Phase differences of near-threshold high-order harmonics generated in atoms and molecules

We present the observations of the phase differences Delta phi((2n))(HH) between adjacent high-order harmonics generated from Ar and N-2 at the near-threshold region. The Delta phi((2n))(HH)'s are extracted from the photoelectron signals resulting from two-color two-photon ionization of rare-gas atoms, which are produced by high-order harmonics to be measured and a part of the fundamental pulse for probing. An analysis method is employed to remove the inevitable modulations in high-order-harmonic intensities based on the underlying mechanism of the production of photoelectrons. We find a significant difference in the Delta phi((2n))(HH) at the nearest-threshold order between Ar and N-2. This difference cannot be reproduced by the model calculation by using the saddle-point method within the strong-field approximation. To elucidate the origin of the difference between the Delta phi((2n))(HH) for Ar and that for N-2, we note the fact that the phase difference Delta phi((2n))(HH) contains information both on the recombination time t(r) of the freed electron and on the phase of the recombination dipole moment d*. With the help of some numerical calculations, we discuss the effect of the potential created by the parent ion on t(r) and d* which are neglected in the strong-field approximation.