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Mixing Times Of Organic Molecules Within Secondary Organic Aerosol Particles: A Global Planetary Boundary Layer Perspective, Adrian M. Maclean, Christopher L. Butenhoff, James W. Grayson, Kelley Barsanti, Jose L. Jimenez, Allan K. Bertram 2017 University of British Columbia

Mixing Times Of Organic Molecules Within Secondary Organic Aerosol Particles: A Global Planetary Boundary Layer Perspective, Adrian M. Maclean, Christopher L. Butenhoff, James W. Grayson, Kelley Barsanti, Jose L. Jimenez, Allan K. Bertram

Physics Faculty Publications and Presentations

When simulating the formation and life cycle of secondary organic aerosol (SOA) with chemical transport models, it is often assumed that organic molecules are well mixed within SOA particles on the timescale of 1 h. While this assumption has been debated vigorously in the literature, the issue remains unresolved in part due to a lack of information on the mixing times within SOA particles as a function of both temperature and relative humidity. Using laboratory data, meteorological fields, and a chemical transport model, we estimated how often mixing times are < 1 h within SOA in the planetary boundary layer (PBL), the region of the atmosphere where SOA concentrations are on average the highest. First, a parameterization for viscosity as a function of temperature and RH was developed for α-pinene SOA using room-temperature and low-temperature viscosity data for α-pinene SOA generated in the laboratory using mass concentrations of ∼ 1000 µg m−3. Based on this parameterization, the mixing times within α-pinene SOA are < 1 h for 98.5 % and 99.9 % of the occurrences in the PBL during January and July, respectively, when concentrations are significant (total organic aerosol concentrations are > 0.5 µg m−3 at the surface). Next, as ...


Doubly-Excited State Effects On Two-Photon Double Ionization Of Helium By Time-Delayed, Oppositely Circularly-Polarized Attosecond Pulses, Jean Marcel Ngoko Djiokap, Anthony F. Starace 2017 University of Nebraska-Lincoln

Doubly-Excited State Effects On Two-Photon Double Ionization Of Helium By Time-Delayed, Oppositely Circularly-Polarized Attosecond Pulses, Jean Marcel Ngoko Djiokap, Anthony F. Starace

Anthony F. Starace Publications

We study two-photon double ionization (TPDI) of helium by a pair of time-delayed (non-overlapping), oppositely circularly-polarized attosecond pulses whose carrier frequencies are resonant with 1Po doubly-excited states. All of our TPDI results are obtained by numerical solution of the two-electron time-dependent Schrödinger equation for the six-dimensional case of circularly-polarized attosecond pulses, and they are analyzed using perturbation theory (PT). As compared with the corresponding nonresonant TPDI process, we find that the doubly-excited states change the character of vortex patterns in the two-electron momentum distributions for the case of back-to-back detection of the two ionized electrons in the polarization ...


Imaging Electronic Motions By Ultrafast Electron Diraction, Hua-Chieh Shao, Anthony F. Starace 2017 University of Nebraska-Lincoln

Imaging Electronic Motions By Ultrafast Electron Diraction, Hua-Chieh Shao, Anthony F. Starace

Anthony F. Starace Publications

Recently ultrafast electron diraction and microscopy have reached unprecedented temporal resolution, and transient structures with atomic precision have been observed in various reactions. It is anticipated that these extraordinary advances will soon allow direct observation of electronic motions during chemical reactions. We therefore performed a series of theoretical investigations and simulations to investigate the imaging of electronic motions in atoms and molecules by ultrafast electron diraction. Three prototypical electronic motions were considered for hydrogen atoms. For the case of a breathing mode, the electron density expands and contracts periodically, and we show that the time-resolved scattering intensities re ect such ...


Energy-Resolved Coherent Diffraction From Laser-Driven Electronic Motion In Atoms, Hua-Chieh Shao, Anthony F. Starace 2017 University of Nebraska-Lincoln

Energy-Resolved Coherent Diffraction From Laser-Driven Electronic Motion In Atoms, Hua-Chieh Shao, Anthony F. Starace

Anthony F. Starace Publications

We investigate theoretically the use of energy-resolved ultrafast electron diffraction to image laser-driven electronic motion in atoms. A chirped laser pulse is used to transfer the valence electron of the lithium atom from the ground state to the first excited state. During this process, the electronic motion is imaged by 100-fs and 1-fs electron pulses in energy-resolved diffraction measurements. Simulations show that the angle-resolved spectra reveal the time evolution of the energy content and symmetry of the electronic state. The time-dependent diffraction patterns are further interpreted in terms of the momentum transfer. For the case of incident 1-fs electron pulses ...


