Collisional Linewidths Research Articles

The ro-vibrational ν2 mode spectrum of methane investigated by ultrabroadband coherent Raman spectroscopy.

We present the first experimental application of coherent Raman spectroscopy (CRS) on the ro-vibrational ν2 mode spectrum of methane (CH4). Ultrabroadband femtosecond/picosecond (fs/ps) CRS is performed in the molecular fingerprint region from 1100 to 2000cm-1, employing fs laser-induced filamentation as the supercontinuum generation mechanism to provide the ultrabroadband excitation pulses. We introduce a time-domain model of the CH4 ν2 CRS spectrum, including all five ro-vibrational branches allowed by the selection rules Δv = 1, ΔJ = 0, ±1, ±2; the model includes collisional linewidths, computed according to a modified exponential gap scaling law and validated experimentally. The use of ultrabroadband CRS for in situ monitoring of the CH4 chemistry is demonstrated in a laboratory CH4/air diffusion flame: CRS measurements in the fingerprint region, performed across the laminar flame front, allow the simultaneous detection of molecular oxygen (O2), carbon dioxide (CO2), and molecular hydrogen (H2), along with CH4. Fundamental physicochemical processes, such as H2 production via CH4 pyrolysis, are observed through the Raman spectra of these chemical species. In addition, we demonstrate ro-vibrational CH4 v2 CRS thermometry, and we validate it against CO2 CRS measurements. The present technique offers an interesting diagnostics approach to in situ measurement of CH4-rich environments, e.g., in plasma reactors for CH4 pyrolysis and H2 production.

Resonant nonlinear refraction of 4–5−μm light in CO and CO2 gas

The resonant nonlinear refraction of $4\text{--}5\text{\ensuremath{-}}\ensuremath{\mu}\text{m}$ light in CO and $\mathrm{C}{\mathrm{O}}_{2}$ gas at a peak intensity of $15\phantom{\rule{0.16em}{0ex}}\mathrm{MW}/\mathrm{c}{\mathrm{m}}^{2}$ was demonstrated using time- and frequency-resolved measurements of self-focusing and self-defocusing. The nonlinearity of these molecular gases exhibits intensity-dependent sign reversals and a $<4\text{\ensuremath{-}}\mathrm{ns}$ response time. A change from self-focusing to self-defocusing or vice versa was observed to occur for Rabi frequencies that are comparable to the collisional linewidth. A density matrix model for the nonlinear susceptibility of a strongly driven two-level system provides a qualitative explanation for these results.

Direct measurements of collisional Raman line broadening in the S-branch transitions of CO perturbed by CO, N2, and CO2.

We report direct measurement of collisional line-broadening coefficients associated with rotational Raman transitions of carbon monoxide (CO), obtained using time-resolved picosecond rotational coherent anti-Stokes Raman scattering spectroscopy. The dependencies of the CO self-broadening coefficients on rotational quantum number, J, and temperature are described for the J=3-16 lines of S-branch (ΔJ=+2) transitions for T=295-600 K at atmospheric pressure. Further, we report collisional linewidths of CO and collision partners N2 and CO2. The obtained S-branch linewidths of self-broadened CO agree well with previously reported frequency-domain experimental spectroscopy results, whereas the mixture-linewidth broadening coefficients differ from reported theoretical results by up to 80%.

Pressure broadening coefficients in the ν6 band of CH3F in collision with He

In this study we report the first measurements of the pressure line broadening coefficients of 219 lines of CH3F in the PP, RP, RQ, PQ, and RR sub-branches of the ν6 band (8.5 µm-spectral region) at room temperature using a high-resolution Fourier transform spectrometer. The collisional line widths are obtained from the non-linear least-square multi-pressure fitting techniques. A Voigt line shape model was used in the present analysis. Measured values are compared with the results of theoretical calculation based on semi-classical formalism of Robert-Bonamy (RB) model with exact isotropic trajectories. Even with a Smith-Giraud-Cooper intermolecular potential model the calculated values show a quite good agreement with experimental results.

Precise predictions of H2O line shapes over a wide pressure range using simulations corrected by a single measurement

In this work, we show that precise predictions of the shapes of H2O rovibrational lines broadened by N2, over a wide pressure range, can be made using simulations corrected by a single measurement. For that, we use the partially-correlated speed-dependent Keilson–Storer (pcsdKS) model whose parameters are deduced from molecular dynamics simulations and semi-classical calculations. This model takes into account the collision-induced velocity-changes effects, the speed dependences of the collisional line width and shift as well as the correlation between velocity and internal-state changes. For each considered transition, the model is corrected by using a parameter deduced from its broadening coefficient measured for a single pressure. The corrected-pcsdKS model is then used to simulate spectra for a wide pressure range. Direct comparisons of the corrected-pcsdKS calculated and measured spectra of 5 rovibrational lines of H2O for various pressures, from 0.1 to 1.2 atm, show very good agreements. Their maximum differences are in most cases well below 1%, much smaller than residuals obtained when fitting the measurements with the Voigt line shape. This shows that the present procedure can be used to predict H2O line shapes for various pressure conditions and thus the simulated spectra can be used to deduce the refined line-shape parameters to complete spectroscopic databases, in the absence of relevant experimental values.

