Uniform supersonic flow sampling for detection by chirped-pulse rotational spectroscopy

Abstract

Chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy is a powerful near-universal detection method finding application in many areas. We have previously coupled it with supersonic flows (CPUF) to obtain product branching in reaction and photodissociation. Because chirped-pulse microwave detection requires monitoring the free induction decay on the timescale of microseconds, it cannot be employed with good sensitivity at the high densities achieved in some uniform supersonic flows. For application to low-temperature kinetics studies, a truly uniform flow is required to obtain reliable rate measurements and enjoy all the advantages that CP-FTMW has to offer. To this end, we present a new setup that combines sampling of uniform supersonic flows using an airfoil-shaped sampling device with chirped-pulse mmW detection. Density and temperature variations in the airfoil-sampled uniform flow were revealed using time-dependent rotational spectroscopy of pyridine and vinyl cyanide photoproducts, highlighting the use of UV photodissociation as a sensitive diagnostic tool for uniform flows. The performance of the new airfoil-equipped CPUF rotational spectrometer was validated using kinetics measurements of the CN + C2H6 reaction at 50 K with detection of the HCN product. Issues relating to product detection by rotational spectroscopy and airfoil sampling are discussed. We show that airfoil sampling enables direct measurements of low temperature reaction kinetics on a microsecond timescale, while rotational spectroscopic detection enables highly specific simultaneous detection of reactants and products.

Radical–Radical Reaction Dynamics Probed Using Millimeterwave Spectroscopy: Propargyl + NH2/ND2

“…Using the C 3 H 3 + NH 2 /ND 2 reaction, we show that chemical dynamics of a wide variety of reactions can be investigated using the CPUF-mmW approach. Applications of CPUF-mmW extend beyond the investigation of reaction dynamics; future generations of CPUF-mmW experiments will enable precise measurements of absolute rate coefficients of complex reactions by exploiting its ability to detect and quantify reactants and products simultaneously at well-defined conditions …”

“…Applications of CPUF-mmW extend beyond the investigation of reaction dynamics; future generations of CPUF-mmW experiments will enable precise measurements of absolute rate coefficients of complex reactions by exploiting its ability to detect and quantify reactants and products simultaneously at well-defined conditions. 48 ■ ASSOCIATED CONTENT * sı Supporting Information…”

Multichannel Radical–Radical Reaction Dynamics of NO + Propargyl Probed by Broadband Rotational Spectroscopy

“…The CPUF experimental setup consisting of a pulsed Laval flow coupled to a chirped pulse millimeter-wave spectrometer is discussed in our previous publications. ,, Therefore, only a brief experimental description relevant to this study is provided below. In this work, propargyl bromide (99%, Sigma-Aldrich) and 1,2-butadiene (98% Chemsampco) were used as precursor molecules for the propargyl radical.…”

“…We apply chirped pulse rotational spectroscopy in a pulsed flow (CPUF) to investigate the gas phase bimolecular reaction dynamics of NO + C 3 H 3 . In previous studies, we demonstrated the successful application of CPUF for product branching determinations in unimolecular and bimolecular reactions with isomer and vibrational level specificity, and we have recently shown application to low-temperature kinetics determination employing sampling. − …”

“…Our present findings are discussed in light of previous unimolecular photodissociation studies of isoxazole 35,36 and prior potential energy surface calculations of the NO/C 3 H 3 system by Danilack and Goldsmith and by Wang et al 30,31 ■ EXPERIMENTAL SECTION The CPUF experimental setup consisting of a pulsed Laval flow coupled to a chirped pulse millimeter-wave spectrometer is discussed in our previous publications. 32,33,37 Therefore, only a brief experimental description relevant to this study is provided below. In this work, propargyl bromide (99%, Sigma-Aldrich) and 1,2-butadiene (98% Chemsampco) were used as precursor molecules for the propargyl radical.…”