Modification Of The G-Phonon Mode Of Graphene By Nitrogen Doping, Pavel V. Lukashev, Liuyan Zhao, Tula R. Paudel, Theanne Schiros, Noah Hurley, Evgeny Y. Tsymbal, Aron Pinczuk, Abhay Pasupathy, Rui He 2017 University of Nothern Iowa

Modification Of The G-Phonon Mode Of Graphene By Nitrogen Doping, Pavel V. Lukashev, Liuyan Zhao, Tula R. Paudel, Theanne Schiros, Noah Hurley, Evgeny Y. Tsymbal, Aron Pinczuk, Abhay Pasupathy, Rui He

Pavel Lukashev

The effect of nitrogen doping on the phonon spectra of graphene is analyzed. In particular, we employ first-principles calculations and scanning Raman analysis to investigate the dependence of phonon frequencies in graphene on the concentration of nitrogen dopants. We demonstrate that the G phonon frequency shows oscillatory behavior as a function of nitrogen concentration. We analyze different mechanisms which could potentially be responsible for this behavior, such as Friedel charge oscillations around the localized nitrogen impurity atom, the bond length change between nitrogen impurity and its nearest neighbor carbon atoms, and the long-range interactions of the nitrogen point defects. We ...


Modification Of The G-Phonon Mode Of Graphene By Nitrogen Doping, Pavel V. Lukashev, Liuyan Zhao, Tula R. Paudel, Theanne Schiros, Noah Hurley, Evgeny Y. Tsymbal, Aron Pinczuk, Abhay Pasupathy, Rui He 2017 University of Nothern Iowa

Modification Of The G-Phonon Mode Of Graphene By Nitrogen Doping, Pavel V. Lukashev, Liuyan Zhao, Tula R. Paudel, Theanne Schiros, Noah Hurley, Evgeny Y. Tsymbal, Aron Pinczuk, Abhay Pasupathy, Rui He

Rui He

The effect of nitrogen doping on the phonon spectra of graphene is analyzed. In particular, we employ first-principles calculations and scanning Raman analysis to investigate the dependence of phonon frequencies in graphene on the concentration of nitrogen dopants. We demonstrate that the G phonon frequency shows oscillatory behavior as a function of nitrogen concentration. We analyze different mechanisms which could potentially be responsible for this behavior, such as Friedel charge oscillations around the localized nitrogen impurity atom, the bond length change between nitrogen impurity and its nearest neighbor carbon atoms, and the long-range interactions of the nitrogen point defects. We ...


Charge State Of Vacancy Defects In Eu-Doped Gan, Brandon Mitchell, N. Hernandez, D. Lee, A. Koizumi, Y. Fujiwara, V. Dierolf 2017 West Chester University of Pennsylvania

Charge State Of Vacancy Defects In Eu-Doped Gan, Brandon Mitchell, N. Hernandez, D. Lee, A. Koizumi, Y. Fujiwara, V. Dierolf

Physics

Eu ions have been doped into GaN in order to achieve red luminescence under current injection, where coupling between the Eu ions and intrinsic defects such as vacancies are known to play an important role. However, the charge state of the vacancies and the impact it would have on the optical and magnetic properties of the Eu ions have not been explored. Through a combination of first-principle calculations and experimental results, the influence of the charge state of the defect environment surrounding the Eu ions has been investigated. We have identified two Eu centers that are related through the charge ...


