Photoionization of Atoms Using High Harmonics
by Lydia Bender
Fort Hays State University, Hays, Kansas
†††††††††††† supervisor: Dr. Artem Rudenko, Associate Professor of Physics
This program is funded by the National Science Foundation through grant number PHYS-1461251. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
My project focused on figuring out what order of harmonics we were retrieving from XUUS (eXtreme Ultraviolet Ultrafast Source). We are able to do this by shooting the high harmonics that we generated at a jet of gas inside a VMI (Velocity Map Imaging) spectrometer. When the high harmonics interact with the atoms electrons, are ejected from the atom; this is called photoionization.† Investigating the properties of the ejected electrons gives us the ability to identify the order or harmonics that were being used. Knowledge of the harmonics is versatile as it can be used for multiple projects in the lab that use high harmonic light.
Photoionization is the process in which a photon interacts with an atom or molecule to form an ion. To be able to ionize, the energy of the photon must be greater than the ionization potential of the atom or molecule. This could be done using one photon with a high energy, or by absorbing multiple photons to reach the threshold.
High Harmonic Generation
A single photon that is capable of reaching the threshold is produced by using high harmonic generation (HHG). HHG is achieved through a process of exciting atoms and causing them to release high energy photons. The flow chart below depicts the process in which high harmonics are created.
multiple photons ∑
to absorb multiple photons before it can overcome the ionization
in light with a high intensity Photoionization with a
single photon ∑
single photon (with a large energy) can overcome the ionization potential
Photoionization with multiple photons
∑ Strong field
∑ Has to absorb multiple photons before it can overcome the ionization potential
∑ Results in light with a high intensity
Photoionization with a single photon
∑ Weak field
∑ A single photon (with a large energy) can overcome the ionization potential
For this experiment we set up our laser to a velocity map imaging (VMI) machine. The laser had to be propagated through a vacuum to avoid ionization. The high harmonics are generated through XUUS. Only certain harmonics are allowed through using filters that can block certain harmonics. The data collection all takes place inside of the VMI. The laser comes through the VMI chamber and interacts with the jet of gas that comes through the bottom of the chamber. All of the electrons that are emitted from this interaction are then accelerated towards the MCP and phosphorus screen by applying voltage to the extractor/repeller plates. The camera that is mounted above the phosphorus screen is able to capture the raw data images of the experiment. The VMI could also capture ions instead of electrons by simply switching the polarity of the plates. Below is a figure of the experimental setup and the VMI.
The experiment was performed with helium, neon, and nitrogen gas with various voltages applied to them. The following images are the data that was collected from the different trial runs at 1 kV with helium and neon.
Raw images: the raw images depict what the camera captured while the experiment was running
Inverted images: the inverted image is a 3-D representation of the raw data
Angular distribution: the angular distribution of the raw data, plotted as radius vs. phase/degree.
The energy of a photon is roughly 1.5 eV so in theory each peak should be spaced 1.5 eV apart, but only odd harmonics can be produced; therefore, the peaks are spaced 3 eV apart. After using a graphing program to find the value of each peak and knowing the ionization potential of each gas we can discover what harmonics were produced by the laser using Einsteinís photoelectric equation.
In this experiment, the 15th, 17th, 19th, and 21st harmonics were used.
My REU Experience
This REU program has taught me what it is like to work in a professional lab and about the process of scientific research. It has also provided me with a lot of information concerning the advancement of my education and careers. Along with providing me with an educational experience it provided me with an opportunity to get to know fellow physicists. I had the opportunity to get to know them in a work setting as well as a relaxed setting. There were some impromptu trips to baseball stadiums and towns around Kansas and some planned trips to see the bison in the Flint Hills or to go canoeing on Tuttle Lake.
Currently I attend Fort Hays State University in Hays Kansas as a rising Junior. Iím involved in my physics department through a peer mentoring program, tutoring, and holding a position in the Physics Club. Outside of physics I enjoy eating cheese, naps, and watching baseball.