Tunable Laser Spectrometer Research Articles

Atlas of the High-Temperature Water Vapor Spectrum in the 3000 to 4000 cm−1 Region

An atlas of the high-temperature (1200 K) absorption spectrum of water vapor in the 3000 to 4000 cm−1 region is presented. The infrared spectrum was recorded at Doppler-limited resolution using a tunable difference-frequency laser spectrometer. The spectral region scanned encompasses the strong OH stretching fundamentals, ν1 and ν3, and the bending overtone, 2ν2, as well as associated hot bands. Almost all the lines have been assigned using a model Hamiltonian which yields very satisfactory agreement between calculated and observed line positions for J up to 27 or Ka up to 14. The calculated eigenvectors applied to the transition moment operator predict the measured line intensities quite closely. This work should serve as a reference for analyzing spectra from high-temperature sources such as combustion exhausts and cool stars.

Rotational structure and vibrational predissociation in the HF stretching bands of the HF dimer

The rotational structure in the HF stretching bands of the HF dimer has been recorded with nearly Doppler-limited resolution using a tunable difference-frequency laser spectrometer and a long-path cell held at low temperatures and pressures. Two bands are observed; the higher frequency band, corresponding principally to the ‘‘free’’ hydrogen stretch, has the appearance of a B-type or perpendicular band with two prominent Q subbranches RQ0 and PQ1; the lower band is an A-type or parallel band arising primarily from the ‘‘bonded’’ hydrogen vibration. The K=0 subbands of both vibrations have been fully assigned and fit with polynomial expansions in J(J+1) to yield ground state constants in excellent agreement with a previous microwave resonance molecular beam study of the HF dimer. The subbands exhibit doubling and 10:6 intensity ratios for alternate J indicative of the internal rotation tunneling motion proposed in the microwave study. A pressure-independent broadening of the lines to about twice the Doppler width, attributed to vibrational predissociation, is observed for the bonded hydrogen stretching band.

The Q branch of the 2 ν1 + ν4 band of CF 4

The Q branch of the 2 ν 1 + ν 4 band of CF 4 was recorded with Doppler-limited resolution at T = 296 and 77 K using a tunable difference-frequency laser spectrometer. The low-temperature spectrum is fit to within 1.1 × 10 −4 cm −1 rms for J ≤ 40 using the simple diagonal approximation terms, m, v, and g, without centrifugal distortion. Extrapolation to higher J for the room-temperature measurement is moderately successful.

Observation of the v = 3←0 band of SO(X 3Σ−) with a difference frequency laser

The second overton band (v = 3→0) of SO(X 3Σ−) near 3 μm has been observed by using a tunable difference frequency laser spectrometer and a 2 m multiple-reflection discharge cell. An analysis of the overtone data yields the band origin and molecular constants of the v = 3 state. (AIP)

Torsional Splittings and Assignments of the Doppler-Limited Spectrum of Ethane in the C-H Stretching Region.

The Doppler-limited absorption spectrum of the C-H stretching region of ethane has been recorded at T≃ 119 K with a tunable difference-frequency laser spectrometer. The strong torsional hot band structure at room temperature is eliminated at 119 K, and the enhanced resolution from the Doppler width reduction allows us to observe small torsional splittings. The two fundamentals in the region, v 7, a perpendicular band and, v 5, a parallel band have been essentially completely assigned as have a large number of transitions in the parallel component of the v 8 + v 11 combination band. A number of perturbations of both global and local nature have been observed. The complete spectrum and a listing of transition wavenumbers, intensities and assignments are presented here to facilitate identification and quantitative analysis of ethane in a variety of monitoring applications. Precise ground state rotational constants have been determined from combination differences.

High-resolution infrared laser spectroscopy of supercooled hexafluorides of heavy elements

A tunable injection laser spectrometer was used to investigate the ν3 band of sulfur, tungsten, and uranium hexafluorides. In order to eliminate hot bands superimposed on the fundamental ν3 transitions, the gases were supercooled in a supersonic jet. The gas under study constituted 1% of a helium mixture. It was found that in a supersonic jet the rotational temperature of the molecules was reduced to 40 °K. The isotopic structure of the ν3 band of supercooled WF6 was resolved and four Q branches corresponding to the 186W, 184W, 183W, and 182W isotopes were observed. The isotope shift was 0.31 cm−1/amu. For uranium hexafluoride, Q branches of the ν3 band of uranium isotopes with atomic weights of 238 and 235 were obtained. The isotope shift was 0.650±0.005 cm−1. The fine structure of the Q branch was resolved, the position of the maximum for U238F6 being 627.680 cm−1.

