2014-2015 AMOP Seminar Abstracts

Imaging of Alignment, Deformation and Dissociation of CS2 Molecules using Ultrafast Electron Diffraction

Jie Yang

The ability to follow molecular dynamics in time and space remains a challenge due to the extreme requirements for spatial and temporal resolution. Ultrafast electron diffraction from aligned molecules (UEDAM) provides atomic resolution and allows for the retrieval of structural information without the need to rely on theoretical models. Here we use UEDAM to investigate the dynamics in carbon disulfide (CS2) following the interaction with an intense femtosecond laser pulse. We demonstrate, for the first time, imaging of transient molecular states that resulted from the interaction with an intense laser pulse. Due to the high spatial and temporal resolution in finding the atomic positions (0.03Å and 1.0 ps, respectively) we were able to observe vibrations and deformation of the molecule along with dissociation, in addition to capturing the angular distribution of the molecules (alignment). We were thus able to build a fairly complete picture of the molecular dynamics over a large range of laser intensities. We have found an upper limit to the degree of alignment that is caused by bending vibrations at low laser intensities and by deformation and dissociation of the molecule at higher laser intensities.

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Similarity between Positronium-Atom and Electron-Atom Scattering

I.I. Fabrikant

Collisions of projectiles, containing loosely-bound electrons, with neutral targets are often controlled by the electron-target interaction. At low electron energies the main contribution to this interaction is given by a long-range force and a short-range repulsion due to the Pauli exclusion principle. For free-electron scattering the long-range force is due to the polarization interaction, and for electron bound in a neutral projectile it is due to the van der Waals interaction. We analyze the role of these forces in collisions of positronium (Ps) with rare-gas atoms. At the energy range above the Ps ionization threshold the exchange electron-target interaction dominates, and the Ps-A cross section, when plotted as a function of Ps velocity, is very similar to the e-A cross section. At lower energies this similarity disappears because of the difference between the polarization and van der Waals interactions. In particular, there is no Ramsauer-Townsend minimum in Ps-Ar and Ps-Kr scattering.

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Difference parametic amplification in QASER - a new light source on the horizon?

Wayne Cheng-Wei Huang, Texas A&M University

In this presentation I would like to discuss what a QASER is and the mechanism that it is based on [1]. Contrasting with an optical parametric amplifier, a QASER is a device that amplifies seeded light using a low-frequency pump. Specifically, the pump frequency should be equal to the difference of the seeded light frequencies. In other words, the QASER should act as a black box where we put low frequency in and get high frequency out. Using femtosecond infrared laser as pump, a QASER could in principle be used as a coherent X-ray light source. This idea is pioneered by Marlan Scullyʼs group.

[1] A. A. Svidzinsky, L. Yuan, and M. O. Scully, Phys. Rev. X 3, 041001 (2013).

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Newtonian Semiclassical Gravity,The Quantum Measurement Problem, and The GRW Theory

Maaneli Derakhshani, University of Nebraska–Lincoln

Semiclassical gravity is a well-studied halfway house between full quantum gravity and classical GR. In recent years, a Newtonian description of semiclassical gravity, known as the Schroedinger-Newton (SN) theory, has suggested the possibility of (nearly) experimentally testable predictions. However, the SN theory also has serious problems of empirical adequacy and internal consistency that potentially undermine its value. The origins of the SN theory, including its problems, and the root of its problems in the quantum measurement problem, will be discussed. A modification of the SN theory will then be introduced, based on an alternative quantum theory known as the Ghirardi-Rimini-Weber (GRW) theory, and the implications of "GRW-Newton" will be worked out. It will be shown that GRW-Newton is both empirically viable and internally consistent, and suggests the possibility of testable new predictions, over and above the predictions of the SN theory.

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Nonlinear Attosecond Dichroism in Double Ionization of He by an Intense elliptically polarized Few-cycle XUV Pulse

J.M Ngoko Djiokap, University of Nebraska–Lincoln

In this presentation, I would like to focus on our latest results: control of He double ionization by means of the polarization and the CEP of an intense few-cycle attosecond XUV pulse. We demonstrate this numerically by solving the six-dimensional two-electron, TDSE for He interacting with an elliptically-polarized attosecond light pulse. Using PT, we predict a new type of CEP-sensitive polarization asymmetry that is normally absent in single photon double ionization of He, but does occur for an elliptically-polarized, few-cycle attosecond XUV pulse. We call this new effect nonlinear dichroism, which is sensitive not only to the ellipticity, peak intensity I, and temporal duration of the pulse, but also to the energy-sharing. This dichroic effect (i.e., the difference of the two-electron angular distributions for opposite helicities of the ionizing XUV pulse) is / I3=2, indicating from a perturbation theory analysis that it originates from interference of first- and second-order amplitudes owing to the broad pulse bandwidth, allowing one to investigate and control S- and D-wave channels of the two-electron continuum. Nonlinear dichroism probes electron correlation on its natural timescale since it vanishes for long pulses.

