Thinh Bui

Mid-Infrared Time-Resolved Frequency Comb Spectroscopy of Transient Free Radicals

We demonstrate time-resolved frequency comb spectroscopy (TRFCS), a new broadband absorption spectroscopy technique for the study of trace free radicals on the microsecond timescale. We apply TRFCS to study the time-resolved, mid-infrared absorption of the deuterated hydroxyformyl radical trans-DOCO, an important short-lived intermediate along the OD + CO reaction path. Directly after photolysis of the chemical precursor acrylic acid-d1, we measure absolute trans-DOCO product concentrations with a sensitivity of 5 × 10(10) cm(-3) and observe its subsequent loss with a time resolution of 25 μs. The multiplexed nature of TRFCS allows us to detect simultaneously the time-dependent concentration of several other photoproducts and thus unravel primary and secondary chemical reaction pathways.

Direct frequency comb measurement of OD + CO → DOCO kinetics

Combing through CO oxidation kinetics Carbon monoxide reacts with OH radicals to produce CO2. This process is central to combustion and atmospheric oxidation chemistry. The reaction sequence is widely assumed to involve the intermediacy of a HOCO adduct that has eluded direct monitoring under thermal conditions. Bjork et al. successfully observed the formation of the deuterated analog of this intermediate, DOCO, while simultaneously monitoring OD by using a multifrequency infrared comb. The results confirm the termolecular nature of the formation mechanism and its sensitivity to the ambient bath gas. Science, this issue p. 444 A broadband frequency comb tracks the kinetics of a previously elusive intermediate in the oxidation of carbon monoxide to carbon dioxide The kinetics of the hydroxyl radical (OH) + carbon monoxide (CO) reaction, which is fundamental to both atmospheric and combustion chemistry, are complex because of the formation of the hydrocarboxyl radical (HOCO) intermediate. Despite extensive studies of this reaction, HOCO has not been observed under thermal reaction conditions. Exploiting the sensitive, broadband, and high-resolution capabilities of time-resolved cavity-enhanced direct frequency comb spectroscopy, we observed deuteroxyl radical (OD) + CO reaction kinetics and detected stabilized trans-DOCO, the deuterated analog of trans-HOCO. By simultaneously measuring the time-dependent concentrations of the trans-DOCO and OD species, we observed unambiguous low-pressure termolecular dependence of the reaction rate coefficients for N2 and CO bath gases. These results confirm the HOCO formation mechanism and quantify its yield.

Observations of Dicke narrowing and speed dependence in air-broadened CO2 lineshapes near 2.06 μm

Frequency-stabilized cavity ring-down spectroscopy was used to study CO2 lineshapes in the (20013) ← (00001) band centered near 2.06 μm. Two rovibrational transitions were chosen for this study to measure non-Voigt collisional effects for air-broadened lines over the pressure range of 7 kPa-28 kPa. Lineshape analysis for both lines revealed evidence of simultaneous Dicke (collisional) narrowing and speed-dependent effects that would introduce biases exceeding 2% in the retrieved air-broadening parameters if not incorporated in the modeling of CO2 lineshapes. Additionally, correlations between velocity- and phase/state changing collisions greatly reduced the observed Dicke narrowing effect. As a result, it was concluded that the most appropriate line profile for modeling CO2 lineshapes near 2.06 μm was the correlated speed-dependent Nelkin-Ghatak profile, which includes all of the physical effects mentioned above and leads to a consistent set of line shape parameters that are linear with gas pressure.

Three-photon absorption in optical parametric oscillators based on OP-GaAs

We report on, to the best of our knowledge, the first singly resonant (SR), synchronously pumped optical parametric oscillator (OPO) based on orientation-patterned gallium arsenide (OP-GaAs). Together with a doubly resonant (DR) degenerate OPO based on the same OP-GaAs material, the output spectra cover 3 to 6 μm within ∼3  dB of relative power. The DR-OPO has the highest output power reported to date from a femtosecond, synchronously pumped OPO based on OP-GaAs. We observed strong three-photon absorption with a coefficient of 0.35±0.08  cm3/GW2 for our OP-GaAs sample, which limits the output power of these OPOs as mid-IR light sources. We present a detailed study of the three-photon loss on the performance of both the SR- and DR-OPOs, and compare them to those without this loss mechanism.

