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The application of terahertz (THz) absorption spectroscopy was developed for chemical characterization in inductively coupled plasmas. Plasma processing is a complex and important tool of the semiconductor manufacturing industry, which makes use of several diagnostic methods for precise process control. Electronically based THz spectroscopy is a technique with favorable attributes for the characterization of plasmas and process control in semiconductor reactors. These attributes include (1) plasmas are transparent and noise-free for THz transmission/detection, (2) concentration and temperatures of molecules can be calculated from first principles without adjustable variables, and (3) the technique has very high resolution and has absolute specificity. However, rotational spectroscopy requires that the molecule have a permanent dipole moment, precluding direct observation of atomic and symmetric species such as fluorine or [Formula: see text]. In this work, an electronically based 500–750 GHz absorption spectrometer and a method to accurately and simultaneously determine number densities and temperatures were developed. Density and temperature measurements of molecular species in Ar/CF[Formula: see text]/CHF[Formula: see text] and N[Formula: see text]/CF[Formula: see text]/CHF[Formula: see text] plasmas as a function of flow ratio, power, and pressure will be discussed. In addition, a quantitative survey of spectroscopically measurable molecules and radicals was conducted for plasma mixtures using varying quantities of CF[Formula: see text], CHF[Formula: see text], N[Formula: see text], and O[Formula: see text] feedstock gases.

Laser-induced plasmas and reactions therein depend on the ablated target's chemical composition and morphology. Recently, we have characterized plasma properties such as spatiotemporal morphology, electron density, species temperature within the plasma, ionization degree, etc. for pure Al under variations in morphology. Here, we utilize a bi-metal powder system to study similar characteristics as a function of powder chemistry. Micron-sized ball milled Al/Zr composites of three chemistries (3Al:2Zr, Al:Zr, and Al:3Zr) are compared to similarly-sized pure Al, pure Zr, and a mixture of Al + Zr powder. We find that the introduction of increasing concentrations of Zr has several interesting effects on the plasma compared to pure Al, including a dramatic increase in the electron density (to ~3–5 × 1020 cm−3), and the earlier onset of ZrO emission bands which have a higher disassociation energy and are thermodynamically preferred over AlO at high temperatures. Differences in plume morphology, the spatial distribution of molecular species, and the emission intensities of ZrO, Zr I and Al I were observed for Al + Zr and ball-milled Al:Zr samples, indicating the influence of microstructure on the chemical reactions. We measured plasma temperatures using the Saha-Boltzmann plot method; the composites demonstrate higher temperatures than pure Zr, Al + Zr mixtures, and pure Al. We observe only minor differences in temperature as a function of increasing Zr-content within the composites. This work continues to build on our understanding of the relationships between plasma properties and microsecond-timescale chemical reactions of reactive materials.

CMOS (Complementary Metal Oxide Silicon) integrated circuits (IC’s) technology is emerging as a means for realization of capable and affordable systems that operate at 300GHz and higher. Despite the fact that the unity maximum available gain frequency, fmax of NMOS transistors has peaked at ~320GHz somewhere between 65 and 32-nm technology nodes, signal generation up to 1.3THz and coherent detection up to 410GHz and incoherent detection up to ~10THz have been demonstrated using CMOS integrated circuits. Furthermore, a highly integrated rotational spectroscopy transceiver operating up to near 300GHz and imaging array operating at 820GHz have been demonstrated in CMOS. The nonlinear devices and circuit techniques that enable the operation at these frequencies beyond fmax will be described. Lastly, emerging applications, electronic nose/smelling using rotational spectroscopy that can detect and quantify concentrations of a wide variety of gases, imaging that can enable operation of autonomous systems in a wide range of weather conditions (rain, dust, snow, fog .. ), high-bandwidth communication over 1-2m long dielectric waveguides that can rival the bandwidth of optical communication systems, and electronic detectors that can make thermal imaging/night vision affordable will be discussed.

