EXCITATION OF SODIUM ATOMS BY 330-NM LASER-PULSES

The evolution of populations of Na(4P) and Na(3P(1/2)) atoms in a dens e sodium vapour irradiated by nanosecond laser pulses tuned near the 3 S --> 4P transition was investigated. It was observed that the populat ion of Na(4P) atoms remained high only within the laser pulse, in spit e of the relatively long lifetime of the 4P level (110 ns). The 3P(1/2 ) level, which is populated as a result of cascade transitions from th e higher levels, reached the highest population several nanoseconds af ter the laser shot. The fast population changes are explained by casca de-stimulated transitions between the excited atomic levels.