Sensitive detection and imaging of H2O density through Rydberg resonant femtosecond laser induced photofragmentation fluorescence.

A femtosecond laser induced photofragmentation fluorescence (fs-LIPF) scheme for the sensitive detection and imaging of water vapor is presented. Two photons of 244.3 nm excite water to the D̃ state and produce hydroxyl radicals in the fluorescing à state. Two more photons promote electrons from the D̃ state to a neutral Rydberg state of the (1b2)-1 ionic core through a 2 + 2 doubly resonant process. The resulting high-lying Rydberg state undergoes neutral dissociation, and the energetic hydrogen fragments are detected from their Balmer series fluorescence. These channels (in the low-pressure limit) have detection sensitivities around 1012 molecules per cubic centimeters, orders of magnitude more sensitive than laser-induced fluorescence based approaches, allowing for sensitive non-invasive detection and imaging of water density for many important processes.

Direct observation of coherence transfer and rotational-to-vibrational energy exchange in optically centrifuged CO2 super-rotors.

Optical centrifuges are laser-based molecular traps that can rotationally accelerate molecules to energies rivalling or exceeding molecular bond energies. Here we report time and frequency-resolved ultrafast coherent Raman measurements of optically centrifuged CO2 at 380 Torr spun to energies beyond its bond dissociation energy of 5.5 eV (Jmax = 364, Erot = 6.14 eV, Erot/kB = 71, 200 K). The entire rotational ladder from J = 24 to J = 364 was resolved simultaneously which enabled a more accurate measurement of the centrifugal distortion constants for CO2. Remarkably, coherence transfer was directly observed, and time-resolved, during the field-free relaxation of the trap as rotational energy flowed into bending-mode vibrational excitation. Vibrationally excited CO2 (ν2 > 3) was observed in the time-resolved spectra to populate after 3 mean collision times as a result of rotational-to-vibrational (R-V) energy transfer. Trajectory simulations show an optimal range of J for R-V energy transfer. Dephasing rates for molecules rotating up to 5.5 times during one collision were quantified. Very slow decays of the vibrational hot band rotational coherences suggest that they are sustained by coherence transfer and line mixing.

Suppression of coherent interference to electric-field-induced second-harmonic (E-FISH) signals for the measurement of electric field in mesoscale confined geometries.

We present spatially enhanced electric-field-induced second-harmonic (SEEFISH) generation with a chirped femtosecond beam for measurements of electric field in mesoscale confined geometries subject to destructive spurious second-harmonic generation (SHG). Spurious SHG is shown to interfere with the measured E-FISH signal coherently, and thus simple background subtraction is not sufficient for single-beam E-FISH approaches, especially in a confined system with a large surface-to-volume ratio. The results show that a chirped femtosecond beam is effective in preventing higher-order mixing and white light generation in windows near the beam focal point which further contaminates the SEEFISH signal. The successful measurements of electric field of a nanosecond dielectric barrier discharge in a test cell demonstrated that spurious SHG detected with a congruent traditional E-FISH approach can be eliminated using the SEEFISH approach.

Numerical study of pure rotational fs/ps CARS coherence beating at high pressure and for multi-species rotation-vibration non-equilibrium thermometry.

Coherent anti-Stokes Raman scattering (CARS) has long been the gold standard for non-intrusively measuring gas temperature in reacting flows such as flames and plasmas. Recently, the development of ultrafast hybrid fs/ps CARS has enabled the exploitation of coherence beating between neighboring spectral lines to simultaneously measure rotational and vibrational temperatures from a single pure rotational spectrum. However, the influence of non-Boltzmann vibrational state distributions and limitations due to collisional dephasing at pressures greater than 1 atm remains unclear. In this work, we use spectral simulations to investigate the effects of non-Boltzmann vibrational state distributions and the applicability of coherence beating at pressures up to 10 atm. We show that short probe pulses can be leveraged to quantify non-Boltzmann vibrational state distributions of N2. Furthermore, we demonstrate that fs/ps CARS coherence beating can simultaneously provide sensitive measurements of rotational and vibrational temperatures of both O2 and N2 in air. A sensitivity analysis was conducted to qualitatively explain the accuracy and precision comparisons between probe delays.