System technologies and sensor applications in the THz largely bifurcate between high-resolution ( 1 GHz) methods. There is an extensive high-resolution literature and many approaches to low-resolution spectroscopy have been proposed and demonstrated. In this paper, we discuss and apply an approach, cavity-based medium resolution spectroscopy (CBMRS) that is applicable continuously across this intermediate resolution gap. CBMRS systems use high brightness and spectrally pure continuous wave electronic sources to determine absolute absorption coefficients by the measurement of 100–1000 mode widths of a high-finesse Fabry–Perot cavity. This eliminates the challenge of distinguishing broad spectral features from a complicated baseline that results from variations in source power, transmission efficiency, and detector response. CBMRS has many fewer data points than observed with high-resolution systems interrogating near Doppler-limited spectra. Thus, CBMRS sacrifices the “absolute” specificity of high resolution Doppler-limited systems, but still can retain excellent specificity. Indeed, this smaller sampling does not set the specificity limit, but rather it is set by whether or not the analyte has a specific THz signature. Results from pressure broadened (including atmospheric pressure) applications and molecules with dense and/or quasi-continuous spectra will be presented. Comparisons with related observations in the literature are discussed.

Species stratification and local plasma composition can affect microsecond-timescale oxidation reaction rates of metals such as Al in an oxidizing atmosphere. Here, we utilize fast, gated emission spectroscopy and a high-speed framing camera to determine the intensity and spatiotemporal evolution of various Al ablation products within a laser-induced plasma. Using a high-purity Al plate, micron- and nano-sized Al powders, and inert micron Al2O3 powder, we studied the effect of Al morphology and reactivity on the oxidation characteristics and plasma hydrodynamics in air at 1 atm over two temporal regimes (2–10 μs and 20–100 μs). We observed an increase in the spatial distribution and intensity of emission from vaporized Al within the plasma for powder-based samples compared to plate Al due to enhanced material dispersion. In nano-Al, AlO emission forms at, and propagates along, the surface of the powder bed from 2–10 μs, whereas for micron powder, there is a delay in AlO formation within the bulk of the plasma until tens of microseconds. We measured electron densities from a variety of spectral lines, which can range from ~ 2 × 1015 to 2 × 1018 cm−3, and which scale inversely with the rate of plasma expansion across morphologies. The Al I and Al II species temperatures from 2 to 10 μs calculated via Boltzmann plots are similar (from ~ 10,000 to 14,000 K), and we performed a suite of local thermodynamic equilibrium (LTE) validity calculations to establish that these two species are in LTE, while H is not. Using image co-registration, we calculated the thickness of the AlO layer surrounding the expanding Al cloud at times > 20 μs, which can range from ~ 50 to 300 μm. These results allow us to begin to understand the complexities of laser ablated metal powder reactions at microsecond timescales.

Methods for facilitating the fast energy release of aluminum to enhance detonation performance will be discussed. The energy release rates of milligram-quantity samples have been compared by measuring the laser-induced shock wave velocities and tracking the formation of AlO on both the microsecond- and millisecond-timescales.

Thioformaldehyde is an abundant molecule in various regions of the interstellar medium. However, available laboratory data limit the accuracies of calculated transition frequencies in the submillimeter region, in particular for minor isotopic species. We aim to determine spectroscopic parameters of isotopologs of H2CS that are accurate enough for predictions well into the submillimeter region. We investigated the laboratory rotational spectra of numerous isotopic species in natural isotopic composition almost continuously between 110 and 377 GHz. Individual lines were studied for most species in two frequency regions between 566 and 930 GHz. Further data were obtained for the three most abundant species in the 1290-1390 GHz region. New or improved spectroscopic parameters were determined for seven isotopic species. Quantum-chemical calculations were carried out to evaluate the differences between ground state and equilibrium rotational parameters to derive semi-empirical equilibrium structural parameters. The spectroscopic parameters are accurate enough for predictions well above 1 THz with the exception of H2(13)C(34)S where the predictions should be reliable to around 700 GHz., Comment: Astron. Astrophys., in press; 12 pages