Direct measurements of DOCO isomers in the kinetics of OD + CO

Frequency comb spectroscopy captures real-time dynamics of cis- and trans-DOCO isomers produced from the OD + CO reaction. Quantitative and mechanistically detailed kinetics of the reaction of hydroxyl radical (OH) with carbon monoxide (CO) have been a longstanding goal of contemporary chemical kinetics. This fundamental prototype reaction plays an important role in atmospheric and combustion chemistry, motivating studies for accurate determination of the reaction rate coefficient and its pressure and temperature dependence at thermal reaction conditions. This intricate dependence can be traced directly to details of the underlying dynamics (formation, isomerization, and dissociation) involving the reactive intermediates cis- and trans-HOCO, which can only be observed transiently. Using time-resolved frequency comb spectroscopy, comprehensive mechanistic elucidation of the kinetics of the isotopic analog deuteroxyl radical (OD) with CO has been realized. By monitoring the concentrations of reactants, intermediates, and products in real time, the branching and isomerization kinetics and absolute yields of all species in the OD + CO reaction are quantified as a function of pressure and collision partner.

Spectral analyses of trans- and cis-DOCO transients via comb spectroscopy

ABSTRACT We use time-resolved direct frequency comb spectroscopy in the mid-infrared to obtain high-resolution rovibrational spectra of products produced from the OD + CO reaction. In this work, we present spectral analyses for isotopologues of the transient DOCO radicals from this reaction in the OD stretch region. The analyses were performed with the aid of two different theoretical approaches based on both perturbation theory and variational calculations used for prediction of rovibrational spectra of polyatomic molecules. We discuss the advantages and challenges of our current approach for studying spectroscopy and dynamics of transient molecules. GRAPHICAL ABSTRACT

PHOTOACOUSTIC SPECTROSCOPY OF PRESSURE- AND TEMPERATURE- DEPENDENCE IN THE O2 A-BAND

MATTHEW J. CICH, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA; ELIZABETH M LUNNY, GAUTAM STROSCIO, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA; THINH QUOC BUI, JILA, National Institute of Standards and Technology and Univ. of Colorado Department of Physics, University of Colorado, Boulder, Boulder, CO, USA; PRIYANKA RUPASINGHE, Physical Sciences, Cameron University, Lawton, OK, USA; DANIEL HOGAN, Department of Applied Physics, Stanford University, Stanford, CA, USA; CAITLIN BRAY, Department of Chemistry, Wesleyan University, Middletown, CT, USA; DAVID A. LONG, JOSEPH T. HODGES, Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD, USA; TIMOTHY J. CRAWFORD, CHARLES MILLER, BRIAN DROUIN, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA; MITCHIO OKUMURA, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA.

Time-resolved frequency comb spectroscopy for studying the kinetics and branching ratio of OD+CO

THINH QUOC BUI, JILA, National Institute of Standards and Technology and Univ. of Colorado Department of Physics, University of Colorado, Boulder, Boulder, CO, USA; BRYCE J BJORK, OLIVER H HECKL, BRYAN CHANGALA, BEN SPAUN, JILA, National Institute of Standards and Technology and Univ. of Colorado Department of Physics, University of Colorado, Boulder, CO, USA; MITCHIO OKUMURA, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA; JUN YE, JILA, National Institute of Standards and Technology and Univ. of Colorado Department of Physics, University of Colorado, Boulder, Boulder, CO, USA.

Time Resolved Frequency Comb Spectroscopy for Studying Gas Phase Free Radical Kinetics

We report the development of a novel technique, mid-Infrared Time Resolved Frequency Comb Spectroscopy (TRFCS) for the high sensitivity, broad-band, detection of trace free radicals and reactive intermediates in gas phase, and the study of their reaction kinetics in real-time.

The Oxygen A Band

D. CHRIS BENNER, V. MALATHY DEVI, JIAJUN HOO, Department of Physics, College of William and Mary, Williamsburg, VA, USA; JOSEPH HODGES, DAVID A. LONG, Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD, USA; KEEYOON SUNG, Jet Propulsion Laboratory, Science Division, California Institute of Technology, Pasadena, CA, USA; BRIAN DROUIN, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA; MITCHIO OKUMURA, THINH QUOC BUI, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA; PRIYANKA RUPASINGHE, Physical Sciences, Cameron University, Lawton, OK, USA.