Chirped-pulse millimeter-wave spectroscopy for dynamics and kinetics studies of pyrolysis reactions

Abstract

A Chirped-Pulse millimeter-Wave (CPmmW) spectrometer is applied to the study of chemical reaction products that result from pyrolysis in a Chen nozzle heated to 1000-1800 K. Millimeter-wave rotational spectroscopy unambiguously determines, for each polar reaction product, the species, the conformers, relative concentrations, conversion percentage from precursor to each product, and, in some cases, vibrational state population distributions. A chirped-pulse spectrometer can, within the frequency range of a single chirp, sample spectral regions of up to ∼10 GHz and simultaneously detect many reaction products. Here we introduce a modification to the CPmmW technique in which multiple chirps of different spectral content are applied to a molecular beam pulse that contains the pyrolysis reaction products. This technique allows for controlled allocation of its sensitivity to specific molecular transitions and effectively doubles the bandwidth of the spectrometer. As an example, the pyrolysis reaction of ethyl nitrite, CH3CH2ONO, is studied, and CH3CHO, H2CO, and HNO products are simultaneously observed and quantified, exploiting the multi-chirp CPmmW technique. Rotational and vibrational temperatures of some product molecules are determined. Subsequent to supersonic expansion from the heated nozzle, acetaldehyde molecules display a rotational temperature of 4 ± 1 K. Vibrational temperatures are found to be controlled by the collisional cooling in the expansion, and to be both species- and vibrational mode-dependent. Rotational transitions of vibrationally excited formaldehyde in levels ν4, 2ν4, 3ν4, ν2, ν3, and ν6 are observed and effective vibrational temperatures for modes 2, 3, 4, and 6 are determined and discussed.

A chirped-pulse Fourier-transform microwave/pulsed uniform flow spectrometer. II. Performance and applications for reaction dynamics

“…The SO 2 (Sigma Aldrich, 99.9%) was seeded at 0.5% in the helium flow and dissociated using an ArF excimer laser. To monitor the appearance of the product, the "fast-frame" capability of the oscilloscope was used, 7,11 such that successive spectra were obtained at 5 μs intervals following an initial 10 μs delay between the laser trigger and the first chirped pulse excitation. Ten frames (i.e., spectra) obtained in this fashion, each an average of roughly 6500 acquisitions, are shown in Fig.…”

“…Moreover, the broadband detection scheme readily allows for comparison of relative line intensities of different species to provide accurate branching ratios if the products have a well-defined rotational temperature. 11 In a recent CP study, a signature of roaming dynamics was observed by quantifying branching into several reaction products aided by accurate kinetic modeling. 14 As a form of rotational spectroscopy, CP-FTMW requires cold samples to produce sufficiently large population differences for sensitive spectroscopic detection.…”

“…Indeed, Pate and co-workers have demonstrated that the broadband feature of CP-FTMW spectroscopy can be used to measure the rates of isomerization in laser-excited molecules, 10 and more recently CP-FTMW/mmW has been used to probe the products generated from pyrolysis and photolysis reactions. [11][12][13][14] Rotational spectroscopy techniques have been used for reaction dynamics and kinetics studies in the past, [15][16][17] but have not been widely adopted. Traditional techniques, such as laserinduced fluorescence (LIF) or resonant multiphoton ionization (REMPI) methods, are still the preferred approaches for investigating reaction dynamics.…”

A chirped-pulse Fourier-transform microwave/pulsed uniform flow spectrometer. I. The low-temperature flow system

“…Moreover, the intensities of the spectral lines are reliable indicators of relative species' populations, making it possible to accurately determine branching ratios and, in some cases, vibrational distributions. 31 Thus, when combined with a PUSF source, this combined CPUF technique is uniquely well-suited for studying polyatomic unimolecular and bimolecular reaction dynamics and kinetics.…”

Edge effects in chirped-pulse Fourier transform microwave spectra

“…Fine structure in the chirpedpulse spectrum of the prism isomer of the water hexamer enables the measurement of hydrogen-bond network rearrangement [3]. Prozument et al [4] have used chirped-pulse millimeter wave spectroscopy to obtain meaningful product branching ratios and vibrational population distributions from a flash pyrolysis reactor. Recent work in the A. G. Suits lab [5,6] has demonstrated the use of time-domain CP-FTMW measurements to measure the vibrationally-specific rate of appearance of unimolecular and bimolecular reaction products in a cold uniform flow.…”