Ionization Enhancement And Suppression By Phase-Locked Ultrafast Pulse Pairs, David B. Foote, Y. Lin, Liang-Wen Pi, Jean Marcel Ngoko Djiokap, Anthony F. Starace, W. T. Hill 2017 University of Maryland at College Park

Ionization Enhancement And Suppression By Phase-Locked Ultrafast Pulse Pairs, David B. Foote, Y. Lin, Liang-Wen Pi, Jean Marcel Ngoko Djiokap, Anthony F. Starace, W. T. Hill

Anthony F. Starace Publications

We present the results of a study of ionization of Xe atoms by a pair of phase-locked pulses, which is characterized by interference produced by the twin peaks. Two types of interference are considered: ordinary optical interference, which changes the intensity of the composite pulse and thus the ion yield, and a quantum interference, in which the excited electron wave packets interfere. We use the measured Xe+ yield as a function of the temporal delay and/or relative phase between the peaks to monitor the interferences and compare their relative strengths. We model the interference with a pulse intensity function ...


"Blinded By The Lines: Mid-Ir Spectra Of Mira Variables Taken With Spitzer", Dana Baylis-Aguirre, Michelle J. Creech-Eakman, Donald G. Luttermoser, Tina Gueth 2017 New Mexico Institute of Mining and Technology

"Blinded By The Lines: Mid-Ir Spectra Of Mira Variables Taken With Spitzer", Dana Baylis-Aguirre, Michelle J. Creech-Eakman, Donald G. Luttermoser, Tina Gueth

Donald G. Luttermoser

We present preliminary analysis of mid-infrared spectra of M-type and C-type Mira variables. Due to the brightness of this sample, it is straightforward to monitor changes with phase in the infrared spectral features of these regular pulsators. We have spectra of 25 Mira variables, taken with phase, using the Spitzer Infrared Spectrograph (IRS) high-resolution module. Each star has multiple spectra obtained over a one-year period from 2008-09. This is a rich, unique data set due to multiple observations of each star and the high signal-to-noise ratio from quick exposure times to prevent saturation of the IRS instrument. This paper focuses ...


Correlated Percolation In Island-Forming Processes: Analysis Of Cooperative Filling On A Square Lattice, D. E. Sanders, James W. Evans 2017 Iowa State University

Correlated Percolation In Island-Forming Processes: Analysis Of Cooperative Filling On A Square Lattice, D. E. Sanders, James W. Evans

James W. Evans

Percolation transitions are analyzed for correlated distributions of occupied sites created by irreversible cooperative filling on a square lattice. Filling can be either autocatalytic, corresponding to island formation, or autoinhibitory. Here percolation problems for occupied and unoccupied clusters are generally distinct. Our discussion focuses on the influence of island formation (associated with correlation lengths of many lattice vectors), and of island perimeter roughness, on percolation. We also discuss the transition to continuum percolation problems as the ratio of island growth to nucleation rates, and thus the average island size, diverges. Some direct analysis of occupied cluster structure is provided, the ...


Analysis Of A Diffusion-Limited Island Growth Mechanism For Chemisorption And Epitaxy, James W. Evans 2017 Iowa State University

Analysis Of A Diffusion-Limited Island Growth Mechanism For Chemisorption And Epitaxy, James W. Evans

James W. Evans

We consider an island growth mechanism wherein species adsorbed on top of two-dimensional islands diffuse to the edge and are then incorporated. A simple matrix formulation is developed for this random-walk problem which allows exact calculation of the growth probability distribution, neglecting island rearrangement. Furthermore, we also show how related trapping probabilities can be calculated recursively for various stages of island growth. The case of imperfect trapping at island edges is also considered, and reduction to Eden-model–type growth in the low-trapping-probability regime is demonstrated. In general, the growing islands are rounder and have fewer defects and narrower active zones ...


Construction And Analysis Of Accurate Exchange-Correlation Potentials, Sviataslau V. Kohut 2017 The University of Western Ontario

Construction And Analysis Of Accurate Exchange-Correlation Potentials, Sviataslau V. Kohut

Electronic Thesis and Dissertation Repository

Practical Kohn–Sham density-functional calculations require approximations to the exchange-correlation energy functional, EXC[ρ], or the exchange-correlation potential, vXC(r), defined as the functional derivative of EXC[ρ] with respect to the electron density, ρ. This thesis focuses on the following problems: (i) development of approximate exchange-correlation potentials by modelling the exchange-correlation charge distribution; (ii) accurate approximation of functional derivatives of orbital-dependent functionals; (iii) generation of exchange-correlation potentials from many-electron wavefunctions; (iv) analysis of accurate exchange-correlation potentials in atoms and molecules.

