J. Boissoles

Collisionally induced population transfer effect in infrared absorption spectra. I. A line‐by‐line coupling theory from resonances to the far wings

The line‐by‐line coupling for a pressure broadened rovibrational band is formulated in the far‐wing limit. The present quasistatic theory assumes that the wave frequency is displaced from the line centers by an amount that is large compared with both the reciprocal duration of a typcial binary collision and frequency separation between strongly coupled lines. This theory generalizes that of Rosenkranz [J. Chem. Phys. 83, 6139 (1985)] where the Fano’s relaxation operator was reduced to a scalar parameter through a band average. The present approach permits computation of far‐wing absorption more specifically tailored to individual lines. Such a line‐by‐line approach is needed for rovibrational bands where some far lines contribute significantly to the total absorption. In order to obtain a qualitative picture of the line coupling as a function of the frequency displacement, calculations for collisions of CO2 with Ar have been performed for some lines. The results are compared with the corresponding variati...

Pressure induced shifts of CO2 lines: Measurements in the 0003–0000 band and theoretical analysis

Self‐broadened and N2, Ar, He‐broadened halfwidth and pressure shift coefficients of the rotational transitions in the 0003–0000 band of 12C16O2 have been measured from laboratory absorption recorded at room temperature with a Fourier transform spectrometer. Comparison is made with the results of a theoretical calculation based on a semiclassical model. Good agreement is obtained for all the IR vibrational bands for which data are available. It has been shown that the shifts mainly originate from vibrational dephasing. Infrared line shifts have been compared with results obtained from stimulated Raman spectroscopy and we show that they are not consistent. A number of possible explanations have been discussed to account for this discrepancy.

IOS and ECS line coupling calculation for the CO-He system - Influence on the vibration-rotation band shapes

Line coupling coefficients resulting from rotational excitation of CO perturbed by He are computed within the infinite order sudden approximation (IOSA) and within the energy corrected sudden approximation (ECSA). The influence of this line coupling on the 1–0 CO–He vibration–rotation band shape is then computed for the case of weakly overlapping lines in the 292–78 K temperature range. The IOS and ECS results differ only at 78 K by a weak amount at high frequencies. Comparison with an additive superposition of lorentzian lines shows strong modifications in the troughs between the lines. These calculated modifications are in excellent quantitative agreement with recent experimental data for all the temperatures considered. The applicability of previous approaches to CO–He system, based on either the strong collision model or exponential energy gap law, is also discussed.

State-to-state rotational phase coherence effect on the vibration-rotation band shape - An accurate quantum calculation for CO-He

Accurate coupled state calculations of line coupling are performed for infrared lines of carbon monoxide perturbed by helium. Such calculations lead to both real and imaginary line couplings. For the first time, the effect of this imaginary line couplings, connected with state‐to‐state rotational phase coherences, on infrared band shape, is analyzed. An extension of detailed balance principle to the complex plane is suggested from the present computed off‐diagonal cross sections. This allows us to understand the physical mechanism underlying the weak effect of phase coherences on CO–He infrared band shape.

Infrared spectroscopy of (CO2)N nanoparticles (30<N<14500) flowing in a uniform supersonic expansion

The infrared signature of carbon dioxide clusters of nanometric size is discussed both in the bending (ν2 mode at 15 μm) and in the asymmetric stretching (ν3 mode at 4.2 μm) spectral region of the monomer. The carbon dioxide nanoparticles were formed using a capillary tube injection inserted upstream of a uniform supersonic flow of argon generated by a Laval nozzle. The size of the formed clusters was varied by changing the stagnation pressure P0 of the capillary. The empirical power law connecting P0 to the number N of monomers per cluster: N∝P02.2 was verified in this work. The cluster mean size was estimated using a Rayleigh scattering experiment showing the formation of nanometric clusters whose radii are in the range 0.7 nm<r<5.3 nm, corresponding to 30<N<14 500. The thermodynamic and kinetic parameters of the flow were determined from the rovibrational absorption lines of the monomer and from a time-of-flight experiment. The measured flow velocity and flow temperature show that CO2 condensation is r...

