Laser cooling cycle

Yb " " is because of both fundamental and practical reasons. First, the simple energy-level structure of Yb is advantageous since it most closely resembles the ideal case of a four-level laser-cooling cycle (see Fig. 9). The absence of any other excited states eliminates competing processes such as ESA and... 

FIGURE 11.20 First atomic levels involved of singly-ionized calcium (Call) for the ion atomic clock at 729 nm (light continuous line). One of the bold continuous line (397nm) indicates the transition used for the laser cooling process and the second other one (866 nm) permits the back pumping from the Dj/j toward the cooling cycle. 

The optical cooling techniques discussed so far are restricted to true two-level systems because the cooling cycle of induced absorption and spontaneous emission has to be performed many times before the atoms come to rest. In molecules the fluorescence from the upper excited level generally ends in many rotational-vibrational levels in the electronic ground state that differ Irom the initial level. Therefore most of the molecules cannot be excited again with the same laser. They are lost for further cooling cycles. 

It should be noted, that while there is nothing non-conservative in the atom-molecule collision, the collisions that produce the cold molecules also produce the hottest atoms. Just as in the case of laser cooling, the absorption and spontaneous emission cycle conserves energy, and while the emitted photons are in random directions, conservation laws determine the energy and direction of the emitted photon on any one absorption and... 

Example 13.1 Lorenz equations The strange attractor The Lorenz equations (published in 1963 by Edward N. Lorenz a meteorologist and mathematician) are derived to model some of the unpredictable behavior of weather. The Lorenz equations represent the convective motion of fluid cell that is warmed from below and cooled from above. Later, the Lorenz equations were used in studies of lasers and batteries. For certain settings and initial conditions, Lorenz found that the trajectories of such a system never settle down to a fixed point, never approach a stable limit cycle, yet never diverge to infinity. Attractors in these systems are well-known strange attractors. 

Block thermal cyclers are based on a conventional 96-well-plate format in a metal block combined with either a laser-induced fluorescence system or a LED/halogen lamp-based optical excitation source. Due to the usual thermal block time-temperature profiles, the ramp rates for cooling and heating are approximately 1.5 to 3°C/s, which results in an overall time of about 2 h for a typical PCR run of 45 cycles. Due to the 96-well-plate format, the usual pipetting and equipment can be used. Of particular interest for food analysis, in which relative quantification is often used, the numbers of reactions per run are of great importance. Therefore, those types of machines may have an advantage over the machine types described below. 

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