Inversion of population

The condition for observing induced emission is that the population of the first singlet state Si is larger than that of So, which is far from the case at room temperature because of the Boltzmann distribution (see above). An inversion of population (i.e. NSi > Nso) is thus required. For a four-level system inversion can be achieved using optical pumping by an intense light source (flash lamps or lasers) dye lasers work in this way. Alternatively, electrical discharge in a gas (gas lasers, copper vapor lasers) can be used. 

Repeating these experiments using the YAG laser fundamental (1064nm, 1.17 eV), adjusting the energy to achieve the same calculated temperature jump, gave essentially identical (Q, J, A)-state distributions. The inversion of population in the two spin-orbit levels, the population plateau for internal energies below 300 cm and the rapid fall-off of rotational population for... 

The mechanism consists of using a conventional energy source (flash-lamp or other) to excite atoms or molecules from the ground state to some excited state, so that an inversion of population occurs in the Arrhenius55 sense this is usually best understood in a three-level laser, although two-level lasers are also discussed (see Problem 10.10.1), and many are four-level lasers (see Fig. 10.14). 

Chemiluminescence from CaX species (X = halogen) arises from the reaction of copper with Xj molecules. For Cu + Fj, ground-state copper atoms ( 5) react to produce A, B, and C states of CuF, with an inversion of population of the C state relative to the B state observed, whereas metastable Cu atoms ( D) are responsible for chemiluminescent reactions with the other halogens. Emission from GaF and InF ( 11) is seen in reactions of the ground-state Group Ilia atoms with F2, with population inversions formed in the vibrational levels of these excited states the products of ground-state reactions of and... 

The core components of soUd-state lasers are laser materials that allow for the inversion of population and amplification of radiation through stimulated emission. The properties of the laser materials determine the ways to design pumping system and laser resonator of a soUd-state laser. Because the characteristics of laser active centers are determined by the physical processes related to the laser materials, while there are various possible interactions between the active centers and the electromagnetic radiations, the interrelationship among the composition, stmcture, properties, and functionality of laser materials is very complicated, leading the research in this field to be unlimited. 

The excited ions of the pumped laser materials in a laser resonator can be de-excited by various radiative (either laser or luminescence) and nonradiative (electron-phonon interaction or energy transfer) processes. Also, the amount of ions participating in laser emission is dependent on the laser emission efficiency ( /i). The laser emission is produced by the excited ions inside the laser mode volume and pumped above the threshold. The excited ions inside the pumped volume but outside the laser mode volume and those that form the inversion of population at the laser threshold can be de-excited by luminescence and nonradiative processes. 

Inversion of population of a medium by application of a Ught pulse whose frequency is swept quickly through that of an absorbing transition... 

Another possible application is related to the problems of gamma laser [I]. The femous dilemma of gamma laser (which is better—the nuclei with short-lived radiation transitions with t< IO s,rT I, maximal amplification coefficient G = An f/2 r(l -I- a) at small inversion of population Ani = — n,[Pg.292]

The connection between this semiclassical description and the transitions between the quantum euergy levels described by Equation (2.5) can be understood, in the simple case of n 2 and n pulses applied to spin 1/2 systems, as an equalization and an inversion of populations, respectively. This means that after a nl2 or a 7t pulse, the populations are not anymore given by the thermal equilibrium expression (Equation 2.6). The return to equilibrium requires that the spin system give up some energy to the environment (generally named the lattice). This process is termed relaxation and is detailed in the next section. 

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