The collisional quenching rate coefficient of metastable nitrogen molecules N
2(A
3Σ
u+) by
m-xylene (C
8H
10) is determined experimentally in the Townsend discharge. The diffusion coefficient of N
2(A
3Σ
u+), and the reflection coefficient of N
2(A
3Σ
u+) at the electrode surface are also determined simultaneously. During the experiment, we find that it is difficult to continue the current measurement necessary to determine the fundamental constants of N
2(A
3Σ
u+) for long period. It is considered that any by-product of xylene would be decomposed by N
2(A
3Σ
u+) is deposited on the cathode, through repeated experiments, then the current-voltage curves consistently shift to the higher-
E/
p0 region. For the purpose of clarifying the reason behind this behavior, we confirm that these changes are caused by the current prevention by the decreases of initial photoemission current from the cathode and the decrease in the secondary ionization coefficient
γ, because the cathode surface is covered by deposition film of a by-product of decomposed xylene.
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