Approximation of the width of the line profile narrowed by collisions

For models of hard and soft (in velocity) collisions, algebraic approximations of the line half-width are determined that depend on parameters of collisional broadening and narrowing and on the Doppler half-width. The average difference between the approximated and the exact values of the half-width do not exceed 0.1%. A simple criterion for determining the correspondence of models of hard and soft collisions to the experimental dependence of the half-width on the buffer gas pressure is proposed.     

Memory effects in speed-changing collisions and their consequences for spectral line shape

The kinetic model accounting for speed-memory effects on the spectral line shape proposed in I [D. Robert, L. Bonamy, Eur. Phys. J. D 2, 245 (1998)] is extended for any density range, within the binary collision framework. The additional Doppler contribution requires to consider the 3D velocity-memory function instead of the 1D speed one, with distinct treatments for the velocity-orientation and velocity-modulus memory mechanisms. Both the collisional confinement narrowing of the Doppler distribution and the radiator speed-dependence of the collisional broadening and shifting parameters are thus conveniently taken into account. In the high density regime, this model leads to the same results as in I. At lower densities, it generalizes the very well-known hard and soft collision models for the Dicke narrowing of the Doppler distribution, but it also includes the second source of inhomogeneity tied to the speed-dependent collisional parameters and, concomitantly, the speed class exchanges. Numerical applications to H₂-N₂ and H₂-Ar gaseous mixtures are in close agreement with experiments. This allows one to clearly analyze the specific role of speed and velocity memory effects on the line profile.Received: 18 June 2004, Published online: 3 November 2004PACS: 34.10. + x General theories and models of atomic and molecular collisions and interactions (including statistical theories, transition state, stochastic and trajectory models, etc.) - 33.70.Jg Line and band widths, shapes, and shifts     

Spectral Intensity and Lineshape Measurements in the First Overtone Band of HF Using Tunable Diode Lasers

High-resolution absorption lineshapes for the P(3) and P(6) transitions of the first overtone (v = 2-0) band of HF at 296 K have been measured using a pair of distributed feedback diode lasers operating near 1.31 and 1.34 μm, respectively. Spectral line intensities and self-broadening parameters were determined by fitting the measured spectra with Voigt, Galatry, and Rautian lineshape models. Voigt profiles fit the low-pressure (<10 Torr) spectra of the P(3) transition reasonably well due to the relatively strong collisional broadening effect. Lineshape measurements of the P(6) transition (for pressures ranging from 5 to 60 Torr) show significant variation from the Voigt lineshape model due to velocity-changing collisions that effectively reduce the Doppler component of the spectral line. Lineshape models that include motional (Dicke) narrowing effects, Galatry (soft collision) and Rautian (hard collision) profiles yield significant improvements in the spectral lineshape fits compared with Voigt profiles. The collisional broadening coefficient (gamma) of the P(6) transition obtained from a Voigt fit is approximately 4% lower than those found with either Galatry or Rautian profile fits. The measured intensities and self-broadening coefficients are compared with values in the HITRAN database and previous measurements. Copyright 1999 Academic Press.     

Xe-broadening coefficients of 12CH3D: a test of theoretical line shapes

We present the Xe-broadening coefficients for six lines belonging to the ν₃ band of ¹²CH₃D measured at room temperature with a diode-laser spectrometer. The collisional widths are obtained from least-squares fitting of each absorption line with theoretical line shapes. We used the well-known Voigt profile, the soft and hard collision models considering the collisional narrowing and also line shapes taking into account the speed dependence of the collisional cross-section. The results derived by these different models are compared with each other and with theoretical broadening coefficients. The calculations are based on a semiclassical impact formalism that includes the atom–atom Lennard–Jones potential for CH₃D–Xe interactions in which CH₃D is considered as a linear molecule.     

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.     

Study of the HF overtone line profile broadened by Ar,Xe, Kr,N2 using near-IR region diode laser spectroscopy

The profile of the 0-2 R(0) overtone vibrational-rotational absorption line of HF molecules in mixtures with Ar, Xe, Kr, N₂ is experimentally studied using near-IR-range diode laser spectroscopy. The coefficients of collisional broadening, shift, and collisional narrowing of the HF line in these buffer gases are determined using the Voigt, Rautian, and Galatry profiles.     

Diode-laser spectroscopy: Temperature dependence of R (0) line in the ν4 band of CH4 perturbed by N2 and O2

In this paper, we present a line profile study of the R (0) line in the ν₄ band of methane diluted in nitrogen and oxygen, from room temperature to 153 K. The measurements were performed over a total pressure range from 14 to 128 mbar. The collisional broadening and narrowing (Dicke effect) coefficients are derived from a fit of the experimental spectra by using the soft and hard collision models, taking into account the Dicke effect. For higher pressures, we have fitted the data with a model taking into account simultaneously the Dicke narrowing and the speed dependence effect. Finally, we have deduced the parameter n of the temperature dependence (inverse power law) of the broadening coefficients for the CH₄-N₂ and CH₄-O₂ gas mixtures.     

