A theory of nonresonant x-ray emission spectra of conjugated molecules
is presented. The role of relaxation effects on the spectral shape of
x-ray emission is advanced and examined in detail for the transpolyen
es. In some contrast to unconjugated systems, the relaxation of valenc
e charge around the core hole is shown to influence the distribution o
f x-ray intensities significantly. Site and size dependences of the sp
ectral shape of x-ray emission are predicted; the site dependence as b
eing alternant among the pi levels. Comparative calculations of x-ray
emission spectra at the ab initio Hartree-Fock and the semiempirical l
evels show that the relaxation qualitatively changes the spectral dist
ribution and shifts intensity from the sigma to the pi parts of the sp
ectra, creating excitonlike features in the latter. The exciton charac
ter is site and size dependent; for longer polyenes it is stronger for
mid-atoms than for end-atoms, and is found stronger at the top than a
t the bottom of the pi band. Complication/simplification trends of the
spectra along the oligomer sequence are predicted for both the relaxe
d and frozen orbital models. (C) 1996 American Institute of Physics.