Optical cooling frequency measurement

The molecular constants that describe the stnicture of a molecule can be measured using many optical teclmiques described in section A3.5.1 as long as the resolution is sufficient to separate the rovibrational states [110. 111 and 112]. Absorption spectroscopy is difficult with ions in the gas phase, hence many ion species have been first studied by matrix isolation methods [113], in which the IR spectrum is observed for ions trapped witliin a frozen noble gas on a liquid-helium cooled surface. The measured frequencies may be shifted as much as 1 % from gas phase values because of the weak interaction witli the matrix. 

The vibrational spectrum of benzene around 1000 cnf has also been measured. IQ. Benzene was physisorbed on a cooled copper substrate in the vacuum chamber. Figure 19 shows the transmission for several thicknesses of benzene and a prism separation of 3 cm. The thickness was determined from the measured transmission in transparent regions using Eg. (7). The solid curves were calculated from Eqs. (5) and (6) using optical constants for benzene obtained from an ordinary transmission experiment.il The benzene film was assumed to be isotropic. Of the two absorption lines seen, one belongs to an in-plane vibrational mode, and one to an out-of-plane vibration. Since the electric field of the SEW is primarily perpendicular to the surface, the benzene molecules are clearly not all parallel or all perpendicular to the copper surface. Also it should be noted that the frequencies are the same (within the experimental resolution) as those of solid benzene22 and of nearly the same width. These features indicate that the benzene interacts only weakly with the copper surface, as would be expected for physisorbed molecules. 

Fig. 6. Density of non-condensed fraction of the gas as the trap depth is reduced along the cooling path. The density is measured by the optical resonance shift, and the trap depth is set by the rf frequency. The lines (dash, solid, dot-dash) indicate the BEC phase transition line, assuming a sample temperature of (l/5th, l/6th, l/7th) the trap depth. The scatter of the data reflects the reproducibility of the laser probe technique and is dominated by alignment of the laser beam to the sample...

Another impressive example is work made at Physikalisch-Technische Bundesanstalt in Braunschweig [38], The optical frequency of the intercombination line, Pi- So, at 657 nm in has been measured, both in an atomic beam as well as with trapping/cooling techniques. This transition was connected all the way to the Cs frequency by a chain giving a total uncertainty of the absolute frequency for their measurements as low as 10 which is the lowest uncertainty in the visible domain. This transition has a linewidth < 400 Hz which gives good prospects of considerably improving the results. 

Both microwave and optical frequency standards have benefited greatly from the development of the laser and the methods of laser spectroscopy in atomic physics. In particular, the ability to determine both the internal and external (that is, motion) atomic states with laser light - by laser cooling for example - has opened up the prospect of frequency standards with relative uncertainties below lO, for example, the Cs atomic fountain clock. The best atomic theories in some cases at starting to match in accuracy that of measurement, providing thereby refined values of the fundamental, so-called atomic constants. Even quite practical measurements (such as used in GPS navigation and primary standards of length) have advanced in recent years. 

The frequency and amplitude of an oscillating quartz crystal are influenced by even a minimal mass load. A sensitive dew sensor was developed by using a Peltier-cooled quartz plate [54]. This sensor is capable of measuring relative humidity with an accuracy better than l.St o of the measured relative humidity in automatic dew-point hygrometry. Since the frequency but not the amplitude of specially cut quartz crystals depends strongly on the temperature, the dew-point temperature can be determined simultaneously with the same sensor element. Compared with optical dew-point hydrometers, this quartz sensor combines dewpoint detection and temperature measurement in a single element. 

A similar ion trap experiment was performed with Be ions. The ions were cooled via the 2s Si/2(M/ = -3/2, Mj = -1/2) - 2p /2( -3/2, -3/2) transition at A = 313 nm with a frequency doubled dye laser, and were additionally optically pumped in the ground state. Measurements of the axial (I z), magnetron (v ), and electric field shifted cyclotron (Vc)frequencies of the stored ions provide the free-space cyclotron... 

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