A chirped-pulse Fourier transform mm-wave spectrometer has been tested in analytical chemistry applications of headspace analysis of volatile species. A solid-state mm-wave light source (260-290 GHz) provides 30-50 mW of power. This power is sufficient to achieve optimal excitation of individual transitions of molecules with dipole moments larger than about 0.1 D. The chirped-pulse spectrometer has near 100% measurement duty cycle using a high-speed digitizer (4 GS/s) with signal accumulation in an FPGA. The combination of the ability to perform optimal pulse excitation and near 100% measurement duty cycle gives a spectrometer that is fully optimized for trace detection. The performance of the instrument is tested using an EPA sample (EPA VOC Mix 6 – Supelco) that contains a set of molecules that are fast eluting on gas chromatographs and, as a result, present analysis challenges to mass spectrometry. The ability to directly analyze the VOC mixture is tested by acquiring the full bandwidth (260-290 GHz) spectrum in a “high dynamic range” measurement mode that minimizes spurious spectrometer responses. The high-resolution of molecular rotational spectroscopy makes it easy to analyze this mixture without the need for chemical separation. The sensitivity of the instrument for individual molecule detection, where a single transition is polarized by the excitation pulse, is also tested. Detection limits in water will be reported. In the case of chloromethane, the detection limit (0.1 microgram/L), matches the sensitivity reported in the EPA measurement protocol (EPA Method 524) for GC/MS.

I will describe a variety of cavity-enhanced spectroscopic techniques, including frequency-agile rapid scanning spectroscopy (FARS) and heterodyne-detected cavity ring-down spectroscopy (HD-CRDS), which we have recently developed for rapid, ultrasensitive absorption measurements. Scanning rates that are limited only by the cavity response time itself as well as noise-equivalent detection limits as low as 6×10⁻¹⁴ cm⁻¹ Hz^−1/2 have been achieved. I will discuss the application of these techniques to current problems in atmospheric science including recent infrared measurements of analytes which are present at ultra-trace concentrations.

High resolution rotational spectroscopy enables chemical sensors that are both sensitive and highly specific, which is well suited for analysis of expired human breath. We have previously reported on detection of breath ethanol, methanol, acetone, and acetaldehyde using THz sensors¹. This paper will outline our present efforts in this area, with specific focus on our ongoing quest to correlate levels of blood glucose with concentrations of a few breath chemicals known to be affected by elevated blood sugar levels. Prospects, challenges and future plans will be outlined and discussed.
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¹Fosnight, A.M., B.L. Moran, and I.R. Medvedev, Chemical analysis of exhaled human breath using a terahertz spectroscopic approach. Applied Physics Letters, 2013. 103(13): p. 133703-5.

Analytical chemical sensing based on high resolution rotational molecular spectra has been recognized as a viable technique for decades. We recently demonstrated a compact implementation of such a sensor ¹. Future generations of these sensors will rely on automated algorithms for quantification of chemical dilutions based on their spectral libraries, as well as identification of spectral features not present in spectral catalogs. Here we present an algorithm aimed at detection of unidentified lines in complex molecular species based on spectroscopic libraries developed in our previous projects. We will discuss the approaches suitable for data mining in feature-rich rotational molecular spectra.
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¹Neese, C.F., I.R. Medvedev, G.M. Plummer, A.J. Frank, C.D. Ball, and F.C. De Lucia, Ä Compact Submillimeter/Terahertz Gas Sensor with Efficient Gas Collection, Preconcentration, and ppt Sensitivity." Sensors Journal, IEEE, 2012. 12(8): p. 2565-2574

Bank of England notes of 20 GBP denomination have been studied using infrared spectroscopy in order to generate a method to identify forged notes. A principal aim of this work was to develop a method so that a small, compact ATR FTIR instrument could be used by bank workers, police departments or others such as shop assistants to identify forged notes in a non-lab setting. The ease of use of the instrument is the key to this method, as well as the relatively low cost. The presence of a peak at 1400 cm⁻¹from the blank paper section of a forged note proved to be a successful indicator of the note’s illegality for the notes that we studied. Moreover, differences between the spectra of forged and genuine 20 GBP notes were observed in the ν(OH) (ca. 3500 cm⁻¹), ν(C-H) (ca. 2900 cm⁻¹) and ν(C=O) (ca. 1750 cm⁻¹) regions of the IR spectrum recorded for the polymer film covering the holographic strip. In cases where these simple tests fail, we have shown how an infrared microscope can be used to further differentiate genuine and forged banknotes by producing infrared maps of selected areas of the note contrasting inks with background paper. Further to this, with an announcement by the Bank of England to produce polymer banknotes in the future, the work has been extended using Australian polymer banknotes to show that the method would be transferable.

