| The amplitude and phase of the complex photoionization/photorecombination dipole matrix element of atoms and simple linear molecules is measured with sub-femtosecond time resolution and sub-electronvolt spectral resolution using High Harmonic Spectroscopy (HHS). The first known measurement of the dipole phase jump at a Cooper minimum is reported for the 3p orbital of argon. Also, the angle-dependent dipole of nitrogen is measured using rotationally-aligned molecular ensembles. In contrast with previous studies, which were limited by traditional shorter-wavelength near-infrared laser sources, only a single orbital is sufficient to explain the nitrogen results, which are in excellent agreement with accurate theoretical scattering-wave dipole calculations. All of these experiments benefit from the extended extreme-ultraviolet cutoff, and improved spectral resolution, afforded by the use of long-wavelength mid-infrared driving laser sources.;This work extends our understanding of the interaction of light and matter on the timescale of the electron's motion, the attosecond (1 as = 10 --18 s). The experimental results presented here lend credence to the methodology of molecular self-imaging by laser-induced ionization and recombination of a molecule's own electron. The successes and limitations of HHS as a tool for ultrafast atomic and molecular imaging are discussed. Finally, the feasibility is examined of using HHS to measure the temporal evolution of complicated chemical dynamics with attosecond precision. |
