Double Imaging Photoelectron Photoion Coincidence Sheds New Light on the Dissociation of State-Selected CH₃F⁺ Ions

Abstract

The vacuum ultraviolet (VUV) photoionization and dissociative photoionization of methyl fluoride (CHF) in the 12.2-19.8 eV energy range were investigated by using synchrotron radiation coupled to a double imaging photoelectron photoion coincidence (iPEPICO) spectrometer. The production of several fragment ions including CHF, CHF, CH, and CH as a function of state and internal energy of CHF ions was identified and analyzed, with their individual appearance energies measured through threshold photoelectron spectroscopy. Dynamical information was inferred from electron and ion kinetic energy correlation diagrams measured at chosen fixed photon energies. The detailed mechanisms governing the dissociation of state-selected CHF ions prepared in the XE, AA, and BE low-lying electronic states as well as outside the Franck-Condon region have been inferred based on the present experimental results and existing theoretical calculations. Both the CHF and CH primary fragment ions have three different channels of production from different electronic states of CHF. The spin-orbit splitting states of the F fragment, P and P, in the CH + F dissociation channels were assigned and adiabatically correlate to the XE ground state and the AA electronic state, respectively, with the aid of previous theoretical results. The CHF ions in the high energy part of the XE ground state are unstable and statistically dissociate to the CHF(1A) and H(S) fragments along the potential energy curve of the XE state. The AA electronic state is a repulsive state and exclusively dissociates to the CH(1A') and F(P) fragments. In addition, the CHF, CHF, CH, and CH fragment ions are also produced in the BE state and in the Franck-Condon gap by indirect processes, such as internal conversion or dissociative autoionization.

Vacuum ultraviolet photodynamics of the methyl peroxy radical studied by double imaging photoelectron photoion coincidences

“…25 The ms-SPES records the signal of photoelectrons with near-zero kinetic energy (KE max = 100 meV) and its energy resolution can be comparable to the traditional TPES. 18,[27][28][29] As shown in Fig. 3, the two ms-SPES mainly exhibits the same structures and their intense peaks are located at hν = 9.838 eV, corresponding to the AIE of the methyl radical.…”

“…2). 18,27 From these matrices the SPES is extracted by integrating the photoelectron signal along a constant ionic state (diagonal line), up to a maximum value, KE max , according to the expression,…”

“…Another explanation for this discrepancy could be the different sensitivity to autoionization resonances of both methods, which are known to affect threshold spectroscopy in general, and leads to significant transition intensities even in wide Franck-Condon gaps. This latter feature of threshold photoionization (SPES and TPES) is routinely utilized in recording breakdown curves even in photon energy regions with no appreciable peaks in the photoelectron spectrum, or to expand the spectroscopic window into the Franck-Condon gap 27,28 .…”

Dissociation of High-Lying Electronic States of NO2+ in the 15.5–20 eV Region

“…Scanning the synchrotron photon energy and collecting the electrons with near zero kinetic energy, the mass-selected TPES corresponding to each mass can be obtained in coincidence. 30…”

“…The coincidence scheme also provides the energy correlation diagram between electrons and ions. Scanning the synchrotron photon energy and collecting the electrons with near zero kinetic energy, the mass-selected TPES corresponding to each mass can be obtained in coincidence …”