Tunable Diode Laser Spectrometer Research Articles

Infrared laser spectroscopy and rovibrational analysis of the ν5 fundamental of cis-1-chloro-2-fluoroethylene

The infrared spectrum of cis-1-chloro-2-fluoroethylene (cis-CHClCHF) has been investigated in the ν5 band region between 1220 and 1250 cm−1, at a resolution of about 0.002 cm−1, using a tunable diode laser spectrometer. This fundamental of symmetry species A′ yields a strong a/b-hybrid band with contributions of comparable intensity from both the components. The rovibrational analysis led to the identification of 3800 lines (J⩽69, Ka⩽25) for the ν5 band of 35Cl species, whose origin is at 1231.849 cm−1. Using Watson's A-reduction Hamiltonian in the Ir representation, a set of spectroscopic parameters up to sextic centrifugal distortion terms was obtained for the v5=1 state, which turned out to be free from perturbations.

N2 Broadening of Carbon Disulfide 12C32S2 in the ν3 and ν3–ν1 Bands

N2 broadening coefficients have been measured for 33 lines in the ν3 fundamental band and 21 lines in the ν3–ν1 band of C32S2, using a tunable diode–laser spectrometer. These lines with J values ranging from 0 to 78 in the ν3 band and from 4 to 70 in the ν3–ν1 band are located in the spectral ranges 1514–1548 and 853–887 cm−1, respectively. The collisional widths are obtained by fitting the spectral lines with a Voigt and a Rautian profile. Semiclassical calculations of these broadening coefficients have been performed by considering, in addition to electrostatic interactions, successively the atom–atom Lennard–Jones model and a simple formulation for the anisotropic dispersion forces leading to more satisfactory results.

High-Resolution Infrared Laser Study of the ν4 Absorption Band of cis-CHClCHF

cis-1-Chloro-2-fluoroethylene was synthesized, and the gas-phase infrared spectrum was investigated in the ν4 band region between 1321–1350 cm−1, at a resolution of about 0.002 cm−1, employing a tunable diode–laser spectrometer. This vibration of symmetry species A′ gives rise to an a/b-hybrid band, even though our analysis pointed out that the intensity of the a-type component is predominant. Most of the J and K structure was resolved in different subbranches, and the rovibrational analysis led to the assignment of about 1900 a-type lines with J ≤ 71 and Ka ≤ 20 for the 35Cl isotopic species. The ν4 fundamental is locally perturbed by a weak Coriolis resonance and some local crossings were found; the 3ν12 state seems to be the most likely interacting level. Using Watson's A-reduction Hamiltonian in the Ir representation, a set of spectroscopic parameters for the v4 = 1 state was obtained.

Inhomogeneous Lineshape Profiles of C2H2 Perturbed by Xe

Several infrared transitions of the ν5 band of C2H2 perturbed by Xe near 700 cm−1 have been recorded using a tunable diode–laser spectrometer, equipped with a thermostatically regulated cell. The pressure and temperature regimes covered during the experiments are respectively ranging between 20 and 200 mbar and 170 and 350 K. The experimental profiles are accurately studied with a recently proposed lineshape model accounting for the collisional confinement narrowing of the Doppler spectral contribution, the inhomogeneous collisional broadening and shifting, and the exchange between the various speed classes of the Maxwell–Boltzmann distribution. The speed dependence of the collisional broadening is either ab initio calculated from a semiempirical potential or deduced from the experimental temperature dependence of the line broadening. It is shown that, for the considered range of temperature, this speed dependence does not significantly modify the lineshape, so that the usual hard and soft models conveniently account for most of the observed effects.

