Arlan W. Mantz

Tunable diode laser spectroscopy in the infrared: some practical considerations of techniques and calibration with ν 2 lines of HCN

A tunable diode laser has been incorporated as a source in an operating high resolution vacuum grating spectrometer. The advantages of such a system for recording molecular spectra have been elaborated. In the process of developing this system, several practical problems came about, and it has been considered useful to document the manner in which they have been resolved. In particular, for work in the region of 15 microm, it was advantageous to develop an air-spaced Fabry-Perot etalon to record fringes simultaneously with the scanning of molecular spectra. This was successfully done, and the upsilon(2) band lines of the HCN molecule at 14 microm have proved to be ideally suited for determining the fringe spacing spectroscopically.

Determination of self- and H2-broadening and shift coefficients in the 2-0 band of 12C16O using a multispectrum fitting procedure

Self- and hydrogen-broadening coefficients and pressure-shift coefficients for the first overtone band transitions of 12C16O at room temperature have been determined through analysis of nine high-resolution (0.0055cm−1) absorption spectra. These spectra were recorded using the 1-m Fourier transform spectrometer (FTS) at the McMath-Pierce facility of the National Solar Observatory on Kitt Peak, Arizona. Because of the short path length of the sample cell (10.0cm), the volume mixing ratios of CO in hydrogen were relatively high, ∼18–22%, to achieve measurable absorption. These large volume-mixing ratios necessitated the simultaneous determination of the CO self-broadening and self-shift coefficients along with the hydrogen-broadening and hydrogen-induced shift coefficients. We have determined these coefficients at room temperature, along with line positions and absolute intensities, for the P(27) through R(27) 12C16O 2-0 transitions by fitting the entire spectral interval from 4130 to 4345cm−1 in all nine spectra simultaneously using our multispectrum nonlinear least-squares procedure. Our intensity measurements are consistently low (1–6%) compared with the HITRAN values but the majority of the intensities are within 4% of the HITRAN values. The values of self-broadening coefficients vary from 0.0452 to 0.0862cm−1atm−1 at 296K and those of hydrogen-broadening coefficient range between 0.0475 and 0.0795cm−1atm−1 at 296K. All of our measured self- and hydrogen-shift coefficients are negative and range from −0.002 to −0.008cm−1atm−1. With the pressure and path length used in our study we did not find evidence of significant line mixing in either the self- or hydrogen-broadened spectra. This study represents the first high-resolution experimental determination of hydrogen-induced pressure broadening and pressure-shift coefficients in the 2-0 band of 12C16O at room temperature.

Pressure broadening study at low temperature: application to methane.

We studied the R(0) line profile in the CH4 v4 band from room temperature to 188 K with N2 as a perturber, to 100 K with O2 as perturber, and from room temperature to 15 K using He as a perturber. The N2 and O2 measurements were performed over a total pressure range of 15-110 mbar, and for the He measurements the maximum sample pressure at 15 K was 1.10 mbar. Broadening parameters were determined, taking into account the confinement narrowing (Dicke effect), and narrowing parameters, deduced from the soft or hard collision model, are compared with the dynamic friction coefficient calculated values. We also obtained preliminary values for the temperature dependence of the N2, O2 and He line broadening parameters for this transition.

Spatial mapping of collisionally cooled carbon monoxide molecules in a cold cell

Abstract We used a collisional cooling cell which was designed to mount directly on the second stage of a CTI-cryogenics Model 22C CRYODYNE CRYOCOOLER. The cell was maintained at approximately 14 K for all measurements reported here. The absorption path length in the cell was 4.2 cm and the window clear aperture was 1.27 cm. The position of the injector was designed to be adjustable, so we could probe the cold gas cloud from the exit plane of the injector along a vertical line to approximately 11 mm downstream from the injector. We also probed the cloud in a horizontal direction, orthogonal to the previous direction. Our beam is imaged to a spot diameter of approximately 1 mm in the center of the cell. Gas temperatures as low as 24 K were measured approximately 11 mm from the nozzle and temperatures of approximately 40 K were measured close to the exit plane of the nozzle. Carbon monoxide was used for all measurements.

