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

The vacuum ultraviolet (VUV) photoionization and dissociative photoionization of methyl fluoride (CH₃F) in the 12.2–19.8 eV energy range were investigated by using synchrotron radiation coupled to a double imaging photoelectron photoion coincidence (i²PEPICO) spectrometer. The production of several fragment ions including CH₂F⁺, CHF⁺, CH₃⁺, and CH₂⁺ as a function of state and internal energy of CH₃F⁺ 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 CH₃F⁺ ions prepared in the X²E, A²A₁, and B²E 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 CH₂F⁺ and CH₃⁺ primary fragment ions have three different channels of production from different electronic states of CH₃F⁺. The spin–orbit splitting states of the F fragment, ²P_3/2 and ²P_1/2, in the CH₃⁺ + F dissociation channels were assigned and adiabatically correlate to the X²E ground state and the A²A₁ electronic state, respectively, with the aid of previous theoretical results. The CH₃F⁺ ions in the high energy part of the X²E ground state are unstable and statistically dissociate to the CH₂F⁺(1¹A₁) and H­(²S) fragments along the potential energy curve of the X²E state. The A²A₁ electronic state is a repulsive state and exclusively dissociates to the CH₃⁺(1¹A₁′) and F­(²P_1/2) fragments. In addition, the CH₂F⁺, CHF⁺, CH₃⁺, and CH₂⁺ fragment ions are also produced in the B²E state and in the Franck–Condon gap by indirect processes, such as internal conversion or dissociative autoionization

Unveiling the Ionization Energy of the CN Radical

The cyano radical is a ubiquitous molecule and was, for instance, one of the first species detected in astrophysical media such as comets or diffuse clouds. In photodissociation regions, the reaction rate of CN⁺ + CO → CN + CO⁺ is one of the critical parameters defining nitrile chemistry. The enthalpy of this charge transfer reaction is defined as the difference of ionization energies (<i>E</i>_I) between CN and CO. Although <i>E</i>_I(CO) is known accurately, the <i>E</i>_I(CN) values are more dispersed and deduced indirectly from thermodynamic thresholds only, all above <i>E</i>_I(CO), leading to the assumption that the reaction is fast even at low temperature. Using a combination of synchrotron radiation, electron/ion imaging coincidence techniques, and supporting ab initio calculations, we directly determine the first adiabatic ionization energy of CN at 13.956(7) eV, and we demonstrate that <i>E</i>_I(CN) < <i>E</i>_I(CO). The findings suggest a very slow reaction in the cold regions of interstellar media

Vacuum Ultraviolet Photoionization Study of Gas Phase Vitamins A and B1 Using Aerosol Thermodesorption and Synchrotron Radiation

Gas-phase studies of biomolecules are often difficult to initiate because of the thermolability of these systems. Such studies are nevertheless important to determine fundamental intrinsic properties of the molecules. Here we present the valence shell photoionization of gas-phase vitamins A and B1 close to their ionization threshold. The study was performed by means of an aerosol thermodesorption source coupled to an electron/ion coincidence spectrometer and synchrotron radiation (SOLEIL facility, France). Ion yield curves were recorded for both molecules over a few electronvolt energy range and the threshold photoelectron spectrum was also obtained for vitamin A. Some fundamental properties were extracted for both ions such as adiabatic and the three first vertical ionization energies of retinol (IE_ad = 6.8 ± 0.2 eV and IE_vert = 7.4, 8.3, and 9.2 eV) and dissociation appearance energies for the main fragment ions of vitamin B1. Analysis of the data was supported by <i>ab initio</i> calculations which show a very good agreement with the experimental observations

Vibrationally Resolved Photoelectron Spectroscopy of Electronic Excited States of DNA Bases: Application to the <i>Ã</i> State of Thymine Cation

For fully understanding the light–molecule interaction dynamics at short time scales, recent theoretical and experimental studies proved the importance of accurate characterizations not only of the ground (D0) but also of the electronic excited states (e.g., D1) of molecules. While ground state investigations are currently straightforward, those of electronic excited states are not. Here, we characterized the <i>Ã</i> electronic state of ionic thymine (T⁺) DNA base using explicitly correlated coupled cluster ab initio methods and state-of-the-art synchrotron-based electron/ion coincidence techniques. The experimental spectrum is composed of rich and long vibrational progressions corresponding to the population of the low frequency modes of T⁺(<i>Ã</i>). This work challenges previous numerous works carried out on DNA bases using common synchrotron and VUV-based photoelectron spectroscopies. We provide hence a powerful theoretical and experimental framework to study the electronic structure of ionized DNA bases that could be generalized to other medium-sized biologically relevant systems