Product channel-specific reaction kinetics at low temperature presents a significant challenge for both experiment and theory but provides essential inputs for astrochemical models o f cold interstellar environments. Reaction kinetics studies using chirped pulse Fourier transform microwave (CP-FTMW) spectroscopy provide a potential solution but require the use of a buffer gas to thermalize the reactants and products; however, collisions with the buffer gas reduce the length of the detected signal, the free induction decay, through pressure broadening. The effect of this on time domain signals is largely unexplored using CP-FTMW spectrometers. A new E-band (60-90 GHz) spectrometer has been constructed using previously unavailable equipment in order to maximize its performance for detecting molecules in collisional environments. The design of this spectrometer is described in detail. The pressure broadening of OCS in He was used to test the spectrometer under collisional conditions similar to those that are routinely used to perform reaction kinetics measurements. The corresponding pressure broadening coefficients for transitions in this frequency range were determined at room temperature and the performance of the spectrometer was assessed in relation to recently reported chirped pulse in uniform flow experiments.
We present a thorough pure rotational investigation of the CH 2 CN radical in its ground vibrational state. Our measurements cover the millimeter and sub-millimeter wave spectral regions (79−860 GHz) using a W-band chirped-pulse instrument and a frequency multiplication chain-based spectrometer. The radical was produced in a flow cell at room temperature by H abstraction from acetonitrile using atomic fluorine. The newly recorded transitions of CH 2 CN (involving N″ and K a ″ up to 42 and 8, respectively) were combined with the literature data, leading to a refinement of the spectroscopic parameters of the species using a Watson S-reduced Hamiltonian. In particular, the A rotational constant and K-dependent parameters are significantly better determined than in previous studies. The present model, which reproduces all experimental transitions to their experimental accuracy, allows for confident searches for the radical in cold to warm environments of the interstellar medium.
Experimental measurements using EM II (see text). Theoretical calculations. Drafting and correction of article. Sidaty Cheikh Sid Ely Experimental measurements using EM I (see text). Ilsa R. Cooke Experimental measurements using EM II (see text). Drafting and correction of article. Théo Guillaume Laser setup for EM II
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