Dispersion studies of the 22 GHz water vapor line shape II. Instrumental correction

Anomalies in the resonance dispersion of the pressure-broadened water vapor line at υ₀ = 22.235 GHz, which were reported in Part I,⁽¹⁾ are resolved. The pressure-scanning differential-refraction spectrometer (microwave dispersometer) gives rise to a signal enhancement as the line center moves within the width of the dispersion discriminator which is a dual-mode transmission cavity. Resonance absorption prevents the frequency of peak transmitted power from coinciding with the resonance condition of zero phase, thus systematically distorting the molecular resonance dispersion. An expression, valid for |υ − υ₀| greater than the cavity half width, is derived which predicts apparent departure from Lorentzian behavior. The results reported in I as well as results for the 23.6 GHz rotational 3_1,2 → 3_2,1 line of C₂H₄O (ethylene oxide) show a Lorentzian molecular line shape. At pressures above 1 torr nonresonant polarization effects become detectable. Instrumental distortion, and Doppler-, wall-, and saturation-broadening effects at low pressures (<50 mtorr) are accounted for only qualitatively to determine the resolving power and to define v₀.     

Influence of line interference on the vibration-rotation band shapes

The shapes of the CO, v₃, CO₂, and v₃ N₂O fundamental vibration-rotation bands have been studied at various temperatures and in the presence of several perturbing gases. Also the half-widths of CO vibration-rotation lines have been measured at 78 K. In the region of line wings, the measured absorption coefficients deviate from those given by the superposition of Lorentzian profiles. These deviations are explained by the collision-induced line interference that causes redistribution of absorption inside the band. A theory of line mixing is formulated which is based on Markov approximation and on the strong collision model. Simple analytical expressions are obtained for the band shape. The computed shapes are in satisfactory agreement with the experimental results. The deviations from the Lorentz absorption observed in pure CO and in CO-N₂ at low temperature are partially ascribed to the formation of van der Waals dimers.     

High-sensitivity CRDS absorption spectroscopy of acetylene between 5851 and 6341 cm−1

The absorption spectrum of acetylene has been recorded at room temperature (297 K) using high-sensitivity cavity ring-down spectroscopy (α_min ～ 5×10^?11 cm^?1) in the 5851 and 6341 cm^?1 interval corresponding to a region of very weak absorption. A list of about 10,700 absorption features with estimated absolute line intensities was constructed. The smallest intensities are of the order of 5×10^?29 cm molecule^?1. The line list includes about 2500 absorption lines of ethylene present at the ppm level in the acetylene sample and identified on the basis of a high-resolution Fourier transform spectrum specifically recorded. A total of more than 2700 lines of ¹²C₂H₂ were rovibrationally assigned in comparison with accurate predictions provided by a global effective operator model. Overall, the present effort adds about 2260 new assignments to the set of about 500 assigned transitions available in the literature. The new assignments correspond to 45 new bands and 17 already-known bands, for which additional J lines were assigned. Spectroscopic parameters were derived for the upper vibrational levels from a band by band fit of the line positions (typical root mean square deviation values are of the order of 0.001 cm^?1). A few of the analysed bands were found to be affected by rovibrational perturbations, which are discussed. The new data will be valuable to refine the parameters of the global effective Hamiltonian and dipole moments of ¹²C₂H₂.     

EPR and optical absorption studies of VO doped l-alanine (C3H7NO2) single crystals

VO²⁺ doped l-alanine (C₃H₇NO₂) single crystals and powders are examined by electron paramagnetic resonance (EPR) and optical absorption spectroscopy. Three magnetically different sites are resolved from angular variations of l-alanine single crystal EPR spectra. In some specific orientations each VO²⁺ line splits into three superhyperfine lines with intensities of 1:2:1 and maximum splitting value of 2.23 mT. The local symmetries of VO²⁺ complex sites are nearly axial. The optical absorption spectra show three bands. Spin Hamiltonian parameters are measured and molecular orbital coefficients are calculated by correlating EPR and optical absorption data for the central vanadyl ion.     