Simultaneous single-shot rotation-vibration non-equilibrium thermometry using pure rotational fs/ps CARS coherence beating.

We report the development of a simple and sensitive two-beam hybrid femtosecond/picosecond pure rotational coherent anti-Stokes Raman scattering (fs/ps CARS) method to simultaneously measure the rotational and vibrational temperatures of diatomic molecules. Rotation-vibration non-equilibrium plays a key role in the chemistry and thermalization in low-temperature plasmas as well as thermal loading of hypersonic vehicles. This approach uses time-domain interferences between ground state and vibrationally excited N2 molecules to intentionally induce coherence beating that leads to apparent non-Boltzmann distributions in the pure rotational spectra. These distortions enable simultaneous inference of both the rotational and vibrational temperatures. Coherence beating effects were observed in single-shot fs/ps CARS measurements of a 75 Torr N2 DC glow discharge and were successfully modeled for rotational and vibrational temperature extraction. We show that this method can be more sensitive than a pure rotational fs/ps CARS approach using a spectrally narrow probe pulse. Lastly, we experimentally measured the beat frequencies via Fourier transform of the time-domain response and obtained excellent agreement with the model.

1-D imaging of rotation-vibration non-equilibrium from pure rotational ultrafast coherent anti-Stokes Raman scattering.

We present one-dimensional (1-D) imaging of rotation-vibration non-equilibrium measured by two-beam pure rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS). Simultaneous measurements of the spatial distribution of molecular rotation-vibration non-equilibrium are critical for understanding molecular energy transfer in low temperature plasmas and hypersonic flows. However, non-equilibrium CARS thermometry until now was limited to point measurements. The red shift of rotational energy levels by vibrational excitation was used to determine the rotational and vibrational temperatures from 1-D images of the pure rotational spectrum. Vibrational temperatures up to 5500 K were detected in a CH4/N2 nanosecond-pulsed pin-to-pin plasma within 2 mm near the cathode. This approach enables study of non-equilibrium systems with 40 µm spatial resolution.

Generation of narrowband pulses from chirped broadband pulse frequency mixing.

We extend an approach based upon sum-frequency generation of oppositely chirped pulses to narrow the bandwidths of broadband femtosecond pulses. We efficiently generate near-transform-limited pulses with durations of several picoseconds, while reducing the pulse bandwidth by a factor of 120, which is more than twice the reduction reported in previous literature. Such extreme bandwidth narrowing of a broadband pulse enhances the effects of dispersion nonlinearities. Precise chirp control enables us to characterize the efficacy of frequency mixing broadband pulses with nonlinear temporal chirps. We demonstrate the use of these narrowband pulses as probes in coherent anti-Stokes Raman spectroscopy.

Rotational coherence beating in molecular oxygen: Coupling between electronic spin and nuclear angular momenta.

Time-resolved pure-rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman spectroscopy (fs/ps RCARS) of oxygen (O2) is performed at pressures from ∼0.04 to 0.4 atm. As the RCARS spectra evolve with probe delay, they exhibit coherence beating between unresolved S-branch triplet transitions (ΔN = 2, ΔJ = 2). The time-domain fitting of the RCARS signal intensity enables the determination of these transition frequency separations, which are as low as 480 MHz (0.016 cm-1). Additionally, we study the underlying pressure-dependent dynamics and the signatures of the time-domain triplet signals compared to the simple decays associated with the O2 self-broadened linewidths. Pressure- and N-dependent O2 linewidths are compared to literature coefficients obtained from experiments and models that have not incorporated the triplet splitting. Our findings are incorporated into a time-domain model for rotational CARS thermometry of O2 and have significant impact for spectral evaluations at probe delays greater than 100 ps for temperature or species concentration determination. The time- and frequency-resolved experiments presented in this work provide insight into the spectroscopic complexities introduced by the electronic ground state of O2 for accurate evaluation of time-resolved coherent Raman spectra.

