Experimental and theoretical threshold photoelectron spectra of methylene

The threshold photoelectron spectrum of methylene (CH₂), produced by consecutive H atom abstractions on methane, has been recorded using synchrotron radiation. The experimental spectrum spans the region of the X ^{+ 2}Π ← X ³ B ₁ ionizing transition. It is modeled starting from ab initio bending potentials and using the bending approach introduced by Coudert et al. [J. Chem. Phys. 148, 054302 (2018)] accounting for the quasilinearity of CH₂ and the strong Renner-Teller interaction in $$ . This first calculation yields a theoretical threshold photoelectron spectrum which is in moderate agreement with the experimental one. A more accurate approach treating the three vibrational modes is developed for computing the threshold photoelectron spectrum of triatomic $$ molecules. This new treatment is tested modeling the already measured threshold photoelectron spectrum of the X ^{+ 2}Π ← X ¹ A ₁ ionizing transition of the water molecule. The threshold photoelectron spectrum of CH₂ computed with the new approach compares more favorably with the experimental spectrum and yields an adiabatic ionization potential of 10.386(6) eV.