Charge transfer by electronic excitation: High resolution measurements via rotationally resolved spectroscopy in the gas phase

Understanding the intricate molecular motions that occur in solvents is a scientific challenge for many fields, including biology, chemistry, and physics. Solvents are ever-present in living organisms, and may play a vital role in the folding of proteins and nucleic acid chains. Currently, ultrafast spectroscopic techniques are able to map long range networks of hydrogen bonds within the universal solvent water, where hindered motions are important. Presented in this dissertation is a detailed study of several highly resolved frequency spectra, each of which makes a unique contribution to the understanding of molecular structure, intermolecular bonding dynamics, and the forces that stabilize hydrogen bonds in the gas phase. It is here, in an isolated environment, that solute-solvent interactions can be dissected, both experimentally and theoretically, void of perturbations from the bulk. Among the molecular systems investigated here, the photoacid β-naphthol was studied in the presence of water and ammonia, and the electric dipole moments of each complex were shown to contain intrinsic contributions from intermolecular charge transfer. This charge transfer is present in the ground electronic state, and increases upon excitation with ultraviolet light. Two rotamers of the donor-acceptor system meta-aminobenzoic acid have been identified by differences in their moments of inertia and dipole moments, and singly and doubly solvated complexes of this system were observed. The ground, S1, and S2 dipole moments of anomalous dual fluorescence molecules, such as DMABN and phenylpyrrole, have also been determined, and their relevance to condensed phase solvatochromism is discussed.The work reported here makes use of two ultraviolet laser spectrometers; a pulsed supersonic jet spectrometer, and a high resolution continuous wave molecular beam spectrometer. A wide variety of ab initio calculations were performed in support of these experiments

Cavity-enhanced optical frequency comb spectroscopy in the mid-infrared - application to trace detection of H2O2

We demonstrate the first cavity-enhanced optical frequency comb spectroscopy in the mid-infrared wavelength region and report the sensitive real-time trace detection of hydrogen peroxide in the presence of a large amount of water. The experimental apparatus is based on a mid-infrared optical parametric oscillator synchronously pumped by a high power Yb:fiber laser, a high finesse broadband cavity, and a fast-scanning Fourier transform spectrometer with autobalancing detection. The comb spectrum with a bandwidth of 200 nm centered around 3.75 {\mu}m is simultaneously coupled to the cavity and both degrees of freedom of the comb, i.e., the repetition rate and carrier envelope offset frequency, are locked to the cavity to ensure stable transmission. The autobalancing detection scheme reduces the intensity noise by a factor of 300, and a sensitivity of 5.4 {\times} 10^-9 cm^-1 Hz^-1/2 with a resolution of 800 MHz is achieved (corresponding to 6.9 {\times} 10^-11 cm^-1 Hz^-1/2 per spectral element for 6000 resolved elements). This yields a noise equivalent detection limit for hydrogen peroxide of 8 parts-per-billion (ppb); in the presence of 2.8% of water the detection limit is 130 ppb. Spectra of acetylene, methane and nitrous oxide at atmospheric pressure are also presented, and a line shape model is developed to simulate the experimental data.Comment: submitted to special FLAIR 2011 issue of Appl. Phys.

Applications of neuroimaging to disease-modification trials in Alzheimer's disease.

Critical to development of new therapies for Alzheimer's disease (AD) is the ability to detect clinical or pathological change over time. Clinical outcome measures typically used in therapeutic trials have unfortunately proven to be relatively variable and somewhat insensitive to change in this slowly progressive disease. For this reason, development of surrogate biomarkers that identify significant disease-associated brain changes are necessary to expedite treatment development in AD. Since AD pathology is present in the brain many years prior to clinical manifestation, ideally we want to develop biomarkers of disease that identify abnormal brain structure or function even prior to cognitive decline. Magnetic resonance imaging, fluorodeoxyglucose positron emission tomography, new amyloid imaging techniques, and spinal fluid markers of AD all have great potential to provide surrogate endpoint measures for AD pathology. The Alzheimer's disease neuroimaging initiative (ADNI) was developed for the distinct purpose of evaluating surrogate biomarkers for drug development in AD. Recent evidence from ADNI demonstrates that imaging may provide more sensitive, and earlier, measures of disease progression than traditional clinical measures for powering clinical drug trials in Alzheimer's disease. This review discusses recently presented data from the ADNI dataset, and the importance of imaging in the future of drug development in AD

MID-INFRARED FREQUENCY COMB SPECTROSCOPY USING A VIRTUALLY IMAGED PHASED ARRAY

Here we present a new mid-infrared frequency comb system for rapid spectral acquisition using a virtually imaged phased array (VIPA) spectrometer.\footnote{L. Nugent-Glandorf et al., \textit{Opt. Lett.} \textbf{37,} 3285 (2012)} A difference-frequency generation comb, tuneable from 4.4 $\mu$m to 4.7 $\mu$m, was used to interrogate a single-pass absorption cell containing either N$_2$O or CO dilute in either N$_2$ or air. Precision molecular spectroscopy capabilities at timescales of less than 1 ms will be presented, and progress toward cavity-enhanced and time-resolved comb spectroscopies\footnote{A.J. Fleisher et. al., \textit{J. Phys. Chem. Lett.} \textbf{5,} 2241 (2014)} will be discussed

QUANTUM-NOISE-LIMITED CAVITY RING-DOWN SPECTROSCOPY IN THE MID-INFRARED

We report a highly sensitive mid-infrared spectrometer capable of recording cavity ring-down events in the quantum (shot) noise limit. A linear optical cavity of finesse 31,000 was pumped by a distributed feedback quantum cascade laser (DFB-QCL) operating at 4.5 $mu$m until a cavity transmission threshold was reached. A fast optical switch then extinguished optical pumping and initiated a cavity decay which exhibited root-mean-square noise proportional to the square root of optical power (quantum noise) for several cavity time constants until a detector noise floor was reached. This spectrometer has achieved a noise-equivalent absorption of NEA = $2.6times10^{-11}$ wn Hz$^{-1/2}$ and a minimum absorption coefficient of $alpha = 2.3times10^{-11}$ wn in 3 seconds. Applications for such a highly sensitive spectrometer operating in the mid-infrared region, including ultra-trace molecular spectroscopy of chem{CO_2} isotopologues and the direct interrogation of weak mirror birefringence and polarization-dependent losses, will be discussed

DIRECT ABSORPTION SPECTROSCOPY WITH ELECTRO-OPTIC FREQUENCY COMBS

The application of electro-optic frequency combs to direct absorption spectroscopyfootnote{D.A. Long et al., textit{Opt. Lett.} textbf{39,} 2688 (2014)} has increased research interest in high-agility, modulator-based comb generation. This talk will review common architectures for electro-optic frequency comb generators as well as describe common self-heterodyne and multi-heterodyne (i.e., dual-comb) detection approaches. In order to achieve a sufficient signal-to-noise ratio on the recorded interferogram while allowing for manageable data volumes, broadband electro-optic frequency combs require deep coherent averaging,footnote{A.J. Fleisher et al., textit{Opt. Express} textbf{24,} 10424 (2016)} preferably in real-time. Applications such as cavity-enhanced spectroscopy, precision atomic and molecular spectroscopy, as well as time-resolved spectroscopy will be introduced