Absorption of millimeter to submillimeter waves by atmospheric water molecules

Using the values of the rotational molecular parameters (including cencrifugal distortion terms) of the H₂ ¹⁶O molecule, which can explain 12 observed transitions below 800 GHz, all rotational energy levels with normalized Boltzmann factors larger than 5×10^–8 at 300°K are calculated. Probabilities of all possible electric dipole transitions among these states, 2277 lines, are calculated using the eigenfunctions thus obtained, and the permanent electric dipole moment of 1.8546 Debye. Assuming the single and full Lorentzian line forms, we calculated the absorption coefficient for millimeter to submillimeter region. Our result, using the single term Lorentzian line form, agrees quite well with experiment for 1 Torr of water vapor in 760 Torr air at 300°K.     

Line strength and halfwidth measurements from far infrared absorption spectra: Carbon monoxide

A method for deriving Lorentzian line shape parameters (line strengths and halfwidths) from absorption spectra taken at two different path lengths is discussed. Far i.r. absorption spectra of carbon monoxide are analysed to yield line strengths and halfwidths for the rotational transitions J″ = 3 through J>″ = 9. Experimental errors are discussed, and the value for the dipole moment derived from the line strengths, 0·108±0·005 D, is in agreement with the microwave value within experimental uncertainty. The Lorentzian halfwidths are compared, where appropriate, with two recent determinations of these quantities. Lorentzian halfwidths are also derived from absorption spectra taken at long path length using theoretical line strengths based upon μ_co = 0·112D.     

Synthetic atmospheric transmittance spectra near 15 and 4.3μm

Synthetic monochromatic atmospheric transmittance spectra are presented for infrared intervals in the vicinity of the 15 and 4.3μm CO₂ bands. The intervals are nominally 20 cm^-1 in width, and the spectra partition the intervals [560, 780] cm^-1 and [2180, 2400] cm^-1, respectively. The spectra are for a vertical atmospheric path. A given spectrum is presented at that pressure for which the mean weighting function $\text{(-d}\text{Tau (P)}\text{?d ln P)}$ is a maximum. In order to indicate the molecular origin of the features in a given spectrum, separate spectra are also presented for those species which make center line contributions to the absorption over the interval. The transmittance model for the calculations is described. In addition to the representation of spectral features for intervals which comprise these two CO₂ bands, the spectra afford a utility in the design of high precision satellite temperature radiometers.     

Tunable diode laser spectroscopy of CO2 in the 10- to 15-μm spectral region—Lineshape and Q-branch head absorption profile

The self-broadened and nitrogen-broadened absorption profiles of the Q-branch head of the 10⁰0 ← 01¹0 band of CO₂ near 618 cm^?1 were studied under high resolution (～10^?4 cm^?1) using tunable diode laser techniques. The intensity and linewidth of several lines in this Q branch, the ν₂ band, 10.4-μm and 9.4-μm laser bands have been measured accurately. The lineshape of a CO₂ line has also been measured to be Lorentzian within experimental accuracy to about 15 half-widths. Preliminary calculated Q-branch head absorption profiles agree reasonably well with the observed profiles.     

Optical thickness in Σ-Σ molecular bands

The single line resonance results of Malinowski⁽¹⁾ and others are extended to include overlapping neighboring lines. This result is combined with the molecular line strengths and energy levels of a permitted Σ-Σ transition to calculate both relative and absolute optical depths of individual lines, multiplets, and vibrational bands within a Σ-Σ molecular band system. An average optical depth is also calculated for the entire band system. The results will apply to self absorption in laboratory and auroral emissions within appropriate ranges of temperature, pressure, optical depth, and molecular constants.     

Spectroscopy of the Tm:Sc<Subscript>2</Subscript>SiO<Subscript>5</Subscript> laser crystal in different orientations

Six absorption bands of Tm³⁺:Sc₂SiO₅ crystal have been investigated by decomposing each absorption band into a series of Lorentzian peaks. Polarized light of two polarizations was transmitted in three perpendicular directions through a crystal shaped as a parallelepiped with polished faces. The results are presented in the form of tables containing the parameters of the found Lorentzian peaks: central wavelength, width, and height.     

