Side band cooling

Fig. 6.1. Laser cooling towards the zero-point energy of the motion using three lasers. The quantum number riy characterizes the quantized motion of a single Hg+ ion in the harmonic effective potential of the confining rf quadrupole trap (eigenfrequency jjjl K — 2.96 MHz, huj = 12 neV). Using side band laser transitions Any = —1 transitions can be pumped. Analysis of side band resolved spectra reveals that the system can be cooled so far that it occupies the ground state ny = 0 for 95% of the time, corresponding to a temperature of 47 fiK. As soon as the cooling laser is switched off, the ion motion is heated with a rate of (dny/dt) = 6/s. After 2 s it reaches a mean value of (riy) = 12 corresponding to T 1.7 mK.

Figure 2.4 displays (a) room temperature PARS spectrum, (b) jet-cooled action spectrum, (c) REMPI spectrum of CH3NH2, and, on the right side of each panel, the respective excitation schemes. The spectra are characterized by a multiple peak structure, related to the (7-branches of different bands, and the peaks of the action spectrum show up whenever the difference frequency of the SRS laser beams matches that of a specific vibrational transition. However, whereas in the PARS spectrum two of the peaks, of the degenerate CH3 stretch, V2 (2961 cm ) and the CH3 symmetric stretch, V3 (2820 cm ), are dominant and the others quite weak, in the action spectrum all peaks carry significant intensity and, in particular, the PARS low-intensity peaks become prominent. An additional dominant peak, in both the PARS and action spectra, due to the NH2 symmetric stretch, Vj (3361 cm ), is beyond the wave number span of the figure. 

A photoconductive detector is a semiconductor whose conductivity increases when infrared radiation excites electrons from the valence band to the conduction band. Photovoltaic detectors contain pn junctions, across which an electric field exists. Absorption of infrared radiation creates electrons and holes, which are attracted to opposite sides of the junction and which change the voltage across the junction. Mercury cadmium telluride (Hg,. Cd/Te, 0 < x < 1) is a detector material whose sensitivity to different wavelengths is affected by the stoichiome-try coefficient, x. Photoconductive and photovoltaic devices can be cooled to 77 K (liquid nitrogen temperature) to reduce thermal electric noise by more than an order of magnitude. 

Very recently, Bailey and Richards (23) have shown that a high degree of sensitivity for adsorbed species can be achieved by measuring the absorption of infrared radiation on a thin sample cooled to liquid helium temperature. The optical arrangement used in these studies is shown in Figure 10. The modulated beam produced by the interferometer is introduced into the UHV sample chamber and reflected off a thin slice of monocrystalline alumina covered on one side by a 1000 k film of nickel or copper. Radiation absorbed by the sample is detected by a doped germanium resistance thermometer. The minimum absorbed power detected by this device when operated at liquid helium temperature is 5 x 10 14 W for a 1 Hz band width. With this sensitivity absorbtivities of 10"4 could be measured. 

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