HELIUM-NEON LASER in engineering physics 1 lecture note download free
Ground state of helium is 1s2. In the first excited state one of the electron goes to 2s level or the excited states are 23s (triplet) and 21s (singlet). Both the states (23s and 21s) of He are metastable states because 23s → 11s is forbidden due to change in multiplicity or flip in the spin and 21s → 11s transition is also forbidden by electric dipole transition. Ground state of Ne is 1s2 2s2 2p6 and the first few excited states are [1s2 2s2 2p5] 3s1, 4s1, 5s1, 3p1 or 4p1. When electric discharge is passed through the gas the electrons collide with the helium and neon atoms and excite them to the higher levels of both helium and neon.