Millimetre-wave and Fourier transform infrared spectra between 325 cm−1 and 4720 cm−1 of BrCN and global rovibrational analysis of the main isotopomers

The infrared absorption spectrum of BrCN has been recorded in the re?ion 325–4720 cm^?1 with a Bruker 120 HR interferometer. More than 30 000 lines have been measured and most of them have been assigned: 10734 lines for 151 bands and 10993 lines for 150 bands of ⁷⁹Br¹²C¹⁴N and ⁸¹Br¹²C¹⁴N, respectively. Also new millimetre-wave spectra were measured for the vibrational excited states lying between 1000 cm^?1 and 1400 cm-′. After a band by band analysis of these transitions, rovibrational global analyses have been performed for both isotopomers using all available experimental data. The rotational I-type resonance and the anharmonic resonance associated with k_l22 are taken into account in one-step diagonalization procedures. Sets of 73 molecular parameters are obtained for both isotopomers. It is shown that the existing equilibrium structure is likely to be reliable. The vibrational dependence of the quartic centrifugal distortion constant is analysed for the linear triatomic molecules studied so far.     

Infrared diode laser absorption features of N2O and CO2 in a laval nozzle

Design and operation of a pulsed Laval nozzle and the characterization of molecular flow through such a nozzle using IR tunable diode laser (TDL) is the central theme of this work. The results here deal with He diluted N₂O and CO₂ gaseous systems. Boltzmann type plots of the spectral intensity data of both N₂O and CO₂ show non-linear behaviour. We have attempted to understand this non-linear behaviour of Boltzmann plots in terms of (1) instability in the jet and (2) a two-temperature model for the flowing gas, a cold central core and a hot boundary layer close to the nozzle walls. The model based on jet instability represents the data somewhat poorer than the two-temperature model. The parameters derived from fitting our experimental data to the former model could be used to calculate the thermodynamic parameters only through further approximations. Measured absorption line profile of the P(15) line of the v ₂ band of N₂O as a function of axial distance from the nozzle exit gradually shifts from a Lorentzian to a Gaussian type. Velocity distribution of N₂O molecules in a Laval nozzle is determined by differentiating the absorption line profile of the P(15) line (v ₀=576.235 cm^–1) of the v ₂ band of N₂O. Translational temperature of N₂O molecules is determined from the observed spectral profiles.     

Linewidths of HCl broadened by CO2 and N2 and CO broadened by CO2

High resolution measurement of the linewidths of HCl broadened by CO₂ and N₂ and CO broadened by CO₂ have been performed in both the 1-0 and 2-0 bands of HCl and the 2-0 band of CO. The data were analyzed by the direct and the peak absorption methods. Values of the linewidths obtained by the two methods are in good agreement. For |m| ≤ 3, for the case of HCl + CO₂, the agreement is good for the values obtained in both bands of HCl. However for |m| > 3, the HCl + CO₂ linewidths in the 1-0 band are smaller than the corresponding lines in the 2-0 band by as much as 11% for |m| = 9. Lines (|m| ≤ 3) of the 1-0 and 2-0 bands of HCl broadened by CO₂ were also analyzed in terms of the super-Lorentzian line profile proposed by Varanasi, S. K. Sarangi, and G. D. T. Tejwani (J. Quan. Spectr. Radiative Transfer12, 857 (1972)) and the Lorentzian profile. The results indicate that near the line center (within 3γ), the shape of HCl + CO₂ lines are Lorentzian.     

An empirical study on the importance of a speed-dependent Voigt line shape model for tropospheric water vapor profile remote sensing

We use high quality ground-based solar absorption spectra measured in close coincidence with Vaisala RS92 radiosonde in situ water vapor profiles to demonstrate that a Voigt line shape model yields systematic errors in the remotely sensed tropospheric water vapor profiles. We analyse absorption signatures of 4 H₂¹⁶O and 2 HD¹⁶O bands situated between 790 and 4710 cm⁻¹. We find that applying a speed-dependent Voigt line shape model instead of a Voigt line shape model significantly improves the agreement between the water vapor profiles obtained by the radiosondes and by infrared remote-sensing in the different bands. An optimal agreement is obtained for a Γ₂ (relaxation rate for speed-dependence) of 6-21% of Γ₀ (Voigt relaxation rate), which is consistent to the values derived from laboratory experiments. Our study suggests that further extensive laboratory investigations of line shape models are a key for improving the quality of modern water vapor remote sensing products.     

