Collision-broadened Line Widths Research Articles

The temperature dependence of lineshifts, linewidths and line intensities of methane at low temperatures

Employing the techniques that we have described previously for measuring infrared (i.r.) lineshifts, linewidths and line intensities with a tunable diode laser, we have measured these parameters for lines in the R(4) and R(7) multiplets of the v 4-fundamental band of 12CH 4 at 161, 209 (or 218) and 295 K using He, Ne, Ar, H 2, N 2, O 2, and air as the perturbers. The temperature dependence of the collision-induced lineshift is markedly different from that of the corresponding collision-broadened linewidth.

Measurements of collision-broadened line widths in the v 4-fundamental band of 12CH 4 at low temperatures

Collision-broadened half-widths of several lines in the v 4-fundamental band of 12CH 4 have been measured at low temperatures between 130 and 295 K using a tunable diode laser and the sweep integration technique. The broadening gases are H 2, N 2, He, and Ar. The temperature dependence of the measured line widths is described in terms of an exponent n, which is shown not only to be different for each broadening gas but also to depend upon the tetrahedral symmetry identification of the lines of CH 4.

Photothermal lensing spectroscopy of supersonic jet expansions of acetylene

Photothermal lensing effects are observed in the early expansion stages (X/D=1–7) of a supersonic free jet and narrow P and R branch rotational lines have been recorded at 100 MHz resolution for the ν2 +5ν3 band of acetylene. Rotational Raman loss spectra were also recorded and, from the collision-broadened linewidths of these, it is estimated that about 100 collisions occur in the beam transit from pump to probe positions. This is sufficient to produce rotational and some vibrational relaxation in forming the lens. The photothermal intensities are consistent with a Boltzmann rotational distribution in the ground state and a rotational temperature of 70 K is deduced for 100 PSI expansions of 20% C2 H2 in helium. Charac- teristics of the photothermal signal in the jet are described and the time and spatial dependence of the signal is used to measure a flow velocity 625 m/s for the jet. This photothermal method may prove useful as a diagnostic probe of beam properties and as a spectroscopic measure of weak one- and two-photon transitions of molecules cooled in jets.

Foreign-gas collision broadening of the far-infrared spectrum of water vapor

The far-infrared rotational spectrum of H2(O-16) has been studied in the spectral range 25-112/cm to measure the foreign-gas collision-broadened linewidths. Measurements of 17 lines broadened by nitrogen and 21 lines broadened by oxygen are reported. The measurements were made at 297 K. From these data, the widths due to air broadening are obtained. The experimental results are compared with recent theoretical calculations and with the case of a constant linewidth, equal to the average experimental width. There is some correlation between the relative experimental linewidths and the theoretical predictions. However, the simple assumption of a constant value for the collision-broadened linewidths gives a better representation for the case of N2- and O2-broadened linewidths than do present detailed theoretical calculations.

Infrared line widths at planetary atmospheric temperatures

A review of published measurements of the variation of collision-broadened line width with temperature in the infrared (i.r.) at temperatures relevant to planetary atmospheres is presented along with an examination of the existent theoretical models.

Estimation of Small Signal Gain and Saturation Intensity in a Waveguide CO2 Laser by a Curve-Fitting Technique

Output power curves as a function of the frequency have been measured by heterodying between a gas flowing waveguide and a conventional sealed-off CO2 laser with a W–Ni point contact diode. The small signal gain, saturation intensity, collision-broadened linewidth and frequency tunability are simultaneously estimated by fitting a theoretical equation to output power curves according to a least-squares technique.

Measurement of the rotational spectra of OH+ and OD+ by laser magnetic resonance

Far IR rotational transitions between the four lowest rotational levels in the X 3Σ− vibronic ground states of OH+ and OD+ have been observed by laser magnetic resonance spectroscopy. Ground state molecular constants, including the three g factors, have been determined and employed in the calculation of a Born–Oppenheimer equilibrium geometry. The centrifugal distortion of the spin-rotation interaction is found to have a significant effect on the determination of other molecular constants. Hyperfine splittings have been resolved and analyzed for OH+, but could not be observed in OD+ spectra with a 6 MHz collision-broadened linewidth.

