Multiphoton and Above-Threshold Ionization

We are currently occupied with collecting and analyzing spectra produced by the ATI of rare gas atoms, especially xenon, argon, and krypton, by 800-nm, 100-fs pulses with intensities in the range 10¹³-10¹⁴ W/cm². We collect the spectra in a field free time-of-flight (TOF) chamber with an aperture near the interaction region (see figure 1 below) to restrict the observed ionization volume, which reduces spectral averaging caused by the distribution of intensities in the laser focus. Flight times are measured using a time-to-digital analyzer, allowing for very high time resolution (< 100 ps), and these can be converted to mass spectra of ions (this requires a static extraction field) or kinetic energy spectra of electrons. See figures 2 and 3 below for examples of such spectra.

 
Figure 1. Schematic of the time-of-flight setup used in our experiments.

Figure 2. Example of a typical time-of-flight mass/charge spectrum for ATI of Xenon.

Figure 3. Example of a typical kinetic energy spectrum of photoelectrons produced by ATI of Xenon.

The collection and study of these types of spectra has been going on for a couple of decades, and is fairly well-developed. Many, if not most, of the features observed in them have been explained at least qualitatively. However, there are some important questions which remain to be answered, and a complete understanding of the features of these spectra and their behavior with changing experimental parameters (e.g. laser intensity, polarization) has yet to be achieved. There are a number of reasons for this, not the least being the difficulty of theoretically modeling the data, given the regime of parameters and the complexity of the atoms studied. Moreover, as experiments become more sensitive (as ours have), new features and behavior are uncovered. What follows will be a short list and description of some of these "unanswered questions" and examples of our experimental data exhibiting these as well as some new features we have seen in our experiments.

Topics in MPI/ATI which (in our humble opinion) still await satisfactory answers include the non-sequential double ionization (NSDI) problem, high-order ATI enhancements (the "resonant hot-electron" problem), and anomalous low energy structures.