The impact of laser-driven flyer plates on energetic materials CL-20, PETN, and TATB has been investigated. Flyer plates composed of 25 μm thick Al were impacted into the energetic materials at velocities up to 1.3 km/s. The flyer plates were accelerated by means of an Nd:YAG laser pulse. The laser pulse generates rapidly expanding plasma between the flyer plate foil and the substrate to which it is adhered. As the plasma grows, a section of the metal foil is ejected at high speed, forming the flyer plate. The velocity of the flyer plate was determined using VISAR, time of flight, and high-speed video. The response of the energetic material to impact was determined by light emission recorded by an infrared-sensitive photodiode. Following post-impact analysis of the impacted energetic material, it was hypothesized that the light emitted by the material after impact is not due to the impact of the flyer itself but rather is caused by the decomposition of energetic material ejected (via the shock of flyer plate impact) into a cloud of hot products generated during the launch of the flyer plate. This hypothesis was confirmed through schlieren imaging of a flyer plate launch, clearly showing the ejection of hot gases and particles from the region surrounding the flyer plate launch and the burning of the ejected energetic material particles.

Quantum monodromy has a dramatic and defining impact on all those physical properties of chain-molecules that depend on a large-amplitude bending coordinate, including in particular the distribution of the ro-vibrational energy levels. As revealed by its pure rotational (a-type) spectrum [B. P. Winnewisser et al., Phys. Chem. Chem. Phys., 2010, 12, 8158-8189] cyanogen iso-thiocyanate, NCNCS, is a particularly illuminating exemplar of quantum monodromy: it clearly shows the distinctive monodromy-induced dislocation of the ro-vibrational energy level pattern for its low-lying bending mode. This dislocation centers on a lattice defect in the energy vs. momentum map of the ro-vibrational levels at the top of the barrier to linearity, and represents an example of an excited state quantum phase transition [D. Larese and F. Iachello, J. Mol. Struct., 2011, 1006, 611-628]. To complete the data, so far limited to ΔJ = +1 transitions, we decided to measure the high-resolution far-infrared band of the large-amplitude bending vibration ν7, and, if possible, mid-infrared bands. This Perspectives article presents our ongoing progress towards this goal, beginning with the description of how to predict line positions and intensities of the a- and b-type bands of the large amplitude bending mode using the Generalized-SemiRigid-Bender (GSRB) Hamiltonian for NCNCS and ab initio dipole moment functions [B. P. Winnewisser et al., Phys. Chem. Chem. Phys., 2010, 12, 8158-8189]. We include background information about synchrotron physics to clarify the advantages and limitations of that radiation source for our experiments. Details of the chemical preparation and sample handling, leading to the realization that NCNCS is 50 kJ mol(-1) lower in energy than its isomer S(CN)2 [Z. Kisiel et al., J. Phys. Chem. A, 2013, 117, 13815-13824] are included. We present the far-infrared and mid-infrared spectrum of NCNCS obtained at the Canadian Light Source synchrotron, using the IFS 125HR Bruker Fourier transform spectrometer. Eight of the fundamental vibrational modes of NCNCS have now been observed at high resolution. Initial analyses of the data confirm band assignments and demonstrate the accuracy of the predictions.

The high resolution Fourier transform spectrum of the chemically challenging sulfur dicyanide, S(CN)2, molecule was recorded at the far-infrared beamline of the synchrotron at the Canadian Light Source. The spectrum covered 50-350 cm(-1), and transitions in three fundamentals, ν4, ν7, and ν8, as well as in the hot-band sequence (n + 1)ν4 - nν4, n = 1-4, have been assigned and measured. Global analysis of over 21,300 pure rotation and rotation vibration transitions allowed determination of precise energies for 12 of the lowest vibrationally excited states of S(CN)2, including the five lowest fundamentals. These results constitute an extensive set of benchmarks for ab initio anharmonic force field calculations and the observed and calculated vibration-rotation constants and anharmonic frequencies are compared. The semiexperimental equilibrium, r(e)(SE), geometry of S(CN)2 has also been evaluated. In the course of the measurements, new information concerning the physical chemistry of S(CN)2 has been obtained.

