The Origin of the Mercury Bands at 2480A

The group of eight mercury bands near 2480A was photographed under varied excitation conditions with the purpose of determining their origin. The source was a discharge through mercury vapor produced in a quartz tube through external electrodes by a low-voltage Tesla coil. Five tubes containing distilled mercury and commercial mercury arc lamp showed this group of bands. These bands were weakened by heat along with known mercury bands. The origin is undoubtedly some form of mercury molecule. The most probable forms are ${\mathrm{Hg}}_2^{+}$ and ${\mathrm{Hg}}_2$. Five observations favor ${\mathrm{Hg}}_2^{+}$ over ${\mathrm{Hg}}_2$. (1) These bands have never been observed in fluorescence. (2) The 2476 band is more intense than the 2345 ${\mathrm{Hg}}_2$ band under strong field excitation but weaker than 2345 under low field excitation. (3) No other bands with properties like those of the 2480 group have been observed in the mercury spectrum and Rayleigh has shown that these bands do not occur in absorption. (4) The bands in this group may be classified as sequences $v^{\prime }-v^{\prime \prime }=0\mathrm{}\pm 1\pm 2\pm 3$, and a lower limit for $D$ of 0.3 volts estimated. (5) In the $v^{\prime }-v^{\prime \prime }=0\mathrm{}$ sequence, emission is observed from state $v^{\prime }=41\mathrm{}$ indicating molecules with very high vibrational energy. This energy may be supplied by the electric field if the emitter is an ion but not if it is a neutral molecule.