Collisional Line Shape Research Articles

Pure rotational R(0) and R(1) lines of CO in Ar baths: experimental broadening, shifting and mixing parameters in a wide pressure range versus ab initio calculations.

The results of a rigorous study of the two first pure rotational transitions of CO perturbed by Ar are presented. The experimental part is based on the use of three different spectrometers covering together the pressure range from 0.02 up to 1500 torr. The measurement results of collisional line shape parameters are supported by fully ab initio calculations, which are in remarkable agreement with retrieved data. A sub-percent uncertainty of line intensity measurements is achieved and the first firm evidence that the resonance spectrum of CO is observed on the continual pedestal is given. We analyze the results of our ab initio calculations on the basis of early analytical theories and demonstrate a good general applicability of the latter to the CO-Ar collisional system.

Simultaneous Temperature and Pressure Measurements in Compressible Flow Using Nanosecond O2 Coherent Anti-Stokes Raman Spectroscopy.

Simultaneous pure-rotational coherent anti-Stokes Raman spectroscopy (PRCARS) and vibrational O2 CARS spectroscopy (VCARS) were performed at elevated pressure and lowered temperature conditions in non-reacting compressible flow. We applied dual-pump CARS in a three-laser, three-color configuration to simultaneously acquire the PRCARS and VCARS spectra of O2. PRCARS spectra provide excellent sensitivity to temperature at relatively low temperatures. Pressure was extracted using the differential response of collisional effects in the PRCARS and the VCARS spectra. We used an under-expanded jet outside a choked converging nozzle as the compressible flow-field. We numerically analyze the pressure sensitivity of the combined CARS technique. Finally, we compare the collisional narrowing lineshape models of rotational diffusion narrowing and modified-exponential-gap model, for fitting the experimental spectrum.

Ab initio calculations of collisional line–shape parameters and generalized spectroscopic cross-sections for rovibrational dipole lines in HD perturbed by He

We report ab initio calculations of generalized spectroscopic cross-sections for hydrogen deuteride perturbed by helium. From these calculations, collisional line-shape parameters are deduced for the HD electric dipole transitions: from R(0) to R(5) and from P(1) to P(6) in the 0–0 to 5–0 bands. These parameters are necessary for a proper interpretation of HD spectra from the atmospheres of gas giants. We demonstrate that the centrifugal distortion cannot be ignored, not only for pure rotational lines but also for rovibrational lines when one aims at sub–percent accuracy of the collisional line–shape parameters.

Tunable Diode Laser Atomic Absorption Spectroscopy for Detection of Potassium under Optically Thick Conditions.

Potassium (K) is an important element related to ash and fine-particle formation in biomass combustion processes. In situ measurements of gaseous atomic potassium, K(g), using robust optical absorption techniques can provide valuable insight into the K chemistry. However, for typical parts per billion K(g) concentrations in biomass flames and reactor gases, the product of atomic line strength and absorption path length can give rise to such high absorbance that the sample becomes opaque around the transition line center. We present a tunable diode laser atomic absorption spectroscopy (TDLAAS) methodology that enables accurate, calibration-free species quantification even under optically thick conditions, given that Beer-Lambert's law is valid. Analyte concentration and collisional line shape broadening are simultaneously determined by a least-squares fit of simulated to measured absorption profiles. Method validation measurements of K(g) concentrations in saturated potassium hydroxide vapor in the temperature range 950-1200 K showed excellent agreement with equilibrium calculations, and a dynamic range from 40 pptv cm to 40 ppmv cm. The applicability of the compact TDLAAS sensor is demonstrated by real-time detection of K(g) concentrations close to biomass pellets during atmospheric combustion in a laboratory reactor.

Hyperfine effects on collisional line shape. I. A self-consistent set of equations

We show that the treatment available in the literature for calculating hyperfine effects on collisional line shape is affected by an error making the theory not self-consistent. By correcting such an error we show that the two hyperfine effects on the line shape, modification of the resolved components and collisional coupling between them, cancel each other when hyperfine structure collapses into a single line.

Experimental and Theoretical Study of Helium Broadening and Shift of HCO+ Rotational Lines

An experimental and theoretical study of pressure broadening and pressure shift of HCO(+) rotational lines perturbed by collisions with He is presented. Results are reported from measurements at 88 K for the lines j=4<--3, 5<--4 and 6<--5 with frequencies ranging from 0.35 to 0.54 THz. Using a new CCSD(T)/aug-cc-pVQZ potential energy surface for the He-HCO(+) interaction, the collisional line shape parameters are studied from fully quantum and semiclassical calculations. Results from the quantum treatment are in satisfactory agreement with experiments whereas the semiclassical approach can lead to appreciable differences. A study of the dependence of line width Gamma and shift s as a function of the translational energy shows the presence of quantum oscillations. Calculations on a previous Hartree-Fock-based potential energy surface lead to quite similar results for the collisional line shape parameters. Using a simplified version of the potential morphing method it is found that the line width Gamma is particularly sensitive to the long-range part of the potential energy surface. This also explains the success of the first line-broadening calculations which date back to the 1950s.