Experimental determination of the temperature dependence of nitrogen-broadened line widths in the 1← 0 band of HCl

Nitrogen-broadened line widths in the 1 ← 0 band of HCl have been measured over the temperature range 215–296 K using Fourier transform absorption spectroscopy at 0.01 cm −1 resolution. The J dependence at room temperature has been compared with data from previous measurements; our values are consistently lower than those currently given in the AFGL compilation. A temperature dependence of the form α( T) = [ α( T 0)]( T/ T 0) n has been fitted to the data. A value for n of −0.85 near band centre gave the best fit, which differs significantly from n = −0.5 predicted using the hard-collision approximation in simple kinetic theory. This difference should be noted when using collision-broadened line widths in the interpretation of stratospheric HCl spectra.

Tunable diode laser measurements of spectral parameters of HCN at room temperature

A tunable infrared diode laser was used to record 17 fully resolved vibration-rotation transitions in the v 1 fundamental band of HCN at 3μ. The experiments were conducted in an absorption cell on room temperature mixtures of HCN diluted by N 2 and Ar. The v 1 fundamental band strength of HCN was determined to be 267±8 cm -2 atm -1 at 273.2 K. Small but significant reductions in the residual errors were obtained by using the Galatry profile rather than the Voigt profile to fit the experimentally recorded line shapes. Collisional broadening and narrowing parameters were determined simultaneously from Galatry profile fits to the data. The collision-broadened linewidths of HCN lines in N 2 and Ar were determined as a function of rotational quantum number of transitions ranging from P(14) to R(14) (3268.22-3353.29 cm -1). The optical diffusion coefficients of HCN in N 2 and Ar at 300 K were determined from the collisional narrowing parameters and were 0.074±0.01 and 0.016±0.03 cm 2s -1 respectively.

Sensitivity of ozone retrievals in limb-viewing experiments to errors in line-width parameters

The variance of atmospheric ozone concentration profiles retrieved in i.r. limb-viewing experiments is examined over the range of published ozone halfwidth values. Simulated retrievals are performed for a typical satellite-based high-resolution absorption experiment and a broad-band emission experiment. The results indicate that, for limb-viewing experiments, inaccurate collision-broadened linewidth values can lead to significant errors in retrieved profiles for broad-band radiometric data or for isolated strong lines in high-resolution spectra.

Analysis of CO_2 bands near 2600 cm^−1

The collision-broadened linewidths and band intensities of three overlapping CO(2) bands near 2600 cm(-1) have been determined from spectra obtained with a Fourier transform spectrometer. These spectra have SNRs in excess of 500 and a spectral resolution of approximately 0.06 cm(-1). Calculated spectra were matched to the observed spectra by nonlinear least squares regression over 100-cm(-1) intervals. More than forty parameters were estimated simultaneously, and the mean differences between the spectra were about one and a half times the mean noise in the experimental spectra.

Energy transfer and quenching rates of laser-pumped electronically excited alkalis in flames

A pulsed, tunable dye laser has been used to excite each of the 3p, 3d, 4p, 5s and 4d levels of sodium atoms seeded into an atmospheric pressure acetylene-air flame. Three of these transitions, 3d, 5s and 4d, involve two-photon absorption from the ground 3s state. Fluorescence measurements furnish the populations of each of the levels populated by collisional transfer, enabling the deduction of relative energy transfer and quench rates among the four higher states. Fluorescence and opto-acoustic measurements under conditions near optical saturation of the 3p transition yield energy transfer rates and quenching rates for the two doublet components. Total collision-broadened linewidths are obtained by scanning the laser in a narrow-line mode. A qualitative experiment seeding the flame with both sodium and lithium suggests the presence of electronic-to-vibrational-to-electronic energy transfer.

Single-mode 3.39 μm laser study of methane absorption lineshape

A single-mode, HeNe 3.39 μm laudy has been made of the overlapping P(7) 1, v 3 methane line. The Galatry lineshape (involving collision-narrowing) is clearly verified, and the temperature dependence of the collision-broadened linewidth is at T −0.69 ± 0.4, as opposed to our previous determination of ∼ T −1.

