1)  n  ←  m  fast pump excitation ll mm  k k nn The general view was that it would be impossible or at least very difficult to achieve population inversion relative to the ground state  m  2)  n  →  l  intersystem crossing 3)  l  →  k  laser line fast(ish) 4)  k  →  m  fast to maintain population inversion fast (often) radiationless transitions Should be easier in a 4-level system where state  k  would not be populated at room temperature 4-Level Laser Scheme

Resonant Collisional Energy Transfer He nm Fast collisional relaxation with the walls of the laser tube down to the G.S. 1 S (1s) 1 (2s) 1 Ne Helium-Neon Laser Energy Level Scheme He is excited by electron impact in discharge 1 S (1s) 2 1 S (1s) 2 (2s) 2 (2p) 6 1 P (1s) 2 (2s) 2 (2p) 5 (3p) 1 1 S (1s) 2 (2s) 2 (2p) 5 (5s) 1 1 S (1s) 2 (2s) 2 (2p) 5 (3s) 1 Fast depletion by an allowed transition to a 1 S state Laser transition