Full-dimensional treatment of short-time vibronic dynamics in a molecular high-order-harmonic-generation process in methane

The method of multiconfigurational strong-field approximation with Gaussian nuclear wave packets is developed to study the effect of nuclear motion for molecules in strong laser fields, where an investigation on the polyatomic molecule methane reveals that the high-order-harmonic generation contains signatures of field-free vibronic dynamics at the conical intersection and signatures of other chemical processes.