  The generation of the below threshold harmonics (BTHs) under different
driving laser intensities is investigated. The linearly shifting of photon
energy and resonantly enhancement of photon yield of the harmonics from 23rd
(H23) to 27th (H27) are found by changing the laser intensity around 18.6
TW/cm2. It is identified that this driving laser intensity dependence is due to
the transient ac Stark-shifted resonance between the first excited state and
the ground state. With this transient ac Stark, the linearly shifting of the
photon energy can be interpreted very well by considering the sub-cycle
electron dynamics for BTH generation, which shows that the generation of the
BTHs is surprisingly similar to the plateau harmonics.
