This process can be enhanced using gallium arsenide/aluminum arsenide heterojunctions, and quantum coherence effects can be used to circumvent the need for it. Water undergoing this process in star-forming regions leads to the super-strong 22GHz spectral line. The 21cm transition in hydrogen can be exploited to facilitate this process by applying a magnetic field and flipping the electron spins. This phenomenon often leads to a negative absolute temperature, and it is usually created in a three or four level system via a pumping process, after which the system escapes it either through spontaneous or stimulated emission. For 10 points, name this phenomenon in which the majority of particles 
This process can be enhanced using gallium arsenide/aluminum arsenide heterojunctions, and quantum coherence effects can be used to circumvent the need for it. Water undergoing this process in star-forming regions leads to the super-strong 22GHz spectral line. The 21cm transition in hydrogen can be exploited to facilitate this process by applying a magnetic field and flipping the electron spins. This phenomenon often leads to a negative absolute temperature, and it is usually created in a three or four level system via a pumping process, after which the system escapes it either through spontaneous or stimulated emission. For 10 points, name this phenomenon in which the majority of particles in a system are in an excited state, which commonly occurs in lasers.
This process can be enhanced using gallium arsenide/aluminum arsenide heterojunctions, and quantum coherence effects can be used to circumvent the need for it.
{Population inversion} (Prompt on {lasing})