Soroush Khosravi Dehaghi,
Department of Physics,
University of Connecticut

Femtosecond Laser Transient Spectroscopy of Carotenoids and Carbon Nanotubes

Carotenoids and semiconducting single-wall carbon nanotubes, which are both nanoscale carbon-based quantum mechanical systems, were studied using different optical spectroscopy techniques.

The lifetime of the S\(_2\) excited state of the carotenoids were estimated using a model-based lifetime analysis routine. It was found that the addition of a conjugated carbonyl group to the carotenoid has several crucial effects on the optical response due to the alteration of the delocalized conjugated \(\pi\) molecular orbital of the molecule. These effects include the shortening of the lifetimes of the excited states, the change in the decay mechanism from the S\(_2\) excited state to the S\(_1\) excited state, and the unresolved vibrational peaks in the steady-state absorption spectrum.

The decay of the E\(_{22}\) excited state of (6,5) semiconducting carbon nanotube dispersed in flavin mononucleotide with water was observed using transient grating spectroscopy. The lifetime of this excited state was estimated to be 450 ± 50 fs. A very strong single-frequency oscillatory component was observed in the transient grating decay profile.

Electric Form Factor of the Neutron from Asymmetry Measurements

Field of Study: Physics

Jonathan Kwolek,
Department of Physics,
University of Connecticut

Studying Charge Exchange in a Hybrid Ion-neutral Trap


Interactions between neutral and ionized atoms are medium-range, and the spiraling nature of these collisions can lead to long interaction times and large cross sections. The field of ion-neutral interactions is bolstered by methods from atomic physics, which allow us to accurately control and determine the quantum-states of the reactants. Atomic physics opens the door to studying these interactions over a large range of energies, in order to observe interesting effects from the ultracold to hot temperature regimes.

The hybrid-trap enables the concentric trapping of a sample of cold atoms and ions. The dissertation discusses our exploration into the controlled reactions in the Na+Ca^+ system, including measurements of sympathetic cooling and a charge-exchange rate which changes as a function of energy and atomic state. The quantum-state distributions of Na in our magneto-optical trap is also explored, as well as its agreement and subsequent deviation based on a predictive model. The measurements of reaction rates indicate a reaction-energy threshold in two different reaction channels, which can be corroborated by theoretical investigation.

Dale L Smith, Department of Physics, University of Connecticut

Velocity Map Imaging of the Single Ionization of
Molecular Iodine

Single ionization in molecules is a critical first step in many higher order process such as high harmonic generation. However, single ionization remains poorly understood even in simple diatomic systems. We have found that molecular iodine, I2, has a complicated ionization processes which does not originate from valance orbitals, as would be expected, but from more tightly bound inner orbitals comprised of 5s electrons. I will discuss two experiments which use ultrafast lasers to investigate the ionization of diatomic I2. Ultrafast laser pulses have a duration in the tens of femtoseconds (fs). Since the vibrational period of I2 is ≈180 fs, we are able to ionize the molecule before it dissociates. Velocity map imaging allows us to observe the kinetic energy release (KER) of charged fragments from dissociation after single ionization. In the first experiment, we have performed a wavelength study from 800 to 400 nm. We have found in the I + I+ dissociation channel (which we denote as the (1,0)) the KER of the fragments was mostly inconsistent with ionization to the X-, A-, or B-states of I2+ which implies ionization through deeper orbitals. A pump-probe Fourier technique also showed that modulation in the cation products only occurs below 0.2 eV, which is consistent with dissociation through the B-state and ionization of high-lying molecular orbitals. This rules out a two-step process through the X- and A-states for much of the observed KER. Employing intensity-, polarization-, and wavelength-dependent experiments we were able to eliminate bond softening, electron rescattering, and photon mediation through the X- or A-states. In the second experiment, we performed a detailed wavelength analysis of I2 around its one-photon B-state resonance at 530 nm where we have identified a strong enhancement in the (1,0) channel. The resonance enhancement is found in both ionization from outer orbitals through the B-state as well as ionization from inner orbitals. Interestingly, the branching ratio for inner orbital ionization reaches over 98% at 519 nm and the peaks in the branching ratios of the two channels occur at slightly different wavelengths. For double ionization into an excited state of I2+ we find that the branching ratio closely follows the branching ratio of the single ionization of deeply bound inner orbitals. This implies that excitation of molecules comes about through inner orbital ionization. The finding of these experiments are inconsistent with current tunneling ionization theory which postulates that ionization arises through the least bound, outer orbital electrons.

Udaya Raj Dahal, Department of Physics, University of Connecticut

Polyethylene Oxide Hydration in Solutions, Nanoconfinement and Nanostructures

Amphiphilic water-soluble polymers are actively used in designing novel nanomaterials and have been the subject of extensive experimental and simulation studies. Polyethylene oxide (PEO) is a water soluble, biocompatible, non-toxic synthetic polymer capable of preventing protein adsorption which is widely used in industry and biomedicine for protein crystallization, control of particle aggregation and drug delivery. Most of the applications of PEO and PEO-based nanoparticles are utilized in an aqueous environment as PEO is highly soluble in water due to its ability to form hydrogen bonds with water and therefore understanding the role of water on the conformation and dynamics of PEO and PEO-based nanomaterials is essential in improving and designing new nanomaterials for a variety of applications.

