Self- and nitrogen-broadened line shape data for the Pe(11) line of the ν1+ν3 band of acetylene, recorded using a frequency comb-stabilized laser spectrometer, have been analyzed using the Hartmann–Tran profile (HTP) line shape model in a multispectrum fitting. In total, the data included measurements recorded at temperatures between 125K and 296K and at pressures between 4 and 760Torr. New, sub-Doppler, frequency comb-referenced measurements of the positions of multiple underlying hot band lines have also been made. These underlying lines significantly affect the Pe(11) line profile at temperatures above 240K and poorly known frequencies previously introduced errors into the line shape analyses. The behavior of the HTP model was compared to the quadratic speed dependent Voigt profile (QSDVP) expressed in the frequency and time domains. A parameter uncertainty analysis was carried out using a Monte Carlo method based on the estimated pressure, transmittance and frequency measurement errors. From the analyses, the Pe(11) line strength was estimated to be 1.2014(50)×10−20 in cmmolecule−1 units at 296K with the standard deviation in parenthesis. For analyzing these data, we found that a reduced form of the HTP, equivalent to the QSDVP, was most appropriate because the additional parameters included in the full HTP were not well determined. As a supplement to this work, expressions for analytic derivatives and a lineshape fitting code written in Matlab for the HTP are available.

Frequency-modulated laser transient absorption has been used to monitor the ground-state rotational energy-transfer rates of CN radicals in a double-resonance, depletion recovery experiment. When a pulsed laser is used to burn a hole in the equilibrium ground-state population of one rotational state without velocity selection, the population recovery rate is found to depend strongly on the Doppler detuning of a narrow-band probe laser. Similar effects should be apparent for any relaxation rate process that competes effectively with velocity randomization. Alternative methods of extracting thermal rate constants in the presence of these non-thermal conditions are evaluated. Total recovery rate constants, analogous to total removal rate constants in an experiment preparing a single initial rotational level, are in good agreement with quantum scattering calculations, but are slower than previously reported experiments and show qualitatively different rotational state dependence between Ar and He collision partners. Quasi-classical trajectory studies confirm that the differing rotational state dependence is primarily a kinematic effect.

Hyperfine-resolved saturation spectra were measured for a selection of low and medium J rotational lines in the A 2Π–X 2Σ+ system of CN using two copropagating laser beams tuned to transitions in the (2–0) and (1–0) bands. A bleach laser was amplitude modulated and fixed in frequency near the center of a rotational line of the (2–0) vibrational band, while a probe laser was frequency-modulated and scanned across selected lines of the (1–0) vibrational band, sharing a common lower state with the bleach laser. Locking the probe laser with a tunable radio frequency offset to a cavity that tracks the slowly drifting bleach laser greatly improved the quality of the double-resonance saturation signals, by stabilizing the relative frequency of the two beams. The sub-Doppler resonances were fit with Lorentzian line shapes having a typical full-width at half maximum of 2–3MHz. The hyperfine spectra observed depend on the hyperfine structure within both rovibronic transitions excited, permitting the determination of hyperfine molecular constants in the υ=2 state and the refinement of previously published values in the υ=1 state. Four nuclear magnetic dipole and two electric quadrupole hyperfine constants were determined for each of the upper states from a fit with a weighted root mean squared error of 0.5MHz. The vibrational dependence of these constants is weak or negligible.

Using an extended cavity diode laser locked to a frequency comb, the line shape of the P(11) line in the ν1 + ν3 combination band of acetylene has been studied as a function of varying concentration of the absorber in nitrogen. Mixture concentrations of 1, 5 and 10% at 296 K and pressures between a few Torr and one atmosphere were made and the measurements analyzed using two different speed-dependent broadening models. These experiments are designed to test the additivity of contributions to pressure broadening and shift in speed-dependent line shape modeling, i.e. whether the lineshape parameters follow partial pressure weighting in the binary mixtures. P(11) is relatively isolated with respect to underlying hot band transitions and neighboring transitions of the same band, but it was found that the accurate positions of underlying hot band transitions were crucial to the successful modeling of the observed line shapes, even though these lines are typically 100-1000 times weaker than P(11) itself and are many Doppler line widths removed from the line center. Positions of the hot band lines quoted in the HITRAN database, which are derived from the analysis of high resolution FTIR spectra, are of the order of 10’s of MHz in error. In parallel work, we have measured the positions of many of these lines by saturation dip spectroscopy. Progress in the analysis of the data and the new saturation dip line center measurements will be reported. Acknowledgments: Work at Brookhaven National Laboratory was carried out under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy and supported by its Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences.