A series of organic polymers and the military explosive cyclotrimethylenetrinitramine (RDX) were studied using the light emission from a femtosecond laser-induced plasma under an argon atmosphere. The relationship between the molecular structure and plasma emission was established by using the percentages of the atomic species (C, H, N, O) and bond types (C-C, C═C, C-N, and C≡N) in combination with the atomic/molecular emission intensities and decay rates. In contrast to previous studies of organic explosives in which C2 was primarily formed by recombination, for the organic materials in this study the percentage of C-C (and C═C) bonds was strongly correlated to the molecular C2 emission. Time-resolved emission spectra were collected to determine the lifetimes of the atomic and molecular species in the plasma. Observed differences in decay rates were attributed to the differences in both the molecular structure of the organic polymers or RDX and the chemical reactions that occur within the plasma. These differences could potentially be exploited to improve the discrimination of explosive residues on organic substrates with laser-induced breakdown spectroscopy.

Preliminary Atacama Large Millimeter/Submillimeter Array (ALMA) science verification data for a single pixel centered on the hot core of Orion KL (R. A. = 05 h 35 m 14.35 s, Dec = −05°22′35″ (J2000)) are available as this special issue on broadband spectroscopy is coming to press. As part of this verification process it is useful to compare simulations based on laboratory spectroscopy with ALMA results. This provides not only a test of instrumentation and analysis, but also a test of astrophysical assumptions such as local thermodynamic equilibrium (LTE) and the temperature variations within telescope beams. However, these tests are spectroscopically limited because it is well known that astrophysical spectra contain large numbers of unknown lines, many of which are presumably due to unanalyzed rotational spectra in excited vibrational states of a relatively few molecules. To address this issue we have previously discussed the use of broadband complete experimental spectra (CES) that is obtained from the analysis of several hundred intensity calibrated spectra taken over a range of temperatures. In this paper we will compare these CES with the similarly complete astrophysical spectra.

The laser induced breakdown spectroscopy (LIBS) technique has been used to analyze a diverse array of materials for several decades. LIBS is ideal for rapid materials analysis since data can be collected in real time with no sample preparation. The coupling of LIBS with multivariate analysis has increased in recent years and allows for rapid processing of spectral information for qualitative or quantitative analysis. We will discuss several examples of how LIBS and multivariate analysis has been used to classify geological and energetic materials at the United States Army Research Laboratory. It is important to understand the parameters that influence the results and the limitations of multivariate analysis for LIBS applications.

There is a broad consensus that many, if not most, of the unidentified spectral lines in astrophysical spectra are due to transitions in excited vibrational states of a relatively small number of molecules, the astrophysical weeds. For these unidentified lines, it is somewhat less well understood that the spectroscopic effort required to characterize them in the traditional quantum mechanical catalog approach is substantially larger because of significant perturbations. We have previously discussed a new experimental approach that addresses this challenge. This approach is based on the analysis of many complete, intensity-calibrated spectra taken over a range of temperatures. However, the spectroscopic completeness of this approach results in a much larger database. These data can be transfer to the astrophysical community in a variety of ways, but because an order of magnitude larger number of lines is included, consideration must be given to implementation strategies.

Using a series of thirteen organic materials that includes novel high-nitrogen energetic materials, conventional organic military explosives, and benign organic materials, we have demonstrated the importance of variable selection for maximizing residue discrimination with partial least squares discriminant analysis (PLS-DA). We built several PLS-DA models using different variable sets based on laser induced breakdown spectroscopy (LIBS) spectra of the organic residues on an aluminum substrate under an argon atmosphere. The model classification results for each sample are presented and the influence of the variables on these results is discussed. We found that using the whole spectra as the data input for the PLS-DA model gave the best results. However, variables due to the surrounding atmosphere and the substrate contribute to discrimination when the whole spectra are used, indicating this may not be the most robust model. Further iterative testing with additional validation data sets is necessary to determine the most robust model.