Pure-rotational H2 thermometry by ultrabroadband coherent anti-Stokes Raman spectroscopy.

Coherent anti-Stokes Raman spectroscopy (CARS) is a sensitive technique for probing highly luminous flames in combustion applications to determine temperatures and species concentrations. CARS thermometry has been demonstrated for the vibrational Q-branch and pure-rotational S-branch of several small molecules. Practical advantages of pure-rotational CARS, such as multi-species detection, reduction of coherent line mixing and collisional narrowing even at high pressures, and the potential for more precise thermometry, have motivated experimental and theoretical advances in S-branch CARS of nitrogen (N2), for example, which is a dominant species in air-fed combustion processes. Although hydrogen (H2) is of interest given its prevalence as a reactant and product in many gas-phase reactions, laser bandwidth limitations have precluded the extension of CARS thermometry to the H2 S-branch. We demonstrate H2 thermometry using hybrid femtosecond/picosecond pure-rotational CARS, in which a broadband pump/Stokes pulse enables simultaneous excitation of the set of H2 S-branch transitions populated at flame temperatures over the spectral region of 0-2200 cm-1. We present a pure-rotational H2 CARS spectral model for data fitting and compare extracted temperatures to those from simultaneously collected N2 spectra in two systems of study: a heated flow and a diffusion flame on a Wolfhard-Parker slot burner. From 300 to 650 K in the heated flow, the H2 and N2 CARS extracted temperatures are, on average, within 2% of the set temperature. For flame measurements, the fitted H2 and N2 temperatures are, on average, within 5% of each other from 300 to 1600 K. Our results confirm the viability of pure-rotational H2 CARS thermometry for probing combustion reactions.

Direct Coherent Raman Temperature Imaging and Wideband Chemical Detection in a Hydrocarbon Flat Flame.

A single-shot coherent Raman imaging technique has been developed for spatially correlated one-dimensional high-fidelity gas-phase thermometry and multiplex chemical detection in flames. The technique utilizes two-beam phase matching, operating a single ultrashort pump/Stokes excitation pulse (7 fs) and a narrowband picosecond probe pulse (70 ps), interrogating a Raman active window of ∼4200 cm(-1) with ∼0.3 cm(-1) spectral resolution. The measurement geometry is formed intersecting the two beams shaped as laser-sheets and the one-coordinate spatial information is retrieved with a linespread function of <40 µm. The advance provides the possibility for the multiplexed measurement of all combustion relevant major species simultaneously with gaseous temperature monitored over a several millimeter field of view. The current technique is optimized in a premixed hydrocarbon flat-flame. At the flame-front, it is shown that direct imaging renders the temperature profile within ∼1% inaccuracy, whereas typical point-wise raster scanning may have relative systematic deviations up to 15% due to spatial averaging effects.

Split-probe hybrid femtosecond/picosecond rotational CARS for time-domain measurement of S-branch Raman linewidths within a single laser shot.

We introduce a multiplex technique for the single-laser-shot determination of S-branch Raman linewidths with high accuracy and precision by implementing hybrid femtosecond (fs)/picosecond (ps) rotational coherent anti-Stokes Raman spectroscopy (CARS) with multiple spatially and temporally separated probe beams derived from a single laser pulse. The probe beams scatter from the rotational coherence driven by the fs pump and Stokes pulses at four different probe pulse delay times spanning 360 ps, thereby mapping collisional coherence dephasing in time for the populated rotational levels. The probe beams scatter at different folded BOXCARS angles, yielding spatially separated CARS signals which are collected simultaneously on the charge coupled device camera. The technique yields a single-shot standard deviation (1σ) of less than 3.5% in the determination of Raman linewidths and the average linewidth values obtained for N(2) are within 1% of those previously reported. The presented technique opens the possibility for correcting CARS spectra for time-varying collisional environments in operando.

