TEA CO2 laser is a laser in which the gas inside the laser tube is at atmospheric pressure.
The pulses out of these lasers have a special pulse shape:
At the beginning of the pulse there is a sharp spike for a short time (100-500 [nsec]), followed by a long (1-2 [sec]) tail with low power (see figure 7.17a).
The leading spike is called Gain Switched Spike, and it results from the same physical reasons of the spikes in the pulse train.
The long trailing tail is a result of excitation of CO2 molecules by molecular collisions.