| More information on this experiment. ||
What are Metastable States?
Atoms can be excited (by lasers, electron collisions, etc.) to energy
levels above their ground states. Normally, these levels are short-lived and
the atoms quickly decay (in 10-7 sec or less) back to the ground state. When
the atom is excited into a level which is not allowed to quickly decay
(i.e., it survives 10-6 sec or longer) it is said to exist in a metastable
level. These metastable atoms contain a lot of energy and are very important in
real-world applications such as gas-discharge lasers and plasma processing of
semiconductors. In our lab we measure cross sections for electron-impact excitation out of the
metastable levels into higher excited levels.
How are Metastable Atoms created?
The study of electron-metastable atom collisions is complicated by
the difficulty of producing targets of metastable atoms in the lab. This
experiment uses a hollow cathode discharge to produce an atomic beam of
metastable atoms. The beam emerging from the hollow cathode discharge has a
metastable atom density of approximately 107 atoms/cm3.
How do we measure the signal?
In this experiment, a low-energy electron beam crosses the atomic
beam at a right angle. The metastables are excited into higher levels, and
when these levels decay, they emit visible radiation. This emitted radiation
is measured using a photomultiplier tube and from this information we can
extract cross sections.
Absolute calibration of cross sections:
We get to use really neat lasers (Ar+ and Ti:Sapphire,
along with some frequency doubling) to obtain absolute values for our
cross sections. We use a laser-induced fluorescence(LIF) technique, which
involves taking the ratio of our electron-excitation signal to a
Some experimental specs:
|| 107 to 108 metastables/cm3
||0eV to ground state onset
|Electron Beam current:
|| 10 µA at 10 eV
|| He(21S) and He(23S) metastables (excitation into more than 25 higher levels studied)
Ar( 1s3 ) and Ar( 1s5 ) metastables (excitation into 8 higher levels studied)
Ne( 1s3 ) and Ne( 1s5 ) metastables (excitation into 10 higher levels studied)
Kr( 1s3 ) and Kr( 1s5 ) metastables (excitation into 7 higher levels studied)
Recent Published Results
- Measurement of electron-impact excitation cross sections out of the neon 3P2 metastable level
John B. Boffard, M. L. Keeler, Garrett A. Piech, L. W. Anderson, and Chun C. Lin,
Phys Rev. A 64 (2001) 032708.
- Measurement of Cross Sections for Electron Excitation out of the Metastable Levels of Argon
Garrett A. Piech, John B. Boffard, Mark F. Gehrke, L. W. Anderson, and Chun C. Lin,
Phys Rev. Lett. 81 (1998) 309-312.
- Electron excitation out of the 23S metastable level of helium into high-n triplet levels
Garrett A Piech, J Ethan Chilton, L W Anderson, and Chun C Lin,
J. Phys. B: At. Mol. Opt. Phys. 31 (1998) 859-872.
- Cross sections for electron excitation of the 23S metastable level of He into higher triplet levels
Garrett A. Piech, Mark E. Lagus, L. W. Anderson, Chun C. Lin, and M. R. Flannery,
Physical Review A 55 (1997) 2842-2856.
- A method for measuring cross sections for electron-impact excitation out of metastable levels of atoms
Ronald B. Lockwood, L. W. Anderson, and Chun C. Lin,
Z. Phys. D- Atoms, Molecules and Clusters 24 (1992) 155-160.
- Cross sections for electron excitation out of the metastable levels of helium into the higher singlet levels
Ronald B. Lockwood, Francis A. Sharpton, L. W. Anderson, and Chun C. Lin,
Physics Letters A 166 (1992) 357-360.
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