Nonresonant interaction of ultrashort electromagnetic pulses with multilevel quantum systems

A theory is derived for the excitation of a multilevel quantum system by an electromagnetic pulse whose length is much shorter than the reciprocal of the frequency of transitions between levels. This interaction is characterized by a selectivity of a new type, which does not involve resonant properties of the substance. It instead stems from selection rules for transitions between levels and from the vanishing of the "area" defined by the integral of the electromagnetic field intensity. This selectivity mechanism could be exploited to create a deviation from equilibrium in a multilevel quantum system. In particular, there is the possibility of a nonresonant excitation and dissociation of molecules by femtosecond electromagnetic pulses.