Broadening of CH3F in presence of Stark fields. II. Collisional coupling between the Stark components

The collisional line shape of methyl fluoride rotational transitions J,K=2,2→3,2 and 5,5→6,5 is observed under the effect of a static electric field. The collisional coupling between partially overlapping Stark components is studied under two particularly favorable conditions: first, the degree of overlapping can be set by both the electric field and the pressure, second, we have a calculated collisional relaxation matrix checked on both well isolated and completely unresolved Stark components. This allows us to obtain better insight into the collisional coupling effect and to show definitely that this effect explains the difference between collisional linewidths of the CH3F rotational transitions and of their Stark components.

A PARTIALLY CORRELATED STRONG COLLISION MODEL FOR VELOCITY- AND STATE-CHANGING COLLISIONS APPLICATION TO Ar-BROADENED HF ROVIBRATIONAL LINE SHAPE

High-resolution tunable laser measurements of rovibrational line shapes in Ar-broadened HF [A.S. Pine, J. Chem. Phys., 1994, 101, 3444] have shown the need of partially correlated Dicke narrowing models to accurately describe the observed asymmetries. A strong collision model accounting for both velocity- and state-changing mechanisms (VCD) and phase-changing ones (D), is presented. The resulting line shape justifies, on a physical basis, the empirical model previously used to interpret the observed features. The present partially correlated strong collisional model leads to a clearer interpretation of the characteristic parameters determined from the experiment for HF–Ar. Further theoretical calculations and experiments show that the residual discrepancies for the first rovibrational lines cannot be attributed to the speed dependence of the collisional line shape parameters.

Satellite structure in the NaXe collisional lineshape observed in four wave mixing

Degenerate four-wave mixing in sodium vapor in the presence of a noble gas is measured and calculated for large detunings from resonance, where the collisional impact approximation is no longer applicable. The measurement thus probes the collisional potential of the Na-noble gas system. When the noble gas is Xe, and the detuning is to the red of the Na D 1 resonance, an avoided crossing between the quasi-molecular potentials results in a satellite in the absorption line shape, which can be detected in the four wave mixing signal. We calculate collisional line shapes with several available sets of NaXe interaction potentials using the quasi-static and semi-classic line broadening theories, and find that the available potentials do not yield line shapes which agree with the measurements. The lineshape derived from the measured degenerate four wave mixing signal shows good agreement with theoretical calculations, only if the non-adiabatic nature of the level crossing is considered. Based on our experimental lineshape and a non-adiabatic level crossing theory, we derive corrections to the NaXe potentials, near the level crossing, that is for interatomic separations in the range of 0.55–0.75 nm.

Air-Broadening of Rotational Lines of Ozone in the 1.5-THz Region

Collisional lineshape parameters of the rotational transitions of ozone in the submillimeter-wave region are required for an accurate determination of the mixing ratio of this molecule in the stratosphere. Moreover, accurate modeling of concentration profiles from atmospheric data requires knowledge of temperature dependence for pressure-broadening parameters. In this note, the authors report air-broadening measurements for rotational lines of [sup 16]O[sub 3] in the frequency range 1.5-1.8 THz. Three lines were chosen for the analysis: 15[sub 8,8] [l arrow] 14[sub 7,7] at 1 776 408.080 MHz, 19[sub 6,14] [l arrow] 18[sub 5,13] at 1 507 007.275 MHz, and 32[sub 4,28] [l arrow] 31[sub 3,29] at 1 505 242.570 MHz. They have been previously selected to retrieve the ozone mixing ratio from balloon-borne experiments. Air pressures ranged from 0 to 400 Pa and measurements were carried out at four different temperatures (233, 263, 295, and 323 K).

Effect of a dressing radiation field on the laser-induced collisional energy-transfer line shape

We describe a new scheme suitable for the study of strong-field effects on the laser-induced collisional energytransfer process. The use of two independently tuned radiation fields, to induce the interatomic transition and to modify the energy levels, is found to give rise to a splitting of the excitation spectrum peak, similar to the Autler-Townes doublet. Owing to the resonant coupling, an observable splitting of a few inverse centimeters can be induced by a dressing laser intensity of the order of 10 MW/cm(2).

Far-infrared self-broadening in methylcyanide: Absorber-perturber resonance.

