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John P. Doering
Experimental Chemical Physics Johns Hopkins University Phone: 410.516.7445 PhD - University of California, Berkeley John Doering does experimental research on collisions of electrons with gas phase atoms and molecules. Both excitation and ionization by electron impact are currently under investigation. The research program is motivated by the need for better understanding of collision phenomena both for their own sake as well as for their relevance to the processes which take place in the atmospheres of planets, stars, and other objects. The program's strong geophysical interest arises from its long-standing involvement with rocket and satellite experiments in planetary atmospheres. The most important contribution of this type was the Atmosphere Explorer satellite program. The laboratory provided electron spectrometers for three missions between 1970 and 1983 which measured the photoelectron spectrum of the earth's atmosphere to a degree of detail which remains unequaled. These satellite data are still an active area of investigation. The laboratory program currently operates two different types of experiments. The first is excitation of atoms and molecules by electron impact. In these experiments, an electron beam collides with the target gas and produces excitation. The electronic, vibrational and rotational transitions which take place during the collision are studied by measuring the energy lost by the electrons in the collisions. These collisions are exactly the same as those by which free electrons in the atmospheres of the earth and other objects lose energy to the gas species present. The second type of experiment is electron impact ionization. The apparatus used for this work has detectors for both electrons which leave the target atom or molecule after an ionization event takes place. The kinematics of the ionization can be completely determined with such an apparatus by measuring the energies and momenta of the final electrons leaving the ionization event. The electrons must be detected in coincidence to ensure that they came from the same event. This requirement adds a number of interesting features to the apparatus. These experiments allow the determination of the final electronic states of the of positive ions produced in the ionization process. Ionization and excitation by electron impact is one of the principal processes which gives rise to the light observed from astronomical objects such as comets. The upper atomospheres of planets, comets, stars, etc. contain many chemically unstable species. In order to study how these atoms and molecules such as atomic oxygen interact with electrons, it has been necessary to develop a number of techniques to produce them in the laboratory apparatus. So far, electron scattering experiments have been carried out on hydrogen, oxygen, nitrogen, carbon, and sulfur atoms. The improvement of sources for these species as well as development of techniques for producing others is a current active area of research. |
