Diode Laser Spectrometer Research Articles

Self-broadening coefficients in the ν7 band of ethylene at room and low temperatures

Using a tunable diode-laser spectrometer, we have measured self-broadening coefficients for a few transitions in the ν7 fundamental band of C2H4 at 298 and 174K. The studied transitions J′, Ka′, Kc′←J, Ka, Kc with 3⩽J⩽17, 1⩽Ka⩽4, and 1⩽Kc⩽14 are located in the spectral range 919–982cm−1. The collisional widths are measured by fitting each spectral line with Voigt, Rautian, and speed-dependent Rautian profiles. The latter model provides larger broadening coefficients than the Rautian profile and still larger coefficients than the Voigt profile. An approximate semiclassical calculation performed by considering only electrostatic interactions leads to reasonable agreement with the experimental data. By comparing the results obtained at room and low temperatures, the temperature dependence of the self-broadening has been determined both experimentally and theoretically.

Argon broadening of the 13CO R(0) and R(7) transitions in the fundamental band at temperatures between 80 and 297 K: comparison between experiment and theory

We present measurements of Ar-broadening parameters for the R(0) and R(7) lines in the fundamental band of 13CO at eight temperatures from 80 to 297K. The broadening parameters are determined by simultaneous least-squares fitting of spectra recorded using a frequency stabilized diode laser spectrometer. The comparison of the broadening parameter values for R(7) derived at room temperature and different pressures from different line profiles shows that an empirical line profile, which takes into account narrowing effects (Dicke narrowing and absorber speed dependence) but neglects any correlation between collisions, is able to describe the observed lines with constant values of the narrowing and broadening parameters over a 10–500Torr pressure range. Starting from a recent ab initio potential energy surface, theoretical thermally averaged close coupling values of the Ar broadening parameter are calculated for the same temperatures. The comparison between experimental and calculated values shows an overall agreement of 1.5%.

High-resolution infrared spectra of carbonyl sulfide solvated with helium atoms

Infrared spectra of HeN–OCS clusters with N up to about 20 have been studied in the 2062 cm−1 region of the O–C stretching vibration using a tunable diode laser spectrometer to probe pulsed supersonic expansions from moderately high-pressure (<35 atm) cooled (>−60 °C) jet sources. Resolved rotation-vibration transitions have been assigned for the clusters with N=2–8, and these assignments have been confirmed in detail by observations of the corresponding pure rotational transitions in the microwave region. The vibrational frequencies (band origins) were observed to move to higher frequencies (blueshift) for N=2–5, continuing the shift previously observed for the binary complex, He–OCS. Then, for N=6–8, the vibrational frequency moves back in the direction of lower frequencies (redshift), leading towards the limiting redshift previously observed in larger helium nanodroplets with N≈103–104. These vibrational shifts are consistent with a model in which the first five helium atoms fill a “ring” around the “equator” of the OCS molecule, with the subsequent heliums then taking positions closer to the ends. The cluster rotational constants decrease monotonically (increasing rotational moments of inertia) for N=1–8, falling below the value previously observed for the “free” rotation of OCS in helium nanodroplets. Strong, sharp spectral lines from clusters in the size range N≈9–20 were also observed, but not assigned. This difficulty in extending the analysis for N>8 may be related to the onset of partially free internal rotation of the OCS within the helium cluster, which in turn has interesting implications for the nature of superfluid-type behavior in this finite scale system.

High-precision determination of the 13CO2/12CO2 isotope ratio using a portable 2.008-μm diode-laser spectrometer

A compact and portable diode-laser spectrometer is developed for high-precision measurements of the 13CO2/12CO2 isotope ratio. Ro-vibrational transitions belonging to different combination bands, around 2.008 μm, are detected for the two isotopomers, using wavelength-modulation spectroscopy with first-harmonic detection. The determination of the isotope ratio is performed through a comparison between the spectra, simultaneously acquired in two different multiple-reflection cells designed to contain the sample and reference gases. The reproducibility of our method is extensively investigated over different time scales. In particular, the short-term precision of the isotope ratio determination is found to be 0.03%. We discuss the presence of possible systematic shifts in the δ-v alue and optimise the operating conditions to ensure accuracy within the precision level. A possible use for in-situ measurements in volcanic gases is also discussed.

