Valence-Shell Photoionization of C₄H₅: The 2-Butyn-1-yl Radical

Area of Science:

• Physical Chemistry
• Chemical Physics
• Molecular Spectroscopy

Background:

• The 2-butyn-1-yl radical is a key intermediate in chemical reactions.
• Understanding its electronic structure is crucial for reaction mechanism elucidation.
• Previous studies on related radicals like propargyl provide context.

Purpose of the Study:

• To investigate the photoionization of the 2-butyn-1-yl radical (C4H5) with high resolution.
• To characterize the electronic states of the C4H5+ cation.
• To determine the adiabatic ionization energy and absolute photoionization cross section.

Main Methods:

• Double-imaging, photoelectron-photoion coincidence (DIPPC) spectroscopy.
• Photoionization spectra recorded from 7.7 to 11 eV.
• Threshold photoelectron spectroscopy with 17 meV resolution.

Main Results:

• Observation of significant resonant autoionizing structure converging to excited states of C4H5+.
• Extended observation of bands corresponding to excited electronic states of the ion.
• Refined adiabatic ionization energy (IE) of 7.93 ± 0.01 eV.
• Absolute photoionization cross section (σion) at 9.7 eV determined as 6.1 ± 1.8 Mb.

Conclusions:

• The photoionization spectrum of the 2-butyn-1-yl radical exhibits complex structures similar to the propargyl radical.
• High-resolution DIPPC spectroscopy provides detailed insights into the electronic states of the C4H5+ cation.
• The determined ionization energy and cross section are valuable data for theoretical and experimental studies.