Photoelectron Spectroscopy of Small Organic Anions Prepared Using a Novel Cold Ion Source Employing Entrainment of Charged Particles

Negative ion photoelectron spectroscopy is utilized to investigate the methyl anions (CH3– and CD3–), the propadienylidene anion (H2CCC–), and the propargylene anion (HCCCH–) prepared using recently developed variations to the entrainment anion sources. Combining experiment and theory allows for further insight into the electronic and vibrational structures of these molecules. To enhance the anion-synthesis capability, multiple, pulsed entrainment valves are used for stepwise chemistry. In addition to operation of multiple valves, pulsed plasma-entrainment anion source is developed. The major feature of this anion source is the additional pulsed valve for perpendicular entrainment of plasma, which is made in an electrical discharge. This anion source provides the capability to make substantial clusters of entrained anions, e.g., OH–(Arn =0−32), as well as to stabilize products of exothermic reactions in the primary expansion, e.g., HOCO– and H3COO–. We expect that more pulsed valves could be used for additional complex syntheses if necessary. The methyl anion CH3– is formed using the pulsed plasma-entrainment source. Upon photodetachment, the out-of-plane bending angle from the pyramidal CH3- anion to the planar CH3 radical changes significantly, leading to an extended Franck-Condon progression in the umbrella mode. The observation of the well-studied v2 bands of CH3 and CD3, coupled with the dramatically improved electron energy resolution, enables us to directly measure the inversion splitting between the 0+ and 0- energy levels in both CH3- and CD3-. The EAs of CH3 and CD3 are measured precisely; using a thermochemical cycle, the methane gas-phase acidity is refined. The photoelectron spectra of H2CCC– and HCCCH– are used to characterize the electronic and vibrational structures of the corresponding neutral molecules. The reaction of O- with allene (H2C=C=CH2) produces nearly pure H2CCC-, which exhibits resolved vibrational progressions. In contrast, the O- + propyne reaction produces both H2CCC- and HCCCH- products. Comparison of the HCCCH– and H2CCC- photoelectron spectra provides information about the electronic states of HCCCH. With the aid of calculations and simulations, we can assign and characterize the electronic states of H2CCC and HCCCH