With the growth in sensitivity and angular resolution of millimeter and submillimeter telescopes, the number of unidentified molecular spectral lines in surveys of the interstellar medium has grown rapidly. While some of these unidentified lines are due to as yet unidentified astrophysical species, it is the general consensus that most are due to lines from a limited number of well-known interstellar species, the interstellar weeds. These unidentified lines do not appear in astrophysical line catalogs, which are based on quantum mechanical models and are incomplete primarily because of the difficulty of performing the usual bootstrap assignment and analysis process in their often highly perturbed low-lying vibrational states. To address this problem, we have proposed and demonstrated an alternative catalog approach that is based on the analysis of intensity-calibrated spectra taken over a range of temperatures in the laboratory. These analyses also make it possible to quantitatively address the astrophysical completeness of existing catalogs. In this Letter, we use extensive new experimental data in the 210-270 GHz window to address this question for eight molecules that are considered to be the leading candidates for astronomical weeds—methyl formate, methanol, dimethyl ether, acetaldehyde, sulfur dioxide, methyl cyanide, vinyl cyanide, and ethyl cyanide. Additionally, for each of the eight molecules, we use these results and knowledge of the molecular vibrational/torsional energy levels to predict completeness as a function of astronomical source temperature.

There is a growing consensus that traditional approaches to submillimeter spectroscopy will continue to be outpaced by the growth of molecular astrophysics and the analytical sciences. We report here a methodology that significantly reduces this challenge by providing three-dimensions of spectroscopic information (frequency, line strength, and lower-state energy), even for complex, perturbed spectra, without the need for spectral analysis. The method is based on the analysis of complete, intensity-calibrated spectra taken over a range of temperatures. The 3D spectral information significantly reduces the assignment space and leads to much more efficient and accurate spectral assignment and quantum-mechanical modeling.

The analysis of the rotational spectrum of HNO 3 has been extended to include the υ 8 = υ 9 = 1 state at 1205.7 cm −1 and the υ 6 = υ 7 = 1 state at 1223.4 cm −1 . Based on 78–519 GHz data, the assignments in the 8 1 9 1 vibrational state have been significantly expanded from the previously reported microwave measurements [T.M. Goyette, F.C. De Lucia, J. Mol. Spectrosc. 139 (1990) 241–243]. A new microwave analysis is also reported for the 6 1 7 1 vibrational state. A simultaneous analysis takes into account the localized ΔK a = ±2 Fermi resonances between the vibrational states, describes the torsional splitting of 3.3 and 1.4 MHz for the 8 1 9 1 and 6 1 7 1 states respectively, and fits to experimental accuracy over 1500 rotational transition frequencies that extend up to J = 59. Infrared energy levels [A. Perrin, J.-M. Flaud, F. Keller, A. Goldman, R. D. Blatherwick, F. J. Murcray, C. P. Rinsland, J. Mol. Spectrosc. 194 (1999) 113-123] were also included in the analysis and fit to experimental accuracy. Measurement of strongly perturbed transitions in each vibrational state provide a determination of the band origin difference of 17.733184(17) cm −1 . The rotational constants agree well with those predicted by vibrational–rotational constants of the fundamental modes. Furthermore, the analysis will provide a very accurate simulation of the infrared spectrum of HNO 3 in the 8.3 μm region.

A series of nitrogen-rich molecules, such as 5-aminotetrazolium nitrate (HAT-NO3) and hydrazinebistetrazole (HBT), bis(2,2,2-trinitroethyl)-hydrazodicarboxylate (BTHC), cyclotrimethylene trinitramine (RDX), trinitrotoluene (TNT), melamine, sucrose, and L-glutamine were studied using laser-induced breakdown spectroscopy (LIBS). The atomic emission intensities and intensity ratios of the constituent elements from the LIBS spectrum were shown to correlate with the mole fractions and stoichiometries of the molecules. In addition, the amount of oxygen present in the molecule influenced the emission intensities of molecular fragments such as C2. Finally, we used principal components analysis to analyze the data from the LIBS spectra and separate the different organic molecules based on the atomic emission intensities and ratios.

The submillimeter spectral region has undergone an explosive growth in the last decade, not only in its impact on the larger scientific and technological world, but also in the high resolution laboratory spectroscopy that makes much of this success possible. Major astronomical and atmospheric instruments are the most visible evidence, but the impact is much broader. This breadth lays the foundation for the impact of submillimeter spectroscopy to extend beyond the scientific community. In this paper we consider the scientific and technical foundations of submillimeter spectroscopy and show how the underlying physics has made possible current successes and shows the path forward to even greater opportunities.

