The electronic states of pyridine-N-oxide studied by VUV photoabsorption and ab initio configuration interaction computations

Abstract

The first vacuum-ultraviolet absorption spectrum of pyridine-N-oxide has been obtained, and has led to the identification of nearly 30 Rydberg states. These states were identified by use of the vibrational envelope ("footprint") of the UV-photoelectron spectrum, and are based on the first to the third ionization energies (IE). The adiabatic IE order, central to the Rydberg state symmetry identification, is confirmed by multi-configuration SCF calculations as: 1 2 B 1 < 1 2 B 2 < 1 2 A 2 < 2 2 B 1 . Several excited valence state equilibrium structures were determined by multi-configuration SCF and coupled cluster procedures. Multi-reference multi-root CI was used to calculate both Rydberg and valence state vertical excitation energies and oscillator strengths, which were correlated with the experimental measurements. © 2013 AIP Publishing LLC. [http://dx

Interpretation of the vacuum ultraviolet photoabsorption spectrum of iodobenzene by ab initio computations

“…32 The SMCD can be used to demonstrate the directionality of the Rydberg states and hence their symmetry. [13][14][15] Expansions of short energy ranges of the UV + VUV spectrum are exhibited below; the dominant three regions of the spectrum are near 5.5, 6.5, and 7 eV; each of these ranges show high f(r) valence states. The most intense individual band lies between 7 and 7.5 eV and can be correlated with two calculated ππ * electronic states, of 1 A 1 (1a 2 2a 2 * ) and 1 B 2 (2b 1 2a 2 * − 1a 2 4b 1 * ) symmetries, together with other weaker states.…”

“…Thus, vertical excitation energies (VEE) for valence and Rydberg states, determined by a MRD-CI, [25][26][27] as implemented in GAMESS-UK, 36 were also utilized; this method is fully described in our previous studies. [13][14][15] In this paper, theoretical oscillator strengths (f(r)), derived from the CI wave-functions, are compared with absorption band intensities. Wide differences in f(r) occur between valence and Rydberg states, generally enabling differentiation between the types of excitation.…”

“…The second moments of the charge distribution (SMCD, x 2 , y 2 , and z 2 ) give additional information both in relation to valence states, which have similar values for SMCD to the ground state, and also concerning the directionality of Rydberg p-, d-, and f-states, where selected directions of the SMCD are large; this procedure was described previously. [13][14][15] In all the CI methods, the 29 core orbitals (15a 1 + 5b 1 + 7b 2 + 2a 2 ) are frozen in occupancy and density; the full valence shell occupied orbitals (8a 1 + 3b 1 + 6b 2 + 1a 2 ) were included in the CI, together with up to 120 virtual MOs (VMOs) in the MRD-CI study. Valence shell numbering is used to enable comparisons with the lower PhX.…”

“…We have used a procedure similar to our previous studies [13][14][15] to enable distinction between the valence and Rydberg types of spectra. In order to help identify features in the VUV spectrum as Rydberg states, an underlying broad structure made up of several Gaussian bands was "subtracted" from the original spectrum.…”

“…PES are important for identification of Rydberg states in the VUV absorption spectrum since the potentials experienced in the ionic (PES) and Rydberg state (VUV) formation, particularly for high-n states, are similar and this leads to similar vibrational profiles in the two types of spectra. [9][10][11][12][13][14][15] Therefore, in the analysis of the absorption spectra, we use the vibronic structure observed in the PES 1 as a fingerprint to identify the ionic core of the observed Rydberg state or series.…”