2013-2014 AMOP Seminar Abstracts

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Aug. 26

Compact Ultra-fast X-ray Diagnostics Design for the Recording of Multiple Simultaneous Experimental Parameters

Poopalasingam Sankar

Albert Einstein, the father of relativity, once said, “Look deep into nature, and then you will understand everything better”. Today, high resolution tabletop tools to look deep into matter and living things employ an x-ray source. We have developed two tabletop ultrabright-keV x-ray sources, one from Xeq+ hollow-atom states (which operates at 1.2-1.6 nm) and the other from Krq+ hollow-atom states (which operates at 600-800 pm) with unique characteristics that make them complementary to currently-available extreme-light sources such as x-ray free-electrons laser, and synchrotron x-ray sources, but at a modest laboratory scale.

We have upgraded an ultra-fast tabletop KrF* laser to pump target-clusters created by a custom-designed double gas sonic-nozzle. The interaction of the pump-laser with target rare-gas clusters produces hollow-atom states, which later coherently collapse to the empty inner-shell and thereby generate keV x-ray radiation. The KrF* pump-laser beam is self-focused and forms a channel to guide the x-ray radiation parallel to the pump-laser beam, which generates a directed x-ray beam.

A unique compact ultra-fast x-ray diagnostics setup was built for the purpose of recording a large number of single-shot calibrated experimental parameters in an extremely fast and reproducible fashion to understand the physics behind the x-ray sources. The carefully constructed diagnostic setup has 8 different calibrated devices built using the latest technologies. Careful design and engineering enabled the devices to be aligned and mounted in permanent locations around a compact target region in optimized positions without interfering with other devices in order to shorten the time needed to conduct experiments and to produce reproducible day to day experimental data sets. The resultant system requires only a 2 hour setup time, which includes amplifier and device warm-ups in order to begin the experiment. The system is capable of recording 16 single-shot calibrated experimental parameters every time pump-laser interacts with target cluster at the rate of 0.1 Hz. Thus far, two terabytes of data, including 200,000 single-shot events have been recorded in 3 years, time, offering a unique insight into the x-ray generation process.

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Sept. 11

Photodesorption and Photodissociation of H2O on Amorphous Ice and a Lunar Surface

Alice DeSimone

The first excited state of water is dissociative in the gas phase, but it does not necessarily lead to dissociation in the condensed phase because the excitation may spread out over many water molecules. Because the excited water molecule’s dipole is nearly reversed from its ground-state orientation, the rest of the surface exerts a repulsive force on the molecule, which can cause desorption. The competition between photodissociation and photodesorption of water has been studied using 157-nm photons on copper covered with thick amorphous solid water (ASW) at 100 K and on a real lunar slab from Apollo 16, using the secondary photoproduct O(3P) as an indicator of photodissociation. Both H2O and O(3P) were detected with resonance-enhanced multiphoton ionization. Translational temperatures were determined by fitting the data with Maxwell-Boltzmann distributions. The cross sections for water and O(3P) depletion near 100 K were calculated for H2O exposures from 0.1 to 30 L on copper and the lunar slab. Because the cross section for water removal is much higher on the lunar slab than on copper at low exposures, the lunar slab appears to enhance photodesorption and/or photodissociation.

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Dec. 2

Non-classical State Revealed in a Fully Classical System

Wayne Huang

In 1947, the measurement of Lamb and Retherford on the hyperfine spectrum of the hydrogen atom gave the first experimental evidence of the electromagnetic vacuum field. The interaction between matter and the vacuum field has since become an important topic in fundamental quantum electrodynamics.

In this presentation, first I would like to discuss the excitation spectrum of a classical harmonic oscillator immersed in the vacuum field. Both the numerical simulation and perturbation analysis indicate that such a classical system exhibits the same excitation spectrum as its quantum counterpart. Second, I would like to show some preliminary results of realizing the “non-classical states” in such a classical system. Namely, upon excitation by one or two laser pulses, the classical harmonic oscillator in the vacuum field demonstrates dynamical behaviors that are analogous to a coherent state, a squeezed state, and a Schrodinger cat state of a quantized light field. Finally, I would like to briefly discuss the seeming “energy quantum jumps” in the single particle excitation/decay process.

The intriguing connection between the classical harmonic system and the quantized light field may find application in generating non-classical light in nano/optomechanical systems.

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Jan. 22

Laser-Driven Proton/Ion Acceleration from Solid Targets

Alex Stamm

An overview of laser-driven proton/ion acceleration from solid targets will be discussed. Applications of this technology include ion beam cancer therapy, laser triggering and control of nuclear reactions, and injectors for synchrotron ion accelerators. Emphasis will be placed on the traditional target normal sheath acceleration (TNSA) mechanism, though alternative acceleration mechanisms will be discussed. Additionally, the important open questions in the field will be discussed in addition to possible ways forward.

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