Spectral intensities in the ν3 bands of 12CH4 and 13CH4

Relative and absolute line intensities for the ν3 bands of the 12C and 13C isotopic varieties of methane have been measured using a tunable difference-frequency laser spectrometer. From these data the integrated band strength of 13CH4 is calculated to be 0.983 ± 0.007 that of 12CH4, with the uncertainty representing three standard deviations. The absolute ν3 bandstrength for 12CH4 is 266.1 ± 3.0 cm−2 atm −1 at 294.7 K where the errors are dominated by the pressure measurement. This band strength corresponds to an effective transition moment 〈μ3〉 = 0.0534(3)D for 12CH4 from which the ν4 band dipole moment and the Herman-Wallis F factor can be estimated using a recent force field model for methane.

Wavenumbers, line strengths, and assignments in the Doppler-limited spectrum of formaldehyde from 2700 to 3000 cm −1

The spectrum of H 2CO from 2700 to 3000 cm −1 has been examined at Doppler-limited resolution using a tunable difference frequency laser spectrometer at Lincoln Laboratory. The wavenumbers and strengths of 4350 absorptions have been determined with an accuracy of 0.001 cm −1 and 5%, respectively. These data have been incorporated into the analysis of lower-resolution data from Florida State University to assign 72% of the observed absorptions to one of seven bands: ν 3 + ν 4 (a C-type band at 2655 cm −1), ν 3 + ν 6 (a B-type band at 2719.156 cm −1), ν 1 (an A-type band at 2782.457 cm −1), ν 5 (a B-type band at 2843.326 cm −1), ν 2 + ν 4 (a C-type band at 2905 cm −1), 2 ν 3 (an A-type band at 2999.5 cm −1) and ν 2 + ν 6 (a B-type band at 3000.066 cm −1). The band ν 3 + ν 4 has been observed for the first time, and the band center for 2 ν 3 has been corrected from a value of 2972 cm −1 to the value listed above. The effects of strong Fermi and Coriolis resonances on the spectra are discussed.

Λ-Doubling in the v = 2 ← 0 overtone band in the infrared spectrum of NO

Λ-doubling in the first overtone, v = 2 ← 0, vibration-rotation band of 14N 16O has been measured with Doppler-limited resolution using a tunable difference-frequency laser spectrometer. P- and R-branch Λ-splittings for 1 2 <- J ≤ 53 2 in the 2 Π 1 2 electronic state and for 47 2 ≤ J ≤ 57 2 in the 2 Π 1 2 state have been determined by least squares fitting of the spectral profiles to Gaussian doublets with a precision of better than ±1.5 MHz for the stronger, well-resolved lines. The Λ-doubling parameters obtained from a least square Hamiltonian analysis of the data for the v = 2 level are p 2 = 349.542 (60) MHz and q 2 = 2.754 (5) MHz with the error limits representing three standard deviations. These precise v = 2 parameters are combined with excellent v = 1 constants available in the literature to calculate the Λ-splittings expected in the v = 2 ← 1 hot band of NO for comparison with recently reported measurements. The effect of nuclear hyperfine structure on the lineshapes of some of the low J overtone transitions has also been observed.

Doppler-limited spectra of the CH stretching fundamentals of formaldehyde

The Doppler-limited spectrum of H 2CO in the 2700 to 3000 cm −1 region has been recorded using a tunable difference-frequency laser system. This region encompasses the ν 1 and ν 5 fundamental CH stretching bands of formaldehyde as well as a number of strongly interacting combination bands. The tunable laser spectrometer affords complete spectral coverage with a calibration precision better than 10 −3 cm −1 for the transition frequencies and with absolute absorption intensity measurements better than 2%. The band centers for the ν 1 and ν 5 vibrations are measured to be 2782.4572 ± 0.0010 cm −1 and 2843.3254 ± 0.0015 cm −1 respectively, independent of the upper-state rotational constants.

Photodetachment of Negative Ions Using a Continuously Tunable Laser and an Ion Cyclotron Resonance Spectrometer

Negative ions photodetachment by continuously tunable laser and ion cyclotron resonance spectrometer