Theoretical and applied researches on measuring line width in wavelength modulation spectroscopy

Line width is the key parameter in tunable diode laser absorption spectroscopy (TDLAS); the dominant line widths are Doppler and collisional broadenings. Under low absorbance, the ratios of 2nd to 4th harmonics at the line center monotonously decrease and intersect at a fixed point, regardless of the changes in Doppler and collisional line widths. Based on this characteristic, a method is proposed which employs the ratios to measure the line width. To validate the reliability and accuracy of the proposed method, the transition of CO2 at 6982.0678 cm-1 is selected to measure the line width, the gas partial and total pressure. Experimental results show that the proposed method can achieve high precision in actual measurements.

Direct measurements of collisional Raman line broadening in the S-branch transitions of acetylene (C2H2)

We report direct measurements of the self- and N2-broadened Raman S-branch linewidths of acetylene (C2H2), obtained by employing time-resolved picosecond rotational coherent anti-Stokes Raman scattering spectroscopy. Using broadband 115-ps pump and Stokes pulses (~135 cm(-1) bandwidth) and a spectrally narrowed 90-ps probe pulse (~0.2 cm(-1) bandwidth), Raman-coherence lifetimes are measured at room temperature for the S-branch (ΔJ = +2) transitions associated with rotational quantum number J = 3-25. These directly measured Raman-coherence lifetimes, when converted to collisional linewidth broadening coefficients, differ from the previously reported broadening coefficients extracted from theoretical calculations by 6%-35% for self-broadening for C2H2 and by up to 60% for N2-broadened C2H2.

Direct measurements of collisionally broadened Raman linewidths of CO2 S-branch transitions

We report direct measurements of S-branch Raman-coherence lifetimes of CO(2) resulting from CO(2)-CO(2) and CO(2)-N(2) collisions by employing time-resolved picosecond coherent anti-Stokes Raman scattering spectroscopy. The S-branch (ΔJ = +2) transitions of CO(2) with rotational quantum number J = 0-52 were simultaneously excited using a broadband (~5 nm) laser pulse with a full-width-at-half-maximum duration of ~115 ps. The coherence lifetimes of CO(2) for a pressure range of 0.05-1 atm were measured directly by probing the rotational coherence with a nearly transform-limited, 90-ps-long laser pulse. These directly measured Raman-coherence lifetimes, when converted to collisional linewidth broadening coefficients, differ from the previously reported broadening coefficients extracted from frequency-domain rotational Raman and infrared-absorption spectra and from theoretical calculations by 7%-25%.

Experimental and theoretical studies of room-temperature sub-millimetre CH3 35Cl line shapes broadened by H2

Hydrogen-broadening coefficients of methyl chloride rotational lines J = 6 → 7, 10 → 11, 17 → 18, 22 → 23 and 31 → 32 at 296 K are measured as functions of the quantum number K using a sensitive frequency-modulation technique. As expected for this light perturber, the observed line shapes are well described by Voigt profile model. A clear dependence of the collisional broadening on K is observed for most transitions. From a detailed study of the K-components of the transition J = 6 → 7 situated at 186 GHz no variation of the broadening of the hyperfine components related to 35Cl quadrupole is stated. Given the absence of refined ab initio computed potential energy surfaces and the impracticality of quantum-mechanical calculations for the considered molecular system, theoretical values of these broadening coefficients are estimated by a semi-classical approach with exact trajectories and a model interaction potential including both long-range and short-range (atom-atom) interactions of the active molecule rigorously treated as a symmetric top. It is shown that the short-range forces yield important contributions to the collisional line width for all values of the rotational quantum numbers J and K. Various models are also tested for the isotropic part of the interaction potential which governs the relative translational motion. It is demonstrated that for the very light perturbing molecule H2 the calculated line widths, practically independent from the rotational quantum number J (for K ≤ J), are particularly sensitive to the position and slope of the repulsive wall. Modifications required in the semi-classical formalism for a correct application of the cumulant expansion are also tested and it is stated that no difference is observed for the CH3Cl–H2 system characterised by quite weak interactions.

