CO2 Laser Lines Research Articles

High Enrichment of 28Si by Infrared Multiple Photon Decomposition of Si2F6.

High enrichment of 28Si has been carried out using the laser isotope separation technique based on the isotopically selective infrared multiple photon decomposition of Si2F6. When about 2 Torr of Si2F6 was irradiated at a fluence of 1.0 J/cm2 per pulse with the 10P(8) line of a TEA CO2 laser at 954.55 cm-1, the compound decomposed very efficiently with high isotope selectivity. The products SiF4 and white solids were enriched with 29Si and 30Si, while the residual Si2F6 was enriched with 28Si. The atomic fraction of 28Si in residual Si2F6 increased with increasing decomposition of Si2F6; 99.9% of 28Si was obtained at a consumption of 50% of initial Si2F6. The large-scale flow experiment yielded 99.7% 28Si at a production rate of 2.5 g/h, where a mixture of 3.3 Torr Si2F6 and 6.6 Torr argon was irradiated with 10P(8) laser pulses at a repetition rate of 5 Hz. Merits and demerits of the present laser separation are discussed in comparison to those of conventional separation methods. Laser enrichment of 28Si seems promising for economical production of large quantity of pure 28Si. Various applications of silicon isotopes are briefly reviewed.

Precision Tunable Infrared Source at 10 μM: CO 2 -Laser/Microwave-Sideband System with an Evenson CO 2 Laser and a Cheo Waveguide Modulator

A CO2-laser/microwave-sideband tunable infrared source system has been newly built at the University of New Brunswick (UNB) in Canada. The system employs a high-resolution CO2 laser of Evenson design that lases on a wide range of lines including hot and sequence band lines as well as high-J lines of the regular 9.6 and 10.6 μm bands. The frequency of the CO2 laser is stabilized to the Lamb dip in the 4.3 μm fluorescence signal in an external CO2 cell. Microwave (MW) sidebands are generated in a Cheo-type infrared waveguide modulator driven by a synthesized sweeper and a traveling wave tube (TWT) amplifier. The MW sidebands appear on either side of the CO2 laser line, and are individually separated from the carrier by a tunable Fabry-Perot etalon. The sidebands currently have a typical power of about 1.4 mW and a continuous frequency tuning range of 22 GHz (from ± to ±18 GHz) when 5 W laser power and 15 W microwave power are delivered to the modulator. Details are given on the source construction and measured performance characteristics. Features of the source and its planned applications are discussed.

Sub-Doppler Measurements of vco=1←0, K=0, A/E Lines of Methanol with Microwave Sidebands of CO2 Laser Lines

Sub-Doppler Measurements of vco=1←0, K=0, A/E Lines of Methanol with Microwave Sidebands of CO2 Laser Lines

Multifrequency Diagnostics of a Vibrationally Equilibrium CO2-Containing Gas Mixture

We present a technique which makes it possible to simultaneously determine the temperature T and the partial pressure of carbon dioxide in a vibrationally equilibrium gas mixture at atmospheric pressure by using the experimentally measured spectral distribution of the absorption factor at the oscillation lines of a tunable CO2 laser. The technique developed can be employed for monitoring both the energy efficiency and the ecological purity of the processes of combustion of large amounts of hydrocarbon fuels accompanied by release to the atmosphere of combustion products containing carbon dioxide.

Sub-Doppler spectroscopy of the C—O stretching fundamental band of methanol by use of microwave sidebands of CO_2 laser lines

Microwave sidebands of CO2 laser lines were used in a sub-Doppler spectrometer to observe sub-Doppler spectrum of the C—O stretching fundamental band (vCO=1←0) of methanol. Frequencies of more than 200 vibration-rotation lines were measured with an accuracy of better than 0.20 MHz. Sixty-one blended spectral lines in the Fourier-transform spectrum were resolved with a resolution of 0.2 MHz. For transitions involving A-species levels with K=2, 3, and 4 in the vt=0 state and K=2 in the vt=1 state, 64 doublet lines arising from asymmetry splittings were observed. From these observed asymmetry splittings and calculated ground-state splittings the asymmetry splittings and asymmetry-splitting constants for the vCO=1 state were determined. The R- and Q-branch transitions for the (vt, E, K)=(1, E, 2) and (1, E, 5) sequences were assigned by observation of their Stark effects and by use of the Ritz’s combination principle. Term values of the levels in the vCO=1 state for these two sequences were given, and their Taylor-series expansion coefficients were determined.

More Sub-Doppler Heterodyne Frequency Measurements on OCS between 56 and 63 THz

By using tunable microwave sidebands added to CO-laser lines, we have made more sub-Doppler heterodyne frequency measurements on OCS. Three new rotational transitions have been measured for each of three absorption bands, 1000–0000, 0201–0000, and 0311–0110. The absolute uncertainties of the measurements are on the order of ±25 kHz. New calibration tables are given for the region 1860–1925 and 2020–2085 cm−1 based on the most recent OCS measurements.

