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Abstract

The generation of high-order harmonics in small diatomic molecules is theoretically investigated with inclusion of the vibrational degree of freedom. The results obtained from the solution of the time-dependent Schrödinger equation for a model H2 molecule are interpreted by analysing the influence of the vibrational motion in the framework of the strong-field approximation. Ionization launches a vibrational wave packet whose motion is correlated with the motion of the continuum electron wave packet. The harmonics are sensitive to a correlation function quantifying the overlap between the vibrational wave packet at the time of recombination and a vibrational target wave packet, i.e. the wave packet for which de-excitation into the ground state is most likely. We show that more intense harmonics are generated in heavier isotopes due to the slower nuclear motion.