Using all-atom molecular dynamics simulations, we studied PEO in bulk solutions, under nanoconfinement in carbon nanotubes and in nanostructures (PEO brushes grafted to a planar surface and nanoparticles). In the bulk solution, we find that PEO forms a globule like structure in hexane, coil-like structure in water or benzene and an extended rod-like or helical structure is isobutyric acid. The conformation and mobility of PEO in water and in isobutyric acid is dictated by the hydrogen bonding with PEO. As part of our confinement studies, we found that PEO is spontaneously encapsulated from aqueous solution into carbon nanotubes and forms rod-like, helical and wrapped chain conformation depending on the size of the carbon nanotube. The stable helix inside the carbon-nanotube is a consequence of the stable water arrangement around PEO.

As part of our studies of PEO brushes we investigated PEO grafted to gold nanoparticles and planar gold surfaces at varying grafting densities. We found that PEO hydration in the brush depends on grafting density and for gold nanoparticles also varies with the radial distance and nanoparticle radius of curvature. Our simulation results agree well with the classical scaling theories and we were able to explain the curvature dependent hydration based on bulk-solution PEO behavior.

Di Shu, Department of Physics, University of Connecticut

Threshold Resonance Effects and the Phase-Amplitude Approach

A fundamental aspect of scattering is the appearance of resonances, which are rather ubiquitous. Although their effect is often lost due to averaging at room temperatures or higher, they can become dominant features at low or ultralow temperatures, where only a few partial waves contribute to the scattering process. Our ability to form and manipulate ultracold molecules provides the seed to study in a precise and controlled fashion the role of single partial waves, and state-to-state processes in chemical systems. We will explore resonances occurring due to the existence of a quasi-bound state in the entrance channel of a scattering system and explain the energy scaling due to these near threshold resonances (NTR) based on the properties of the Jost functions. We will also investigate the threshold resonance effects in the Efimov systems which have been studied in a variety of context, such as three-body Coulomb systems and nuclear three-body systems. Numerical schemes based on Milne's phase-amplitude approach are invented to analyze NTR effects. We will present a simple and practical approach to solve the long-standing problem of finding the non-oscillatory solution to Milne's equation. Our numerical approach also gives an integral representation of scattering phase shift and allows us to compute ultra-narrow shape resonances.

Sahan Handunkanda, Department of Physics, University of Connecticut

Lattice dynamics studies of negative thermal expansion due to two low-temperature lattice instabilities

A material's tendency to shrink upon heating is known as negative thermal expansion (NTE). A class of materials in which the NTE phenomenon arises from the lattice degrees of freedom has gained much attention over the past couple decades. Simple cubic ScF3 is a prominent candidate of this class of materials as it displays strong, isotropic NTE over a 1000 K temperature range. In addition, no structural phase transition has been reported above 0.4K and it retains the simple cubic structure up to its high melting point of 1800 K, which is unusual compared with other transition metal trifluorides.

Here I present a combined inelastic x-ray scattering (IXS), x-ray diffraction (XRD) and thermal diffuse scattering (TDS) study of ScF3, revealing some exciting features of this material such as clean-limit structural quantum phase transition (SQPT), evidence of essential dimensional reduction, disorder phase diagram and nanoscale correlation of NTE modes. Further investigation of the zone center (ZC) dynamics of ScF3 by using infrared (IR) reflectivity measurement reveals evidence of multi-phonon absorption involving low energy NTE modes resides at Brillouin zone boundary. The IR result motivates us to consider a new approach to study NTE dynamics by populating corresponding modes using optical pump. I then address whether the anomalously strong and thermally persistent NTE behavior of ScF3 is a consequence of the SQPT? We carried out an IXS study of a second system Hg2I2 also tuned near SQPT while retaining stoichiometric composition and high crystallinity. We find similar behavior and significant NTE below 100 K for dimensions along the body-centered tetragonal c axis, bolstering the connection between NTE and zero-temperature structural transitions.

Anees Ahmed, Department of Physics, University of Connecticut

Resurgence and Large N: the Gross-Witten-Wadia model

I present a detailed study of parametric resurgence in the Gross-Witten-Wadia unitary model. I show how trans-series expansions in different sectors transmute into each other as they pass through the large N phase transition. This transition is well-studied in the immediate vicinity of the transition point. Here I present a complementary analysis of the transition at all coupling and all finite N, in terms of a differential equation, using the explicit Tracy-Widom mapping of the Gross-Witten-Wadia partition function to a solution of a Painleve III equation. This mapping provides a simple method to generate trans-series expansions in all parameter regimes. A surprising result is uncovered: the strong coupling expansion is convergent and yet there is a non-perturbative trans-series completion. I also define a uniform large N strong-coupling expansion (a non-linear analogue of uniform WKB), which is much more precise than the conventional large N expansion through the transition region, and apply it to the evaluation of Wilson loops and the Beta function.