Line-pair selections for remote sensing of atmospheric ammonia by use of a coherent CO_2 differential absorption lidar system

Research on wavelength selection of CO2 laser lines for range-resolved remote sensing of atmospheric ammonia by use of a coherent differential absorption lidar system is described. Four laser line pairs are suggested for different levels of ammonia concentrations from approximately a few parts per billion to 1 part per million in a polluted atmosphere. The most suitable line for measuring ambient ammonia concentrations is 9R(30), because it has the highest absorption coefficient. 10R(14) has the lowest absorption coefficient, making it suitable for strong source mapping. 10R(8) and 10P(32) are best for intermediate levels of ammonia concentration. Absorption coefficients of ammonia calculated from the HITRAN96 database are in good agreement (mostly within +/-10% )with other experimental results.Sensitivity of measurement, interference from water-vapor lines with typical humidity in the summer,and sensitivity of ammonia absorption cross section to temperature and pressure are analyzed and calculated for the four wavelength pairs. The results show that the interference from water-vapor lines is easily correctable to a negligible amount, and errors caused by uncertainties in temperature and pressure are insignificant.

Infrared–microwave double-resonance spectroscopy of CH_3OH by use of sidebands of CO_2 laser lines

An infrared–microwave double-resonance technique using microwave sidebands of CO2 laser lines as an infrared source has been applied for observation of rotational lines of the methanol molecule. Frequencies of more than 50 rotational lines in the excited C—O stretching vibrational state (vco=1) have been measured with good precision and have been compared with those reported in infrared studies. Many of them agree within several megahertz, although some lines show differences of >10 MHz. The pressure dependence of the double-resonance signals for two low-J microwave transitions belonging to the ground and the vco=1 states, respectively, have been observed for sample pressures as high as 0.4 Torr. For the former transition the signal has been observed to change its sign at higher pressures. Rate equation analysis explains the observed pressure dependence quantitatively and allows us to understand the physical processes involved in the double resonance.

High-Resolution Infrared Spectra and Simultaneous Rovibrational Analysis of the ν2, ν3, ν5, and ν6Bands of H3SiF

A high-resolution FT infrared spectrum of H3SiF (resolution 0.0024 cm−1) in the region 620–1130 cm−1was measured and used to analyze the fundamental bands ν2(A1), 990.851 cm−1; ν3(A1), 875.011 cm−1; ν5(E), 962.213 cm−1; and ν6(E), 729.528 cm−1. A total number of 7241 transition wavenumbers (including 53 perturbation-allowed transitions) withJ′ ≤ 49 have been fitted by taking into account various Coriolis interactions, α-resonance terms, and ℓ-type interactions between and within the vibrational levelsv2= 1,v3= 1,v5= 1, andv6= 1. The strongest interaction in this system of levels is thex-yCoriolis coupling between thev2= 1 andv5= 1 vibrational states. However, it turns out that thex–yandz-type Coriolis interactions also have to be introduced explicitly between thev5= 1 andv6= 1 states as well as thex–yCoriolis interactions betweenv3= 1 andv5= 1, andv3= 1 andv6= 1 states, in order to fit the data quantitatively. The standard deviation of the fit was ς = 8.6 × 10−5cm−1for 7241 transition wavenumbers and 68 fitted parameters. The sign relations between the fitted parameters and the possibility of fitting the parameters in different schemes are discussed. The results have been used to find close coincidences between the frequencies of the CO2laser lines and the transition wavenumbers of the ν2and ν5bands of H3SiF for the main natural isotopomer. We report integrated absorption band strengths which are important for infrared laser chemistry.

18O-Selective Infrared Multiple Photon Decomposition of Natural and 18O-Enriched Diisopropyl Ethers

18O-Selective infrared multiple photon decomposition of diisopropyl ether has been studied in detail using samples with 18O at oxygen atomic fractions of 0.20% (natural abundance ratio), 4.5%, and 17.3%. The 1022 cm-1 band due to the 12C−16O−12C antisymmetric stretching vibration was found to shift toward a low-energy side by 24 cm-1 for 12C−18O−12C. The 18O-selectivities and yields of the main products containing oxygen, acetaldehyde, and 2-propanol were examined as functions of experimental parameters. The irradiation of a few Torr (1 Torr = 1 mmHg = 133.322 Pa) of diisopropyl ether with TEA (transversely excited atmospheric) CO2 laser lines in the vicinity of 950 cm-1 resulted in considerably high enrichment of 18O in the products. The fluence at focus was more or less than 8 J cm-2. Acetaldehyde and 2-propanol containing 18O at about 20% were obtained from natural ether, and those containing more than 70% were obtained from 17.3% 18O-enriched ether. The observed results can be explained in term of the mechanism involving radical and molecular decomposition channels.