Figure Vibrational sum frequency generation (SFG) spectroscopy was used to probe electrochemical processes taking place at the interface between metal anodes and the liquid phase in model lithium ion systems. Lithium ion batteries have been extensively studied and characterized by numerous techniques. However, the mechanisms behind many properties are still unclear due to the lack of techniques that can directly probe them in situ. The formation of the electrode passivating layer known as solid-electrolyte interphase (SEI) is one such example. During the first charging cycle of a battery, some of the electrolyte undergoes reduction at the electrode surface forming an electrically isolating barrier that prevents the subsequent reduction of more electrolyte molecules. The SFG selection rules suppress signals from molecules in centrosymmetric environments such as electrolyte layers, so SFG is a selective probe of interfacial environments such as the SEI. In this study, ethylene carbonate’s (EC) response to potential cycling was observed. EC is commonly used as a high permittivity solvent in batteries and is widely believed to be the main component of the SEI in its reduced form, lithium ethylene dicarbonyl. EC’s carbonyl stretch (1850 cm⁻¹) was measured in conjunction with cyclic voltammetry experiments. The SFG intensity showed remarkable agreement with the changing potential, as seen in the figure below. The shoulders on each side of the peaks in (a) are especially interesting, as they correspond to the potentials where lithium metal is oxidized and reduced. Vibrational modes found at 1300-1400 cm⁻¹, usually assigned to the reduced form of EC, are also being studied in order to provide more information on the nature of the SEI.

Figure Metal oxides are widely used in gas sensor applications due to their low cost, easy production and selectivity. Tungsten Oxide (WO₃) is one of the most used metal oxides in the detection of Nitrogen gases (NO_x). The purpose of this research is to determine if the Raman features of a metal oxide gas sensor can serve as tools to make estimates regarding the sensor capabilities related to the target gases. This research will be used for gas sensing of oxidizing/reducing toxic gases (i.e. H₂S, NO_x, SO₂, etc.) and finding the effect that temperature, gas concentration, type of gas, exposure time and other variables have on the Raman spectra of metal oxides. In this experiment, the temperature was increased from 30−160 ^°C and the Raman data was taken using a 780 nm infrared laser. In two of the samples, WO₃ on Silicon substrate and WO₃ nanopowder, we found vibrational modes at  807,  716 and  271 cm⁻¹, which are indicators of a monoclinic WO₃ structure. The WO₃ nanowires samples exhibit the O-W-O bond stretching feature is present and asymmetric stretching of the W-O bonds occurs, resulting in a 750 cm⁻¹ band. The intensity of Raman features such as 750 cm⁻¹ for nanowires and 492 and 670 cm⁻¹ for WO₃ on Silicon substrate begins to decay as temperature increases. Additionally, the vibrational modes related to O-H and W-OH become more pronounced as temperature increases due to those bonds reacting more strongly to the temperature change than the normal W-O bonds related to the original lattice structure. Finally, all samples have low-frequency phonon mode markers associated with temperature change, and in most cases these change as temperature increases. The understanding of the thermal effects will help develop theoretical models for the identification of specific metal oxide-gas relationships and provide a supplemental way of observing gas adsorption in addition to current conductivity measurements.

A methodology to probe localized vibrational transitions of self-assembled monolayers (SAMs) adsorbed on gold films using vibrational sum-frequency generation (SFG) is described. The gold film is subjected to heating from a 400nm pump laser, exposing the adsorbed molecules to a temperature jump in the 30-175 ^° K range, calibrated using ultrafast reflectance measurements of the gold compared to steady state oven heating . SAMs of alkyl thiols as well as nitro functionalized aryl thiols were deposited and temperature jumped while be observed with SFG, monitoring the symmetric and asymmetric methyl vibrations as well as nitro vibrations. The amplitude, center, and width of the transitions were measured and provide information about delay and orientation of the molecules, as well as providing an indicator of the overall monolayer state. All transitions probed exhibited overshoot decay plateau patterns, attributed to a fast hot electron process directly exciting the probed transitions, followed by a slower bulk heating process causing monolayer disordering. This leads to a shift in the average angle of the terminal methyl, manifesting itself as a change in the amplitude of the vibration. These techniques will be applied to thin films of energetic materials to study reactions to temperature jumps. HMX is known to have a peak in sensitivity as δ-HMX transitions to β-HMX at high temperatures, but fairly little information about the reason for this is known. This technique should be able to probe that process and provide data that can be used with computational models to gain some understanding of the process.

