Synchrotron-based valence shell photoionization of CH radical

Abstract

We report the first experimental observations of X(+) (1)Σ(+)←X (2)Π and a(+) (3)Π←X (2)Π single-photon ionization transitions of the CH radical performed on the DESIRS beamline at the SOLEIL synchrotron facility. The radical was produced by successive hydrogen-atom abstractions on methane by fluorine atoms in a continuous microwave discharge flow tube. Mass-selected ion yields and photoelectron spectra were recorded as a function of photon energy using a double imaging photoelectron/photoion coincidence spectrometer. The ion yield appears to be strongly affected by vibrational and electronic autoionizations, which allow the observation of high Rydberg states of the neutral species. The photoelectron spectra enable the first direct determinations of the adiabatic ionization potential and the energy of the first triplet state of the cation with respect to its singlet ground state. This work also brings valuable information on the complex electronic structure of the CH radical and its cation and adds new observations to complement our understanding of Rydberg states and autoionization processes.

The absolute photoionization cross section of the mercapto radical (SH) from threshold up to 15.0 eV

“…Vibrationally hot species have already been produced in a similar manner via H-abstractions by fluorine radicals, e.g. CH, 76 As for the b 1 Σ + state, it was found to exhibit broader peaks as compared with the rest of the bands in their PES study. The accuracy in the determination of the ionization onset of the state can be called into question due to the aforementioned spectral congestion.…”

Experimental and theoretical threshold photoelectron spectra of methylene

“…32 The methylene spectrum has been recorded simultaneously with that of the CH radical presented elsewhere. 33 Experimental conditions should be found in this reference. Both species were produced by consecutive H atom abstractions on methane.…”

Communication: On the first ionization threshold of the C2H radical

“…32,33 All the experimental conditions can be found in a recent work dealing with the CH radical, 34 since the C 2 H radical photoelectron spectrum presented below has been recorded simultaneously in the same a) Author to whom correspondence should be addressed. Electronic mail:…”

“…First, the origin band seems to present a shoulder on its left side (approximately 280 cm 1 below the origin band), which might be attributed to a hot band since the lowest vibrational state of the neutral species can be populated, as it was observed in the same experiment for the CH radical. 34 Indeed, the C 2 H radical can be produced, for instance, by the CH + CH → C 2 H + H reaction (∆H r = −409 kJ/mol) or the CH + CH 2 → C 2 H 2 + H (∆H r = −541 kJ/mol) reaction followed by C 2 H 2 + F → C 2 H + HF (∆H r = −14 kJ/mol only but with C 2 H 2 issued from CH + CH 2 and then potentially highly excited), the CH and CH 2 radical being produced through consecutive H abstractions by fluorine atoms on the CH 4 molecule. However, the lowest vibrational excited level of the X 2 Σ + ground electronic state of C 2 H is located 493 cm 1 above the = 0 ground state.…”

“…This point needs further investigations at higher resolution (e.g., with the pulsed-fieldionization zero-kinetic-energy photoelectron spectroscopy technique) to be confirmed and precisely described. The presented photoelectron spectrum was limited to the [IE, IE + 0.4 eV] range because of experimental issues 34 and we plan to reinvestigate the ionizing transitions of the C 2 H radical up to the next excited states (for instance, the 3 Σ − state is predicted to lie only 0.5 eV above the cationic ground state). …”