Collisional line widths of autoperturbed N 2: Measurements and quantum calculations

In this work we present new experimental and theoretical values for the line broadening coefficients of the Q-branch Raman lines of autoperturbed N 2. For the experimental determination of the coefficients, high resolution stimulated Raman spectra of the Q-branch of N 2 at different pressures were obtained at 77, 194 and 298 K. Simultaneously, quantum dynamical calculations, performed on two potential energy surfaces, were carried out for the system between 77 and 298 K, rendering a set of theoretical line broadening coefficients that could be directly compared to those obtained from the present measurements and the previous ones. Within the limit of considering the colliding molecules distinguishable we discuss the ortho and para contributions to the pressure broadening cross-sections. Because such calculations are time consuming we indicate routes to circumvent this difficulty. We observe a reasonable agreement between theoretical and experimental values of the collisional line broadening coefficients at all the studied temperatures.

Comparison of quantum, semiclassical and classical methods in hydrogen broadening of nitrogen lines

We use an accurate N 2–H 2 ab initio potential energy surface (PES) in order to inter-compare various methods commonly employed to calculate pressure broadening coefficients. Close-coupling (CC) calculations of the collisional linewidths of the isotropic Raman lines of N 2 perturbed by H 2 are performed for temperatures between 77 and 2000 K. The CC results compare well with available experimental values. Three less exact methods of calculation are also used: the full classical (FC) model of Gordon, the semiclassical (SC) formalism of Robert and Bonamy and the quantum dynamical coupled states (CS) method. The CS method provides good agreement with CC calculations for all studied temperatures, FC calculations can be considered as accurate above room temperature while the SC method gives overestimated values by about 20–30% in all cases. The temperature dependences of pressure broadening coefficients provided by each method are very similar at elevated (above room) temperatures.

On the temperature dependence of collisional linewidths of the 1000 — 0001 laser transition in the CO2 molecule

Unsaturated absorption coefficients in pure carbon dioxide and CO2 — N2 and CO2 — He binary mixtures are measured at a pressure of 100 Torr in the temperature rangeof 300–700 K using a frequency-stabilised tunable CO2 laser. The relative coefficients of collisional broadening caused by N2 and He buffer gases and their temperature dependence are determined for the R(22) absorption line (1000 — 0001 transition) of the CO2 molecule.

Possibilities of the observation of the discrete spectrum of the water dimer at equilibrium in millimeter-wave band

Attempts of experimental observations of the water dimer spectrum at equilibrium conditions have lasted for more than 40 years since the dimeric hypothesis for extra absorption, but have not yielded any positive confirmed result. In the present paper a new approach is considered: using a high-resolution millimeter-wave spectrum of the water dimer at equilibrium, calculated by a rigorous fully quantum method, we show the potential existence of discernible spectral series of discrete features of the water dimer, which correspond to J+1← J, K lines of E 1 symmetry, already observed in cold molecular beam experiments and having, therefore, well-defined positions. The intensity of spectral series and contrast to the remaining continuum-like spectrum of the dimer are calculated and compared with the monomer absorption. The suitability of two types of microwave spectrometers for observing these series is considered. The collisional line-width of millimeter lines of the dimer at equilibrium is estimated and the width of IR dimer bands is discussed. It is pointed out that the large width of IR dimer bands may pose difficulties for their reliable observation and conclusive separation from the rest of absorption in water vapor. This situation contrasts with the suggested approach of dimer detection in millimeter-waves.

Ne- and Kr-broadening coefficients in the [formula omitted] band of [formula omitted] studied by diode-laser spectroscopy

We report in this work the Ne- and Kr-broadening coefficients of C 2 H 4 for 36 lines in the P , Q and R branches of the ν 7 fundamental band. These rovibrational lines with J values ranging from 3 to 23 are located in the spectral range 919–1024 cm −1. The measurements were carried out at room temperature with a tunable diode-laser spectrometer. The collisional line widths obtained by fitting Rautian profiles to the measured lineshapes are slightly larger than those derived from Voigt profiles. The C 2 H 4 line-broadening by Kr is more important than the broadening by Ne.

