title: Beyond the electric-dipole approximation: A formulation and implementation of molecular response theory for the description of absorption of electromagnetic field radiation.
abstract: We present a formulation of molecular response theory for the description of a quantum mechanical molecular system in the presence of a weak, monochromatic, linearly polarized electromagnetic field without introducing truncated multipolar expansions. The presentation focuses on a description of linear absorption by adopting the energy-loss approach in combination with the complex polarization propagator formulation of response theory. Going beyond the electric-dipole approximation is essential whenever studying electric-dipole-forbidden transitions, and in general, non-dipolar effects become increasingly important when addressing spectroscopies involving higher-energy photons. These two aspects are examined by our study of the near K-edge X-ray absorption fine structure of the alkaline earth metals (Mg, Ca, Sr, Ba, and Ra) as well as the trans-polyenes. In following the series of alkaline earth metals, the sizes of non-dipolar effects are probed with respect to increasing photon energies and a detailed assessment of results is made in terms of studying the pertinent transition electron densities and in particular their spatial extension in comparison with the photon wavelength. Along the series of trans-polyenes, the sizes of non-dipolar effects are probed for X-ray spectroscopies on organic molecules with respect to the spatial extension of the chromophore.
authors.affiliation: Department of Physics, Chemistry and Pharmacy, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark.
authors.forename: Nanna Holmgaard
authors.initials: NH
authors.lastname: List
authors.affiliation: Department of Physics, Chemistry and Biology, Linköping University, Linköping SE 58183, Sweden.
authors.forename: Joanna
authors.initials: J
authors.lastname: Kauczor
authors.affiliation: Laboratoire de Chimie et Physique Quantiques, UMR 5626-CNRS/Université Toulouse III (Paul Sabatier), 118 route de Narbonne, F-31062 Toulouse Cedex, France.
authors.forename: Trond
authors.initials: T
authors.lastname: Saue
authors.affiliation: Department of Physics, Chemistry and Pharmacy, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark.
authors.forename: Hans Jørgen Aagaard
authors.initials: HJ
authors.lastname: Jensen
authors.affiliation: Department of Physics, Chemistry and Biology, Linköping University, Linköping SE 58183, Sweden.
authors.forename: Patrick
authors.initials: P
authors.lastname: Norman
mesh_terms: D065638:Absorption, Radiation; D004574:Electromagnetic Fields; D008274:Magnesium / Q000737:chemistry; D008673:Metals, Alkaline Earth / Q000737:chemistry; D008958:Models, Molecular; D008962:Models, Theoretical; D008968:Molecular Conformation; D017785:Photons; D011090:Polyenes / Q000737:chemistry; D011789:Quantum Theory
chemical_list: D008673:Metals, Alkaline Earth; D011090:Polyenes; D008274:Magnesium