Atomic and Molecular dynamics in intense mid-infrared fields

摘要

In the low optical frequency limit, photoionization processes in atoms and molecules irradiated with intense ultrafast laser fields can be described within the quasi-static tunneling framework. Higher order processes of the photoelectron dynamics were observed and attributed to the recollision of the tunneled electrons. Development of the quantitative rescattering theory laid a foundation for retrieving field-free differential cross section (DCS) from the distribution of rescattered photoelectrons. Due to technological reasons, the majority of strong field physics experiments were carried out at near-infrared wavelengths 0.8 mum or 1 mum. Retrieved DCS' at these wavelengths were not very accurate since these laser frequencies are high and the systems do not strictly satisfy the quasi-static tunneling condition. Here we present the accurate retrieval of noble gas atom DCS' from photoelectron distributions under mid-infrared laser radiation. These lower frequency fields not only result in better quasi-static tunneling initial conditions, but also higher photoelectron recollision energies, core penetrating collisions and improved spatial resolutions, suitable for molecular imaging. Bond lengths of nitrogen and oxygen molecules were extracted from the rescattered photoelectron distributions. This method is called laser-induced electron diffraction (LIED). Alternatively, utilizing the broadband nature of recolliding photoelectrons, bond length of the nitrogen molecule was extracted from a Fourier transform of the backscattered photoelectron spectrum along the laser polarization direction. This is the fixed-angle broadband laser-driven electron scattering (FABLES). Both LIED and FABLES rely on well defined recollision geometries, so molecular alignment is needed to generalize both imaging methods for more complex molecules.;Theoretical and experimental evidence shows that the Coulomb potential of the parent ion can be safely ignored during the high-energy recollisions in LIED and FABLES. However, this is not true for the case of low-energy, small-angle recollisions. Evidence of this is the photoelectron low-energy structure (LES). Soon after the discovery of the LES, theoretical studies attributed its origin to an interplay between the Coulomb field of the parent ion and the electric field of the laser. The type of electron trajectories responsible for the LES are those leading to soft-recollisions, taking place about one and half laser cycles after tunneling ionization. So far, most LES experiments were conducted with multi-cycle pulses where the envelope of the laser electric field is practically constant. Here we present LES measurements with few-cycle laser pulses of various pulse durations. Since in this case the envelope of the laser electric field changes drastically from cycle to cycle and hence during the soft-recollision motion of the LES photoelectrons, changes in the LES peak position as a function of laser pulse duration were expected and observed. Classical trajectory Monte Carlo simulations reproduced experimental data and lead to more insight about the mechanism behind LES. Universal scaling of the LES peak position was also observed, which could be calibrated to obtain in situ measurements of few-cycle mid-infrared laser pulses' duration and carrier-envelope phase offset.