Heterodyne frequency measurements of carbonyl sulfide transitions at 26 and 51 THz. Improved OCS,O13CS,and OC34S molecular constants

Heterodyne frequency measurements were made on selected absorption features of carbonyl sulfide (OCS) near 26 THz (860 cm^?1) and 51 THz (1700 cm^?1). Frequency differences were measured between a tunable diode laser (TDL) locked to carbonyl sulfide absorption lines and either a stabilized ¹³CO₂ laser or a CO laser which was referred to stabilized CO₂ lasers. These measurements are combined with conventional TDL measurements and published microwave measurements to obtain new, more reliable molecular constants for OCS, O¹³CS, and OC³⁴S. New frequency measurements are given for nine CO laser transitions between 1686 and 1726 cm^?1.     

Heterodyne frequency measurements on N2O between 1257 and 1340 cm−1

Frequency measurements are given for the 00⁰1-00⁰0 and 01¹1-01¹0 bands of N₂O from 1257 to 1340 cm^?1. The measurements utilize heterodyne techniques by measuring small frequency differences between a tunable diode laser locked to the center of an N₂O absorption line and harmonic combinations of frequencies of radiation from two CO₂ Lamb-dip-stabilized lasers. The measurements are facilitated by the use of the CO laser as a transfer laser whose frequency is also measured. These measurements have been combined with other data to provide new band constants and frequency calibration tables for several band systems of N₂O in the following regions; 1215 to 1340, 1816 to 1930, and 2135 to 2268 cm^?1. A correction factor is also provided for existing calibration tables near 590 cm^?1.     

High-resolution laser Stark and Fourier transform spectroscopy of DBr at 5.5 μm

A very close coincidence between the P(1) transition of the 1-0 band of D⁷⁹Br and a CO laser line has allowed the observation of a DBr laser Stark spectrum. The observed inverse Lamb dips resolve the Br hyperfine structure of the transition, and are used to determine the value of the dipole moment of DBr in the v = 1 state. A high-resolution Fourier transform spectrum of DBr in the region 1750 to 1900 cm^?1 has also been recorded and accurately measured. These new data, together with the best previous microwave and infrared measurements, are used to determine molecular parameters for DBr, and also result in improved secondary wavenumber standards for the 5.5-μm region.     

Sub-Doppler Heterodyne Frequency Measurements near 5 μm with a CO-Laser Sideband System: Improved Calibration Tables for Carbonyl Sulfide Transitions

We present a series of sub-Doppler frequency measurements of carbonyl sulfide (OCS) rovibrational transitions covering the spectral region around 2050 cm⁻¹(61 THz). The absolute experimental uncertainties are between 21 and 84 kHz (Δν/ν = 3–14 × 10⁻¹⁰). In our spectrometer, tunable microwave sidebands are added to CO-laser lines and are used to saturate OCS transitions at low absorber pressure. The CO laser, plus a fixed-frequency sideband, is stabilized to the center of the OCS Lamb-dip signal and the laser frequency is measured against combination frequencies of two saturation-stabilized CO₂lasers. The determined OCS transition frequencies are combined with other data in a least-squares fit. These measurements improve the accuracy of several carbonyl sulfide bands for calibration purposes by one to two orders of magnitude. New molecular constants and detailed calibration tables between 860 and 3100 cm⁻¹are given.     

Laser magnetic resonance spectra of NH2 in the 9-μm region: Observation of weak ΔKa = −3 transitions in the ν2 band

Laser magnetic resonance spectra of the ν₂ band of the NH₂ radical have been observed with a CO₂ laser in the 1030–1108-cm^?1 range. The measurements of these weak ΔN = ?1, ΔK_a = ?3 transitions, involving levels with 5 ≤ N ≤ 10, should complement measurements in the higher frequency half of the band (1500–1900 cm^?1) made using CO lasers.     