Spectral properties of gaseous biomarkers and choice of the optimal analytical line at interference of the spectra of detected gases

A method for comparative investigation of the absorption line properties at interference of vibration-rotation bands of the detected and interfering gases is proposed. This method is intended for highly sensitive analysis of complex gas mixtures when measurement of absorption in a weak analytical line is hindered by a stronger, closely located interfering line. Disappearance of the analytical line extrema in the spectrum studied is proposed as an assessment criterion. The concentration ratio of the analytical and interfering gases under this condition could be used for quantitative analysis and comparison of the properties. This criterion can be applied to the first and second derivatives of the absorption spectra. To demonstrate the potential of the approach proposed, it was applied to the NO and NH₃ lines that are promising for monitoring the contents of these compounds in atmospheric and exhaled air. The interference of the absorption bands of H₂O, CO₂, and several gaseous biomarkers was analyzed in the near-IR spectral region using the HITRAN2000 database. It was demonstrated that the problem of the vibration-rotation band interference for detected and interfering gases is aggravated in this spectral region. This aggravation is caused not only by the decrease in the absolute band intensity for overtones and combination bands of the molecular vibrations but also by the relative increase in the absorption in H₂O lines and high density of CO₂ lines in the near-IR spectral region.     

Spectral line parameters in the (2 ← 0) overtone band and the dipole moment function of the HI molecule

We present here new high-resolution experimental data on the linestrengths and pressure-broadened Lorentzian widths for the P₂(13) to R₂(12) vibration-rotation lines in the first overtone absorption band of hydrogen iodide. By combining the measured linestrengths with our previous results for the fundamental band [J. Mol. Spectrosc. 218 (2003) 75] and with other published data for the higher-overtone bands of this molecule, an improved dipole moment expansion as a function of the dimensionless reduced nuclear displacement x is obtained: μ(x)=0.4471(5)−0.0772(2)x+0.542(3)x²−1.90(2)x³. This experimental dipole moment function is compared with the results of a few recent non-relativistic and relativistic ab initio calculations. The agreement between theoretical and experimental Herman-Wallis coefficients for the first two vibrational bands of HI is found best as ever reported before.     

N2- and O2-broadening of CO2 for the (301)III ← (000) band at 6231 cm

The N₂- and O₂-broadening effect have been investigated for 10 absorption lines of the CO₂ (30⁰1)_III ← (00⁰0) band centered at 6231 cm⁻¹, in the range from P(28) to R(28) by a near-infrared diode-laser spectrometer. We have analyzed the observed line profiles with the Galatry function, and determined the N₂- and O₂-broadening coefficients precisely. The air-broadening coefficients for these lines have been derived. The present results are compared with those of the previous studies for this band and with some of the other bands.     

Calculation of the sound absorption in superfluid helium below 0.6 K

The sound absorption in superfluid helium is calculated using the renormalized perturbation expansion of the phonon self-energy. The second-orderωT ⁴-result is derived without the assumption of a Lorentzian form of the phonon spectral function. The fourth-order contribution leads to better agreement with experimental data. These results are compared with simple perturbation theory. A discussion of the Boltzmann-equation approach is also given.     

Relation between the EPR linewidths and the exchange integrals in ACrO2 (A = Li, NaandK)

The temperature dependence of the peak-to-peak EPR linewidth (ΔH_p-p) of the ACrO₂ (A = Li, Na, K) compounds in the X-band is typical for the previously observed antiferromagnetic interactions. All three oxides show a single absorption line with a lorentzian shape. As expected the room temperature values of ΔH_p-p in the Q-band are very close to those obtained in the X-band. Using the Anderson-Weiss model for exchange-narrowed Lorentzian lines and the high-temperature limit values for ΔH_p-p 's of all three oxides it was possible to estimate the coefficient relating the exchange integrals between the adjacent Cr³⁺-ions in the cationic sublattice to the corresponding exchange fields and the coefficient relating the crystal-field parameters of Cr³⁺-ions to the second moments of the spectral lines.