There is a general consensus that many of the unidentified features in astrophysical spectra are due to low lying excited vibrational and torsional states of a few molecules—commonly referred to as the astrophysical weeds. This is a challenging spectroscopic problem not only because there are many such states, but also because these states are often highly perturbed and difficult to analyze. We have previously described an alternative approach based on experimental, intensity-calibrated spectra taken at many temperatures. In this paper, we describe the procedures and results obtained with this approach for ethyl cyanide, strategies for archiving and disseminating these results, and the prospects for using these results to reduce the confusion limit in the powerful new observatories that are coming online.

A large suite of natural carbonate, fluorite and silicate geological materials was studied using laser-induced breakdown spectroscopy (LIBS). Both single- and double-pulse LIBS spectra were acquired using close-contact benchtop and standoff (25 m) LIBS systems. Principal components analysis (PCA) and partial least squares discriminant analysis (PLS-DA) were used to identify the distinguishing characteristics of the geological samples and to classify the materials. Excellent discrimination was achieved with all sample types using PLS-DA and several techniques for improving sample classification were identified. The laboratory double-pulse LIBS system did not provide any advantage for sample classification over the single-pulse LIBS system, except in the case of the soil samples. The standoff LIBS system provided comparable results to the laboratory systems. This work also demonstrates how PCA can be used to identify spectral differences between similar sample types based on minor impurities.

In this review we discuss the application of laser-induced breakdown spectroscopy (LIBS) to the problem of detection of residues of explosives. Research in this area presented in open literature is reviewed. Both laboratory and field-tested standoff LIBS instruments have been used to detect explosive materials. Recent advances in instrumentation and data analysis techniques are discussed, including the use of double-pulse LIBS to reduce air entrainment in the analytical plasma and the application of advanced chemometric techniques such as partial least-squares discriminant analysis to discriminate between residues of explosives and non-explosives on various surfaces. A number of challenges associated with detection of explosives residues using LIBS have been identified, along with their possible solutions. Several groups have investigated methods for improving the sensitivity and selectivity of LIBS for detection of explosives, including the use of femtosecond-pulse lasers, supplemental enhancement of the laser-induced plasma emission, and complementary orthogonal techniques. Despite the associated challenges, researchers have demonstrated the tremendous potential of LIBS for real-time detection of explosives residues at standoff distances.

R.A.H.Butler et al., J. Mol. Spectrosc. , bf 243R.A.H.Butler et al., J. Mol. Spectrosc. , bf 213

Since methyl formate (HCOOCH3) is found to have a high abundance in hot molecular cores and other types of clouds in the galactic center, it is reasonable to search among such sources for detectable abundances of the more complex analog ethyl formate (HCOOC2H5). Following a previous study of the millimeter-wave spectrum of ethyl formate, we have extended the analysis of the vibrational ground state of the trans and gauche conformers of ethyl formate into the submillimeter-wave range. Over 2200 new spectral lines have been measured and analyzed at frequencies up to 380 GHz. Fitting the data for each conformer to a Watson A-reduced asymmetric-top Hamiltonian has allowed us to predict the frequencies and intensities of many more transitions through 380 GHz.

The remote sensing of gases in complex mixtures at atmospheric pressure is a challenging problem and much attention has been paid to it. The most fundamental difference between this application and highly successful astrophysical and upper atmospheric remote sensing is the line width associated with atmospheric pressure broadening, ~ 5 GHz in all spectral regions. In this paper, we discuss quantitatively a new approach that would use a short pulse infrared laser to modulate the submillimeter/terahertz (SMM/THz) spectral absorptions on the time scale of atmospheric relaxation. We show that such a scheme has three important attributes: (1) The time resolved pump makes it possible and efficient to separate signal from atmospheric and system clutter, thereby gaining as much as a factor of 10^6 in sensitivity, (2) The 3-D information matrix (infrared pump laser frequency, SMM/THz probe frequency, and time resolved SMM/THz relaxation) can provide orders of magnitude greater specificity than a sensor that uses only one of these three dimensions, and (3) The congested and relatively weak spectra associated with large molecules can actually be an asset because the usually deleterious effect of their overlapping spectra can be used to increase signal strength., 8 pages, 4 figures at end of manuscript, accepted by IEEE Journal of Quantum Electronics

Several satellite series of rotational spectra have been observed in addition to the spectral lines arising from the ground torsional state for the normal and 13C1 species of methyl formate in the 110–380 GHz frequency region. They have been assigned to the first and second excited torsional state for both species. Combined least-squares analyses of selected lines from the A and E substates in each excited torsional state have been undertaken independently from the ground state with the effective rotational Hamiltonian procedure for the internal rotation analysis. Intensity ratios for the same rotational transitions in different torsional states have been compared to estimate relative energies of the excited torsional states and to confirm their assignments.

