Reinvestigation of the rotation-tunneling spectrum of the CH₂OH radical

Abstract

Context. The hydroxymethyl radical (CH2OH) is one of two structural isomers, together with the methoxy radical (CH3O), that can be produced by abstraction of a hydrogen atom from methanol (CH3OH). In the interstellar medium (ISM), both CH2OH and CH3O are suspected to be intermediate species in many chemical reactions, including those of formation and destruction of methanol. The determination of the CH3O/CH2OH ratio in the ISM would bring important information concerning the formation processes of these species in the gas and solid phases. Interestingly, only CH3O has been detected in the ISM so far, despite the recent first laboratory measurement of the CH2OH rotation-tunneling spectrum. This lack of detection is possibly due to the non-observation in the laboratory of the most intense rotation-tunneling transitions at low temperatures. Aims. To support further searches for the hydroxymethyl radical in space, we present a thorough spectroscopic study of its rotation-tunneling spectrum, with a particular focus on transitions involving the lowest quantum numbers of the species. Methods. We recorded the rotation-tunneling spectrum of CH2OH at room temperature in the millimeter-wave domain using a frequency multiplication chain spectrometer. A fluorine-induced H-abstraction method from methanol was used to produce the radical. Results. About 180 transitions were observed, including those involving the lowest N and Ka quantum numbers, which are predicted to be intense under cold astrophysical conditions. These transitions were fitted together with available millimeter-wave lines from the literature. A systematic observation of all components of the rotational transitions yields a large improvement of the spectroscopic parameters allowing confident searches of the hydroxymethyl radical in cold to warm environments of the ISM.

Pure Rotational Spectroscopy of the CH2CN Radical Extended to the Sub-Millimeter Wave Spectral Region

“…The partial pressures of the F 2 /He mixture and CH 3 CN, optimized on CH 2 CN lines, were set to about 30 and 2 μbar, respectively. Compared to our previous publication using this setup, the multiplication of fluorine injection sites leads to an increase of the concentration of radicals in the cell while limiting multiple abstraction reactions for a given partial pressure of the F 2 /He mixture.…”

“…The CH 2 CN radical was synthesized using a relatively selective production method based on hydrogen abstraction from CH 3 CN by fluorine atoms, a method described in Garcia et al Fluorine atoms, produced upstream by a 2450 MHz discharge (20–100 W) in a F 2 /He mixture at 5% dilution, were injected into the reaction cell, where they collided and reacted with CH 3 CN to produce the CH 2 CN radical. The process is similar to the one employed in our previous work on the CH 2 OH radical produced from CH 3 OH . In this work, two reaction cells, adapted to either the CP and frequency multiplication chain-based spectrometers, have been used.…”

“…The process is similar to the one employed in our previous work on the CH 2 OH radical produced from CH 3 OH. 38 In this work, two reaction cells, adapted to either the CP and frequency multiplication chain-based spectrometers, have been used. To limit recombination reactions, both cells were covered with a fluorinated wax (Halocarbon Wax 1500) and were pumped using a roots blower (Edwards, 250 m 3 /h) backed by a chemically graded pump (PFPE-E2M28, Edwards, 30 m 3 /h).…”

“…Nowadays, radiotelescopes such as ALMA and NOEMA offer a large spectral coverage (∼80–950 and ∼80–360 GHz, respectively). Considering that the pure rotational spectrum of CH 2 CN remains intense at frequencies above 300 GHz, even at low temperatures, and that for species with only a few heavy atoms, the frequency extrapolation toward high frequencies is usually not reliable, new measurements over the sub-millimeter range are necessary . In the laboratory, the relatively high reactivity of radicals hampers their pure rotational spectroscopic studies.…”

“…Considering that the pure rotational spectrum of CH 2 CN remains intense at frequencies above 300 GHz, even at low temperatures, and that for species with only a few heavy atoms, the frequency extrapolation toward high frequencies is usually not reliable, new measurements over the sub-millimeter range are necessary. 38 In the laboratory, the relatively high reactivity of radicals hampers their pure rotational spectroscopic studies. In the centimeter wave spectral region, extremely high sensitivity is achieved using cavity-based Fourier transform microwave (FTMW) spectrometers associated with pulsed-discharge supersonic molecular beams.…”