Photoelectron-Photofragment Coincidence Spectroscopy With Ions Prepared in a Cryogenic Octopole Accumulation Trap: Collisional Excitation and Buffer Gas Cooling

Photoelectron photofragment coincidence spectroscopy of carboxylates

Photoelectron–photofragment coincidence (PPC) spectroscopy is a powerful technique for studying the decarboxylation dynamics of carboxyl radicals. Measurement of photoelectron and photofragment kinetic energies in coincidence provides a kinematically complete measure of the dissociative photodetachment (DPD) dynamics of carboxylate anions. PPC spectroscopy studies of methanoate, ethanoate, propanoate, 2-butenoate, benzoate, p-coumarate and the oxalate monoanion are reviewed. All of the systems studied undergo decarboxylation via a two-body DPD channel i.e., driven by the thermodynamic stability of CO₂. Additionally, decarboxylation is observed via a three-body ionic photodissociation channel for p-coumarate. In some cases photodetachment also results in a stable carboxyl radical (RCO₂). The branching ratio for DPD, the threshold detachment energy and the peak of the kinetic energy release spectrum are compared for different carboxylates, as a probe of the character of the potential energy landscape in the Franck–Condon region.

Photoelectron photofragment coincidence spectroscopy studies of a range of carboxylate anions are reviewed, revealing details of the decarboxylation dynamics of carboxyl radicals.

Approximate equation-of-motion coupled-cluster methods for electron affinities of closed-shell molecules

We investigate performance of the equation-of-motion coupled-cluster method at the single and doubles level (EOM-CCSD) and a series of approximate methods based on EOM-CCSD on electron affinities (EA) of closed-shell cations and neutral molecules with positive and negative EAs in this work. Our results confirm that P-EOM-MBPT2 can provide reasonable EAs when molecules with significant multireference character are not considered and its mean absolute error on EAs of these molecules is around or less than 0.2 eV. Its accuracy is comparable to that of the more expensive EOM-CCSD(2) method. Results of EOM-CCSD(2), P-EOM-MBPT2, and CIS(D_∞) indicate that the [[H, a_c ⁺], T₂] term in the 1h2p-1h block is more important on EAs than the term neglected in the 1h2p-1h2p block in P-EOM-MBPT2. We proposed an economical method where EAs from CIS(D_∞) are corrected by treating this [[H, a_c ⁺], T₂] term in the 1h2p-1h block perturbatively [corr-CIS(D_∞)]. EAs with corr-CIS(D_∞) agree very well with those of P-EOM-MBPT2 with a difference of less than 0.02 eV. Computational scaling of this method is N⁴ for the iterative part and N⁵ for some non-iterative steps. Its storage requirement is only of OV³. Corr-CIS(D_∞) is an economical and reliable method on EAs, and it can be applied to EAs of large molecules.