While earning my undergraduate degree at Calvin College I was fortunate enough to spend my summers (as well as some time during the year) working with Professor Haan doing simulation and analysis of nonsequential double ionization of helium atoms. I learned a lot on this project, including some FORTRAN and a lot of Mathematica.
Sequential vs. Non-sequential Ionization in Noble Gases
Very intense, very short laser pulses are capable of stripping electrons from the nuclei of noble gas atoms. One possibility is that the electrons are ejected one by one, with the second electron not being influenced by the first. This is sequential ionization. At lower intensities, an overabundance of doubly-ionized atoms occurs. The extra ionizations are due to atoms whose first electron returns to the (now singly-ionized) nucleus and collides with it. Non-sequential ionization occurs when this collision caused a second electron to escape, either by directly expelling it in the collision, or exciting the ionic core to an auto-ionizing state.
Classical Studies
While these processes are certainly governed by quantum mechanics, classical simulations do a very good job of producing results that agree with experiment. Classical ensemble simulation allow post-selection of interesting events to see what caused them, while a quantum simulation is much harder to ‘rewind’.
The group was able to identify mechanisms for many interesting experimental results, including:
- Higher than expected electron energies and momenta
- Anticorrelated electron
- Frustrated tunneling/trustrated ionization