Communication: two-dimensional gas-phase coherent anti-Stokes Raman spectroscopy (2D-CARS): simultaneous planar imaging and multiplex spectroscopy in a single laser shot.

Coherent anti-Stokes Raman spectroscopy (CARS) has been widely used as a powerful tool for chemical sensing, molecular dynamics measurements, and rovibrational spectroscopy since its development over 30 years ago, finding use in fields of study as diverse as combustion diagnostics, cell biology, plasma physics, and the standoff detection of explosives. The capability for acquiring resolved CARS spectra in multiple spatial dimensions within a single laser shot has been a long-standing goal for the study of dynamical processes, but has proven elusive because of both phase-matching and detection considerations. Here, by combining new phase matching and detection schemes with the high efficiency of femtosecond excitation of Raman coherences, we introduce a technique for single-shot two-dimensional (2D) spatial measurements of gas phase CARS spectra. We demonstrate a spectrometer enabling both 2D plane imaging and spectroscopy simultaneously, and present the instantaneous measurement of 15,000 spatially correlated rotational CARS spectra in N2 and air over a 2D field of 40 mm(2).

Hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman scattering temperature and concentration measurements using two different picosecond-duration probes.

A hybrid fs/ps pure-rotational CARS scheme is characterized in furnace-heated air at temperatures from 290 to 800 K. Impulsive femtosecond excitation is used to prepare a rotational Raman coherence that is probed with a ps-duration beam generated from an initially broadband fs pulse that is bandwidth limited using air-spaced Fabry-Perot etalons. CARS spectra are generated using 1.5- and 7.0-ps duration probe beams with corresponding coarse and narrow spectral widths. The spectra are fitted using a simple phenomenological model for both shot-averaged and single-shot measurements of temperature and oxygen mole fraction. Our single-shot temperature measurements exhibit high levels of precision and accuracy when the spectrally coarse 1.5-ps probe beam is used, demonstrating that high spectral resolution is not required for thermometry. An initial assessment of concentration measurements in air is also provided, with best results obtained using the higher resolution 7.0-ps probe. This systematic assessment of the hybrid CARS technique demonstrates its utility for practical application in low-temperature gas-phase systems.

Communication: Simplified two-beam rotational CARS signal generation demonstrated in 1D.

We explore a novel phase matching scheme for gas-phase rotational coherent anti-Stokes Raman spectroscopy (CARS). The scheme significantly simplifies the employment of the technique in general. Two laser beams, one broadband and one narrowband, are crossed at arbitrary angle and the generated rotational CARS signal, copropagating with the probe beam, is isolated using a polarization gating technique. The effect of phase-vector mismatch for various experimental implementations was measured experimentally and compared to calculations. The spatial resolution of the current technique is improved by more than an order of magnitude over standard gas-phase CARS experimental arrangements, providing an interaction length of less than 50 µm when desired. Both the pump and Stokes photons originate from the broadband pulse, and are therefore automatically overlapped temporally and spatially. Significantly improved signal levels are achieved because of both the ease of alignment and the higher pulse energy available to the pump and Stokes fields. We demonstrate the technique for single-laser-shot 1D rotational CARS signal generation over approximately a 1 cm field in a flame.

High-spatial-resolution one-dimensional rotational coherent anti-Stokes Raman spectroscopy imaging using counterpropagating beams.

A counterpropagating phase-matching geometry is employed for high-spatial-resolution one-dimensional (1D) imaging of temperature and O2-to-N2 concentration ratio using picosecond pure-rotational coherent anti-Stokes Raman spectroscopy (RCARS) over a large field (20 mm). A single-shot 1D RCARS image of more than 20 mm in length is thus acquired at 300 K in air. High-resolution 1D RCARS flame measurements are demonstrated using a custom-built burner and a premixed methane/air flame (Φ=0.6). This phase-matching scheme improves the spatial resolution by approximately 1 order of magnitude when compared to the standard small-angle BOXCARS phase-matching schemes typically employed in CARS measurements. Additionally, for a 20 mm 1D image, signal levels are increased by 10(2) because of the higher irradiance provided in the current scheme.

