Photoionization
of Atoms Using High Harmonics by Lydia
Bender Fort
Hays State University, Hays, Kansas supervisor: Dr. Artem Rudenko, Associate
Professor of Physics Kansas State University Physics Department REU Program 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 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.
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
Helium
1kV Neon
1kV
Inverted
images: the inverted image is a 3-D representation of the raw data
Helium
1kV Neon
1kV Angular
distribution: the angular distribution of the raw data, plotted as radius vs.
phase/degree.
Neon
1kV Helium
1kV
Neon
1kV Helium
1kV
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
Einstein’s photoelectric equation. In this experiment, the 15th, 17th,
19th, and 21st harmonics were used.
Helium
1kV Neon
1kV
Final Presentation: Click here to download my
presentation in PowerPoint and here
for pdf. 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. About Me Currently
I attend Fort Hays State University in Hays Kansas as a rising Junior. I’m
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. |