Sometimes achieved via pumping, this state is required for an optical gain greater than one, via stimulated emission.
Techniques such as PC-SPI and adiabatic RF pulses can be used to accomplish this during NMR spectroscopy. Intersystem crossing can trap materials in this state, as in phosphorescence, and these states are usually stabilized by selection rules preventing rapid spontaneous decay from a triplet spin-state. This condition cannot exist in a two-state quantum system according to Einstein's A and B coefficients, and this state is sometimes described by a negative temperature. Sometimes achieved via pumping, this state is required for an optical gain greater than one, via stimulated emission. For 10 points, name this situation commonly found in lasers in which the majority of particles exist in an excited state.
Techniques such as PC-SPI and adiabatic RF pulses can be used to accomplish this during NMR spectroscopy.
{Population inversion} [prompt on "{lasing}"]