Influence of CO_2-laser linewidth on the measured absorption coefficients of atmospheric water vapor and ammonia

We describe the results of analysis of the experimental and calculated data on the water vapor and ammonia absorption coefficients of CO2-laser radiation. We believe that the different halfwidths of CO2-laser lines, caused by different pressures of the working mixtures in specific experiments, are probably the reason for discrepancies between data presented in various papers.

High-resolution CO-laser sideband spectrometer for molecular-beam optothermal spectroscopy in the 5–6.6 μm wavelength region

We have set up a new CO-laser sideband spectrometer for high-resolution molecular-beam optothermal spectroscopy in the mid infrared. By mixing CO-laser lines with tunable microwave radiation, a spectral coverage of about 50% is achieved. Using a microwave resonator, a typical output power of 3 mW is realized in the region of 5–6.6 μm (1500–2000 cm−1). The resolution in our molecular-beam apparatus with optothermal detection is 2.8 MHz (full width at half maximum). The new setup allowed the observation of the first high-resolution rotationally resolved spectrum of formic acid dimer (HCOOH)2.

Frequency-agile bandpass filter for direct detection lidar receivers

We discuss the development of a frequency-agile receiver for CO(2) laser-based differential absorption lidar (DIAL) systems. The receiver is based on the insertion of a low-order tunable etalon into the detector field of view. The incorporation of the etalon into the receiver reduces system noise by decreasing the instantaneous spectral bandwidth of the IR detector to a narrow wavelength range centered on the transmitted CO(2) laser line, thereby improving the overall D* of the detection system. A consideration of overall lidar system performance results in a projected factor of a 2-7 reduction in detector system noise, depending on the characteristics of the environment being probed. These improvements can play a key role in extending the ability of DIAL systems to monitor chemical releases from long standoff distances.

Differential absorption Mueller matrix spectroscopy and the infrared detection of crystalline organics

The complete 16-element Mueller matrices for backscattering from amino acids, sugars, and other enantiomorphic compounds pressed into wafer form were measured at infrared wavelengths. For each compound a pair of CO(2) laser lines was selected from the 9.1-11.6-mum region such that one line excited an absorption band in the compound, whereas the other did not. It was observed that at least some of the matrix elements differed significantly depending on which of the two wavelengths was used in the measurement. We propose that a neural network pattern recognition system can be trained to detect the presence of specific compounds based on multiwavelength backscatter Mueller matrix measurements.

Wideband Tuning of a High-Pressure CO2 Laser by means of Intracavity Active Frequency Shift

Intracavity active frequency shift, a unique method of shifting the frequency of pulsed lasers, was applied to a high-pressure CO2 laser which has a continuous gain profile over a wide range of the spectrum. In order to extend the amount of frequency shift, the dependence of the maximum frequency shift on the laser gas pressure was investigated. As a result, it was found that the maximum frequency shift was limited by the gain duration time rather than by the gain bandwidth at a pressure of 8–10 atm. By shortening the cavity length, the maximum frequency shift of 9 GHz was obtained at a pressure of 8 atm. Then, the grating-mirror cavity was applied to demonstrate the frequency shift starting from each lasing line of the CO2 laser. According to numerical calculations, the maximum frequency shifts starting from most lasing lines of a multi-isotope cw CO2 laser can exceed 10 GHz, resulting in coverage of almost the entire gain spectrum in the high-pressure CO2 laser.

IR multiphoton absorption of SF 6 in flow with Ar at moderate energy fluences

in flow with Ar (SF6: Ar=1:100) in conditions of a large vibrational/rotational temperature difference (TV≃230 K, TR≃60 K) was studied at moderate energy fluences from ≃0.1 to ≃100 mJ/cm2, which are of interest for isotope selective two-step dissociation of molecules. A 50 cm Laval-type slit nozzle for the flow cooling, and a TEA CO2-laser for excitation of molecules were used in the experiments. The laser energy fluence dependences of the SF6 MPA were studied for several CO2-laser lines which are in a good resonance with the linear absorption spectrum of the ν3 vibration of SF6 at low temperature. The effect of the laser pulse duration (intensity) on MPA of flow cooled SF6 with Ar was also studied. The results are compared with those obtained in earlier studies.

