Spectroscopic Studies of Arcs

Emission in the near ultraviolet from transient arcs between gold electrodes breaking resistive circuits carrying currents in the range 0.8-1.75 amperes was studied. Analysis of the intensities of the AuI lines was carried out in terms of a model involving two distinct zones, a relatively large zone at
\sim
6000°K (the positive column) and a zone at an apparent temperature of
\sim
23 000°K (identified with a region close to the cathode surface), which is 34 orders of magnitude smaller in volume. There is intense emission from the second positive system of N
2
. Analysis of the rotational fine structure of the (0, 0) band also yields a temperature of
\sim
6000°K. Estimation of the electron density (
\sim
3 X 10
¹⁴
cm
^-3
) and the assumption of thermal equilibrium in the positive column allows one to calculate (N
⁺
/ N
0
)
Au
= 3 X 10
^-2
from Saha's equation. From this it follows that the relative vapor pressure of gold in the positive column is
\sim
0.0025 atmosphere, assuming electroneutrality in the plasma. An estimate based on simple gaskinetic considerations of the loss of gold atoms from the plasma column to the surrounding atmosphere and to the anode is within a factor of 4 of the loss observed in practice.