Narrow Results

The above threshold dissociation (ATD) of the HD⁺ molecular ion in a linearly polarized femtosecond laser field is theoretically studied using three-dimensional time-dependent quantum wave packet method. Based on the Born–Oppenheimer approximation (BOA), calculations are performed on two electronic states, the ground state 1sσ and the excited state 2pσ. The energy-dependent distributions of the dissociated fragments, resulting from the ATD, are calculated by using an asymptotic-flow expression in the momentum space. The numerical results demonstrate that, in the laser field of wavelength λ = 800 nm and full-width at half-maximum (FWHM) τ = 30 fs, only two-photon dissociation is observable at a weaker pulse peak intensity, 5.0 × 10¹²W cm^-2, while at an intense intensity, 1.5 × 10¹⁵W cm^-2, the dissociated fragments resulting from four-photon absorption dominates over the photodissociation process. These results are consistent with the experimental observation of Orr et al. [Orr PA et al., Phys Rev Lett98:163001, 2007]. The ac Stark-shift caused by intense laser field will change the kinetic energies of the fragments. The ATD phenomena are quantitatively interpreted in terms of the concept of light-induced potential. The molecular rotation and alignment have some effects on the kinetic energy spectrum of the dissociated fragments. The molecular rotation reduces the ac Stark-shift and broadens the peaks of kinetic energy spectra of the dissociated fragments. However, the intense laser field can effectively align the molecule and is helpful to increase the ATD probability. The ATD spectrum is related to the initial quantum numbers J₀ and M₀ of the molecule. The ATD spectrum of HD⁺ is calculated at a limited thermal temperature.

Exploration of a new ultrafast-ultrasmall frontier in atomic and molecular physics has begun. Not only is it possible to control outer-shell electron dynamics with intense optical lasers, but now control of ultrafast inner-shell processes has become possible by combining strong optical laser fields with tunable sources of X-ray radiation. This marriage of strong-field laser and X-ray physics has led to the discovery of methods to control reversibly resonant X-ray absorption in atoms and molecules on ultrafast timescales. Here we describe three scenarios for control of resonant X-ray absorption: ultrafast field ionization, electromagnetically induced transparency in atoms and strong-field molecular alignment.