Using tunable far-infrared spectrometers with high-frequency stability and accuracy, the self-pressure broadening and shift of CH3CN are measured. Evidence of absorber-perturber resonance effects on the collisional line shape are obtained. This tests the theoretical model and its possible improvements and also allows predictions of broadening and shift for a large class of molecules. Moreover, the resonance effect produces a theoretical temperature dependence of self-broadening that is different from what is commonly assumed.

A review of experiments and theory for collisional line shape effects in the rotovibrational ammonia spectrum

455 self-broadening, self-shift and foreign gas (N2 and CO2) broadening measurements forv 2 andv 4 ammonia vibrational bands are compared to theoretical calculations performed in the frame of the semi-classical impact approximation. On the whole the agreement is satisfactory. However, a detailed analysis of the experimental and theoretical results shows some shortcomings in the accuracy of the experimental data and in the validity of the theoretical approach. General indications are given in order to improve further the description of the collisional line shape effects which is of great interest both for basic knowledge and for various applications.

Collisional line shape for the rotational spectrum of methylcyanide: Experiments and theory

A collisional line-shape study specialized in the rotational spectrum of CH3CN is presented. This system provides a test for pressure broadening and shift theories which has until now been hindered by inadequate theory and by experimental results at variance. New accurate measurements are presented which allow an explanation of the problem related to the reliability of experimental data. The thorough comparison made between measurements and an improved theory—obtained inside the frame of the Anderson–Tsao–Curnutte approximation—demonstrates the ripeness of the study of such rotational spectrum from both the experimental and theoretical points of view.

Collisional redistribution of light: Far-wing line shapes and polarizations for the Ba-(Ar,Xe) systems

We have measured the far-wing collisional redistribution line shape, the redistributed fluorescence polarization, and the collisional alignment decay rates for barium perturbed by argon and xenon. The experiment was performed with a heated gas cell ($T\ensuremath{\sim}900$ K) with perturber-gas densities in the 1---30-Torr range and laser light detuned \ifmmode\pm\else\textpm\fi{} 3--- 1000 ${\mathrm{cm}}^{\ensuremath{-}1}$ from the 5535-\AA{} Ba I $6^{1}S\ensuremath{-}6^{1}P$ resonance line. Metal-vapor densities were determined by a Rayleigh scattering technique, which is outlined in detail. By correlating structure in the line shape with that of the polarization, we are able to make definitive statements about the interatomic potentials. The Ba data show strong similarities with previous experimental results for similar two-electron atoms, namely Ca, Sr, and Hg, and so our conclusions are of relevance to these systems as well.

The spectrum of titan in the far-infrared and microwave regions

The pressure-induced absorptions of gaseous nitrogen (N 2) and methane (CH 4) are computed on the basis of the collisional lineshape theory of G. Birnhaum and E.R. Cohen [ Canad. J. Phys. 54, 593–602 (1976)]. Laboratory data at 300 and 124°K for N 2 and at 296 and 195°K for CH 4 are used to determine the collisional time constant and their temperature dependence. The spectrum of Titan from the microwave to the far-infrared region (0.1–600 cm −1) is then modeled using these opacities and a temperature profile of Titan's atmosphere derived from the Voyager 1 radio occultation experiment. The model atmosphere is composed of N 2 and CH 4, their relative proportions being determined by the vapor pressure law of CH 4. A model with gaseous opacity alone is ruled out by the far-infrared observations. An additional opacity, thought to be associated with a methane cloud, is confirmed. The effective temperature of Titan is estimated at T e = 83.2 ± 1.4°K.

Collisional narrowing of HF fundamental band spectral lines by neon and argon

Collisional narrowing is observed for high J transitions in the fundamental absorption band of HF in the presence of neon and argon buffer gases, with neon providing the most pronounced Doppler width reduction. Detailed line profiles are measured using a high-resolution tunable laser difference-frequency spectrometer in order to test various collisional lineshape models. The measured lineshapes and the pressure dependence of the widths are least-squares fit in the limits of strong and weak velocity-changing collisions. Though both limits qualitatively explain the data, the systematic discrepancies indicate that an intermediate collision model would be more appropriate. In the case of HF/argon collisions, the resulting line profiles have a slight, but definite, asymmetry, implying a correlation between the velocity- and state-changing collisions.

Comparison of collisional redistribution and emission line shapes

The authors have studied collisional redistribution of near-resonance radiation in thallium vapour caused by collisions with argon buffer gas. Electronic Raman scattering and collision-induced fluorescence were spectrally resolved, both to the weakly populated metastable 62P3/2 level in thallium. Thus the problem with radiative trapping encountered in previous experiments is partially avoided. By measuring the total collision-induced fluorescence as a function of the incident laser frequency, the absolute collisional redistribution line shape is obtained.