  We study effects of nonadiabatic couplings in a model of a diatomic molecule
in the context of attosecond transient absorption spectroscopy. By using a
model system consisting of four diabatic electronic states and with a variable
strength of the diabatic coupling, we can explore attosecond transient
absorption spectra in different regimes of the vibronic couplings between the
electronic and nuclear dynamics, and determine when nonadiabatic couplings can
be ignored or when they have a substantial effect. The findings are
rationalized in terms of a multilevel model, which captures aspects of both
electronic and nuclear degrees of freedom.