Self-broadening coefficients in the ν4 band of CH4 by diode-laser spectroscopy

With a diode-laser spectrometer, we have measured self-broadening coefficients of 42 lines of the P-, Q- and R-branches in the ν₄ fundamental band of CH₄ at room temperature, with J values ranging between 1 and 12. For the determination of self-broadening parameters, we have fitted to the experimental lineshape two theoretical line profiles: the Voigt profile taking into account Doppler and collisional broadenings, and the hard collision model developed by Rautian and Sobel’man incorporating Dicke narrowing.     

Spectrum line profiles: A generalized Voigt function including collisional narrowing

Reduction of collision-narrowed line profile functions to dimensionless form analogous to the standard form of the Voigt profile simplifies mathematical treatment. New quadratures are given which allow the efficient calculation of collision-narrowed profiles over the full physically realizable range of both collisional broadening and narrowing.     

Probe field spectroscopy in three-level Λ systems under arbitrary collisional relaxation of low-frequency coherence

We investigate theoretically the spectrum of weak probe field absorption by three-level atoms with the Λ configuration of levels in the field of a strong electromagnetic wave acting on an adjacent transition and colliding with buffer gas atoms. Analysis is carried out for the general case of arbitrary collisional relaxation of low-frequency coherence at a transition between two lower levels. It is shown that, in the absence of collisional relaxation of low-frequency coherence, the probe field spectrum always exhibits clearly manifested anisotropy with respect to mutual orientation of wavevectors of the strong and probe radiation (even under small Doppler broadening). It is found that the probe field spectrum may acquire under certain conditions supernarrow resonances with a width proportional to the diffusion coefficient for atoms interacting with radiation. This fact may form the basis for a spectroscopic method for measuring transport frequencies of collisions between absorbing and buffer particles. A large-amplitude supernarrow resonance (with an amplitude much larger than the amplitude of the resonance near the line center), which is observed in the far wing of the absorption line, exhibits collisional narrowing (a nonlinear spectroscopic analog of the Dicke effect) at collision frequencies several orders of magnitude lower that the Doppler linewidth. Simple working equations proposed for describing the probe field spectrum are convenient for experimental data processing.     

Evidence for collisional narrowing in the infrared laser Stark spectrum of CH3CN

The infrared laser Stark lineshape of the Jkm = 211 ← 111 transition in the ν₄ band of CH₃CN was recorded under computer control at several pressures. Analysis of the lineshapes by means of the usual Voigt profile gives a plot of collision broadening vs pressure that shows marked curvature in the low-pressure region. In addition, extrapolation to zero pressure of the best straight line through the high-pressure points gives a negative intercept. By choosing an appropriate narrowing parameter in either of the limiting soft-collision or hard-collision models of collisional narrowing, it is possible to obtain plots of collision broadening vs pressure that are linear and have acceptable intercepts. The pressure broadening and collisional narrowing parameters derived are 69.4 ± 0.7 and 17.2 ± 1.7 MHz/Torr, respectively, for the soft-collision model and 68.6 ± 0.7 and 11.5 ± 1.2 MHz/Torr, respectively, for the hard-collision model.     

Narrowing and broadening parameters for H2O lines perturbed by helium,argon and xenon in the 1170–1440 cm−1 spectral range

Collision effects on water vapor at low concentration in a mixture with noble gases (helium, argon and xenon) have been studied by Fourier transform spectroscopy in the pressure range where line narrowing by dynamic confinement (Dicke effect) and collision broadening are observable, i.e. when the Voigt function cannot reproduce the observed profiles. Precise values of the broadening parameter have been obtained for the P and Q branches of the H₂O ν ₂ band taking into account molecular confinement (hard or soft collisions). The broadening parameter value derived from a Voigt profile for H₂O lines perturbed by helium is smaller by about 10% than values determined using the soft or hard collision model. For H₂O lines perturbed by argon or xenon this difference can reach more than 50% for the narrowest lines.     

Line Broadening and Shifting Studies of the J=5←4 Transition of Carbon Monoxide Perturbed by CO, N2, and O2

The collisional relaxation of the J=5←4 rotational transition of CO induced by carbon monoxide, nitrogen, and oxygen has been studied at room temperature. Pressure-broadening parameters were determined as 3.29(2), 2.61(2), and 2.30(2) MHz/Torr for CO, N₂, and O₂ buffer gases, respectively. Experimental deviations from the Voigt line shape profile have been observed which are mostly the effect of a narrowing in the spectral line core. The difference between the model profile and the experimental profile is less than 0.5% of the maximum line amplitude in the investigated pressure range 0.2-5 Torr. In addition, a small positive collision-induced shift of the line center frequency was observed for the pure gas, corresponding to a pressure self-shift parameter of 6(3) kHz/Torr.     

