We investigate the focus conditioning effects on molecular field-free alignment observed with high-order harmonic generation(HHG) from CO2 molecules.We also experimentally demonstrate that both the spectral shape and alignment signal of HHG significantly vary with changing focus position.A maximal alignment signal is achieved at a given focus position because of the optimal intensity of the driving laser.This intensity is related to the ionization potential of the molecules.These results indicate that a unique focus position provides an optimal alignment signal for practical applications.
A semi-classical model is utilized to explain the dissociation control of the hydrogen molecular ion （H^-）. By ana- lyzing the curve of the dissociation asymmetry parameter as a function of the time delay between the exciting and steering pulses, we find that the dissociation control is dependent not only on the peak intensity and direction of the electric field of the steering pulse, but also on the peak intensity of the exciting pulse.
We experimentally investigate the high-order harmonic generation in argon gas cell driven by a multi-cycle broadband infrared laser pulse from a tunable optical-parametric-amplifier （OPA） source. The generation of high-order harmonic continuum with the cut-off photon energy up to 110 eV is observed by tuning the chirp of the 800-nm laser pulse which pumps OPA source. The generation of harmonic continuum is understood in terms of the two-hump structure of the OPA output spectrum and the optimal relative phase of the two humps. The demonstrated scheme is of importance for the generation of extreme ultraviolet （XUV） continuum at higher photon energy region.
The molecular dissociation with a two-laser-pulse scheme is theoretically investigated for the hydrogen molecular ion（H2^＋） and its isotopes（HD^＋and HT^＋）. The terahertz pulse is used to steer the electron motion after it has been excited by an ultrashort ultraviolet laser pulse and an unprecedented electron localization ratio can be achieved. With the coupled equations, the mass effect of the nuclei on the effective time of the electron localization control is discussed.
A new polarization gating is demonstrated by our principle-of-proof experiment, which is theoretically proposed to generate the isolated or double attosecond pulses with the multi-cycle driving laser pulse in the previous work [Optics Express. 20, 5196 （2012）]. In the experiment, the polarization gating is formed by two orthogonally-polarized linearly chirped multi-cycle laser pulses, and the spectral bandwidths of tile harmonics are broadened by more than two times, which agree with the previous theoretical work.