Lineshape analysis of the J = 3 ← 2 and J = 5 ← 4 rotational transitions of room temperature CO broadened by N 2, O 2, CO 2 and noble gases

Collisional relaxation has been considered for millimeter lines of carbon monoxide at room temperature. Accurate measurements of carbon dioxide- and rare gases-broadened widths have been performed on the J = 3 ← 2 rotational line of 12CO by using a video-type spectrometer. Measurements of nitrogen-, oxygen-, and xenon-broadened widths of the J = 5 ← 4 rotational line of 13CO were also carried by using a frequency-modulated spectrometer. A lineshape study performed on all the investigated binary systems provide confirmation that Voigt profile is not a suitable model to analyse experimental lines in the millimeter-waves region. On one hand, using this profile in the low pressure range, i.e. in the Doppler regime, the retrieved collisional linewidths do not follow a linear variation with the perturbing gas pressure. On the other hand, regardless of the pressure, lineshapes exhibit a narrowed profile. An accurate analysis of the pressure dependence of relaxation rates show that the Galatry profile is not appropriate and that experimental lineshapes are actually Speed Dependent Voigt profiles. Accurate broadening parameters were retrieved from this profile and compared to previous reported values and predictions calculated from the Robert–Bonamy formalism. Finally a variation of the ratio of relaxation speed dependence to broadening parameters versus relative masses of the collision partners is presented.

Detailed profile analysis of millimetre 502 and 602 GHz N2O–N2(O2) lines at room temperature for collisional linewidth determination

Deviations of experimental millimetre-absorption line shapes from the usual Voigt profile are evidenced for two room-temperature rotational transitions J = 20 ← 19 (502 GHz) and J = 24 ← 23 (602 GHz) of nitrous oxide perturbed by nitrogen and oxygen. These deviations are shown to be due to collisional narrowing processes encompassed by more sophisticated speed-dependent Voigt or Galatry models. The deduced collision-regime line broadening coefficients are compared to their theoretical estimations by a semi-classical formalism. Even with a rough intermolecular potential composed of electrostatic and pair atom–atom interactions, the theoretical linewidths compare favourably with experimental results for both millimetre and infrared absorption.

Electron-impact broadening of the 3 s–3 p lines in low- Z Li-like ions

The collisional electron-impact line widths of the 3 s–3 p transitions in Li-like ions from B III to Ne VIII are calculated with the convergent close-coupling (CCC) method from the atomic collision theory. The elastic and inelastic contributions to the line broadening and their Z-scaling are discussed in detail, and comparisons with recent experimental and theoretical results are also presented. It is found that similar to our previous study of line broadening in Be-like ions, the difference between experimental and CCC results monotonically increases with the spectroscopic charge of an ion.

Electron collisional broadening of 2s3s–2s3p lines in Be-like ions

We present quantum–mechanical calculations for the electron collisional Stark linewidths of the 2s3s–2s3p singlet and triplet lines of the beryllium-like ions from B II to O V. The impact approximation is used with the convergent close-coupling method, which is shown for inelastic cross sections to agree well with the Coulomb–Born-exchange approximation. Similar to previous comparisons, we find our calculated widths to be generally smaller than both measured and (most) theoretical linewidths from semiclassical calculations. A possible contribution from non-thermal Doppler widths is discussed as well.

Atmospheric transmission at microwaves (ATM): an improved model for millimeter/submillimeter applications

We present a model of the longwave atmospheric spectrum that improves in many respects widely used older models such as the microwave propagation model (MPM), since it is based on broadband measurements and calculations. According to our data, the model is fully applicable from 0 to 2 THz while including lines up to 10 THz. Its primary goal is to simulate the millimeter/submillimeter region accessible from the ground (frequencies up to /spl sim/2 THz at most, with a few windows between 1 and 2 THz accessible only under exceptional conditions at very dry sites). Line-by-line calculations of the absorption are performed using a line database generated from the latest available spectroscopic constants for all relevant atmospheric species. The collisional line widths are obtained from published laboratory data. The excess of absorption in the longwave range that cannot be explained by the line spectrum is modeled by introducing two different continuum-like terms based on FTS measurements between 170 and 1100 GHz: collision-induced absorption of the dry atmosphere due to transient dipoles in symmetric molecules (N/sub 2/ and O/sub 2/) and continuum-like water vapor opacity. All H/sub 2/O lines up to 10 THz are included in order to correctly account for the entire H/sub 2/O far-wing opacity below 2 THz for a given line-shape. Hence, this contribution does not need to be part of a pseudocontinuum term below that frequency cutoff (still necessary, as shown in this paper) in contrast to other models used to date. Phase delays near H/sub 2/O and O/sub 2/ resonances are also important for ground-based astronomy since they affect interferometric phase. The frequency-dependent dispersive phase delay function is formally related to the absorption line shape via the Kramers-Kronig dispersion theory, and this relation has been used for modeling those delays. Precise calculations of phase delays are essential for the future Atacama large millimeter array (ALMA) project. A software package called atmospheric transmission at microwaves (ATM) has been developed to provide the radioastronomy and aeronomy communities with an updated tool to compute the atmospheric spectrum in clear-sky conditions for various scientific applications. We use this model to provide detailed simulations of atmospheric transmission and phase dispersion for several sites suitable for submillimeter astronomy.

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.