Line profile study from diode laser spectroscopy in the 12CH 4 2 ν3 band perturbed by N 2, O 2, Ar, and He

We present a line profile study for two lines in the 2 ν 3 band of CH 4 recorded with a frequency stabilized tunable diode laser spectrometer. The broadening and narrowing (Dicke effect) parameters of the R(0) line perturbed by N 2, O 2, and He are derived from a simultaneous fitting of spectra at pressures from 20 to 300 Torr by using the soft and hard collision models. These parameters are determined for the A and F components of the unresolved R(3) manifold perturbed by N 2, Ar, and He from the line profile analysis of spectra at pressures between 50 and 500 Torr. The line mixing effect between the two F components is also taken into account and the absorber speed dependent effect on broadening is estimated for N 2 and Ar.

Isotope analysis of water by means of near infrared dual-wavelength diode laser spectroscopy

A novel diode laser spectrometer was developed using dualwavelength multiplexing, ensuring ideal conditions for high-precision and simultaneous measurements of the 2H/1H, 17O/16O, and 18O/16O isotope ratios in water. A 1.4-m diode laser probed a H16OH / HO2H line pair near 7198 cm-1, while a similar laser observed H16OH, H17OH, and H18OH rovibrational lines around 7183 cm-1, or a H16OH / H18OH line pair near 7200 cm-1. The 1-sigma standard deviation is 0.2 per thousand for 18O/16O, and 0.5 per thousand for the 2H/1H and 17O/16O isotope ratios. Preliminary experiments with repeated injections of a natural abundance sample point to an accuracy of about 1 per thousand for all three isotope ratios in natural samples.

Cluster dynamics in the range N=2–20: High resolution infrared spectra of HeN–CO

Infrared spectra of HeN–CO complexes with N up to about 20 have been observed in the 2145 cm−1 region of the C–O stretch vibration using a tunable diode laser spectrometer to probe pulsed supersonic expansions from moderately high pressure (⩽40 atm) cooled (>−150 °C) jet sources. Cooler (⩽0.2 K) or warmer (⩽0.5 K) effective rotational temperatures were obtained using pinhole or slit jet nozzles, respectively. Two series of R(0) transitions were observed, each correlating smoothly with the known a-type (K=0←0) and b-type (K=1←0) R(0) lines of the binary complex, He–CO. Although the b-type series starts off about 7 times stronger for N=1, it was observed to lose intensity to the a series with increasing N-value. The numbering of cluster size was reliably established up to N=14 for the a-type and N=6 for the b-type series. Some warmer lines due to higher J-value transitions [e.g., R(1)] were observed and tentatively assigned, but these were not sufficient to enable rotational analysis. Thus it has not yet been possible to separate the effects of vibrational shifts and rotational dynamics on the line positions. Two critical regions were observed in the cluster size evolution around N=7 and 15, and these may be related to the theoretically calculated maximum and minimum, respectively, in the incremental binding energy per helium atom.

High-resolution IR spectroscopy of a high lying K a = 0 mode of the weakly bound van der Waals complex Ar—CO

The infrared spectrum of the weakly bound van der Waals complex Ar-CO has been studied in the 2174cm−1 region using a lead salt diode laser spectrometer. We assigned the observed lines to a K a = 0 ← 0 subband. The upper state corresponds to the highest known van der Waals mode of this prototype complex which has been detected so far.

