UC Berkeley

Slow electron velocity-map imaging (SEVI), a high-resolution variant of anion photoelectron spectroscopy, has proven to be a powerful and versatile spectroscopic technique capable of measuring the vibronic structure of a wide range of molecular and cluster species with exquisite detail. The extension of SEVI to study anions cooled to their ground vibronic state (cryo-SEVI) through the addition of a cryogenic ion trap dramatically improved spectral resolution and enabled larger species to be probed. This, however, also greatly limits the number of neutral states accessible via photodetachment. To overcome this, anions are resonantly excited to a selected vibrational state using infrared (IR) radiation prior to photodetachment. This new technique, dubbed IR cryo-SEVI, offers the potential to probe the vibrational structure of both anionic and neutral species to an even greater extent.The cryo-SEVI spectrum without vibrational pre-excitation was collected for the acetyl anion (CH3CO¯). This spectrum reveals a significant vibrational progression along the CCO bending mode with transitions up to Δv=11 clearly observed. The measured electron affinity for the acetyl radical is also used to calculate a refined value for the gas-phase acidity of acetaldehyde (CH3CHO). Cryo-SEVI with vibrational pre-excitation was first applied to the hydroxide anion (OH¯) by exciting the well-characterized R(0) rovibrational transition of the anion. The large rotational constant of the diatomic paired with the high resolution of cryo-SEVI enabled the rotational fine structure to be fully resolved. Upon excitation, depletion of the ground state features is observed along with new features that appear corresponding to transitions from vibrationally excited anions. Additionally, the IR absorption profile of the anion was measured by monitoring the growth of new features as the IR energy is varied, precluding the need for messenger-tagged species that perturb vibrational frequencies. IR cryo-SEVI was extended to polyatomic systems with the vinoxide anion (CH2CHO¯) excited along the CO (ν4) and lone CH (ν3) stretching modes. Excitation of the lower frequency ν4 fundamental results in photoelectron spectra that can be fully explained within the harmonic approximation. Excitation of the higher frequency ν3 fundamental, however, results in a more complicated spectrum consisting of several unexpected transitions. Theoretical considerations reveal the ν3 fundamental is anharmonically coupled to nearby vibrational states in both the anion and neutral manifold, leading to newly allowed transitions appearing in the spectrum. The nitrate radical (NO3) has been the focus of several theoretical and experimental investigations owing to its complex electronic structure arising from strong vibronic interactions between electronic states. IR cryo-SEVI is used to definitively settle a decades-long controversy over the fundamental frequency of the degenerate stretching mode (ν3), wherein the ν3 and 2ν3 vibrational states of the NO3¯ anion are accessed prior to detachment. Through comparison to theory, assignment of transitions from the 2ν3 state allow for unambiguous determination of the neutral ν3 frequency. Vinylidene (H2CC), a high-energy isomer of acetylene, represents a model system to study how isomerization affects the vibrational structure of molecular species. The small barrier for isomerization to acetylene (HCCH) as well as the asymmetric shape of the potential energy surface allows for low-lying vibrational states of vinylidene to interact with highly excited HCCH states, resulting in a complex vibrational structure. Cryo-SEVI spectra collected with and without vibrational pre-excitation probe vinylidene’s complicated structure with unprecedented detail, giving insight into which vibrational modes drive the isomerization reaction. The high-resolution of cryo-SEVI also reveals the nature of vinylidene-acetylene coupling in the energy region surrounding the predicted isomerization barrier height.