R.R. Gamache

N2 and O2 Induced Halfwidths and Line Shifts of water vapor transitions in the (301)?(000) and (221)?(000) Bands

Abstract Using a complex version of Robert–Bonamy theory (CRBF), the role of the intermolecular potential in the pressure broadening of water perturbed by nitrogen and oxygen is studied. Investigation focuses on questions surrounding the convergence of calculated line widths, i.e., (i) why certain spectral lines are more sensitive than others to short-range interactions, and (ii) whether converged calculations containing short-range interactions represent an improvement over other treatments. Comparison with a large number of experimentally determined halfwidths and line shifts in the (301)←(000) and (221)←(000) bands is provided. It is found that the atom–atom component of the intermolecular potential plays an important role in determining the halfwidth and line shift. To obtain good agreement with measurement, the atom–atom potential needs to be expanded to at least eighth order for all water vapor transitions broadened by oxygen and many broadened by nitrogen.

Pressure‐induced widths and shifts for the ν3 band of methane

Widths and shifts of methane lines perturbed by nitrogen are calculated using a complex‐valued implementation of Robert–Bonamy (RB) theory. The static intermolecular potential is described as a sum of electrostatic forces and Lennard‐Jones (6‐12) atom–atom terms, using literature values for all physical parameters. Vibrational dependence of the isotropic potential is obtained from the polarizability of methane assuming a dispersion interaction. The repulsive part of the Lennard‐Jones accounts for the greatest part of widths, while dispersion interactions are largely responsible for shifts. Although the average error between calculated and observed linewidths (up to J=8) is less than 6%, their distribution suggests the influence of interactions not described in the present theory.

Einstein A coefficient, integrated band intensity, and population factors: application to the a O2 band

Abstract Literature values of the Einstein A coefficient for spontaneous emission for the a 1 Δ g →X 3 Σ g − (0,0) band of O 2 differ by as much as a factor of 2. The corresponding integrated band intensity values found in the literature show maximum differences of a factor of 4. The reason for these differences is not only the spread in the measured absorption band intensities but also in the improper use of population factors relating the Einstein A value to the line intensity or integrated band intensity. Here, the relationships between the Einstein A coefficient for spontaneous emission and the integrated band intensity and line intensity are developed with application to the a 1 Δ g ←X 3 Σ g − bands of O 2 . These relationships serve to eliminate the previously demonstrated confusion about the proper use of the lower and upper state population factors, with some using a ratio of 3 : 1 and the others 3 : 2 . A slightly modified expression for the line intensity in terms of the integrated band intensity is proposed which corrects for some approximations used in earlier expressions. Finally, recommendations are made for the most accurate values of the electronic–vibrational Einstein A coefficient and integrated band intensity for the a 1 Δ g (0)←X 3 Σ g − (0) band of O 2 .

PRESSURE BROADENING OF H2O IN THE (301) (000) BAND : EFFECTS OF ANGULAR MOMENTUM AND CLOSE INTERMOLECULAR INTERACTIONS

Abstract Two spectroscopic lines of water vapor, broadened by N 2 , are examined within the Complex Robert–Bonamy formalism in an effort to understand their sensitivity to short-range interactions. The two lines, both of which occur in the (301)←(000) band, are compared in terms of their dependence on impact parameter, sensitivity to convergence of the short-range potential, and the symmetry terms of the intermolecular potential.

The 1- bands and the O2 (0–1) and (1–0) X3Σg−−b1Σg+ bands in the Earth atmosphere

Abstract The CO2 triad of bands in the 9300– 9700 cm −1 region have been observed in near infrared 0.05 cm −1 resolution ground-based solar absorption spectra. This interval is a portion of spectra taken in the 9000– 12,000 cm −1 region, at large solar zenith angles. Considering the available line positions and pressure line shifts for CO2, H2O and O2 in this region as of 2000, it was concluded that these observations show significant inconsistencies among the line positions of the species as listed in the atmospheric spectroscopy databases. The spectra allow a better definition of the O2 (0–1) X3Σg−−a1Δg band, with the discrete (0–1) transitions observable in the 9300– 9450 cm −1 , superimposed on a collision-induced continuum covering the 9200– 9700 cm −1 region. This continuum, as well as the (0–0) continuum in the 7900 cm −1 region, have been previously studied only from atmospheric spectra with much lower spectral resolution. The discrete O2 (1–0) transitions of the X3Σg−−b1Σg+ atmospheric B-band are observed in the 11,500– 11,600 cm −1 region, but no evidence is found for an underlying continuum. A recent laboratory study of the 2ν 1 +3ν 3 12 CO 2 triad significantly improves the consistency between the O2, H2O and CO2 lines in the atmospheric spectra.