A short, intense laser pulse may be employed to create a spatially aligned molecular sample that persists after the laser pulse is over. We theoretically investigate whether this impulsive molecular alignment technique may be exploited for experiments using x-ray pulses from a third-generation synchrotron radiation facility. Using a linear rigid rotor model, the alignment dynamics of model molecular systems with systematically increasing size is calculated utilizing both a quantum density matrix formalism and a classical ensemble method. For each system, the alignment dynamics obtained for a 95 ps laser is compared with that obtained for a 10 ps laser pulse. The average degree of alignment after the laser pulse, as calculated quantum mechanically, increases with the size of the molecule. This effect is quantitatively reproduced by the classical calculations. The average degree of impulsive alignment is high enough to induce a pronounced linear dichroism in resonant x-ray absorption using the intense 100 ps x-ray pulses currently available. However, for structural studies based on elastic x-ray scattering, bright x-ray pulses with a duration of 1 ps or shorter will be required in order to make full use of impulsive molecular alignment.

We theoretically investigate the use of an isolated attosecond vacuum ultraviolet (VUV) pulse to control the emergence of multiple wavepacket rescatterings in the process of high harmonic generation (HHG). Through numerical solution of the time-dependent Schrödinger equation for a helium atom driven by 0.8 − 2.0 µm light, we establish the relationship between evidence of multiple rescatterings in HHG and the time delay between the VUV and infrared pulses. We find features of multiple rescatterings present in both the time and frequency domains of emitted HHG, and demonstrate the use of VUV-induced multiple rescatterings for generating trains of ultrashort light pulses.

We theoretically explore a new mechanism resulting in a minimum in the high harmonic spectrum of a hydrogen molecular ion driven at extended internuclear distances by a mid-infrared laser source. Our analysis identifies this minimum to be a signature of the transient localization of the electron upon alternating nuclear centers and is representative of dynamics occurring exclusively at the time of ionization. We further demonstrate the sensitivity of this spectroscopic feature to driving field parameters as well as its robustness to distributions of laser field intensities and internuclear distances. Finally, we show how variations in the nonadiabatic dynamics induced by the ramping driving field can be imaged through changes in the number and locations of minima in the spectra. II. HIGH-ORDER HARMONIC GENERATION OF EXTENDED H + 2 An example is shown in Fig. 1 (a) for H + 2 prepared in its ground state with internuclear distance R 0 = 7 a.u.,