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FACS Newsletter 1/2000
Recent Advanced in the Investigation of Elementary
Chemical Reactions by Lasers and Molecular Beams
Dr. Yuan T. Lee
Academia Sinica, Taiwan
This talk will cover the recent experimental developments in the chemical dynamics in Taiwan.
The Arrhenius equation, k(T)=Ae-Ea/kT, offers people a
good way to understand
elementary chemical reactions.
In order to make reaction take
place, molecules need enough
energy to overcome the
potential energy barrier. The
pre-exponential factor, A,
which relates to the molecular
alignment is also very important. In more detail,
energies with various forms,
for example, the translational
energy, rotational energy, or
vibrational energy, have
different effects in overcoming
the potential energy barrier due
to different shapes of the
potential energy surfaces.
The reaction, F2+ D2 --> 2DF, can be written to the elementary reactions as following,
F2 --> 2F
F +D2 --> DF + D
D + F2 --> DF + F |
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From the crossed beam study of F + D2-->DF + D, the reaction is a direct reaction faster than the rotational period. The vibration distribution of the DF products is highly inverted. The v=3 and v=4 DF are the dominant products. This vibrational population inversion can make chemical laser.
The crossed beam apparatus built in Taiwan has very low background of 1 x
10-12 torr total pressure in the detector. The low background increases the sensitivity of the crossed beam experiments. We are interested in the
O(1D) reaction with polyatomic molecules.
In the O(1D) + SiH4 reaction, there are about 10 product channels which are energetically allowed. It is difficult to guess what the product is. By the crossed beam study, the picture could be quite clear for the most part of the reaction. The OH product is forward scattered with respect to the
O(1D) beam, which indicates the OH if formed by the direct
pick up of the H atom by the O(1D) atom. The 2H formation, and 2
H2 formation channels all observed to have isotropic angular distribution, indicating the formation of long-lived
SiH3OH intermediate. Backward SiH3O product is also observed. In this single reaction, different mechanisms take place which lead to different angular distributions of the products.
The O(1D) + H2 reaction is studied by using the high resolution H atom Rydberg tagging technique in Xueming Yang's lab. Very high sensitivity of detecting H atom is achieved by Lyman-α excitation. The translational resolution of H atom is very high, which can resolve the OH rotational and vibrational states. In the low J states of OH, clear forward and backward peaking is shown in the experimental data due to orbiting angular momentum conservation,
L~L'.
Multiple mass ion imaging machine was built in C. K. Ni's lab. This machine can look at the photolysis products by VUV ionization. Velocity resolved fragments can be etected simultaneously in a single image. Also the life time of vibrationally excited molecules can be measured in microsecond range in this apparatus. The lifetime of the S0 state of benzene after 193.3nm excitation is measured to be 20 us. Although 20 us is amazingly long, it is similar to the RRKM calculation results done by A.
Mebel.
The excitation function of the crossed beam F + HD reaction is studied in Kopin Liu's lab. Interesting resonances of D + HF product channel is found at very low collisional energy, in which the reaction takes place only by tunneling.
In Ralf Kaiser's lab, some C atom reactions are also studied.
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