Temperature-Dependent, Nitrogen-Perturbed Line Shape Measurements in the ν1 + ν3 Band of Acetylene Using a Diode Laser Referenced to a Frequency Comb

The P(11) line of the ν1 + ν3 combination band of C2H2 was studied using an extended cavity diode laser locked to a frequency comb. Line shapes were measured for acetylene and nitrogen gas mixtures at a series of temperatures between 125 and 296 K and total pressures up to 1 atm. The data were fit to two speed-dependent line shape models and the results were compared. Line shape parameters were determined by simultaneously fitting data for all temperatures and pressures in a single multispectrum analysis. Earlier pure acetylene measurements [Cich et al. Appl. Phys. B 2012, 109, 373-38] were incorporated to account for self-perturbation. The resulting parameters reproduce the observed line shapes for the acetylene-nitrogen system over the range of temperatures and pressures studied with average root-mean-square observed-calculated errors of individual line measurement fits of approximately 0.01% of maximum transmission, close to the experimental signal-to-noise ratios. Errors in the pressure measurements constitute the major systematic errors in these measurements, and a statistical method is developed to quantify their effects on the line shape parameters for the present system.

Line profile study down to 80 K of R(7) in the 13CO (1-0) band perturbed by Ar and 13CO collisional cooling with he at 6.9 K

We present a line profile study at 150, 110 and 80 K for the R(7) line in the fundamental band of 13CO perturbed by Ar in a new stabilized low-temperature cell cooled by liquid nitrogen. The broadening, shifting and narrowing parameters are determined taking into account the absorber speed dependence by simultaneous least-squares fitting of spectra over a 4–300 Torr pressure range recorded using a frequency-stabilized diode laser spectrometer. When the cell is cooled by liquid helium the (1–0) R(2) line of 13CO in collision with helium is observed at a temperature of 6.9 K.

Corrections to measured sample pressures in low temperature collisional cooling experiments.

Three different thermal transpiration models have been tested for a range of pressures and temperatures normally encountered in collisional cooling experiments. The calculated corrections for two of these models follow similar patterns and differ only in magnitude at a fixed sample temperature for sample pressures between 0 and 8 Torr. Corrections calculated from the third model follow a pattern similar to the other two models only at pressures above about two Torr.

Self broadening and linestrength determinations in the ν2 and ν5 bands of CH3F

Abstract Self broadening coefficients and line strengths were determined for several transitions in CH 3 F. The data reported here are the result of a new analysis of data originally recorded in 1993. This new analysis uses a Voigt profile to model the line shape for the previously recorded data. There is good agreement between the results reported here and results reported elsewhere using Rautian and Galatry line profiles to analyze a different data set collected for the same lines.

Performance characteristics of a low-temperature cell for collisional cooling experiments

We fabricated and tested a low temperature cell which is mounted directly on the second stage of a CTI-Cryogenics Model 22C CRYODYNE CRYOCOOLER. The vacuum system consists of a room temperature vacuum shroud, a radiation shield maintained at 77K and the cell which is mounted directly to the second stage of the cryocooler. The ultimate cell temperature is 12.4 Kelvin, and the low temperature limit increases at a rate of 5.6 Kelvin/Watt. We achieve a cell temperature of 22 Kelvin under typical experimental conditions of approximately 29 milli Torr helium, slow flowing gas, and a heated injector. The absorption path length of the cell is 3.35 cm, and the window clear aperture is 1.27 cm. We preformed a series of experiments in which we determined the translational temperatures of vibration- rotation transitions in the band of CO for different cell temperatures. The results of our tests are discussed in this paper.

PROGRESS IN THE MEASUREMENT ON TEMPERATURE-DEPENDENCE OF H2-BROADENING OF COLD AND HOT CH4

KEEYOON SUNG, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA; V. MALATHY DEVI, D. CHRIS BENNER, Department of Physics, College of William and Mary, Williamsburg, VA, USA; TIMOTHY J. CRAWFORD, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA; ARLAN MANTZ, Department of Physics, Astronomy and Geophysics, Connecticut College, New London, CT, USA; MARY ANN H. SMITH, Science Directorate, NASA Langley Research Center, Hampton, VA, USA.