The rotational spectrum of acetone in its second torsional excited state (ν17, 125 cm−1 above the ground state) has been identified and assigned. The interaction between overall rotation and the internal rotation motions causes rotational transitions to split into four components, one for each torsional substate. The splittings in this torsional excited state are significantly larger than any splittings assigned yet for molecules with two methyl internal rotors, making assignments rather difficult. Over 700 frequencies between 72 and 372 GHz in the sub-millimeter wave spectrum have been assigned to bR-transitions in all four torsional substates. Eventually, 32 parameters of an effective rotational Hamiltonian for a molecule with two periodic internal motions were fit to observed frequencies of 612 transitions from all torsional substates with a standard deviation of 0.485 MHz.

Pure rotational transitions have been measured for the normal isotopologue of thioformaldehyde, -->H212C32S , in the ground vibrational state in the 110-370 GHz, 570-670 GHz, and 850-930 GHz frequency ranges. A global data set has been constructed consisting of prior microwave and millimeter-wave transitions, the submillimeter-wave and terahertz (THz) transitions reported here, and far-infrared data and ground state combination differences. The 783 different transition frequencies in the data set were fit to Watson's S-reduced Hamiltonian with 20 parameters, resulting in a dimensionless weighted root-mean-square deviation of 0.78. New frequency predictions have been made for astronomical observations based on the molecular parameters obtained in the present study.

A large number of rotational transitions of 13C1-methyl formate in its ground A and E torsional substates were measured in the 110-380 GHz region with the Fast Scan Submillimeter Spectroscopic Technique (FASSST). Besides strong a-type transitions, many b-type transitions through rotational quantum numbers -->J'' = 59 and -->Ka'' = 16 were newly measured, and some forbidden c- and x-type transitions in the E substate were also assigned. The new lines were analyzed together with previous submillimeter-wave data using an effective rotational-torsional Hamiltonian method designated ERHAM developed for a molecule with one or two internal rotors. In total, over 4900 transitions of 13C1-methyl formate were analyzed. Systematic deviations due to perturbations were found in Q-branch transitions at high J, and higher order Coriolis-type interaction terms were included to fit the data better. A standard deviation of 86 kHz was obtained by varying 33 parameters. In addition to 13C1-methyl formate, the normal isotopologue was studied anew and some high- -->Ka transitions were newly assigned. The previous data of the normal species in the ground state were reanalyzed with the ERHAM method together with new transitions measured in the present study. The same deviations in Q-branch transitions, not observed in previous studies, were also found in the normal species. Using spectroscopic parameters obtained in this study, predictions have been made for many strong lines of 13C1-methyl formate through 620 GHz in frequency not analyzed in the fit.

The ability to detect trace amounts of explosives and their residues in real time is of vital interest to Homeland Security and the military. Previous work at the US Army Research Laboratory (ARL) demonstrated the potential of laser-induced breakdown spectroscopy (LIBS) for the detection of energetic materials. Our recent efforts have been focused on improving the sensitivity and selectivity of LIBS for residue explosives detection and on extending this work to the standoff detection of explosive residues. One difficulty with detecting energetic materials is that the contribution to the oxygen and nitrogen signals from air can impede the identification of the explosive material. Techniques for reducing the air entrainment into the plasma—such as using an argon buffer gas or a collinear double-pulse configuration—have been investigated for this application. In addition to the laboratory studies, ARL’s new double-pulse standoff system (ST-LIBS) has recently been used to detect explosive residues at 20 m. The efficacy of chemometric techniques such as linear correlation, principal components analysis (PCA), and partial least squares discriminant analysis (PLS-DA) for the identification of explosive residues is also discussed. We have shown that despite the typical characterization of LIBS as an elemental technique, the relative elemental intensities in the LIBS spectra are representative of the stoichiometry of the parent molecules and can be used to discriminate materials containing the same elements. Simultaneous biohazard and explosive residue discrimination at standoff distances has also been demonstrated.

