Populations, optical pumping

Some examples illustrate the achievable sensitivity and accuracy ERTMER and HOFER [10.27c] measured the hyperfine structure of metastable states of scandium which were populated by electron impact. The population achieved in these states was only about 1% of the ground state population. Optical pumping to a higher electronic state with a single-mode dye laser allowed selective depletion of the single hfs sublevels of the metastable F state. The hfs constants could be determined with an accuracy of a few kHz. 

Figure lb shows a four-level system. The terminal level, level 2, is ordinarily empty. Atoms are optically pumped to level 4. From level 4, the atoms make a rapid radiationless transition to level 3. The first few atoms to arrive begin to contribute to the population inversion. Therefore, laser operation can begin with much less intense pumping light. After the laser transition, the atoms return to the ground state (level 1) by a radiationless transition. 

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. 

If an atomic transition is optically pumped by a beam of laser radiation having the appropriate frequency, the population in the upper state can be considerably enhanced along the path of the beam. This causes an intensification of the spontaneous emission from this state, which contains information about the conditions within the pumped region, since the exponential decay time for the intensified emission depends upon both the electron number density and the electron temperature. The latter can be obtained from the intensity ratio of the fluorescence excited from two different lower levels, if local thermal equilibrium is assumed. This method has been dis-... 

The lowest excited states in luminescent conjugated polymers are very similar to the dyes used in dye lasers (Chapter 3, section 3.5.3). Therefore the conjugated polymers can be used to achieve population inversion by optical pumping as with the laser dyes. 

Optical pumping lifts many molecules from the ground state to the excited state so that the latter becomes occupied by more molecules than the ground state. This is referred to as population inversion.. ... 

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