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

Abstract

A novel chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer has been constructed to cover the for use in the CRESUCHIRP project, which aims to study the branching ratios of reactions at low temperatures using the chirped-pulse in uniform flow (CPUF) technique. The design takes advantage of recent developments in radio-frequency components; notably high-frequency, highpower solid-state amplifiers. The spectrometer had a flatness of 5.5 dB across the spectral range, produced harmonic signals below -20 dBc, and the recorded signal scaled well to 6 million averages. The new spectrometer was used to determine pressure broadening coefficients with a helium collider at room temperature for three molecules relevant to astrochemistry, applying the Voigt function to fit the magnitude of the Fourier-transformed data in the frequency domain. The pressure broadening coefficient for OCS was determined to be (2.45 ± 0.02) MHz mbar -1 at room temperature, which agreed well with previous measurements. Pressure broadening coefficients were also determined for multiple transitions of vinyl cyanide and benzonitrile. Additionally, the spectrometer was coupled with a cold, uniform flow from a Laval nozzle. The spectrum of vinyl cyanide was recorded in the flow, and its rotational temperature was determined to be (24 ± 11) K. This temperature agreed with a prediction of the composite temperature of the system through simulations of the experimental environment coupled with calculations of the solution to the optical Bloch equations. These results pave the way for future quantitative studies in low-temperature and high-pressure environments using CP-FTMW spectroscopy.

Collisional excitation of HNC by He found to be stronger than for structural isomer HCN in experiments at the low temperatures of interstellar space

“…The laser was aligned along the center of the flow and constrained in size to specifically interact with molecules only in the isentropic core and not the surrounding warmer boundary layers. 51 The ground vibrational state j=1-0 transitions of HCN and HNC were probed in the CRESU flows using an E-band CP-FTmmW spectrometer which has been recently described in detail. 27 Here, it was adapted for spectroscopy under CRESU conditions in a specially constructed vacuum chamber.…”

“…The arbitrary waveform generator (Keysight M8195A) and digitizer (Teledyne SP Devices ADQ7DC-PCIE) used in these experiments were the same as in previous works 27,51 but were reconfigured for time resolved acquisition following pulsed laser photolysis in the uniform flow. 400 µs pulse train sequences were used with 2.5 µs long pulse/collection cycles, resulting in 160 individual pulse -FID (Free Induction Decay) cycles per train, with the laser firing after the first half of each sequence.…”

Rotational Spectrum of the Phenoxy Radical

“…This catalog enables studies of phenoxy and its chemistry in both the laboratory and space to be undertaken with considerable confidence. Narrow- and broadband microwave spectroscopy in the centimeter wave region is ideally suited for isomer- and isotopologue-specific detection of phenoxy in complex reactive mixtures at the rotationally cold laboratory conditions achieved in supersonic expansions, ,, uniform flows, − and cryogenic buffer-gas-cooled cells …”

Hyperfine-Resolved Rotational Spectroscopy of Phenyl Radical

“…This degree of accuracy enables new spectroscopic applications for investigating the phenyl radical and its chemistry in both the laboratory and in space. Microwave spectroscopy provides sensitive isomer- and isotopologue-specific detection of C 6 H 5 in complex reactive mixtures in a frequency range particularly well suited for low-temperature laboratory environments, including cryogenic buffer-gas cooled cells, supersonic expansions, and uniform flows. − …”