NONLINEAR BEHAVIOR OF ATOMIC FLUORESCENCE IN MERCURY VAPORS FOLLOWINGDOUBLE-RESONANCE LASER EXCITATION

When mercury atoms, present with a buffer gas (N2 or Ar) in a quartz c ell or in a graphite furnace are excited into the 7S-3(1) level by mea ns of two pulsed dye lasers tuned at 253.652 nm (6S-1(0) --> 6P-3(1)) and 435.835 nm (6P-3(1) --> 7S-3(1)), a laser-like, collimated emissio n is observed along the axis of the cell at the green mercury line (7S -3(1) --> 6P-3(2)), 546.074 nm). This radiation exhibits a pronounced non-linear dependence upon the number density of the mercury atoms in the cell. This behaviour is interpreted as being due to amplified spon taneous emission (ASE), which occurs as a result of the transient popu lation inversion between the 7sS-3(1) and 6pP-3(2)o levels. The parame ters governing the population inversion can be deduced by applying a r ate equation analysis to the atomic system. A time-resolved observatio n of both ASE and spontaneous fluorescence signals confirms the totall y different characteristics of the two emission processes.