  We develop a trajectory-based approach for excited-state molecular dynamics
simulations of systems subject to an external periodic drive. We combine the
exact-factorization formalism, allowing to treat electron-nuclear systems in
nonadiabatic regimes, with the Floquet formalism for time-periodic processes.
The theory is developed starting with the molecular time-dependent Schroedinger
equation with inclusion of an external periodic drive that couples to the
system dipole moment. With the support of the Floquet formalism, quantum
dynamics is approximated by combining classical-like, trajectory-based, nuclear
evolution with electronic dynamics represented in the Floquet basis. The
resulting algorithm, which is an extension of the coupled-trajectory mixed
quantum-classical scheme for periodically driven systems, is applied to a model
study, exactly solvable, with different field intensities.