H 2-broadening coefficients in the ν2 and ν4 bands of PH 3

H 2-broadening coefficients are measured for 41 transitions of PH 3 in the QR branch of the ν 2 band and the PP , RP , and PQ branches of the ν 4 band, using a tunable diode-laser spectrometer. The recorded lines with J values ranging from 2 to 16 and K from 0 to 11 are located between 995 and 1106 cm −1 . The collisional widths are determined by fitting each spectral line with a Voigt profile, a Rautian profile, and a speed-dependent Rautian profile. The latter model provides larger broadening coefficients than the Voigt model. These coefficients γ 0( J, K) are found to decrease slightly on the whole as J increases and they decrease significantly for K values approaching or equal to J (J⩾4) . The H 2-broadenings are also calculated on the basis of a semiclassical model of interacting linear molecules, using an atom–atom Lennard-Jones potential in addition to the weak electrostatic contributions. The theoretical results are in satisfactory agreement with the experimental data and reproduce the J and K dependencies of the broadenings, but the decrease observed for the QR(J,K) transitions with K= J is notably overestimated.

H 2-broadening coefficients in the ν3 band of CH 3D at low temperatures

Using a diode-laser spectrometer, we have measured H 2-broadening coefficients of CH 3D at low temperatures (153.5, 183.5, and 223.5 K) for four lines in the ν 3 band. The collisional widths are obtained by fitting each absorption line with three lineshape models: the Voigt, Rautian, and Galatry profiles. The broadening coefficients are also calculated on the basis of a semiclassical model of interacting linear molecules by considering an atom–atom Lennard–Jones potential in addition to electrostatic contributions. By comparing the broadening coefficients at room and low temperatures the temperature dependence of these broadenings has been determined both experimentally and theoretically.

The high-resolution spectrum of the Ar–CH 4 complex in the [formula omitted]m region: measurement and ab initio calculation

The ν 4 fundamental vibration spectrum of the weakly bound complex Ar–CH 4 in the 7 μm region was discovered, analyzed, and compared with a synthesized spectrum, derived from ab initio calculations. The measurements were made by probing a supersonic gas expansion with a tunable diode laser (TDL) spectrometer. Several bands of Ar–CH 4 associated with different rovibrational transitions of the ν 4 vibration (asymmetric bending vibration) of CH 4 were recorded and analyzed in a spectral region from 1295 to 1330 cm −1. In particular the following transitions were studied: j=1←0 at 1311 cm −1 reported in Z. Naturforsch. 53 (1998) 725, j=0←1 at 1301 cm −1, j=1←1 at 1306 cm −1, j=2←1 at 1316 cm −1, and j=3←2 transitions at 1322 cm −1. Here, j denotes the angular momentum of the methane unit inside the complex. The densest part of the experimental spectrum located in the region between 1300 and 1315 cm −1 is compared in depth with the synthesized ab initio spectra. The close agreement between observed and theoretical ab initio spectra is convincingly demonstrated with respect to the gross spectral features, including also many details of the spectra. The ab initio spectra were computed as described in J. Chem. Phys. 110 (1999) 5639 for the ν 3 excited Ar–CH 4 complex. Briefly, a potential was obtained from symmetry adapted perturbation theory and used as input for a variational solution of the rovibrational problem. After the levels were calculated the line intensities were computed with the ν 4 transition dipole as unit of dipole strength. A Boltzmann distribution was assumed at 7 K.

Diode laser spectroscopy of CO 2 in the [formula omitted] region for the in situ sensing of the middle atmosphere

A diode laser spectrometer was used in the laboratory to study CO 2 line intensities and pressure-broadening coefficients near 1.6 μm . The spectral region ranging from 6230 to 6250 cm −1 which is suitable for the in situ sensing of carbon dioxide in the lower stratosphere was studied using a commercial telecommunication-type diode laser. Thirteen lines of the (30 01) III←(000) band of CO 2 have been studied. The results of intensity measurements are compared to previous determinations and available databases. Furthermore the broadening coefficients by N 2 and O 2 for the strongest transitions are also reported and analyzed. Finally, preliminary measurements of stratospheric CO 2 achieved in 2002 with the “SDLA” balloonborne TDL-spectrometer are discussed.

