Photoelectron Velocity Map Imaging Spectroscopy and its Application to Small Beryllium-Containing Molecules. Photoelectron velocity map imaging spectroscopy (also called slow electron velocity map imaging spectroscopy, SEVI) combines the best properties of photoelectron spectroscopy and zero electron kinetic energy spectroscopy (ZEKE) by employing the velocity map imaging (VMI) detection method, enabling it to overcome many inherent drawbacks of other commonly used photoelectron spectroscopy methods. Eppink and Xxxxxx developed the VMI optics in 1997 by replacing the grid electrodes that were commonly used in ion imaging with open-hole electrodes.89 When used together with electron photodetachment techniques, these electrodes create an electrostatic lens that maps photodetached electrons, called photoelectrons, with the same initial electron kinetic energy (eKE) onto the same point on a position sensitive detector, regardless of their initial spatial position or ejection angle. These optics improve the eKE, spatial, and temporal resolution over classic electron detection techniques, allowing for vibrationally resolved spectra to be collected for larger molecules than other commonly used methods, such as photoelectron spectroscopy. This technique can achieve single wavenumber resolution, 100% photoelectron collection efficiency, and can record photoelectron angular distributions (PADs). PADs yield information about the electronic and vibronic structure of the molecule being studied.90-94 Photoelectron velocity map imaging spectroscopy has been extensively described elsewhere95-103, and thus only a brief overview of the technique will be discussed herein. Gas-phase ions are mass-selected before being directed into the center of the VMI optics (see Figure 1.1). When they reach the interaction region within the VMI lens the molecules are intersected by a laser fixed at a frequency that lies above the detachment threshold of the ion, causing electron photodetachment. The photoelectrons detach with quantized eKE as described by equation 1.1, eKE = hυ - EA - EInternal (1.1) where hυ is the photon energy of the photodetachment laser, EA is the electron affinity, EInternal represents the internal state energy of the neutral after photodetachment occurs, and eKE is the electron kinetic energy of the photoelectrons. The photoelectrons expand outwards after detachment, forming a cloud-like distribution in space. This cloud consists of Xxxxxx spheres104 whose unique radii correspond to the eKEs (velocities) of the