The advantage of modelling the exchange-correlation potential through the exchange-correlation charge distribution, qXC(r), is ...


Experimental Studies Of The Nacs 53Π0 And 1(A)3Σ+ States, Seth T. Ashman, Brett McGeehan, Christopher Wolfe, Carl Faust, John Huennekens 2017 Providence College

Experimental Studies Of The Nacs 53Π0 And 1(A)3Σ+ States, Seth T. Ashman, Brett Mcgeehan, Christopher Wolfe, Carl Faust, John Huennekens

Seth Ashman

We present experimental studies of the NaCs molecule that are currently underway in our laboratory. The opticaloptical double resonance method is used to obtain Doppler-free excitation spectra for several excited states. Selected data from the 53Π0 electronic state are used to obtain Rydberg-Klein-Rees (RKR) and Inverse Perturbation Approach (IPA) potential curves. We are also mapping the repulsive wall of the 1(a) 3Σ +potential using many resolved bound-free fluorescence spectra from individual ro-vibrational levels of the 53Π0 electronic state to the 1(a) 3Σ + state. Using the determined 53Π0 state potential we fit the repulsive wall of the 1(a ...


Experimental Studies Of Nacs, Seth T. Ashman, C. M. Wolfe, J. P. Huennekens 2017 Providence College

Experimental Studies Of Nacs, Seth T. Ashman, C. M. Wolfe, J. P. Huennekens

Seth Ashman

We present experimental studies of excited electronic states of the NaCs molecule that are currently underway in our laboratory. The optical-optical double resonance method is used to obtain Doppler-free excitation spectra for several excited states. These data are being used to obtain RydbergKlein-Rees (RKR) or Inverse Perturbation Approach (IPA) potential curves for these states. We are also trying to map the bound portion of the 1(a) 3Σ + potential using resolved laser-induced fluorescence and Fourier transform spectroscopy to record transitions into the shallow well. Bound-free spectra from single ro-vibrational levels of electronically excited states to the repulsive wall of the ...


Collisional Transfer Of Population And Orientation In Nak, Seth T. Ashman, C. M. Wolfe, J. Huennekens, B. Beser, J. Bai, A. M. Lyyra 2017 Providence College

Collisional Transfer Of Population And Orientation In Nak, Seth T. Ashman, C. M. Wolfe, J. Huennekens, B. Beser, J. Bai, A. M. Lyyra

Seth Ashman

We report current work to study transfer of population and orientation in collisions of NaK molecules with argon and potassium atoms using polarization labeling (PL) and laser- induced fluorescence (LIF) spectroscopy. In the PL experiment, a circularly polarized pump laser excites a specific NaK A1Σ +(v 0=16, J 0 ) ← X1Σ +(v 00=0, J 0 ± 1) transition, creating an orientation (non-uniform MJ0 level distribution) in both levels. The linearly polarized probe laser is scanned over various 31Π(v, J 0±1) ← A1Σ +(v 0=16, J 0 ) transitions. The probe laser passes through a crossed linear polarizer before detection ...


Polarization Spectroscopy And Collisions In Nak, Seth T. Ashman, C. M. Wolfe, J. Huennekens, B. Beser, J. Bai, A. M. Lyyra 2017 Providence College

Polarization Spectroscopy And Collisions In Nak, Seth T. Ashman, C. M. Wolfe, J. Huennekens, B. Beser, J. Bai, A. M. Lyyra

Seth Ashman

We report current work to study transfer of population and orientation in collisions of NaK molecules with argon and potassium atoms using polarization labeling (PL) and laser-induced fluorescence (LIF) spectroscopy. In the PL experiment, a circularly polarized pump laser excites a specific NaK A1Σ +(v=16, J) ← X1Σ +(v=0, J ± 1) transition, creating an orientation (non-uniform MJ level distribution) in both levels. The linear polarized probe laser is scanned over various 3 1Π(v=8, J 0 ± 1) ← A1Σ +(v=16, J 0 ) transitions. The probe laser passes through a crossed linear polarizer before detection, and signal is recorded ...


