Open AccessReexamining the Lyman-Birge-Hopfield band of N2
Author(s) -
J. A. Bradley,
Arto Sakko,
Gerald T. Seidler,
Ángel Rubio,
Mikko Hakala,
K. Hämäläinen,
Glyn Cooper,
Adam P. Hitchcock,
K. Schlimmer,
K. P. Nagle
Publication year - 2011
Publication title -
physical review. a, atomic, molecular, and optical physics/physical review, a, atomic, molecular, and optical physics
Language(s) - English
Resource type - Journals
eISSN - 1094-1622
pISSN - 1050-2947
DOI - 10.1103/physreva.84.022510
Subject(s) - physics , discrete dipole approximation , dipole , born approximation , energy (signal processing) , resonance (particle physics) , atomic physics , spectroscopy , scattering , quantum mechanics
Motivated by fundamental molecular physics and by atmospheric and planetary sciences, the valence excitations of N2 gas have seen several decades of intensive study, especially by electron-energy-loss spectroscopy (EELS). It was consequently surprising when a comparison of nonresonant inelastic x-ray scattering (NIXS) and nonresonant EELS found strong evidence for violations of the first Born approximation for EELS when leaving the dipole scattering limit. Here we reassess the relative strengths of the constituent resonances of the lowest-energy excitations of N2, encompassed by the so-called Lyman-Birge-Hopfield (LBH) band, expanding on the prior, qualitative interpretation of the NIXS results for N2 by both quantifying the generalized oscillator strength of the lowest-energy excitations and also presenting a time-dependent density functional theory (TDDFT) calculation of the q dependence of the entire low-energy electronic excitation spectrum. At high q, we find that the LBH band has an unexpectedly large contribution from the octupolar w 1Δu resonance exactly in the regime where theory and EELS experiment for the presumed-dominant a 1Πg resonance have previously had substantial disagreement, and also where the EELS results must now be expected to show violations of the Born approximation. After correcting for this contamination, the a 1Πg generalized oscillator strength from the NIXS results is in good agreement with prior theory. The NIXS spectra, over their entire q range, also find satisfactory agreement with the TDDFT calculations for both bound and continuum excitations.This work was supported by the US Department of Energy, the Natural Sciences and Engineering Research Council (NSERC) of Canada, the Australian Research Council, the Research\udFunds of the University of Helsinki, and the Academy of Finland (Contract No. 1127462, Centers of Excellence Program 2006-2011, and National Graduate School in Materials Physics). A.R. acknowledges support by MICINN (FIS2010-21282-C02-01),ACI-promociona (ACI2009-1036), Grupos Consolidados UPV/EHU del Gobierno Vasco (IT-319-\ud07), and the European Community through e-I3 ETSF project (Contract No. 211956).Peer Reviewe