is carried away by the third body, an electron. De-excitation of the high-lying energy levels of Sr+ then occurs, due to collisions with the remaining free electrons in the plasma. This cascade of recombined electrons, down the Sr+ energy levels, continues freely until the 62S level is reached. The downward transition across the relatively large energy gap, 62S-52P, acts as a bottleneck for the electron de-excitation process, which generally proceeds faster for closely spaced levels. A population inversion therefore builds up in the 62S1/2 upper laser level. Inversion occurs between this and the 52P3/2 lower laser level, which is cleared to the metastable and ground levels also by collisions with electrons. References Gas lasers Strontium