Detecting trace explosive residues at standoff distances in real-time is a difficult problem. One method ideally suited for real-time standoff detection is laser-induced breakdown spectroscopy (LIBS). However, atmospheric oxygen and nitrogen contributes to the LIBS signal from the oxygen- and nitrogen-containing explosive compounds, complicating the discrimination of explosives from other organic materials. While bathing the sample in an inert gas will remove atmospheric oxygen and nitrogen interference, it cannot practically be applied for standoff LIBS. Alternatively, we have investigated the potential of double pulse LIBS to improve the discrimination of explosives by diminishing the contribution of atmospheric oxygen and nitrogen to the LIBS signal. These initial studies compare the close-contact (< 1 m) LIBS spectra of explosives using single pulse LIBS in argon with double pulse LIBS in atmosphere. We have demonstrated improved discrimination of an explosive and an organic interferent using double pulse LIBS to reduce the air entrained in the analytical plasma.

Laser-induced breakdown spectroscopy (LIBS) provides an alternative chemical analytical technique that obviates the issues of sample preparation and sample destruction common to most laboratory-based analytical methods. This contribution explores the capability of LIBS analysis to identify carbonate and silicate minerals rapidly and accurately. Fifty-two mineral samples (18 carbonates, 9 pyroxenes and pyroxenoids, 6 amphiboles, 8 phyllosilicates, and 11 feldspars) were analyzed by LIBS. Two composite broadband spectra (averages of 10 shots each) were calculated for each sample to produce two databases each containing the composite LIBS spectra for the same 52 mineral samples. By using correlation coefficients resulting from the regression of the intensities of pairs of LIBS spectra, all 52 minerals were correctly identified in the database. If the LIBS spectra of each sample were compared to a database containing the other 51 minerals, 65% were identified as a mineral of similar composition from the same mineral family. The remaining minerals were misidentified for two reasons: 1) the mineral had high concentrations of an element not present in the database; and 2) the mineral was identified as a mineral with similar elemental composition from a different family. For instance, the Ca–Mg carbonate dolomite was misidentified as the Ca–Mg silicate diopside. This pilot study suggests that LIBS has promise in mineral identification and in situ analysis of minerals that record geological processes. © 2007 Elsevier B.V. All rights reserved.

We have developed a double-pulse standoff laser-induced breakdown spectroscopy (ST-LIBS) system capable of detecting a variety of hazardous materials at tens of meters. The use of a double-pulse laser improves the sensitivity and selectivity of ST-LIBS, especially for the detection of energetic materials. In addition to various metallic and plastic materials, the system has been used to detect bulk explosives RDX and Composition-B, explosive residues, biological species such as the anthrax surrogate Bacillus subtilis, and chemical warfare simulants at 20 m. We have also demonstrated the discrimination of explosive residues from various interferents on an aluminum substrate.

Results are reported of a comprehensive investigation of an almost continuous rotational spectrum of S(CN)2 recorded over the frequency region 110–374 GHz by means of the FASSST spectroscopic technique. The spectrum was analysed in detail and over 22 000 transitions were assigned in total. Precise, octic order spectroscopic constants in the asymmetric rotor Hamiltonian have been determined for the ground state and 12 different vibrationally excited states of the parent isotopologue, including first excited states of five different normal modes. Three states near 370 cm−1 and four states near 490 cm−1 above the ground state were found to be mutually interacting and were successfully analysed in terms of a triad and a tetrad of coupled states, respectively. Rotational transitions in the 34S, 13C, and 15N isotopologues of S(CN)2 have also been assigned and fitted, and newly determined rotational constants were used to derive the r m ( 1 ) geometry of the molecule. The complex multistate analysis of the spectrum was carried out with the newly developed AABS software package for Assignment and Analysis of Broadband Spectra.