The ground state of molecular hydrogen

The v = 0?0 quadrupole spectrum of H₂ has been recorded using a 0.005-cm^?1 resolution Fourier transform spectrometer. The rotational lines S(1) through S(5) are observable in the spectra, in the region 587 to 1447 cm^?1. The spectral position for S(0) was also obtained from its v = 1-0 ground-state combination difference. The high accuracy of the H₂ measurements has permitted a determination of four rotational constants. These are (in cm^?1) B₀ = 59.33455(6); D₀ = 0.045682(4); H₀ = 4.854(12) × 10^?5; L₀ = ?5.41(12) × 10^?8. The hydrogen line positions will facilitate studies of structure and dynamics in astrophysical objects exhibiting infrared H₂ spectra. The absolute accuracy of frequency calibration over wide spectral ranges was verified using 10-μm CO₂ and 3.39-μm CH₄ laser frequencies. Standard frequencies for 5-μm CO were found to be high by 12 MHz (3.9 × 10^?4 cm^?1).     

Precision infrared spectroscopy in CF379Br by means of infrared-microwave double resonance

From the observation of double resonance effects on the microwave spectrum two coincidences between 9.4 μm CO₂ laser lines and infrared transitions of the ν₆ → (ν₆ + ν₁) band of CF₃⁷⁹Br have been determined: R(30) laser line coincident with ^qR₂(7), F = 17/2→17/2 transition, R(28) laser line coincident with all four ΔF = 0 hyperfine components of the ^qQ₈(13) transition. In both cases other infrared transitions lay within the tuning range of the laser. The frequencies of these two laser lines allowed calculations of the band center frequency ν₀ = 1083.530 ± 0.001cm^?1 and α_A = 11.93 ± 0.3MHz, for the ν₆ → (ν₆ + ν₁) band.α_B constants were determined for the vibrational states v₆, (v₆ + v₁), v₁, and v₃.     

Doppler-limited spectra of the ν3 vibration of 12CH4 and 13CH4

The tetrahedral splittings in the P and R branches of the ν₃ band of natural methane have been examined with Doppler-limited resolution using a difference-frequency spectrometer. The spectra obtained by this difference-frequency mixer are compared to recent high-resolution grating spectrometer studies of ¹²CH₄ and enriched ¹³CH₄. The resolution, selectivity and precision are improved over the conventional methods. The mixing spectrometer utilizes tunable, narrow linewidth infrared radiation generated in the nonlinear optical crystal, LiNbO₃ as the beat frequency between a CW argon ion and a tunable dye laser. This spectrometer covers the 2.2 to 4.2 μm infrared spectrum with an instrumental resolution of 5 × 10^?4 cm^?1 and continuous scans up to ～1 cm^?1 and with ir power ～1 μW.     

On-line monitoring of cyclotron target-gas output by means of tunable lead salt diode laser absorption spectroscopy

We demonstrate what is, to our knowledge, the first use of mid-infrared laser absorption spectroscopy for trace-gas measurements of cyclotron target outputs used for the generation of radioactive carbon-11 in positron emission tomography (PET). The spectrometer was based upon a liquid-nitrogen-cooled lead salt diode laser generating single-mode radiation in the wavenumber range of 2230–2240 cm^?1. The sample flowed to a multiple-pass optical cell with a total path length of 15.23 m and the laser radiation was detected by two liquid-nitrogen-cooled InSb photodetectors. We present the results of CO, N₂O and CO₂ measurements on PET trace cyclotron output and discuss future work on ¹¹CO and ¹¹CO₂ detection.     

Heterodyne frequency measurements on carbonyl sulfide near 1050 cm

Heterodyne techniques have been used to measure the frequency differences between carbonyl sulfide (OCS) absorption lines and CO₂ laser transitions. A tunable diode laser was used both to scan the OCS absorption spectrum and to provide a beat signal against a CO₂ laser. Frequency differences as great as 8.6 GHz were measured. Many different OCS hot-band transitions were measured near 1050 cm⁻¹, and the measurements on the 02⁰0-00⁰0 band have been extended to such high J levels (J′ = 86) that higher-order centrifugal distortion terms are needed to fit the data.     

