Population inversion of ground and excited states

The classical technique to find these two times is to subject the spin system to a magnetic pulse with the direction of the pulse at 180° to the primary field, that is, in the opposite direction. Such a pulse inverts the populations of the ground and excited states (parallel and antiparallel to the field, respectively). For this reason, the technique is referred to as inversion recovery. Measurement of the magnetization associated with the return to equilibrium gives Tx. A 180° pulse can be followed by a pulse at 90° to induce a transverse magnetization. (This does not orient the dipoles at 90°, but merely biases the... 

The emitted photon travels in phase with and in the same direction as the initial photon. These two photons can similarly interact with additional excited states, stimulating the emission of further photons, resulting in light amplification (laser is an acronym for light amplification by stimulated emission of radiation), For stimulated emission to occur, the probability (or rate) of stimulated emission must exceed that of absorption. At thermal equilibrium, the relative population of the ground and excited state in a two-level system has A n > Nm- As discussed in Chap. 1, for stimulated emission to dominate absorption, a population inversion is required, such that To create this... 

In the process of amplification with population inversion, spontaneous emission imposes a serious restriction in creating an inversion between atomic levels. For example, in a two-level system with ground state g) and excited state e), the stationary absorptive and emissive processes are governed by the balance condition... 

Another example is the excimer laser, where ion-molecule and excited state reactions produce a population inversion e.g. in the KrF laser, an electric discharge through a mixture of Kr and F2, diluted in He, produces Kr ions, Rydberg-excited Kr, and F atoms, which subsequently react, yielding excited-state KrF. Since the ground-state potential is repulsive (i.e. the ground state is unbound) the molecule... 

If the temperature were raised, more molecules would attain the excited state, but even at 50,000°C there would be only one excited-state atom for every two ground-state atoms, and stimulated emission would not produce a large cascade effect. To reach the excess of stimulated emissions needed to build a large cascade (lasing), the population of excited-state molecules must exceed that of the ground state, preferably at normal ambient temperatures. This situation of an excess of excited-state over ground-state molecules is called a population inversion in order to contrast it with normal ground-state conditions. 

A third pumping method (Fig. Ic) uses an electrical discharge in a mixture of gases. It relies on electronic excitation of the first component of the gas mixture, so that those atoms are raised to an upper energy level. The two components are chosen so that there can be a resonant transfer of energy by collisions from the upper level of the first component to level 3 of the second component. Because there are no atoms in level 2, this produces a population inversion between level 3 and level 2. After laser emission, the atoms in the second component return to the ground state by collisions. 

Lasers are devices for producing coherent light by way of stimulated emission. (Laser is an acronym for light amplification by stimulated emission of radiation.) In order to impose stimulated emission upon the system, it is necessary to bypass the equilibrium state, characterized by the Boltzmann law (Section 9.6.2), and arrange for more atoms to be in the excited-state E than there are in the ground-state E0. This state of affairs is called a population inversion and it is a necessary precursor to laser action. In addition, it must be possible to overcome the limitation upon the relative rate of spontaneous emission to stimulated emission, given above. Ways in which this can be achieved are described below, using the ruby laser and the neodymium laser as examples. 

Sorokin and Lankard illuminated cesium and rubidium vapors with light pulses from a dye laser pumped by a ruby giant-pulse laser, and obtained two-step excitation of Csj and Rbj molecules (which are always present in about 1 % concentration at atomic vapor pressures of 10" - 1 torr) jhe upper excited state is a repulsive one and dissociates into one excited atom and one ground-state atom. The resulting population inversion in the Ip level of Cs and the 6p level of Rb enables laser imission at 3.095 jum in helium-buffered cesium vapor and at 2.254 pm and 2.293 /zm in rubidium vapor. Measurements of line shape and frequency shift of the atomic... 

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