We present a method for recovering Doppler broadened absorption line shapes from frequency modulated ͑FM͒ line spectra. The method of analysis is calibrated and demonstrated with thermalized CN radicals produced by photodissociation of cyanogen ͑NCCN͒, probed on the A -X system near 800 nm with a frequency modulated Ti: sapphire ring laser. Nonthermal, Doppler broadened lines from translationally nascent photofragments can also be recovered by direct transformations of experimental FM line profiles acquired with a time resolution exceeding 100 ns. The superior signal-to-noise afforded by FM spectroscopy, relative to other direct absorption methods, should encourage the application of transient FM spectroscopy to problems in photoinitiated reaction dynamics.
Doppler-resolved laser-induced fluorescence spectroscopy of the OH reaction products of the photoinitiated reaction H + O2 → OH + O has been applied at collision energies near 1.8 eV, where an anomalous peak in the total reaction cross section has been reported by Kessler and Kleinermanns (J. Chem. Phys. 1992, 97, 374). Although the product state distributions show no remarkable variation as the collision energy varies through this region, analysis of the Doppler line shapes suggests a sideways scattering component to the differential cross sections near 1.8 eV that is not present at higher collision energies. We propose that the qualitative change in the differential cross sections as well as the local peak in the integral cross section can be a consequence of the dynamics in the vicinity of the C 2v conical intersection.
ultraviolet. One widely used scheme is laser induced fluorescence (LIF) using a pulsed probe laser to measure the disappearance of CN, scanning the photolysis-probe delay to obtain a time profile [7 -9]. Durant and Tully have used a cw LIF scheme for CN kinetics measurements [10] which is capable of impressive signal-to-background measurements. More recently, Hanson and co-workers have used direct absorption of a cw ultraviolet laser beam to determine timedependent CN concentrations in shock tube experiments [11].The general features of our FM spectrometer have been described in detail elsewhere [2,4]. Briefly, narrow band cw light in the near-infrared is generated by an Ar ϩ laser pumped Ti : sapphire ring laser. The nearinfrared beam is passed through an electro-optic phase modulator driven at 200 MHz, generating radio frequency sidebands above and below the optical carrier frequency. The differential absorption or retardation of the sidebands by a transient chemical species is detected as power at the radio frequency on a fast photodiode. Phase-sensitive detection with a doublebalanced mixer followed by a 20 MHz low-pass filter gives a time-dependent signal proportional to the instantaneous CN concentration. The CN radicals were probed using the R 1 8.5 line in the 2-0 vibrational band of the A-X transition near 800 nm. Premixed Frequency modulation (FM) of a continuous laser with heterodyne detection provides a zero-background absorption method most often applied to sensitive detection of sharp spectral features in stable molecules [1]. We have recently been using an explicitly time-resolved version of FM spectroscopy for measuring spectra of transient molecules [2,3] and Doppler-broadened line shapes [4,5]. The method can also be applied with advantage to the measurement of gas-phase kinetics, as we demonstrate here, for the reaction of CN with ethylene at 298 K: CN ϩ C 2 H 4 9: products.(1)The reaction of CN radicals with unsaturated hydrocarbons is relevant to combustion modeling and the rate constants have been extensively reported over a wide range of temperatures [6]. There have been several techniques used to follow the disappearance kinetics of photolytically generated CN radicals in the presence of reaction partners. Most involve optical detection using the B-X transition of CN in the near ABSTRACTTime-resolved frequency-modulation spectroscopy is shown to be an effective method for measuring the rates of gas-phase reactions. As an example, the rate constant for the reaction of CN with C 2 H 4 at 298 K is measured to be 2.5 Ϯ 0.2 ϫ 10 Ϫ10 cm 3 s Ϫ1 , in good agreement with other literature values. The efficiency and sensitivity of this technique will be of interest to the chemical kineticist.
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