The tunable diode laser: a versatile spectroscopic tool

The tunable diode laser (TDL) has become a very versatile and indispensable tool for basic research and analytical applications alike. The fairly monochromatic beam emitted by the TDL qualifies it to be used for high resolution spectroscopy in the Doppler and sub-Doppler domain. At the Cologne laboratories, TDLs have been used to carry out research in mid IR-spectroscopy in four areas where precisely controlled wavelength stabilized lasers (∼1 part in 10 6) are required. These four areas of fundamental research are: (a) a wavelength stabilized TDL spectrometer, where an internally coupled Fabry–Perot interferometer (icFPI) of 90 MHz free spectral range is used for wavelength stabilization, which enables a frequency accuracy of better than 1 MHz. Line profile measurements with pressure broadening and pressure shift parameters have been performed, e.g. on molecular hydrogen, H 2, and other species; (b) an IR heterodyne receiver with a TDL as local oscillator (LO) and an achieved spectral resolution of λ/Δ λ>10 6 is used for measurement of the Earth’s and other planetary atmospheres. Atmospheric ozone has been recorded; (c) an IR supersonic jet spectrometer with multireflection optics is used to study the low temperature spectra of van der Waals (vdW) complexes, such as Ar–CH 4, Kr–CH 4, and Ne–CH 4. Very recently, in collaboration with A.R.W. McKellar from the National Research Council, Ottawa, Canada, the spectra of paraH 2–CH 4 have been recorded and analyzed; (d) the carbon cluster TDL spectrometer combines the advantages of high resolution and high sensitivity laser spectroscopy with an intensive cluster source. The carbon clusters are produced by laser vaporization of a graphite rod target and flushed into the absorption cell by a supersonic jet expansion of He. Several carbon clusters have been recorded, e.g. C 9, C 13, and others the assignments of which are in progress, providing the possibility to monitor the growth and structure of small carbon clusters. Each of these four research areas will be discussed in turn, by presenting first a short overview of the field, followed by a brief experimental outline. Each section will be closed by reviewing the highlights of the most recent results obtained at the Cologne laboratories.

Infrared laser spectrum of cis-CHCI=CHF near 1060cm−1: rovibrational analysis of the perturbed ν6 fundamental and determination of the constants of the dark vibrational state ν9 + ν10

Cis-1-chloro-2-fluoroethylene has been synthesized, and the gas-phase infrared spectrum has been investigated in the ν6 band region around 1060 cm−1, at Doppler-limited resolution, using a tunable diode laser spectrometer. This fundamental of symmetry species A′ presents an a/b-hybrid band with contributions from both the components. The rovibrational analysis led to the identification of more than 2000 transitions (J ≤ 84, Ka ≤ 19) for the ν6 band of the 35C1 species, whose origin is at 1062.224cm−1. Different excited state perturbations occur throughout the band, caused by the first-order b-type Coriolis resonance with the ν9 + ν10 vibrational state. Using Watson's A-reduction Hamiltonian in the I r representation, a set of spectroscopic constants for both the upper states ν6 and ν9 + ν10 has been obtained. Parameters for the perturber were determined from the interaction effects near the observed crossings in the ν6 band, using a dyad model including first-order b-type Coriolis interaction.

Isotopic probing of very weak intermolecular forces: Microwave and infrared spectra of CO-He isotopomers

The pure rotational transition (J,j,l)=(101)←(000) of the weakly bound CO-He complex has been observed in the 17 GHz region for five isotopomers, namely C12O-16He4, C13O-16He4, C12O-18He4, C13O-18He4, and C13O-17He4, using a pulsed-jet cavity Fourier-transform microwave spectrometer. Hyperfine structure due to the quadrupolar O17 (I=5/2) nucleus has been observed and analyzed to yield the quadrupole coupling parameters. A new microwave-terahertz double resonance spectrometer has been used to carry out an experiment on C13O-16He4 in which a submillimeter-wave pump transition (J,j,l)=(110)←(000) and a microwave signal transition (211)←(110) were detected. Infrared spectra of C13O-16He4 and C12O-18He4, have been obtained in the 2100 cm−1 region of the C-O stretch using a tunable diode laser spectrometer and a long-path (200 m), low-temperature (46 K) equilibrium gas cell. The combined data are sufficient to construct essentially complete experimental energy level schemes for the bound states of the C12O-16He4, C13O-16He4, and C12O-18He4 forms of the complex. These energies are compared with calculated levels derived from two intermolecular potential models, V(3,3,3) and XC (fit). It is shown that the new spectra, in particular those of C13O-He16, discriminate between the two models and indicate that V(3,3,3) provides a better representation of the potential in the region of the attractive well probed by the bound states. A quasibound state of CO-He was observed for the first time, and its width due to predissociation was approximately determined, providing a sensitive measure of the depth of the potential.

