Single-electron structure and dynamics in the strong-field photoionization of noble gas atoms and diatomic molecules.

摘要

We present high-resolution photoelectron kinetic energy spectra of xenon, argon, krypton, and molecular oxygen and nitrogen ionized by intense ultrashort (120fs) linearly-polarized laser pulses in the range 10 13–1014$Wcm2$ of peak intensities at 800nm wavelength. At these intensities, excited atomic states and the ionization threshold are ac Stark shifted by several times the photon energy. This brings the ground state into multiphoton resonance with them multiple times during the pulse, at which point enhancements are observed in the kinetic energy peak amplitudes on both large and small energy scales. It has been shown recently by comparison of experiment to simulations in argon that the large-scale effects are well-explained in the single active electron approximation. However there are several features which although reproduced by the simulations remain unexplained or unassigned. We show evidence of a relationship between narrow peaks observed within high and low ATI orders and attempt to explain the connection between large and small-scale enhancements in terms of the motion of electron wavepackets which pass through a series of stages on their way to being observed at either high or low kinetic energy. Also we report what seems to be the common origin for the broad unexplained peaks which occur in the spectra of all three atoms, and which were not easily identified in the simulations.; In addition we have performed the first high-resolution measurements on the photoelectron spectra produced by ionization of diatomic nitrogen and oxygen with 800nm pulses. The measurement of ion yields of nitrogen using 800nm light have been reported to be similar to those of argon, which has the same ionization potential. Oxygen however has been reported to exhibit a rate an order of magnitude lower than its “companion” atom xenon. We have measured electron spectra of these gases around the intensities where these effects were observed, since they provide a more careful probe of the ionization mechanism in many cases. We present the electron spectra and compare them to the spectra of the companion atoms for similar intensities.