Vibrational State Resolved Lifetimes Of The Na2 21Σ+U Double Well State, Seth T. Ashman, Lutz Huwel, Roy Anunciado, Nadeepa Jayasundara 2017 Providence College

Vibrational State Resolved Lifetimes Of The Na2 21Σ+U Double Well State, Seth T. Ashman, Lutz Huwel, Roy Anunciado, Nadeepa Jayasundara

Seth Ashman

Lifetimes of individual Na2 ro-vibrational levels of the 21Σ + u double well-state have been measured using a delayed photoionization technique. Ground state Na2 produced in a molecular beam is excited resonantly by the doubled output of a pulsed dye laser in the range 333 – 357 nm and then ionized by a 532 nm photon from a time-delayed Nd:YAG laser. By appropriate excitation laser tuning and systematic variation of the probe laser delay, ro-vibrational level resolved lifetimes are obtained for v=25- 49. The double well state lifetime values are found to decrease from about 50 ns at v=25 ...


Comparison Of Absorption, Fluorescence, And Polarization Spectroscopy Of Atomic Rubidium, Seth T. Ashman, Cayla Stifler, Joaquin Romero 2017 Providence College

Comparison Of Absorption, Fluorescence, And Polarization Spectroscopy Of Atomic Rubidium, Seth T. Ashman, Cayla Stifler, Joaquin Romero

Seth Ashman

An ongoing spectroscopic investigation of atomic rubidium utilizes a two-photon, single-laser excitation process. Transitions accessible with our tunable laser include 5P1/2 (F ′ ) ← 5S1/2 (F) and 5P3/2 (F ′ ) ← 5S1/2 (F). The laser is split into a pump and probe beam to allow for Doppler-free measurements of transitions between hyperfine levels. The pump and probe beams are overlapped in a counter-propagating geometry and the laser frequency scans over a transition. Absorption, fluorescence and polarization spectroscopy techniques are applied to this basic experimental setup. The temperature of the vapor cell and the power of the pump and probe beams ...


Adiabatic-Limit Coulomb Factors For Photoelectron And High-Order-Harmonic Spectra, M. V. Frolov, N. L. Manakov, A. A. Minina, S. V. Popruzhenko, Anthony F. Starace 2017 Voronezh State University, Russia

Adiabatic-Limit Coulomb Factors For Photoelectron And High-Order-Harmonic Spectra, M. V. Frolov, N. L. Manakov, A. A. Minina, S. V. Popruzhenko, Anthony F. Starace

Anthony F. Starace Publications

A momentum-dependent Coulomb factor in the probability for nonlinear ionization of atoms by a strong low-frequency laser field is calculated analytically in the adiabatic approximation. Expressions for this Coulomb factor, valid for an arbitrary laser pulse waveform, are obtained and analyzed in detail for the cases of linear and circular polarizations. The dependence of the Coulomb factor on the photoelectron momentum is shown to be significant in both cases. Using a similar technique, the Coulomb factor for emission of high-order harmonics by an atom in a bichromatic laser field is also calculated. In contrast to the case of a single-frequency ...


Raman Spectroscopy Of Oxygen Evolution Catalysts And Psii Manganese Model Compounds, Sergei Shmakov, Daniel A. Hartzler, Alireza Karbakhsh Ravari, Yulia Pushkar 2017 Purdue University

Raman Spectroscopy Of Oxygen Evolution Catalysts And Psii Manganese Model Compounds, Sergei Shmakov, Daniel A. Hartzler, Alireza Karbakhsh Ravari, Yulia Pushkar

The Summer Undergraduate Research Fellowship (SURF) Symposium

Photosynthesis is the basis of life on earth, and oxygen evolution catalysts are key components of this complicated, yet not fully understood process. Photosystem II, a large membrane bound pigment-protein complex, is the key system that facilitates oxygenic photosynthesis via the oxygen evolving complex (a natural oxygen evolving catalyst). It is a key component in oxygen producing catalysts, which can be used in fields such as energy production and biomimetic catalysts. The oxygen evolution cycle, or Kok cycle going within it is still not studied completely. In this project, we were studying the vibrational (and structural) state of a Manganese ...


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