Ab initio study of structures and energies of Al2H4 and Al2H 4 −

Electron Propagator Theory of Vertical Electron Detachment Energies of Anions: Benchmarks and Applications to Nucleotides

A new generation of ab initio electron-propagator self-energy approximations that are free of adjustable parameters is tested on a benchmark set of 55 vertical electron detachment energies of closed-shell anions. Comparisons with older self-energy approximations indicate that several new methods that make the diagonal self-energy approximation in the canonical Hartree-Fock orbital basis provide superior accuracy and computational efficiency. These methods and their acronyms, mean absolute errors (in eV), and arithmetic bottlenecks expressed in terms of occupied (O) and virtual (V) orbitals are the opposite-spin, non-Dyson, diagonal second-order method (os-nD-D2, 0.2, OV²), the approximately renormalized quasiparticle third-order method (Q3+, 0.15, O²V³) and the approximately renormalized, non-Dyson, linear, third-order method (nD-L3+, 0.1, OV⁴). The Brueckner doubles with triple field operators (BD-T1) nondiagonal electron-propagator method provides such close agreement with coupled-cluster single, double, and perturbative triple replacement total energy differences that it may be used as an alternative means of obtaining standard data. The new methods with diagonal self-energy matrices are the foundation of a composite procedure for estimating basis-set effects. This model produces accurate predictions and clear interpretations based on Dyson orbitals for the photoelectron spectra of the nucleotides found in DNA.

Quantitative Absorption Spectroscopy of Aluminum Atoms in Cryogenic Parahydrogen Solids

We report results of quantitative ultraviolet (UV) and infrared (IR) absorption spectroscopy experiments on Al-atom-doped cryogenic parahydrogen (pH₂) solids produced by codeposition of Al vapor and pH₂ gas. For Al-atom concentrations [Al] ≲200 parts-per-million (ppm), the Al/pH₂ solids are optically transparent and primarily contain isolated Al atoms, with a small admixture of AlH, Al₂H₂, and Al₂H₄ molecules formed by UV irradiation and Al-atom recombination/reaction. We assign the Al/pH₂ UV absorption spectrum by invoking a large (≈0.6 eV) gas-to-matrix blue shift to accompany the increase in principal quantum number in the 4s ²S ← 3p ²P_1/2 transition, as previously discussed for boron-atom-doped pH₂ solids. We assign a series of sharp features observed in the 4140-4155 cm^-1 IR region to Al-atom-induced Q₁(0) and Q₁(1) absorptions of the pH₂ solid. We use the solid pH₂ Q₁(0) + S₀(0) absorption to determine the sample thickness and to establish a constant pH₂ deposition efficiency independent of the flow rate. Using all of these absorption features in concert, we show that the Al-atom flux delivered by the effusive source is well described by the Knudsen-Langmuir equation, calculate an absolute Al-atom deposition yield per mass of aluminum evaporated, and demonstrate both constant Al-atom deposition and isolation efficiencies for [Al] ≲200 ppm. We discuss this unexpected constant Al-atom isolation efficiency in detail and speculate that it indicates nonuniform Al-atom recombination/reaction on the surface of the accreting sample, perhaps dominated by processes occurring near pH₂ crystallite grain boundaries. We demonstrate "control" over the deposition process, which we define as the ability to set, achieve, and verify "targets" for the final Al-atom concentrations and pH₂ solid thicknesses. This ability is key to sorting out the very different phenomena observed in samples targeting [Al] ≳300 ppm, which are described in the immediately following companion manuscript.