Quantitative one-dimensional imaging using picosecond dual-broadband pure-rotational coherent anti-Stokes Raman spectroscopy.

We employ picosecond dual-broadband pure-rotational coherent anti-Stokes Raman spectroscopy (CARS) in a one-dimensional (1D) imaging configuration. Temperature and O(2):N(2) concentration ratios are measured along a 1D line of up to 12 mm in length. The images consist of up to 330 individual rotational CARS (RCARS) spectra, corresponding to 330 spatially resolved volume elements in the probe volume. Signal levels are sufficient for the collection of single-laser-pulse images at temperatures of up to approximately 1200 K and shot-averaged images at flame temperatures, demonstrated at 2100 K. The precision of picosecond pure-rotational 1D imaging CARS is assessed by acquiring a series of 100 single-laser-pulse images in a heated flow of N(2) from 410 K-1200 K and evaluating a single volume element for temperature in each image. Accuracy is demonstrated by comparing temperatures from the evaluated averaged spectra to thermocouple readings in the heated flow. Deviations from the thermocouple of <30 K in the evaluated temperature were found at up to 1205 K. Accuracy and single-shot precision are compared to those reported for single-point nanosecond dual-broadband pure-RCARS and nanosecond 1D vibrational CARS.

Furan hydrogenation over Pt(111) and Pt(100) single-crystal surfaces and Pt nanoparticles from 1 to 7 nm: a kinetic and sum frequency generation vibrational spectroscopy study.

Sum frequency generation surface vibrational spectroscopy and kinetic measurements using gas chromatography have been used to systematically study the adsorption and hydrogenation of furan over Pt(111) and Pt(100) single-crystal surfaces and size-controlled 1.0-nm, 3.5-nm and 7.0-nm Pt nanoparticles at Torr pressures (10 Torr of furan, 100 Torr of H(2)) to form dihydrofuran, tetrahydrofuran, and the ring-cracking products butanol and propylene. As determined by SFG, the furan ring lies parallel to all Pt surfaces studied under hydrogenation conditions. Upright THF and the oxametallacycle intermediate are observed over the nanoparticle catalysts under reaction conditions. Butoxy increases in surface concentration over Pt(111) with increasing temperature in agreement with selectivity trends.

Suppression of Raman-resonant interferences in rotational coherent anti-Stokes Raman spectroscopy using time-delayed picosecond probe pulses.

We measure time-dependent pure-rotational coherent anti-Stokes Raman spectroscopy (CARS) spectra for room-temperature N(2), O(2), CO(2), C(2)H(4), and C(3)H(8), as well as in a C(3)H(8) diffusion flame, using picosecond lasers. Because Raman coherences for N(2) and O(2) persist significantly longer than those for the other species, delayed probing can significantly reduce unwanted resonant contributions to rotational coherent anti-Stokes Raman spectroscopy spectra, enabling temperature and relative O(2)/N(2) concentration determination in fuel-rich gas mixtures. Delayed probing also eliminates interference from smeared vibrational CARS. Probe delay affects both the temperature and relative O(2)/N(2) concentrations inferred from rotational spectra when using a standard frequency-domain analysis.

Picosecond time-resolved pure-rotational coherent anti-Stokes Raman spectroscopy for N(2) thermometry.

Time-resolved pure-rotational coherent anti-Stokes Raman spectroscopy using picosecond-duration laser pulses is investigated for gas thermometry. The use of picosecond laser pulses significantly reduces background caused by scattering of the probe beam, and delayed probing of the Raman coherence enables elimination of interference from nonresonant four-wave mixing processes. Temperatures inferred from rotational spectra are sensitive to the probe delay because of the rotational-level dependence of collisional dephasing of Raman coherences. The sensitivity decreases, however, with increasing temperature, and accurate temperature measurements in a flame are demonstrated using a standard frequency-domain analysis of the spectra.