Laser ablation is a popular method for generating metal atoms so that metal clusters, complexes, and molecules may be investigated in gas phase spectroscopic studies. However, the initial production of a highly energetic metal plasma from the surface of a solid metal target can produce atoms which are not in their ground electronic state, and consequently atomic spectra can become quite complicated due to transitions arising from metastable atomic excited states which remain populated on the experimental timescale. Presented herein are details of the laser vaporisation source in use by our group. Spectra of atomic copper are presented, recorded via (1+1') and (2+1) resonance enhanced multiphoton ionisation (REMPI) spectroscopy. The energetic regions examined are expected to correspond to the (4s²4p) ²P ← ²S and the (4s²nd) ²D ← ²S Rydberg series respectively, but the observed spectra also exhibit many additional contributions which are found to arise from electronically excited states, and these will be discussed.

OH and HO₂ are key radical species that control the autoignition and flame chemistry of fuels. Quantification of these radicals in the low-temperature oxidation of fuels is challenging due to their low concentrations. Strong spectral interference from more abundant non-radical species can further complicate accurate quantification of OH and HO₂. Faraday Rotation Spectroscopy (FRS), a laser-based diagnostic that exploits magneto-optical properties of paramagnetic radical species, can overcome these technical challenges to provide sensitive and selective in situ quantification of radicals. Previously we have been able to illustrate the strengths of FRS in quantification of HO₂ radicals in the low-temperature oxidation of dimethyl ether^1,2,3. Recently we have constructed a dual-wavelength FRS system capable of simultaneous in situ measurement of OH and HO₂. A DFB diode laser operating at 2.8 μm is used to target the Q(1.5e) and Q(1.5f) transitions in the fundamental vibrational band of the ²Π_3/2 ground electronic state of OH. An EC-QCL operating at 7.1 μm is used to target a Q-branch spectral feature in the ν₂ vibrational band of HO₂. Concentrations of the target species are extracted from the measured spectra through fitting of an FRS spectral model. Based on preliminary retrievals, 3σ detection limits of < 1 ppmv for OH and HO₂ have been estimated from the non-linear least-squares fitting results. In this talk I will discuss the application of dual-wavelength FRS for sensitive measurement of OH and HO₂ radicals generated by oxidation of dimethyl ether in a flow reactor over a 520 K - 1050 K temperature range.
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¹B. Brumfield et al., J. Phys. Chem. Lett., 4, 872 (2013). ²B. Brumfield et al., "Dual Modulation Faraday Rotation Spectroscopy of HO₂ in a Flow Reactor’’ Accepted in Optics Letters (2014). ³N. Kurimoto et al., "Quantitative Measurements of HO₂ / H₂O₂ and Intermediate Species in Low and Intermediate Temperature Oxidation
[12pt] of Dimethyl Ether’’, Submitted to The 35th International Symposium on Combustion.

Chirped probe pulse (CPP) femtosecond (fs) coherent anti-Stokes Raman scattering (CARS) thermometry at 5 kHz has been successfully applied for single-laser-shot flame temperature measurements in a mildly turbulent hydrogen-air jet diffusion flame, sooting methane-air jet diffusion flame, and most recently a turbulent combustor of practical interest. Measurements were performed at various heights and radial locations within each flame and resulted in temperatures ranging from 300 K to 2400 K. In the turbulent combustor every laser shot produced some resonant CARS signal; no loss of signal due to beam steering, pressure fluctuations, or shear layer density gradients was noticeable. Furthermore, the measurement volume spatial resolution is better than has previously been reported for other CARS experiments. Flame temperature measurements compare well with those previously reported in similar flames. These results indicate high repetition rate CPP fs-CARS is an excellent technique for the study of turbulent combustion.