Measurement and analysis of the ν2 and ν4 infrared bands of 13CD4

A Fourier transform infrared spectrum of 91% ¹³CD₄ has been recorded between 885 and 1193 cm^?1 with a resolution of 0.04 cm^?1. The frequencies of 600 lines were measured with an accuracy of ±0.003 cm^?1. Of these, approximately 368 are assigned as allowed transitions in ν₄, 95 are forbidden ν₄ transitions, and 137 belong to ν₂; maximum upper state J values are 20 for ν₄ and 19 for ν₂. The ground state tensor constants D_t, H_4t, and H_6t were obtained by fitting them to four rotational transitions observed by Kreiner and Opferkuch in the infrared-microwave double-resonance spectrum. An interacting-band analysis of the $\text{ν}{}_2\text{ν}{}_4$ diad then yields 22 spectroscopic constants for these Coriolis-coupled fundamentals and fits the experimental frequencies with an rms deviation of 0.0055 cm^?1 for 432 unblended lines that were assigned unit weight. A ν₄P⁺(12) transition at 943.3812 cm^?1, nearly coincident with the 10P(22) emission of the ¹²C¹⁶O₂ laser, has been investigated by heterodyne spectroscopy and its detuning (?64 MHz) and absorption coefficient have been determined. Such coincidences may lead to the development of laser analytical techniques for ¹³CD₄, which is a useful nonradioactive atmospheric tracer. ¹³CD₄ transitions that are within 300 MHz of isotopic CO₂ laser lines are tabulated for this purpose and for use in double-resonance experiments.     

The adsorption sites of CO on Ni(111) as determined by infrared reflection-absorption spectroscopy

The adsorption of CO on Ni(111) has been studied using infrared reflection-absorption spectroscopy combined with LEED, Auger electron spectroscopy, thermal desorption spectroscopy and work function measurements. At low CO coverage (θ = 0.05) CO adsorbs on threefold sites with a strecthing frequency given by ω_CO = 1817 cm^?1. At θ = 0.30 all molecules have shifted to two-fold sites, and θ = 0.50, where a c(4 × 2) structure is observed, ω_CO = 1910 cm^?1. At θ = 0.57, with a (√7/2) × √7/2)R19.1° structure, one quarter of the molecules are adsorbed on top of the nickel atoms with the others in two-fold sites. Molecules bonded on the top sites give rise to a band at 2045 cm^?1. The frequency shift due to dipole-dipole interactions is small compared with the shift resulting from bonding to different crystallographic sites.     

Diode laser spectroscopy of the ν9 band of ethyl chloride

The vibration-rotation spectrum of the ν₉ band of C₂H₅Cl was recorded using a Nicolet 7199 Fourier Transform Infrared Spectrometer; the Q-branch, R(5)-R(6) multiplets, and P(21)-P(22) multiplets were investigated using a tunable semiconductor diode laser spectrometer. Constants derived from these assignments are (cm^?1) ν₉ = 973.8379(2), A⁹ = 1.039516(3), B⁹ = 0.183081(7), C⁹ = 0.164505(8). The line which coincides with the CO₂ laser 10R(6), which has been employed in multiphoton dissociation studies, is tentatively assigned to the 20_{3 18}-21_{3 19} transition. In addition, a very strong coincidence of transitions 19_{9 11}-19_{9 10} and 19_{9 10}-19_{9 11} with the CO₂ 10R(16) line is found, which can provide a good candidate for laser chemistry and infrared double resonance spectroscopy.     

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

The Doppler-limited spectrum of H₂CO in the 2700 to 3000 cm^?1 region has been recorded using a tunable difference-frequency laser system. This region encompasses the ν₁ and ν₅ 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 ν₁ and ν₅ 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.     

