a spatially homogeneous laser beam. This is due to three main effects: (i) the spatial profile of the laser beam, (ii) the cluster size distribution in the experiment, and (iii) possible saturation effects in the cluster ionization. Each of these effects leaves a characteristic fingerprint in the ion kinetic energy spectrum and is easily identifiable in the spectrum. All three effects together fully account for the shape of a typical spectrum. The simple model provides a quantitative link between observable ion spectra under experimental conditions and the ideal single cluster result of a typical calculation. [Preview Abstract] |
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