Spin–Orbit and Vibronic Coupling in the Ionic Ground State of Iodoacetylene from a Rotationally Resolved Photoelectron Spectrum

The X^+ 2Π ← X ¹Σ⁺ photoionizing transition of iodoacetylene (HC₂I) has been investigated by pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy. The resolution of the rotational structure of the spectra and its analysis provided information on the structure of the HC₂I⁺ cation and the photoionization dynamics of HC₂I. In the ground electronic ²Π state, the HC₂I⁺ cation is found to be linear and subject to a strong spin–orbit coupling. The first adiabatic ionization energy of HC₂I and the spin–orbit splitting of the X^+ 2Π ground state of HC₂I⁺ were determined to be <i>E</i>_I(HC₂I)/<i>hc</i> = 78296.5(2) cm^–1 and Δν̃_SO = 3257(1) cm^–1, respectively. The large spin–orbit interaction almost entirely masks the Renner–Teller effect, which is only detectable through the observation of the nominally forbidden transition to the first excited level (5¹) of the HCC-I bending mode ν₅. The interaction of ∼2 cm^–1 observed between the 5¹ levels of ²Σ_1/2 and ²Δ_5/2 symmetry is attributed to a vibronic interaction with the B ²Σ⁺ electronic state of HC₂I⁺. The spin–orbit energy level structure of tri- and tetra-atomic molecules subject to the Renner–Teller effect and spin–orbit coupling is discussed for the two limiting cases where the spin–orbit-coupling constant is much smaller and much larger than the bending-mode frequencies

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

A Near-Threshold Shape Resonance in the Valence-Shell Photoabsorption of Linear Alkynes

The room-temperature photoabsorption spectra of a number of linear alkynes with internal triple bonds (e.g., 2-butyne, 2-pentyne, and 2- and 3-hexyne) show similar resonances just above the lowest ionization threshold of the neutral molecules. These features result in a substantial enhancement of the photoabsorption cross sections relative to the cross sections of alkynes with terminal triple bonds (e.g., propyne, 1-butyne, 1-pentyne, ...). Based on earlier work on 2-butyne [Xu et al., <i>J. Chem. Phys.</i> <b>2012</b>, <i>136</i>, 154303], these features are assigned to excitation from the neutral highest occupied molecular orbital (HOMO) to a shape resonance with g (<i>l</i> = 4) character and approximate π symmetry. This generic behavior results from the similarity of the HOMOs in all internal alkynes, as well as the similarity of the corresponding gπ virtual orbital in the continuum. Theoretical calculations of the absorption spectrum above the ionization threshold for the 2- and 3-alkynes show the presence of a shape resonance when the coupling between the two degenerate or nearly degenerate π channels is included, with a dominant contribution from <i>l</i> = 4. These calculations thus confirm the qualitative arguments for the importance of the <i>l</i> = 4 continuum near threshold for internal alkynes, which should also apply to other linear internal alkynes and alkynyl radicals. The 1-alkynes do not have such high partial waves present in the shape resonance. The lower <i>l</i> partial waves in these systems are consistent with the broader features observed in the corresponding spectra