Rovibrational intensities in the ν4 band of ammonia

Absolute line intensities of sixteen rovibrational transitions in the ν4 band of 14NH3 were measured around 1800cm−1 using a tunable diode laser spectrometer with a sweep modulation of the laser frequency. Individual laser scans over the line region were recorded to disk storage separately and the spectrum accumulation was performed off-line using a computer to reduce a significant line broadening because of a lower accuracy of the hardware on-line accumulation.From integrated absorption coefficients obtained by the numerical analysis of the measured line profile, the effective transition moments of individual rovibrational lines were derived. Using the second order perturbation theory of the transition moment of rovibrational transitions of the symmetric top molecules, the effective vibrational transition moments for the ν4 fundamental band of 14NH3 were estimated.

Oxygen Broadening of Acetylene Lines in the ν5Band at Low Temperature

O2-broadening coefficients have been measured for 29 lines of C2H2at 173.2 K in thePandRbranches of the ν5band near 14 μm, using a tunable diode–laser spectrometer. The collisional widths obtained by Rautian profiles closely fitting the measured spectral shape of the lines are slightly larger than those derived from Voigt profiles. The broadening coefficients are compared with results calculated from a semiclassical theory, performed by considering, in addition to electrostatic interactions involving the hexadecapole moments of C2H2and O2, the atom–atom Lennard–Jones model. By comparing broadening coefficients at 297 and 173.2 K from a simple power law, the temperature dependence of these broadenings has been determined both experimentally and theoretically.

Seasonal variation in methane emission from a temperate Phragmites‐dominated marsh: effect of growth stage and plant‐mediated transport

AbstractThe eddy covariance technique was employed with a tunable diode laser spectrometer to quantify methane flux from a prairie marsh dominated by Phragmites australis in north‐central Nebraska, USA. The observations spanned the entire growing season (April to October) and a wide range of weather conditions, allowing a quantitative assessment of the physical and biological controls of methane emission in this ecosystem. Diel patterns in methane emission varied markedly depending on plant growth stage. Prior to plant emergence above water, the rate of methane emission from the marsh was fairly constant throughout the day. After emergence above water, there was a gradual increase in methane emission after sunrise with a peak in late afternoon. Significant changes in diel patterns were observed after tillering. Then, the diel pattern was characterized by a mid‐ to late‐morning peak and a 2‐to 4‐fold increase in methane emissions from night to daytime. In early stages of plant growth, molecular diffusion through dead/live plants and the standing water column seemed to be the primary transport mechanism. After tillering, a transition occurred in the transport mechanism from a molecular diffusion to a convective throughflow, which is a rapid and active gas transport driven by pressure differences. The role of convective throughflow became less important as the plants senesced. Integrated methane emission over the six‐month measurement period (April–October) was about 64 g CH4 m–2. On an annual basis, we estimate the annual methane emission from this ecosystem to be ≈ 80 g CH4 m–2 and that about 80% of the total methane emission occurred between late April and late October.

Infrared Laser Spectroscopy of the ν4and ν9Band System of CH2F37Cl

The spectra of CH2F37Cl (95% isotopically pure sample) have been recorded at high resolution with a tunable diode laser spectrometer in the regions 979–1030 and 1108–1136 cm−1. The spectral analysis allowed us to assign about 2000 lines (J≤ 61,Ka≤ 26) of the ν9band (ν0= 1001.9360 cm−1), which, besides the expectedc-type component, shows a more intense Coriolis-induced parallel component deriving its intensity from mixing with the ν4vibrational state. In addition, the assignments of the ν4band (ν0= 1067.7138 cm−1), previously investigated, have been extended to higherKavalues and about 1300 new lines (J≤ 69, 17 ≤Ka≤ 26) have been identified. The ν4and ν9bands have been fitted simultaneously to account for the first-ordera- andb-type Coriolis coupling; interaction constants and excited state parameters up to the sextic terms have been obtained using Watson'sA-reduction Hamiltonian in theIrrepresentation.

Infrared laser spectroscopy of CCO: The ν1 band of the ã1Δ electronic state

The rotationally resolved infrared spectrum of the ν1 band of the long-lived ã1Δ electronic state of the CCO radical has been observed between 1918 and 1970 cm−1 using a tunable diode laser spectrometer. Metastable CCO was produced in a discharge through a flowing mixture of carbon suboxide and helium. Fifty eight rovibrational transitions in the P and R branches and the five lowest J lines in the Q branch were measured. The band origin was determined to be 1942.8515(19) cm−1. This is in close agreement to the values determined previously from photoelectron spectroscopy of CCO−.

