Introduction to
electron-molecule collision
The
process of electron-molecule scattering can be described by:
elastic
scattering
inelastic
excitation
ionization (dissociation)
These processes extensively exist in the atmosphere, nuclear fusion confined by magnetic field or inertia and gases discharge. Therefore, the data of cross sections and oscillator strengths for electron-molecule impact are significant not only for the development of atomic and molecular physics but also for the advancement of astrophysics, plasmas physics, confined nuclear fusion, X-ray laser and the safety of aerocraft.
The accurate measurement
of oscillator strength is always one of the most important contents for
electron-molecule impact, and this kind of data have already been extensively
used in astrophysics [1, 2]. For example, the Hubble Space Telescope (Fig. 1)
and space satellites have observed a number of spectra for different interstellar
clouds (Fig. 2), and from these spectra the information of element
constituents can be determined. In this procedure, the data
of absolute oscillator strengths and cross sections must be used, and large numbers
of data are provided by the experiment of electron-molecule impact [1]. We know
that carbon monoxide is the second most abundant molecule, after H2,
in interstellar clouds. In diffuse clouds, the amount of CO is mainly derived
from measurements of absorption at UV wavelengths. But due to the inaccuracy of
the data measured by early experiments, the observed spectra are hard to be
explained. However, the recently measured oscillator strengths by electron
impact have resolved this problem, and the measured spectra for the stars Ophiuchi A and
have been well simulated
[1].
The
data of cross sections also have significant application in planet science. For
example, of all of the fascinating questions in contemporary astrophysics,
those relating to the origin and formation of the solar system must rank high
in the curiosity of everyone. Comets,often lumped in texts under the
heading ‘solar system debris’, are pursued because their chemical composition
holds unique clues to the early history of our solar system, this is accessible
by analyzing its constitute [3].
Another
application of the cross-section data determined by electron impact is the gases
discharge process, which is directly related to the development of applied
techniques such as laser, plasma deposition and etching of semiconductors, and
plasma display [4]. Our everyday life is benefited from this great progresses
of these techniques [Fig. 4].
In
order to obtain the differential cross sections and oscillator strengths of
electron-molecule impact, the energy dispersion and angular distribution of
scattering electron intensity should be determined and this can be achieved by
electron-energy-loss spectrometer. In recent years, the important progresses in
experiment technique are: (1) the progress of relatively flux technique [4];
(2) the experimental realization of high energy resolution [5]; (3) the spread of
multi-channel measurement [6]; (4) the development of preparation technique of
excitation target [7]; (5) the realization of the large angular measurement
technique [8] and so on. The accuracy and extent of experimental data are
greatly improved with the progresses of these experimental techniques. With the
progress of the experiment technique, there will be more development for
electron-molecule impact, and the data will be also updated constantly.
[1]
S. R. Federman , D. L. Lambert, J. Electro. Spec. Relat. Phenom., 123, 161(2002)
[2]
B. L. Rachford et al., Astrophys. J. 555, 839(2001)
[3]
M. J. Mumma, H. A. Weaver, H. P. Larson, M. Williams and
D.
[4]
M. J.
Brunger and S. J. Buckman, Phys. Rep. 357, 215(2002) and the references therein
[5]
G. Knoth, M. Gote, M. Radle, K. Jung and H. Ehrhardt, Phys. Rev. Lett. 62, 1735 (1989)
[6]
X. J.
Liu, L. F. Zhu, X. M. Jiang, Z. S. Yuan, B. Cai, X. J. Chen and K. Z. Xu, Rev.
Sci. Instrum., 72,3357-3361(2001)
[7]
S. Trajmar and J. C. Nickel, Adv. At. Mol. Opt. Phys. 30,
5(1992)
[8]
F. H. Read and J. M. Channing, Rev. Sci. Instrum. 67,
2372(1996)