Absorbed pump light at 532 nm excites the Cr ion into excited states. The lifetime of these levels is short 3+ (50 ns), so that the excited ion quickly relaxes by making a transition to the long-lived metastable state. The energy which is lost in this process is nonradiative and goes into heating the crystal by generating phonons or vibrational excitations of the crystal atoms. The metastable energy level must have a lifetime which is long enough to enable the Cr ions to remain excited until a photon having the precise energy comes along to de-excite it.The lasing line in ruby is the so-called "R1-line" having a wavelength of 694.3 nm. The fluorescence lifetime of the R1-line is several ms.
With increasing pressure, this R1 fluorescence shifts to lower energies which provides us with an accurate estimation of pressure inside the sample chamber of a DAC.