Temperature-Dependent Line Shift and Broadening of CO Infrared Transitions

The temperature dependence of lineshift and broadening of the rovibrational transitions R(18) and R(20) of the CO fundamental band, perturbed by Ar, N2, O2, and H2, have been measured with high frequency accuracy and at temperatures between 160 and 270 K in steps of 20 K. A wavelength stabilized tunable diode laser spectrometer has been combined with a low temperature long path cell of 134 m absorption length and 1 m basis length. For all measurements the CO pressure was below 0.1 mbar to avoid self-shift and self-broadening. In case of line broadening the temperature dependence is quite well reproduced by an exponential relation, b(T) = b(T0)(T/T0)−n. For all foreign gases, the exponent n has been obtained (0.53 ≤ n ≤ 0.71) and a value for air has been calculated from the weighted mean values of N2and O2. Within the error limits the magnitudes of all shifts decrease with increasing temperatures, but there is no exponential behavior of the shift versus temperature. The line broadening and shift for CO with Ar and the broadening of CO by N2and O2have been compared to calculations from the semi-classical theory of Robert and Bonamy. Sufficient agreement has been achieved for the line broadening, while the calculated shifts are for all temperatures larger than the measured values.

Collisional-Broadening Coefficients in the ν6Band of12CH3F Perturbed by Ar

Ar-broadening coefficients are measured for 29 lines of CH3F in theRQ0andRQ6branches of the ν6band, using a tunable diode–laser spectrometer. The collisional widths obtained by fitting Rautian profiles to the measured shapes of the lines are significantly larger, especially for the overlapping lines at low-J values, than those obtained from Voigt profiles. The broadening coefficients for the lines belonging to the same low-J transitions (J = 7–10) appear to be greater in theRQ0branch than in theRQ6branch. Comparisons have been made with calculated results based on a semiclassical model including adjustable potential parameters.

Analysis of O2-Broadening of12CH3F Lines in the 6.8-μm Spectral Region

Using a tunable diode-laser spectrometer, we have measured the O2-broadening coefficients for 45 lines of methyl fluoride in theQPandQRbranches of the ν2parallel band, and in thePP,PQ,PR,RP,RQ, andRRbranches of the ν5perpendicular band. These lines withJvalues ranging from 1 to 23 andKfrom 0 to 9 (K ≤ J) are located in the spectral range 1416–1573 cm−1. The collisional halfwidths are obtained by modeling each spectral line with a Voigt and a Rautian profile. The noncorrelated Rautian model leads to larger values of broadening coefficients than the Voigt model. These coefficients have also been calculated from a semiclassical model in which the intermolecular potential includes electrostatic, induction, and dispersion energy contributions. The results in theP,Q, andRbranches of the ν2and ν5bands are in overall agreement with the experimental data.

Diel variation in methane emission from a midlatitude prairie wetland: Significance of convective throughflow in Phragmites australis

Methane flux was measured at a Phragmites‐dominated marsh in the Sandhills of north‐central Nebraska from late July to September 1993. The eddy covariance technique employing a tunable diode laser spectrometer was used to measure spatially integrated fluxes of methane. Rates of methane emission increased rapidly after sunrise and peaked (at up to 50 mg CH4 m−2 h−1) between midmorning and noon. The emission rates during midday hours were 2 to 5 times higher than the relatively constant rates observed at night. This marked diel variation was attributed to plant‐mediated transport of methane by convective throughflow which typically accounted for about 60% of the total methane emission during the daylight hours. Our analysis suggests that the enhanced rate of plant‐mediated methane emission during daytime is significantly correlated with changes in photosynthetically active radiation and the temperature difference between the canopy and the ambient air. Data on windy nights showed enhanced methane emission with increasing wind speeds, perhaps indicating an occurrence of Venturi‐induced convection and/or enhanced ebullition. When averaged over the study period of 65 days, the methane flux was 25 mg CH4 m−2 h−1 during daytime and 8 mg CH4 m−2 h−1 during nighttime. Changes in the daily averaged methane flux were strongly correlated with sediment temperatures at 0–0.25 m depth.

