Design and performance of an E-band chirped pulse spectrometer for kinetics applications: OCS – He pressure broadening

Abstract

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.

A novel Ka-band chirped-pulse spectrometer used in the determination of pressure broadening coefficients of astrochemical molecules

“…14 Numerous other transitions of OCS have also had their broadening coefficients with helium determined. 15,16,17,6 Vinyl cyanide (acrylonitrile, CH 2 CHCN) 18 and benzonitrile (c-C 6 H 5 CN) 19 are both molecules that have been detected in interstellar space, but little is known about their pressure broadening behavior. Cyanide-containing molecules are ideal for rotational spectroscopy as they often have a strong dipole moment, examples that have had their collisions with helium studied include: methyl cyanide (CH 3 CN), 20,21 cyanoacetylene (HC 3 N), [22][23][24][25] and hydrogen cyanide (HCN) [26][27][28] .…”

“…The output of the amplifier is coupled to a 25 dBi gain horn antenna (Sage SAR-2507-28-S2), which directs radiation through a Teflon window into a 1.5 m long by 0.15 m diameter vacuum chamber, containing the species of interest. 6 The interior of the vacuum chamber is lined with microwave-absorbent material (Laird Eccosorb HR-10) in order to dampen reflections of the excitation pulse within the cell.…”

“…The recorded FIDs were analyzed in the manner previously detailed for a time-domain Voigt profile. 6 However, transitions occurring in crowded spectral regions were difficult to fit in the time domain. In these cases, recorded FIDs were converted to frequency-domain spectra by taking the absolute value of a fast Fourier transform (FFT).…”

“…Figure 6 shows the plot of pressure broadening against pressure for the two different methods. The recorded FIDs were filtered using a correlation filter 6 and a digital bandpass filter to remove any DC offset and out-of-band noise before a window was selected over the range of the FID for fitting to the timedomain Voigt profile. If the Doppler broadening rate was allowed to float, it returned a value, on average, of This is the author's peer reviewed, accepted manuscript.…”

“…The Ka-band spectrometer complements an existing E-band spectrometer, which was recently built for the CRESUCHIRP project. 6 Rotational spectroscopy is a powerful tool for product identification: each molecule has a unique rotational spectrum which is specific to a particular molecular isomer, conformer, isotopologue and vibrational mode. Furthermore, the CP-FTMW method can cover a wide spectral range with a single scan and can rapidly target specific transitions.…”