Plasmas are often used as ionization sources for ambient mass spectrometry (AMS). Here, the flowing afterglow of a novel high-energy spark discharge system, operated in nitrogen at high repetition rates, is investigated as a source for AMS. The spark discharge here is the same as that of an automobile ignition circuit.Combustion in automobile engines is initiated by a spark ignition system that is designed to deliver short-duration,high-voltage sparks to multiple engine cylinders. The arrangement utilized in this study is a modified discharge configuration designed to produce similarly short-duration, high-voltage discharges. It consists of an automotive ignition coil that is activated by a spark initiation circuit that discharges in turn into a cell with neutral gas input flow and ultimately into the collection orifice of a mass spectrometer. The discharge voltage is approximately 40kV at 800 Hz. High-frequency spark discharges in a nitrogen flow produce reagent ions such as NO+. In order to better evaluate the effectiveness of the discharge in producing reagent ions, an FTIR is utilized to measure IR active species such as nitric oxide, hydroxide, ozone, and water in the afterglow of the spark discharge during variation of discharge parameters. Time-resolved IR emission spectra provide additional insight into the reagent ion production mechanisms.

Thermoacoustic combustion instability results from the coupling between oscillating heat release and fluctuating pressure inside of a combustion chamber. In the current work, thermoacoustic instability in a low swirl burner is investigated for lean premixed conditions. Measurement of the heat release is a very important aspect of thermoacoustic instability, and in the current experiment the local heat release information is captured with a method based on Planar Laser Induced Fluorescence of the OH radical (OH-PLIF). This is then combined with the pressure signal to quantify the level of thermal-acoustic coupling. The specific goal is to examine the global and local flame response to velocity (5 – 10 m/s) and driving pressure amplitude (up to 1.12% of atmospheric pressure) changes. The root mean square of a non-dimensional Rayleigh index (R_RMS) was analyzed as the indicator of the global response of flame to acoustic perturbation with different amplitudes. The result shows that the coupling level increases with the forcing amplitude in the beginning. However, when the forcing level is high enough, the coupling saturates. Local response is also examined using a locally-weighted R_RMS, focusing on the contribution of the positive and negative coupling regions to the global response. At low velocities, the positive and negative structures play similar roles. However, as velocity is increased, the positive structures become more dominant.

An instrument has been developed to make 10 kHz in situ combustion gas measurements of carbon monoxide (CO) and carbon dioxide (CO₂) concentrations. Operating in both the 4.40 and 4.58 μm wavelength ranges allows for the fundamental molecular absorption bands of both molecules to be utilized.
Such concentration measurements allow for the determination of total combustion efficiency of a particular process, which has engineering implications when considering the energy available from a combustion process to be utilized for propulsion purposes.
A brief discussion of the initial calibration of the sensor with a calibrated diffusion flame, Hencken burner, and pressure-concentration cell is made with the main focus of the current work being the application of the instrument to examine the structure of propagating detonation waves.

Atmospheric-pressure, low-temperature plasmas exist in nature in the form of ball lightning, and last year a natural ball lightning event was finally observed with scientific equipment.¹ Production of ball lightning in the laboratory dates back to Tesla's work at Colorado Springs.² Today, Tesla's "fireballs" are easily produced in the laboratory by discharging kiloJoules of energy slightly above an electrolyte solution via a metal electrode.³ For the sake of clarity, those plasmas produced using this technique are referred to as "plasmoids." Valuable information is obtained from previous experiments, such as the identification of water clusters and the temperature of the interior of plasmoids. We perform mass spectrometry and Fourier-transform infrared emission spectroscopy in an effort to characterize these plasmoids. We present, to our knowledge, the first mass spectrometric data and infrared emission spectra of plasmoid discharges. Mass spectrometry reveals the presence of small protonated water clusters [H(H₂O)₂, H(H₂O)₃] and nitrogen-containing molecules [NO, NO-H₂O]. IR spectra exhibit signals observed in the water emission region (1300-2000 cm, 3000-4000 cm), and signals in several other regions of interest. Fundamental properties of these plasmoids including the electron energy distribution function, component densities, and collisional cross sections will be discussed.
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¹Cen, J.; Yuan, P.; Xue, S. Phys. Rev. Lett. 2014, 112, 035001 ²Tesla, N. Colorado Springs Notes 1899-1900; Marinci\'c, A., Ed.; Nolit: Beograd, Yugoslavia, 1978; pp 368-370 ³Friday, D.M.; Broughton, P.B.; Lee, T.A.; Schutz, G.A.; Betz, J.N.; Lindsay, C.M. J. Phys. Chem. A 2013, 117 (39), 9931-9940