Infrared spectrum of the water–carbon monoxide complex in the CO stretching region

The CO-stretching vibration–rotation spectra of water–carbon monoxide complexes (H2O–CO, D2O–CO, and HDO–CO) in the 2150 cm−1 region have been studied in a supersonic slit jet expansion using a rapid-scan tunable diode laser spectrometer. Accurate band origins and rotational parameters were determined in order to characterize the CO stretching dependence of the bonding in this system. The symmetric isotopes, H2O–CO and D2O–CO, exhibited two distinct nuclear spin components arising from proton- (deuteron-) exchange tunneling, as observed previously in the microwave spectrum. The tunneling splittings were found to increase by about 10% upon excitation of the CO stretching vibration, indicating a lowering of the tunneling barrier in the excited state. The D-bonded isomer, HOD–CO, of the unsymmetrical isotope was observed prominently, and the higher-energy H-bonded isomer, DOH–CO, was also detected for the first time. From the intensities of their spectra, the energy difference between these two forms of the complex was estimated to be 12.4±2.5 cm−1.

A tunable diode-laser spectrometer for the MIR region near 7.2 μm applying difference-frequency generation in AgGaSe 2

and two diode lasers as pump sources are presented. A single-mode Fabry–Perot-type tunable diode laser (TDL) and an external-cavity diode laser (ECL) were combined to generate radiation in the mid-infrared region near 7.2 μm. With a TDL at a wavelength of approximately 1290 nm and an ECL emitting between 1504 and 1589 nm it was possible to carry out spectroscopic experiments concerning SO2 at five different phasematching points between 1350 and 1400 cm-1 by fixing the wavelength of one pump laser and tuning the wavelength of the other. With an input power of 8 mW for the single-mode Fabry–Perot-type diode laser and 6 mW for the external-cavity laser an output power of about 10 nW was generated. Using the tuning capabilities of the external-cavity laser a spectral region up to 5 cm-1 could be covered within one scan. Measurements of SO2 absorption lines at low pressure demonstrate the high-resolution features of the spectrometer. Moreover, these data provide new direct experimental phasematching data for the rarely investigated spectral region at 7.2 μm.

Tropical entrainment time scales inferred from stratospheric N2O and CH4 observations

Simultaneous in situ measurements of N2O and CH4 were made with a tunable diode laser spectrometer (ALIAS II) aboard the Observations from the Middle Stratosphere (OMS) balloon platform from New Mexico, Alaska, and Brazil during 1996 and 1997. We find different compact relationships of CH4 with N2O in the tropics and extra‐tropics because mixing is slow between these regions. Transport into the extra‐tropics from the tropics or the polar vortex leads to deviations from the normal compact relationship. We use measured N2O and CH4 and a simple model to quantify entrainment of mid‐latitude stratospheric air into the tropics. The entrainment time scale is estimated to be 16 (+17, −8) months for altitudes between 20 and 28 km. The fraction of tropical air entrained from the extra‐tropical stratosphere is 50% (+18%, −30%) at 20 km, increasing to 78% (+11%, −19%) at 28 km.

Diode Laser Spectroscopy of Trifluoroethylene in the 8.6-μm Region

The two mid-infrared bands of the CF2CHF molecule, ν5centered at 1172.673 cm−1and ν6+ ν9at 1155.105 cm−1, were measured on a tunable diode laser spectrometer with a resolution near the Doppler limit. These vibrations ofA′ species give rise toa/bhybrid bands, even though our analysis has pointed out that the intensity of thea-type component is predominant. Most of theJandKstructure has been resolved in different subbranches, and the rovibrational analysis led to the assignment of about 1400 (J≤ 60,Ka≤ 22,Kc≤ 60) and 90 (J≤ 56,Ka≤ 5,Kc≤ 56) lines of the ν5and ν6+ ν9bands, respectively. Using Watson'sA-reduction Hamiltonian in theIrrepresentation, a set of accurate spectroscopic constants for the upper states has been derived from transitions free of major resonance effects. The rotational structure of the ν5vibration also exhibits effects of Coriolis perturbation by a state identified as ν7+ ν11. Parameters for the perturber were determined from the interaction effects near the observed crossings, using a dyad model including first-orderb-Coriolis interaction.