Spark spectroscopy and the emission spectrum of air made easy (Note)

摘要

Sparks are fascinating, familiar evidence of the action of electrostatic fields on matter. When a large enough electric potential is applied, for example, to a space filled by air and enclosed within two electrodes, the background electrons are strongly accelerated and eventually ionize atoms/molecules in their trajectories. The resulting ions and electrons are also accelerated (in opposite directions) and in a very short time (a few nanoseconds) an avalanche of electric charges builds up between the two electrodes. Such an ionized plasma channel is characterized by rapid recombination of ions and electrons, which gives rise to radiation emissions, the frequencies of which are associated with the discrete energy levels of the participating atoms and ions. This light emission is called a spark or,in the case of very large events in which noticeable amounts of air and water are involved, such as those happening in thunderclouds during a storm, the electric discharge becomes lightning [1]. The threshold for the spark effect in a gas is referred to as its dielectric strength and it is measured in units of electric field, i.e. volt/m. A typical value for air (depending on the amount of humidity) is about 3 MV/m or 3 kV/mm. This implies that, in order to ignite a spark in the air at a distance of, say, 3 cm, at least 90 kV is needed at the electrodes.