Author Institution: Department of Chemistry, Stony Brook University, Stony Brook, New York 11794; Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973; Department of Physics, Astronomy, and Astrophysics, Connecticut College, New London, CT 06320

Author Institution: Chemistry Department, Brookhaven National Laboratory, Bldg. 555A, P.O. Box 5000, Upton, NY 11973, USA

The P(11) line of the ν1 + ν3 combination band of C2H2 was studied using an extended cavity diode laser locked to a frequency comb. Line shapes were measured for acetylene and nitrogen gas mixtures at a series of temperatures between 125 and 296 K and total pressures up to 1 atm. The data were fit to two speed-dependent line shape models and the results were compared. Line shape parameters were determined by simultaneously fitting data for all temperatures and pressures in a single multispectrum analysis. Earlier pure acetylene measurements [Cich et al. Appl. Phys. B 2012, 109, 373-38] were incorporated to account for self-perturbation. The resulting parameters reproduce the observed line shapes for the acetylene-nitrogen system over the range of temperatures and pressures studied with average root-mean-square observed-calculated errors of individual line measurement fits of approximately 0.01% of maximum transmission, close to the experimental signal-to-noise ratios. Errors in the pressure measurements constitute the major systematic errors in these measurements, and a statistical method is developed to quantify their effects on the line shape parameters for the present system.

Selected isolated rotational transitions in the 1-0 band of the red A(2)Π-X (2)Σ(+) system in CN have been recorded with transient frequency modulation spectroscopy as a function of argon pressure up to 0.2 atm at room temperature. Line shapes were fit using Fourier transforms of a parametrized time correlation function, including Doppler and velocity-dependent collisional broadening, and collisional shifts. Deviations from Voigt line shapes can be equally well fit by modeling the narrowing with a speed-dependent collision model or with a velocity-changing collisional narrowing model. Pressure broadening coefficients were observed with little rotational state dependence, in the range of 0.070-0.075 cm(-1) atm(-1). In contrast, stronger and qualitatively different rotational state dependences are observed for both pressure-dependent blue shift coefficients and the narrowing parameters. No asymmetry in the pressure broadened lines was observed.

Hyperfine splittings were measured in selected rotational lines of the (1–0) vibrational band of the B∼3Πg-A∼3Σu+ transition of 14N2 in a low pressure discharge. Frequency modulated saturation dip spectroscopy produced hyperfine resolved, sub-Doppler lines with typical full-width at half-maximum line widths of 7MHz. Hyperfine transitions within 27 rotational lines in the R33, Q33, Q21, Q22, R11, and Q11 branches were recorded for J between 2 and 12, and fit with the same Hamiltonian used previously by Geisen, Neuschafer and Ottinger [Z. Phys. D 4, 263 (1987)]. The resulting parameters are (in MHz, with one standard deviation in parentheses): α=13.19(13), β=−12.36(18), e2q0Q=−1.19(25) for the (v″=0)A∼3Σu+ state; and K11=98.52(50), D11=13.09(59), D1−1=−44.11(16), G11=70.00(52), Q11=1.390(98) and Q1−1=1.65(11) for the (v′=1) B∼3Πg state.

Author Institution: Chemistry Department, Brookhaven National Laboratory, Bldg. 555A, P.O. Box 5000, Upton, NY 11973, USA

Author Institution: Department of Physics, The Ohio State University, Columbus Ohio, 43210-1106, USA; Department of Physics and Centre for Laser, Atomic, and; Molecular Sciences, University of New Brunswick, P.O. Box 4400; Fredericton NB E3B 5A3, Canada; Canadian Light Source, Inc., University of Saskatchewan; Saskatoon, SK, Canada