Collisionally induced population transfer effect in infrared absorption spectra. II. The wing of the Ar‐broadened ν3 band of CO2

The absorption beyond the ν3‐band head of CO2 broadened by argon has been measured at room temperature. The absorption exhibits a strong sub‐Lorentzian behavior (several orders of magnitude) resulting from collisionally induced line interferences which transfer intensity from this wing region to the ν3‐band center. This wing absorption region implies detuning frequencies from resonances much larger than the reciprocal duration of collision. Consequently, finite duration of collisions in rotational energy transfers and initial correlations must be included in absorption calculation. A line‐by‐line coupling theory accounting for both these effects has been recently proposed [J. Chem. Phys. 89, 625 (1988)] and is applied here to a detailed study of the CO2–Ar collisional system. A convenient generalized detailed balance correction is introduced in this theory to overcome the limitation of the assumed resonant character of the energy transfer in the short time limit with respect to the thermal time ( βℏ)−1. T...

Theoretical calculation of the translation-rotation collision-induced absorption in N2–N2, O2–O2, and N2–O2 pairs

Abstract The translation-rotation collision-induced spectra of N 2 –N 2 , O 2 –O 2 and N 2 –O 2 mixtures are calculated theoretically. For N 2 –N 2 , using the matrix elements for the quadrupole and hexadecapole moments and the isotropic and anisotropic polarizabilities obtained previously from a global analysis of the fundamental band spectra, we obtain numerical values for the zeroth moment that are smaller than the measured values by 9–14%, depending on the temperature. By increasing the value for the matrix element of the isotropic polarizability slightly, good agreement with experiment is obtained. For O 2 –O 2 , the theoretical spectrum is significantly smaller than the experimental result. By increasing the matrix element of the hexadecapole moment by a factor of 1.7, we can obtain good agreement. This larger value for the hexadecapole moment will not appreciably affect the agreement found previously in the fundamental region because the hexadecapole contribution to the intensity is very small, unlike the translation-rotation band where it is larger than the contribution due to the quadrupole moment. Using these parameters, we then calculate the collision-induced absorption for N 2 –O 2 mixtures for which no experimental data exist. Finally, we calculate the collision-induced absorption for air, and compare our results with previous work; we express the results for the ratio of the absorption coefficient of air to that of N 2 –N 2 as a function of wavenumber and temperature, R ( ω , T ), which can easily be implemented in atmospheric models.

Calculation of absorption in the microwindows of the 4.3 μm CO2 band from an ECS scaling analysis

Abstract Line coupling induced by collisions leads to drastic modifications of the absorption profile in the microwindows of the self- and N2-broadened v3 CO2 fundamental vibration-rotation band. Calculations of these modifications have been performed by using the energy-corrected sudden (ECS) scaling law. Linewidths have been inverted to obtain effective rotation-translation basis rate constants and to deduce R-R, P-P and R-P line couplings. The characteristics of these couplings are presented and discussed. Calculated ECS profiles in the microwindows are compared with experimental data and also with previous results based on a statistical fitting law. The ECS approach is particularly suitable for practical infrared spectroscopic applications including line-coupling effects.

Line mixing effects in the 0003–0000 band of CO2 in helium. III. Energy corrected sudden simultaneous fit of linewidths and near wing profile

Line coupling induced by collisions leads to drastic modifications of the spectral profile of the 0003–0000 of CO2 pressurized by helium. Calculation of these modifications have been performed by using two recent energy corrected sudden (ECS) formalisms. The two formulations lead to theoretical predictions rather similar and in good agreement with the available data over extended ranges of frequency and perturber pressure. It has been shown that a simultaneous fit of the pressure broadened linewidths and the near wing profile allows a more accurate determination of the basic ECS parameters. For that purpose, it has been necessary to extend the measurement of the broadened widths to high J values (up to J≊90).

Line mixing effects in the 00°3–00°0 band of CO2 in helium. II. Theoretical analysis

In paper I of this series, important deviations from an additive superposition of Lorentzian profiles were experimentally evidenced in the 00°3–00°0 band of CO2 in He. All the observed deviations are explained by the collision‐induced line mixing effects which schematically transfer intensity from the wing of the band to its central part. The IOS approximation has been found to be insufficient while, the ECS approximation leads to theoretical predictions in good agreement with the experimental data over extended ranges of frequency and perturber pressure. However it must be emphasized that it has been necessary to resort to the method in current use for the determination of the fundamental rates, an ad hoc adjustement starting from the observed linewidths.