Argon-broadening coefficients in the ν5 band of acetylene at room and low temperatures

Ar-broadening coefficients have been measured in the P- and R-branches of the ν 5 fundamental band of C 2H 2 for 30 lines at room temperature and 8 lines at −100 °C, using a tunable diode-laser spectrometer. These lines with J values ranging from 2 to 27 are located in the spectral range 665–795 cm −1. The collisional widths are obtained by fitting each absorption line with three lineshape models: the Voigt profile, the Rautian profile accounting for the Dicke narrowing effect, and a general Rautian profile including the absorber speed-dependent collisional broadening. The latter model provides significantly larger broadening coefficients than the Voigt model. These coefficients are also calculated from a semiclassical theory performed by using a simple intermolecular potential with two adjustable parameters. Finally, the temperature dependence of the broadening coefficients has been determined both experimentally and theoretically.

The near infrared overtone spectrum of propyne taken by diode laser in the 12700 cm−1region

Detailed regions of the near infrared spectrum of propyne between 12737 cm-1 and 12778cm-1 have been measured using a simple and inexpensive home-made laser diode spectrometer. This part of the spectrum covers the overlapping 3v1 + v3 + v5 and 3v1 + v3 + v5 + v10—v10 bands. Combining a global fit and an individual profile analysis made possible the determination of the vibrational and rotational constants in each band and in some cases the resolution of the individual K structure in each transition cluster.

Formula omitted]-broadening in the [formula omitted] band of ethylene by diode-laser spectroscopy

Using a tunable diode-laser spectrometer, we have measured H 2 -broadening coefficients for 34 lines of C 2 H 4 in the ν 7 fundamental band. The studied transitions J ′ , K a ′ , K c ′ ← J ″ , K a ″ , K c ″ with J ″ ranging from 2 to 23, K a ″ from 0 to 4, and K c ″ from 2 to 22 in the P, Q, and R branches are located in the spectral range 919– 1023 cm - 1 . The collisional widths are obtained by modeling each spectral line with a Voigt and a Rautian profile. The measured broadening coefficients belonging to transitions with the same J ″ generally increase with increasing K a ″ and these coefficients decrease slightly on the whole with increasing J ″ . These behaviors can be roughly predicted by an approximate semiclassical calculation performed by using in addition to the weak electrostatic contributions a simple formalism for the contribution of the intermolecular potential.

Collisional broadening and shifting of ammonia absorption lines at 790 nm

By using a diode laser spectrometer based on a commercial heterostructure diode laser operating in free-running mode, line shape parameters of some ammonia ro-vibrational overtones at 790 nm have been measured at room temperature. These weak absorption lines have been detected by using the wavelength modulation spectroscopy technique with second-harmonic detection. The broadening and shifting coefficients have been obtained for ten absorption lines by fitting the collected second-harmonic absorption features and varying the pressure of different buffer gases.

Rovibrational analysis of the ν1fundamental of CF3Cl from diode laser spectra in a supersonic slit-jet expansion

Spectra of the ν1 fundamental band of CF3Cl (CFC-13), with natural isotopic abundance, were recorded with a tunable diode laser spectrometer in the range 1100–1116 cm−1 using a supersonic pulsed slit-jet expansion. A rotational temperature of about 50 K was reached and a significant simplification of the spectra was achieved. The reduced Doppler broadening obtained using the slit source led to a FWHM up to 0.0007 cm−1 and so it was possible to resolve almost completely the K-structure in the P(J) and R(J) clusters for the first time. From the final fit of the assigned transitions a reliable set of molecular constants including the quartic centrifugal distortion coefficients for the v1 = 1 state of CF335Cl and CF337Cl has been obtained.

