  Multidimensional spectroscopy unveils the interplay of nuclear and electronic
dynamics, which characterizes the ultrafast dynamics of various molecular and
solid-state systems. In a class of models widely used for the simulation of
such dynamics, field-induced transitions between electronic states result in
linear transformations (Duschinsky rotations) between the normal coordinates of
the vibrational modes. Here we present an approach for the calculation of the
response functions, based on the explicit derivation of the vibrational state.
This can be shown to coincide with a multimode squeezed coherent state, whose
expression we derive within a quantum-optical formalism, and specifically by
the sequential application to the initial state of rotation, displacement and
squeeze operators. The proposed approach potentially simplifies the numerical
derivation of the response functions, avoiding the time integration of the
Schr\"odinger equation, the Hamiltonian diagonalization, and the sum over
infinite vibronic pathways. Besides, it quantitatively substantiates in the
considered models the intuitive interpretation of the response functions in
terms of the vibrational wave packet dynamics.
