24 research outputs found
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.
DIRECT MEASUREMENT OF OD+CO→ cis-DOCO, trans-DOCO, AND D+CO2 BRANCHING KINETICS USING TIME-RESOLVED FREQUENCY COMB SPECTROSCOPY
The kinetics of the reaction OH+COH+CO has attracted experimental and theoretical studies for more than 40 years due to its importance in atmospheric and combustion environments. This reaction proceeds on a rich potential energy landscape, first by forming vibrationally excited HOCO*; subsequently, HOCO* either back reacts to OH+CO, dissociates to H+CO, or is stabilized to ground state HOCO by collisions with a third body. Due to the formation of the HOCO intermediate, the rate coefficient displays anomalous temperature and strong pressure dependences. Time-resolved Frequency Comb Spectroscopy (TRFCS) combines a mid-IR mode-locked femtosecond laser, a broadband optical enhancement cavity, and spatially dispersive detection system to simultaneously provide broad spectral bandwidth, high spectral resolution, high absorption sensitivity, and microsecond time resolution. We have applied this powerful technique to identify the deuterated analogues of HOCO isomers, {it trans}-DOCO and {it cis}-DOCO, for the first time in the reaction OD+CO under ambient conditions. By directly monitoring the concentrations of OD (reactant), {it trans}-DOCO, {it cis}-DOCO (intermediates), and CO(product), we unambiguously measure all pressure-dependent branching rates of the OD+CO reaction
Direct measurements of DOCO isomers in the kinetics of OD+CO
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 analogue 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.Comment: 19 pages, 4 figure
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-d_1, 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
Mid-infrared VIPA Spectrometer for Rapid and Broadband Trace Gas Detection
We present and characterize a 2-D imaging spectrometer based on a
virtually-imaged phased array (VIPA) disperser for rapid, high-resolution
molecular detection using mid-infrared (MIR) frequency combs at 3.1 and 3.8 \mu
m. We demonstrate detection of CH4 at 3.1 \mu m with >3750 resolution elements
spanning >80 nm with ~600 MHz resolution in a <10 \mu s acquisition time. In
addition to broadband detection, rapid, time-resolved single-image detection is
demonstrated by capturing dynamic concentration changes of CH4 at a rate of
~375 frames per second. Changes in absorption above the noise floor of 5\times
10-4 are readily detected on the millisecond time scale, leading to important
future applications such as real time monitoring of trace gas concentrations
and detection of reactive intermediates
Time-resolved Frequency Comb Spectroscopy Of Transient Free Radicals In The Mid-infrared Spectral Region
The chemical kinetics of transient free radicals, such as HOCO and Criegee intermediates, play important roles in combustion and atmospheric processes. Establishing accurate kinetics models for these complex systems require knowledge of the reaction rates and lifetimes of all molecules along a particular reaction pathway. However, standard spectroscopic techniques lack a combination of sensitivity, frequency resolution, and adequate temporal resolution to survey these reactions on the s timescale. To answer this challenge, we have developed time-resolved frequency comb spectroscopy (TRFCS). This novel technique allows for the detection of transient intermediates with high time-resolution and sensitivity while also permitting the direct determination of rotational state distributions of all relevant molecules. We demonstrate this technique in the mid-infrared spectral region, at 3.7 m, by studying the photolysis of deuterated acrylic acid. We simultaneously observe the time-dependent concentrations of photoproducts trans-DOCO, HOD, and DO, identified through their unique rovibrational structure, with molecules cm sensitivity, and with a time resolution of 25 s. We aim to apply this technique to detect directly the formation of the DOCO intermediate in the OD + CO chemical reaction at atmospherically relevant pressures, in order to validate statistical rate models of this reaction