Collision-broadened linewidths of tetrahedral molecules—III. Dispersion and induction interactions

Expressions for the interruption functions S 2( b) have been derived for the dispersion interaction between a tetrahedral molecule and a linear molecule, and for the interaction between the octopole moment of a tetrahedral molecule and the octopole-induced dipole moment in a perturbing molecule.

Collision-broadened linewidths of tetrahedral molecules—II. Computations for CH 4 lines broadened by N 2, O 2, He, Ne and Ar

Pressure-broadened widths of CH 4 lines have been calculated, using the recent formulation for tetrahedral molecules by Varanasi in the framework of the Anderson-Tsao-Curnutte theory. The present work deals mainly with lines of the R-branch of the ν 3-fundamental at 300°K, for the range 1≤ J≤30 and 0≤ K≤18, and for the F species. In the case of CH 4-N 2 and CH 4-O 2 collisions, octopole-quadrupole, octopole-hexadecapole, hexadecapole-quadrupole, hexadecapole-hexadecapole, dispersion and overlap interactions have been included. Broadening by noble gases was treated in terms of dispersion and overlap interactions. The value |Ω CH 4 |=2·25×10 −34 esu cm 3 obtained in the present work by comparing theoretical and experimental results for γ 0 CH 4−N 2 is not far from the magnitude deduced in studies of the far-i.r. collision-induced absorption spectra of methane. A magnitude for the hexadecapole moment of | Φ CH 4 |=3·0×10 −42 esu cm 4 was also determined. The calculated and measured linewidths are in good agreement, except in CH 4-Ar collisions. In the latter case, the theoretical values are appreciably lower than the experimental ones, which suggests that induced interactions may have to be included.

Collision-broadened line widths of tetrahedral molecules—I. Theoretical formulation

A theoretical formulation is presented within the framework of Anderson-Tsao- Curnutte theory for calculating the half-widths of spectral lines belonging to tetrahedral molecules. The importance of tetrahedral symmetry in the change distribution and in the wave functions is emphasized. Expressions for the interruption functions S 2(b) have been derived for octopole-dipole, octopole-quadrupole, octopole-octopole, octopole-hexadecapole, hexadecapole- hexadecapole, anistropic dispersion interactions as well as for an assumed overlap interaction potential. The earlier theoretical work of Tejawni and Yamamoto and Hirono is shown to be inappropriate for molecules of tetrahedral symmetry.

The role of higher-multipolar and repulsive forces in the calculation of collision-broadened line-widths of linear molecules

Collision-broadened line widths in CO-CO 2 and CO-O 2 collisions have been calculated by incorporating interactions due to octopoles and hexadecapoles and short range repulsive interactions into Anderson's theory. It is shown how these higher-order interactions can be manipulated to yield good agreement with experimental data. A critical evaluation of this totally empirical manipulation suggests that a thorough revision of the theory is required for all but simple dipole-dipole interactions. In the process of the evaluation, the values of the multipole moments are discussed.

Calculation of Collision-Broadened Linewidths in the Infrared Bands of Methane

Anderson–Tsao–Curnutte theory of collision-broadened spectral lines has been extended to octopolar molecules, and the results are applied to the rotational lines in the ν3 band of methane. Calculated half-widths are in excellent agreement with the high-resolution measurements of Varanasi for self-broadening and broadening by N2, O2, H2, and He. The theory has also been applied to CO lines broadened by CH4 and to CH4 lines broadened by HCl. The excellent agreement with experimental data in both the cases leads to the estimate of Ω = 5.0 × 10−34esu·cm3 for the octopole moment of CH4.

Linewidth and Saturation Parameters for the 6328-Å Transition in a He–Ne Laser

Experimental measurements of the collision-broadened linewidth and saturation parameters for the 6328-Å transition of Ne in a He–Ne laser are reported as a function of total gas pressure in the laser tube. The measurements are made from observations of the single-mode output power of a laser as a function of frequency. The experimental results are found to fit well a form of Lamb's theory for single-mode operation which has been modified to include gas-collision effects. A theoretical calculation of the saturation parameter is made and the result is found to be in fair agreement with the experimental measurements.