The ν1 + ν3 combination band of 238UF6

The ν₁ + ν₃ band of ²³⁸UF₆ has been observed at Doppler-limited resolution between 1291.5 and 1297.2 cm^?1 in a statically cooled longpath cell at 226 K. The frequencies of 21 transitions belonging to P(22, 28, 30), 14 belonging to R(12, 30, 31), and 28 belonging to the Q branch with 41 ≤ J ≤ 71, were measured with an accuracy of ±0.002 cm^?1. In addition, 53 frequency differences were measured in the Q branch between lines having different J assignments from Q(14) to Q(76). Five spectroscopic constants were fitted to these 116 data points with a standard deviation of 0.0006 cm^?1. The rotational constants have nearly the values expected from a consideration of ν₁ and ν₃ themselves, and there is little evidence for interaction of ν₁ + ν₃ with other levels.     

The CO fundamental-band laser as secondary frequency standard at 5 µm

We present a very high-resolution heterodyne spectrometer based on a CO laser which operates down to fundamental-band transitions of the molecule. This allows us to detect saturated absorption signals on these transitions at very low pressure (0.4 Pa) and laser intensity (< 1 mW/cm²), yielding a linewidth of about 250 kHz. With the CO fundamental-band laser stabilized to these saturation signals we have measured the transition frequencies of the fundamental bands of three isotopic species to an accuracy of typically 20 kHz (v/v 3 × 10^–10), referenced to the CO₂ frequency standard. Together with additional frequency measurements of the first hot bands, these provide the first heterodyne frequency data of sub-Doppler accuracy for transitions in low lying bands of CO. They now represent the most accurate secondary frequency standard in the spectral region around 5 µm (60 THz).     

High-resolution measurement and analysis of the infrared spectrum of nitric acid near 1700 cm−1

The infrared spectrum of the ν₂ band of nitric acid (HNO₃) has been measured with a tunable diode laser in the frequency interval from 1690 to 1727 cm^?1. A total of 430 assigned transitions have been analyzed to yield a set of nine rovibrational constants for the upper state with a standard deviation of 0.0012 cm^?1. The band is primarily B type with a band center at 1709.568 ± 0.005 cm^?1. Because of the absence of perturbations, the band constants can be used to calculate transition frequencies and relative intensities with a high degree of accuracy.     

Laser Stark spectroscopy of OCS in the 9.5 μm region

Laser Stark spectra of carbonyl sulfide have been measured with parallel and perpendicular polarization of a 9.4 μm band CO₂ laser with the Stark field up to 90 kV/cm. Components of the P(2) and R(1) transitions of the 03¹0–01¹0 band and those of the P(1) transition of the 04⁰0–02⁰0 band for ¹⁶O¹²C³²S have been analyzed. Band origins, 1052.9446 (4) cm^?1 for 03¹0–01¹0 and 1057.7860 (5) cm^?1 for 04⁰0–02⁰0, and dipole moments, 0.7035 (16) D for 01¹0 and 0.6810 (24) D for 03¹P, are obtained.     

Tunable laser Stark-difference spectra of CD3OH

We present Doppler resolution limited spectra of the P(J) and R(J) multiplets for J ≦ 10 of the 10-μm CO stretch band of ¹²CD₃¹⁶OH using a tunable diode laser. Relative frequencies within the multiplets accurate to ±0.0002–0.0005 cm^?1 are obtained, but no absolute frequencies are given. We are able to assign most of the hindered rotation and K substructure in these multiplets. The assignments are based on analyses of Stark-difference spectra combined with the ground-state microwave data and the intensity variations which are expected theoretically. The ground and excited state A, K = 1 asymmetry splitting parameters are measured to be δ₁″ = (8.5450 ± 0.0080) × 10^?3cm^?1 and δ₁′ = (9.7706 ± 0.0080) × 10^?3cm^?1, respectively. The ground-state value agrees well with the microwave results. A rapid-scan system for recording data and a computer-aided technique for calibrating and plotting the spectra are described.