Characterization of the $I2−$ anion ground state using conventional and femtosecond photoelectron spectroscopy

The $X̃ 2Σu+$ state of the $I2−$ anion has been fit to a Morse potential using data from two techniques: conventional and femtosecond photoelectron spectroscopy (FPES). Conventional photoelectron spectroscopy is used to determine the adiabatic electron affinity of $I2$ as well as the well depth and equilibrium nuclear geometry of $I2−.$ In the FPES experiment, the pump pulse induces coherent nuclear motion on the ground state of $I2−$ by resonant impulsive stimulated Raman scattering (RISRS), and the vibrational frequency of the anion is determined from the resulting oscillatory structure in the time-dependent photoelectron spectra. We find the electron affinity (EA) of $I2$ to be $2.524±0.005 eV,$ the well depth $(De)$ for $I2−$ to be $1.014±0.005 eV,$ the equilibrium internuclear separation $(Re)$ to be $3.205±0.005 Å,$ and the vibrational frequency to be $110±2 cm−1.$ These values for the $I2−$ potential parameters differ significantly from previous results.