New view of the ICN A continuum using photoelectron spectroscopy of ICN⁻

Negative-ion photoelectron spectroscopy of ICN⁻ (⁠|${\rm \tilde X}$|$X̃$ ²Σ⁺) reveals transitions to the ground electronic state (⁠|${\rm \tilde X}$|$X̃$ ¹Σ⁺) of ICN as well as the first five excited states (³Π₂, ³Π₁, |$\sideset{^3}{_{0^-}}{\Pi}$|$Π0−3$⁠, |$\sideset{^3}{_{0^+}}{\Pi}$|$Π0+3$⁠, and ¹Π₁) that make up the ICN A continuum. By starting from the equilibrium geometry of the anion, photoelectron spectroscopy characterizes the electronic structure of ICN at an elongated I–C bond length of 2.65 Å. Because of this bond elongation, the lowest three excited states of ICN (³Π₂, ³Π₁, and |$\sideset{^3}{_{0^-}}{\Pi}$|$Π0−3$⁠) are resolved for the first time in the photoelectron spectrum. In addition, the spectrum has a structured peak that arises from the frequently studied conical intersection between the |$\sideset{^3}{_{0^+}}{\Pi}$|$Π0+3$ and ¹Π₁ states. The assignment of the spectrum is aided by MR-SO-CISD calculations of the potential energy surfaces for the anion and neutral ICN electronic states, along with calculations of the vibrational levels supported by these states. Through thermochemical cycles involving spectrally narrow transitions to the excited states of ICN, we determine the electron affinity, EA(ICN), to be 1.34₅ (+0.04/−0.02) eV and the anion dissociation energy, D₀(⁠|${\rm \tilde X}$|$X̃$ ²Σ⁺ I–CN⁻), to be 0.83 (+0.04/−0.02) eV.