Quenching of hadron spectra due to the collisional energy loss of partons in the quark-gluon plasma

We estimate the energy loss distribution and investigate the quenching of hadron spectra in ultrarelativistic heavy-ion collisions due to the collisional energy loss of energetic partons from hard parton collisions in the initial stage.     

Collisional parameters of H2O lines: Velocity effects on the line-shape

This paper is devoted to the effects of velocity on the shapes of six R(J) lines of the ν₃ band of water vapor diluted in N₂. The experiments have been made at room temperature for total pressures between 0.1 and 1.2 atm using a tunable infrared laser frequency difference spectrometer. These measurements, which study broad and narrow lines of low and high J values, are first analyzed using the Voigt and the hard collision (HC) model. It is shown that both lead to unsatisfactory results, the Voigt profile being unable to account for the line narrowing whereas the friction (narrowing) parameter deduced using the HC approach has an unphysical dependence on pressure. Furthermore, at elevated pressure where Dicke narrowing and Doppler effects are negligible, deviations between experimental and fitted profiles are still observed, indicating inhomogeneous effects due to the speed dependence of collisional parameters. In order to go further, an approach based on the kinetic impact equation accounting for both the Dicke narrowing and the speed dependence has been applied. It uses velocity-dependent broadening and shifting coefficients calculated with a semi-classical approach and two parameters. The latter, which govern the memory functions of the modulus and orientation of the H₂O velocity are considered as free parameters and determined from experiments. The results show that all profiles, regardless of pressure and of the transition, can be correctly modeled using a single set of memory parameters. This demonstrates the consistency of the approach, which is then used to analyze the different regimes that monitor velocity effects on the line profile.     

A kinetic theory of spectral line shapes

The methods of kinetic theory are used to describe the radiation from an atom immersed in a gas of perturbing particles. It is shown that the line shape can be expressed in terms of a one-particle distribution function. The appropriate BBGKY hierarchy of equations is derived. This hierarchy is then truncated by assuming that only two-body collisions are important. The resulting equations are solved to obtain a non-Markovian kinetic equation which describes the combined effects of Doppler and pressure broadening. When the Markovian assumption is applied, a generalized linear Boltzmann equation is obtained which describes the line shape in the region where the impact limit is valid and which also describes the phenomenon of collisional narrowing.This research was supported in part by the Advanced Research Projects Agency of the Department of Defense, monitored by Army Research Office-Durham under Contract No. DA-31-124-ARO-D-139.     

Theory of collision effects on atomic and molecular line shapes

A review of recent developments in the theory of the effects of binary collisions on the spectral profiles associated with atomic and molecular systems is presented. To consistently account for collisional perturbations of both the internal energy levels and the velocity of active (emitting or absorbing) atoms or molecules, one must use a theory in which the center-of-mass motion of the active atoms has been quantized. Following this procedure general equations for absorption or emission line shapes are obtained. The line shapes may exhibit narrowing or broadening with increasing perturber pressure, depending upon the nature of the collision interaction. The physical significance of the collision mechanisms giving rise to such behavior is discussed, as is the experimental evidence in support of the theory. Various applications of the theory are presented.     

Collisional parameters of N2 broadened methane lines in the R9 multiplet of the 2ν3 band. Multispectrum fittings of the overlapping spectral lines

A high-sensitive two-channel photo-acoustic (PA) spectrometer with a near infrared diode laser was used to make precise measurements of N₂-broadened methane spectra in the R9 multiplet of 2ν₃ band. For the accurate retrieval of spectroscopic line parameters (intensities, positions at zero pressure, pressure-shifting and -broadening, collisional narrowing and line mixing coefficients) from the methane PA spectra, a program based on a multispectrum fitting procedure has been used. The multispectrum least squares fitting procedure is based on a relatively simple line profile model and on the linear pressure dependences of the line profile parameters. The line parameters were determined for 11 lines of the methane 2ν₃ R9 multiplet. The results have been compared with other available data.     

Collisional broadening and shifting of saturated absorption resonances in silicon tetrafluoride

The broadening and shifting of 12 saturated absorption resonances of the SiF₄ gas induced by molecular collisions have been measured in the lasing region of a low-frequency CO₂ laser at the P(30) line of the 9.4-μm band. Values of the collisional broadening are identical accurate to the measurement uncertainty. The collisional shift does not exceed 10% of the collisional broadening. This means that collisions with changes in the internal energy of molecules prevail over dephasing collisions.