Fourier transform spectrum of the in-plane CH3-rocking fundamental and vibrational coupling in C-13 methanol

The Fourier transform infrared spectrum of the in-plane CH3-rocking fundamental of CH313OH has been investigated at 0.002 cm−1 resolution. The rocking band is principally of parallel character and has a double-peaked Q branch and relatively wide spread subbands indicative of a substantial change in torsional barrier height. All A subbands from K=0 to 11 and all but one E subband from K=0 to 9 have been assigned in the n=0 torsional state and fitted to J(J+1) power-series expansions to obtain the subband origins and excited-state energy structure. The effects of vibrational interactions between the CH3-rocking and CO-stretching modes are prominent in the spectrum. Coriolis coupling between rocking (K−1) and CO-stretching K levels is observable for K⩾6, and makes significant contributions to the subband origins and effective B values. Several J-localized perturbations due to level-crossing resonances with CO-stretch states have been observed and characterized. Two reported strong far-infrared laser lines optically pumped by the 10R(26) CO2 laser line have been found to arise through such a ΔK=3 level-crossing resonance. Modeling of the rocking-state torsion-K-rotation energies yields a height of V3r=469.2(38) cm−1 for the torsional potential barrier, a 26% increase over the ground state. The asymmetry K-doubling pattern in the excited state is qualitatively consistent with this barrier for K=2 to 4, but the K=5 rocking substate displays strongly enhanced splitting.

CO2 laser stabilized on narrow saturated absorption resonances of CO2, improved absolute frequency measurements

We have stabilized a CO2 laser on narrow saturated absorption resonances of CO2 detected on the transmitted beam of an external Fabry-Perot cavity in the 30 THz spectral region. By using another CO2 laser stabilized on a close OsO4 line whose absolute frequency is precisely known, and measuring the beat frequency 4 with a precision of 100 Hz, we determined the absolute frequency of twenty CO2 laser lines, with uncertainties between 100 Hz and 1 kHz mainly limited by the accuracy of the OsO4 grid. The set of measurements reveal some discrepancies with data based on measurements performed with the widely used saturated fluorescence technique; we attribute these to pressure shifts of several kilohertz under the conditions of this last method.

Calibration of a pyroelectric detector at 106 µm with the National Institute of Standards and Technology high-accuracy cryogenic radiometer

The National Institute of Standards and Technology (NIST) is establishing an infrared detector calibration facility to improve radiometric standards at infrared wavelengths. The absolute response of the cryogenic bolometer that serves as the transfer standard for this facility is being linked to the NIST high- accuracy cryogenic radiometer (HACR) at a few laser wavelengths. At the 10.6-microm CO(2) laser line, this link is being established through a pyroelectric detector that has been calibrated against the HACR. We describe the apparatus, methods, and uncertainties for the calibration of this pyroelectric detector.

Combined high resolution infrared and microwave study of bromochlorofluoromethane

We report a detailed spectroscopic investigation of the chiral molecule bromochlorofluoromethane (CHBrClF) with rotational resolution using a pulsed nozzle beam Fourier transform microwave (FTMW) and a waveguide FTMW spectrometer as well as a supersonic jet interferometric Fourier transform infrared (FTIR) and infrared diode laser spectrometer. The rotational spectrum of CHBrClF has been measured between 8 and 18 GHz. The quadrupole hyperfine components have been fully resolved for the assigned rotational transitions with J⩽18. Three ground state rotational constants, five centrifugal distortion constants, and all five independent elements of the bromine and chlorine quadrupole coupling tensors have been determined for each of the four isotopomers CH79Br35CIF, CH81Br35CIF, CH79Br37CIF, and CH81Br37CIF from about 500 measured transition frequencies of the hyperfine components. The quadrupole coupling tensor has been transformed to its principal axes. The determinable sign combinations of the off-diagonal elements of the coupling tensor have been evaluated. Rotational transitions involving high J were measured by FTIR spectroscopy between 15 and 40 cm−1 (450–1200 GHz) using a light pipe cell, providing an estimate of the permanent dipole moment |μ|=(1.5±0.3) D from intensities. In the midinfrared, we have fully analyzed the rovibrational line structure of supersonic jet spectra of the CF-stretching fundamental ν4, giving band centers for the isotopomers CH79Br35CIF [ν̃ 40=1077.178 43(4) cm−1], CH81Br35CIF [ν̃=1077.133 06(4) cm−1], CH79Br37CIF [ν̃ 40=1076.7914(4) cm−1], and CH81Br37CIF [ν̃ 40=1076.730 26(5) cm−1]. A combined analysis of about 20 microwave frequencies, more than 100 infrared ground state combination differences, and about 70 infrared transition frequencies for each of the35Cl isotopomers finally provide accurate ground and excited state rotational parameters as well as structural parameters, which may be compared to ab initio calculations. The results are discussed in relation to the molecular structure as well as coincidences of ν4 absorptions with CO2 laser lines in view of CO2–laser pumping and possible spectroscopic studies of this chiral molecule at ultrahigh resolution.