  Spectral densities are calculated for the dimerized and frustrated S=1/2
chain using the method of continuous unitary transformations (CUTs). The
transformation to an effective triplon model is realized in a perturbative
fashion up to high orders about the limit of isolated dimers. An efficient
description in terms of triplons (elementary triplets) is possible: a detailed
analysis of the spectral densities is provided for strong and intermediate
dimerization including the influence of frustration. Precise predictions are
made for inelastic neutron scattering experiments probing the S=1 sector and
for optical experiments (Raman scattering, infrared absorption) probing the S=0
sector. Bound states and resonances influence the important continua strongly.
The comparison with the field theoretic results reveals that the sine-Gordon
model describes the low-energy features for strong to intermediate dimerization
only at critical frustration.
