Wydział Chemii

Michael H. Palmer

Marcello Coreno

Monica de Simone

Søren Vrønning Hoffmann

Nykola C. Jones

Cesare Grazioli

Kirk A. Peterson

Alberto Baiardi

Teng Zhang

Małgorzata Biczysko

2017

Journal of Chemical Physics

146

084302/1-084302/9

10.1063/1.4975672

Naukowa

Angielski

Artykuł

A new synchrotron radiation photoelectron spectral (PES) study of iodopentafluorobenzene, together with a theoretical analysis of the spectrum, where Franck-Condon factors are discussed, gives detailed insight into the ionization processes, and this exposes the need for a reinvestigation of the vacuum ultraviolet spectral (VUV) assignments. We have calculated adiabatic ionization energies (AIEs) for several ionic states, using the equation-of-motion coupled cluster method for ionic states combined with multi-configuration self-consistent field calculation study. The AIE sequence is: X²B1 < A²A₂ < B²B₂ < C2²B₁ < D²A₁ < E3²B₁. This symmetry sequence has a major impact on previous VUV spectral assignments, which now appear to be to optically forbidden states. Changes in the equilibrium structures for these ionic states are relatively small, but a significant decrease and increase in the C–I bond length relative to the X¹A₁ structure occurs for the X²B₁ and C²B₁ states, respectively. The PES shows major vibrational overlaps between pairs of ionic states, X with A, and A with B. The result of these overlaps is the loss of vibrational structure and considerable broadening of the higher energy PES state. Although the baseline is nearly re-established between the A and B states, where the two bands are nearly separate, the B state is also broadened by the A state. Only the C ionic state, which shows the most highly developed vibrational structure, can be regarded as free from vibrational coupling to a neighbor state. The Franck-Condon analysis of the PES bands X, A, B, and C is described in detail; the apparent simplicity of some of these bands is illusory, since almost all the observed peaks arise from super-position of several calculated vibrational states. The experimental AIE of the A state, which is submerged under the X state envelope, has been determined by the subtraction of the calculated X state envelope from the observed PES spectrum. The overlap of these PES bands and the apparent closeness of the potential energy curves describing them have been investigated, using the state-averaged, complete active space self-consistent field method. We have identified two structures, one where the potential energy curves for the X and A states cross and another for the A and B states. At these two conical intersections (ConInts), there is zero-energy difference within each pair of states. Although similar in energy, the ConInt for the crossing of the X with A states, and that for the A with B states, shows that the open-shell occupancies correspond to the 4 lowest AIE states, and all four states that are quite different from each other.

Multi-configurational self-consistent field, Coupled-cluster methods, Complete-active space self-consistent field, Potential energy surfaces, Vibrational states, Vacuum ultraviolet radiation, Franck Condon principle, Franck Condon factors, Ions and properties, Gas phase

http://dx.doi.org/10.1063/1.4975672

https://www.aip.org/