Steven W. Sharpe

Kilohertz linewidth from frequency-stabilized mid-infrared quantum cascade lasers

Frequency stabilization of mid-IR quantum cascade (QC) lasers to the kilohertz level has been accomplished by use of electronic servo techniques. With this active feedback, an 8.5-microm QC distributed-feedback laser is locked to the side of a rovibrational resonance of nitrous oxide (N(2) O) at 1176.61cm (-1) . A stabilized frequency-noise spectral density of 42Hz/ radicalHz has been measured at 100 kHz; the calculated laser linewidth is 12 kHz.

Anhydrous nitric acid integrated absorption cross sections: 820–5300 cm−1

Abstract Fourier transform infrared absorbance measurements of small aliquots of anhydrous nitric acid were used to determine regional, integrated cross sections at 278.2, 298.22 and 323.15 K . Spectra were recorded with pressure broadened samples (1 atmosphere nitrogen), in a 20 cm path length cell at a spectral resolution of 0.112 cm −1 . Spectral regions measured include the vibrational bands: ν 1 (∼3552 cm −1 ) , ν 2 (∼1710 cm −1 ) , ν3, ν 4 (∼1320 cm −1 ) , and ν5, 2ν 9 (∼890 cm −1 ) and regions of weaker absorption between 820 and 5300 cm −1 . We observe expected changes in the distribution of rovibrational intensities with temperature, but to the accuracy of our measurements, cross-sections integrated over entire vibrational bands are independent of temperature.

Creation of 0.10-cm-1 resolution quantitative infrared spectral libraries for gas samples

The National Institute of Standards and Technology (NIST) and the Pacific Northwest National Laboratory (PNNL) are independently creating quantitative, approximately 0.10 cm-1 resolution, infrared spectral libraries of vapor phase compounds. The NIST library will consist of approximately 100 vapor phase spectra of volatile hazardous air pollutants (HAPs) and suspected greenhouse gases. The PNNL library will consist of approximately 400 vapor phase spectra associated with DOEs remediation mission. A critical part of creating and validating any quantitative work involves independent verification based on inter-laboratory comparisons. The two laboratories use significantly different sample preparation and handling techniques. NIST uses gravimetric dilution and a continuous flowing sample while PNNL uses partial pressure dilution and a static sample. Agreement is generally found to be within the statistical uncertainties of the Beers law fit and less than 3 percent of the total integrated band areas for the 4 chemicals used in this comparison. There does appear to be a small systematic difference between the PNNL and NIST data, however. Possible sources of the systematic difference will be discussed as well as technical details concerning the sample preparation and the procedures for overcoming instrumental artifacts.

Multispectrum analysis of self- and N2-broadening, shifting and line mixing coefficients in the ν6 band of 12CH3D

Abstract A multispectrum nonlinear least-squares fitting technique has been applied to determine accurate zero-pressure line center positions, Lorentz self- and N2-broadening coefficients and self- and N2-induced pressure shift coefficients of transitions in the ν6 (E) perpendicular band of 12CH3D between 1035 and 1270 cm −1 . Seventeen high-resolution (0.002– 0.006 cm −1 ) room temperature absorption spectra recorded with two Fourier transform spectrometers (FTS) were analyzed together. Self-broadening coefficients for over 700 transitions and self-shift coefficients for more than 600 transitions were determined. Measurements of nitrogen-broadening coefficients for more than 480 transitions and nitrogen pressure induced shift coefficients for nearly 430 transitions were also made. The measurements include transitions with rotational quantum numbers as high as J″=18 and K″=15 and several forbidden transitions. The measurements were made in all six sub-bands ( P P , P Q , P R , R P , R Q and RR). The measured self-broadening coefficients vary from 0.025– 0.096 cm −1 atm −1 at 296 K and the N2-broadening coefficients from 0.02 to 0.08 cm −1 atm −1 at 296 K . Self-induced-shift coefficients range from −0.014 to +0.004 cm −1 atm −1 while the nitrogen-induced-shift coefficients vary from −0.013 to +0.008 cm −1 atm −1 . Very few of the measured pressure-shift coefficients were positive, and the positive shift coefficients were mostly associated with the J″=K″ transitions in the PQ sub-bands. The off-diagonal relaxation matrix element coefficients of a number of transitions with K″=3 doublets for both self- and N2-broadening were also obtained. The results obtained with the two different broadening gases are compared and discussed. Comparisons with previous values from the literature are also made.

A multispectrum analysis of the ν1 band of H12C14N: Part I. Intensities, self-broadening and self-shift coefficients

Abstract The infrared spectrum of HCN in the region between 3150 and 3450 cm −1 has been recorded at 0.005 and 0.008 cm −1 resolution using two different Fourier transform spectrometers, the McMath–Pierce Fourier transform spectrometer located at the National Solar Observatory (on Kitt Peak) and the Bruker-120HR Fourier transform spectrometer situated at the Pacific Northwest National Laboratory at Richland, Washington. Room temperature measurements were made of positions, absolute intensities, self-broadening and self-shift coefficients for individual lines belonging to the HCN ν1 band centered near 3311 cm −1 . These are to our knowledge the first extensive set of self-broadening and self shift measurements in the band. In addition, intensities, self-broadening and self-shift coefficients for several lines of the ν1+ν21−ν21 hot band and several intensities for lines in the H13C14N and H12C15N isotopomers were also determined. A multispectrum nonlinear least-squares fitting algorithm was used to fit the entire spectral region covering the 3200– 3400 cm −1 region of up to 27 spectra simultaneously. The measured line intensities in the ν1 band were further analyzed to derive the vibrational band intensity and the Herman–Wallis coefficients. Differences in line intensities between 5 and 10% are found with respect to present measurements and the values given in the HITRAN database for m values beyond 25 in the P branch and 5 in the R branch.