Tunable Diode Laser Spectroscopy of cis-1,2-Difluoroethylene: Rovibrational Analysis of the ν9 and ν5+ν10 Bands and Anharmonic Force Field

cis-1,2-Difluoroethylene was synthesized, and the gas-phase infrared spectrum was investigated in the ν9 and ν5+ν10 band region (1350−1391 cm-1), at a resolution of about 0.002 cm-1, employing a tunable diode laser spectrometer. These vibrations of symmetry species B1 yield strong a-type bands. Most of the J and K structure was recorded in different subbranches and the rovibrational analysis led to the assignment of 2313 (J ≤ 64, Ka ≤ 26) and 1172 (J ≤ 57, Ka ≤ 20) transitions of the ν9 and ν5+ν10 bands, respectively. Using Watson's A-reduction Hamiltonian in the Ir representation a set of accurate spectroscopic parameters, up to four sextic centrifugal distortion terms, was obtained for both the excited states v9 = 1 and v5 = v10 = 1. A Fermi resonance coupling constant W = 4.2(5) cm-1 was estimated for the perturbation between the ν9 and ν5+ν10 levels. Correlated harmonic force constants were obtained from coupled cluster CCSD(T) calculations with the cc-pVTZ basis set while the anharmonic force constants were computed at MP2 level using the same basis set. There is a good agreement with the available experimental data, in particular the equilibrium geometry compared with the experimental r0 structure and the fundamental modes. In addition, the ab initio anharmonic force field of cis-1,2-difluoroethylene provided a critical assessment of the experimental spectroscopic parameters, especially in the treatments of strong Fermi interactions.

Diode-laser measurements and calculations of H 2-broadening coefficients in the ν3 band of CH 3D

Using a tunable diode-laser spectrometer, we have measured at room temperature the H 2-broadening coefficients of 12 CH 3 D for 36 lines belonging to QP and QR branches in the ν 3 parallel band. The recorded lines with J values ranging from 1 to 15 and K from 0 to 9 ( K⩽ J) are located between 1196 and 1412 cm −1. The H 2-broadening coefficients were determined by fitting each spectral line with Voigt, Rautian, and Galatry profiles. They were also calculated on the basis of a semiclassical model of interacting linear molecules performed by considering in addition to the weak electrostatic contributions the atom–atom Lennard-Jones potential. The theoretical results are in reasonable agreement with the experimental data, except for high J transitions where they are overestimated and for K approaching or equal to J with J⩾3 where they are underestimated. The latter discrepancy may be caused by the assumption to consider only Δ K=0 collision-induced transitions, associated with |Δ J| transitions up to 4.

Molecular-Gas-Pressure-Induced Line-Shift and Line-Broadening in the ν 2-Band of H 2S

The pressure-induced shift and broadening of H 2S absorption lines in the ν 2-band due to collisions between H 2S molecules and quadrupole molecules, such as O 2, H 2, D 2, N 2, and CO 2, were studied in the spectral region between 1050 and 1325 cm −1. The measurements were carried out using a pulse-driven diode laser spectrometer with two multipass Herriott cells. The data concerning the collisional broadening and shift coefficients, γ and δ, respectively, and their dependencies on the rotational quantum number J″ and the quadrupole moment Q of the molecular perturber are presented for 14 P-branch transitions (3≤ J″≤8,0≤ K a Prime;≤3, 2≤ K c Prime;≤8), 7 Q-branch transitions (7≤ J Prime;≤10, 1≤ K a Prime;≤2, 6≤ K c Prime;≤9), and 18 R-branch transitions (2≤ J Prime;≤11, 0≤ K a Prime;≤4, 0≤ K c Prime;≤11). The broadening coefficients γ were determined with an accuracy to within 2% and shift coefficients δ were determined with an uncertainty of less than 10 −3 cm −1/atm for the majority of lines and broadening gases.