This work is supported in part by DOE, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Grant No. DE-FG03-96ER14646.

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Action Principles and Numerical Simulation, or Hamilton Meets a Computer

Brad Shadwick, University of Nebraska–Lincoln

Hamilton's Principle (often referred to as the "Principle of Least Action") is central to modern physics; all microscopic physical theories can be formulated in terms of action principles. Systems whose dynamics are governed by action principles generally have intrinsic properties that are of qualitative importance. Such properties can often be expressed as conservation laws, for example energy and momentum conservation or phase-space volume conservation in Hamiltonian systems. Often the only recourse for understanding the behavior of complex systems is numerical simulation. Are these intrinsic properties present in numerical solutions of a system's dynamical equations? As we will see in this talk, the answer is "it depends."

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Generation and Measurement of Femtosecond Electron Pulses

Omid Zandi, University of Nebraska–Lincoln

To investigate the ultrafast dynamics of molecules in gas phase, steps are taken to conduct Time-Resolved Electron Diffraction Experiments with sub-relativistic electrons. Highly charged electron pulses are generated and accelerated to be scattered by the laser-excited molecules in the gas phase. To compensate for the space charge effect that increases the pulse dimensions, magnetic lenses as well as a microwave cavity are employed to compress it both transversely to sub-millimeter radii and longitudinally to femtosecond durations. Then a streak camera has been fabricated that uses the time-varying electric field of a discharging parallel plate capacitor to measure the pulse duration in situ.

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Probing Molecular Structure with a Molecule’s Own Electrons

Carlos Trallero, Kansas State University

We use high-order harmonic generation to probe molecular structure through the photoionization dipole. In particular, we use two-source interferometry, which is equivalent to the famous double-slit interference experiment. We also make use of elliptically-polarized light to generate harmonics, which yields very interesting results. A comparison between the processes of strong field ionization and high-order harmonic generation is also presented. Finally I will discuss our advances in developing ultrafast optics sources.

This seminar will take place at 2:30 p.m.

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A Cylindrically Symmetric “Micro-Mott” Electron Polarimeter

Nathan Clayburn, University of Nebraska–Lincoln

A small, novel, cylindrically-symmetric Mott electron polarimeter has been recently commissioned in our laboratory. The effective Sherman function, or analyzing power, for 20 kV target bias with a 1.3 keV energy loss window is 0.156(011). The device's maximum efficiency at 20 keV, defined as the detected count rate divided by the incident particle current, is 2.13(51) x 10-4. The figure-of-merit of the device, which is inversely proportional to the square of the time required to measure polarization to a given statistical accuracy, is 5.17(73) x 10-6. Potential sources of false asymmetries due to positive ion contamination and beam misalignment have been investigated. The new polarimeter is compared to previously published data for similar compact retarding-field Mott polarimeters. It has state-of-the-art analyzing power but poor efficiency.

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Frequency comb generation from nonlinear Compton and Thomson scattering

Katarzyna Krajewska, University of Warsaw and University of Nebraska–Lincoln

I will discuss similarities and discrepancies between the classical nonlinear Thomson scattering and its quantum counterpart – nonlinear Compton scattering. I will demonstrate how the quantum recoil of electrons during the emission of photons changes the energy and temporal power distributions of generated radiation. The main part of my presentation will focus on frequency comb generation, which occurs when the electrons scatter off a laser pulse train. I will identify the mechanism responsible for the formation of frequency combs and describe their potential applications (such as a diagnostic tool for laser pulse characterization and ultra-short pulse generation).

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Projectile Coherence Effects & Few-Body Dynamics in Proton–H2 Collisions

Sachin Sharma, University of Missouri–Rolla

The most fundamental requirement to understand nature is to obtain a thorough understanding of the forces acting in nature. The mediation of the force is a two-body process because the gauge bosons can only be emitted or absorbed by one particle at a time. This leads to the question of how systems containing more than two particles develop under the influence of these pairwise acting forces, which is known as the few-body problem (FBP). The essence of the FBP is that the Schrödinger Equation in not analytically solvable even if the underlying forces are precisely known.

In order to advance our understanding of the FBP, we have performed various kinematically complete experiments on ionization p + H2 and He collision systems. Earlier, even for the most simple systems, puzzling discrepancies between theory and experiment were observed which were vividly debated for more than a decade. Only a few years ago, we reported experimental results which may offer an explanation for these discrepancies, suggesting that the projectile coherence properties may have an impact on the collision dynamics of a system. In this talk a series of kinematically complete experiments, performed since then, will be reported and analyzed. These results provide strong support for the important, and overlooked, role of the projectile coherence effects in the atomic fragmentation processes.

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