Improved multipass optics for diode laser spectroscopy

Feedback between optical elements can be a major source of noise when trying to attain high sensitivity in infrared absorption experiments. We find that a conventional White‐cell optical arrangement introduces etaloning fringes that modulate the peak‐to‐peak amplitude of our signals by 1 part in 16 666, a fractional change of 6×10−5. Although relatively small, this ‘‘noise’’ is systematic and adds coherently with averaging, obscuring interesting absorption features. An easily constructed multipass optical system suited for performing high‐resolution infrared spectroscopy in molecular beams is described. The design is based on a variation of the White cell and has been optimized for use with lead salt diode lasers. One of the key components in the improved design is the addition of an oscillating mirror for spoiling optical feedback generated by laser scatter and/or poor mode coupling of the laser to the multipass optics.

The PNNL quantitative infrared database for gas-phase sensing: a spectral library for environmental, hazmat, and public safety standoff detection

Pacific Northwest National Laboratory (PNNL) continues to expand its library of quantitative infrared reference spectra for remote sensing. The gas-phase data are recorded at 0.1 cm-1 resolution, with nitrogen pressure broadening to one atmosphere to emulate spectra recorded in the field. It is planned that the PNNL library will consist of approximately 500 vapor-phase spectra associated with the U.S. Department of Energy’s environmental, energy, and public safety missions. At present, the database is comprised of approximately 300 infrared spectra, many of which represent highly reactive or toxic species. For the 298 K data, each reported spectrum is in fact a composite spectrum generated by a Beer’s law plot (at each wavelength) to typically 12 measured spectra. Recent additions to the database include the vapors of several semi-volatile and non-volatile liquids using an improved dissemination technique for vaporizing the liquid into the nitrogen carrier gas. Experimental and analytical methods are used to remove several known and new artifacts associated with FTIR gas-phase spectroscopy. Details concerning sample preparation and composite spectrum generation are discussed.

Disseminator for rapid, selectable, and quantitative delivery of low and semivolatile liquid species to the vapor phase

Nelson and Griggs [Rev. Sci. Instrum. 39, 927 (1968)] introduced a quantitative method for disseminating liquid samples to the vapor phase using a lead screw to depress the plunger of a syringe whose body was heated and whose ambient tip was placed into the flow of a carrier gas. In order to measure quantitative vapor-phase infrared spectra, we have modified a commercial device to improve the accuracy and precision for quantitative vapor delivery. Design changes have focused on disseminating reactive or low volatility liquids by heating only the syringe tip and dispensed liquid. Performance features include quantitative vapor-phase generation with greater than three orders of magnitude concentration range, including low volatility species, with most equilibration times <40s. The method has been vetted by comparing the derived gas-phase infrared data versus IR spectra taken using both gravimetric (National Institute of Standards Technology) and passive vapor generation (Pacific Northwest National Laborator...

Quantitative infrared spectra of vapor phase chemical agents

Quantitative, high resolution (0.1 cm-1) infrared spectra have been acquired for a number of pressure broadened (101.3 KPa N2), vapor phase chemicals including: Sarin (GB), Soman (GD), Tabun (GA), Cyclosarin (GF), VX, nitrogen mustard (HN3), sulfur mustard (HD) and Lewisite (L). The spectra are acquired using a heated, flow-through White cell of 5.6 m optical path length. Each reported spectrum represents a statistical fit to Beers law, which allows for a rigorous calculation of uncertainty in the absorption coefficients. As part of an ongoing collaboration with the National Institute of Standards and Technology (NIST), cross-laboratory validation is a critical aspect of this work. In order to identify possible errors in the Dugway flow-through system, quantitative spectra of isopropyl alcohol from both NIST and Pacific Northwest National Laboratory (PNNL) are compared to similar data taken at the Dugway Proving Ground (DPG).

Temperature-dependent absorption cross-sections in the thermal infrared bands of SF5CF3

Abstract Absorption cross-sections have been measured at five temperatures between 213 and 323 K in the infrared bands of SF5CF3. The spectra were recorded at a resolution of 0.112 cm −1 using a commercial Fourier transform infrared spectrometer and a 20 cm temperature-controlled sample cell. Samples of SF5CF3 were pressurized with high-purity nitrogen to a total pressure of 1013.3 hPa (760 Torr ) . Six or more spectra with varying SF5CF3 column amounts were analyzed at each temperature. The full spectral range of the measurements was 520– 6500 cm −1 , with only weak bands observed beyond 1400 cm −1 . Absorption of thermal radiation in the 8– 12 μm atmospheric window region being important for climate change, we report here the integrated cross-sections of the significant absorption bands in that spectral region. Our results closely match room temperature values reported previously. Only small variation of the integrated absorption cross-sections with temperature was found. Our results confirm the accuracy of the previous measurements, which find SF5CF3 important for global climate change on a per molecule basis. Absorption cross-sections derived from a single, near Doppler-limited spectrum recorded at room temperature do not show any rotational fine structure in the 700– 950 cm −1 region.