Ruby fluorescence

K. T. V. Grattan, R. K. Selli, and A. W. Palmer, Ruby fluorescence wavelength division fiber-optic temperature sensor, Rev. Sei. Instrum. 57, 1231-1234 (1987). 

Perhaps the first detailed discussion of such a technique in fluorescent thermometry (shown in Figure 11.10) was given by Zhang et al. in their work(36) based on both mathematical analysis and experimental simulation. Examples of the electronic design of the corresponding system and the application of the technique in a ruby fluorescence-based fiber-optic sensor system are also listed. This shows that there is no difference in the measurement sensitivity between a system using square-wave modulation and one using sinusoidal modulation. However, the former performs a little better in terms of the measurement resolution. 

Figure 11.18. The ruby fluorescence lifetime-based fiber optic thermometer system. Fi short-pass optical filter Ft /f-line band pass optical filter.

Figure 11.21. Characteristic calibration curve for the ruby fluorescence-based thermometer in the region from room temperature to 550°C.

The intensity of the fluorescence emission detected at the photodetector stage was plotted as a function of temperature over the same range, and this is shown in Figure 11.22. It falls off rapidly with temperature increase over the whole temperature region. This does not contradict the experimental evidence of Burns and Nathan(56) who showed that the fluorescence quantum efficiency of the ruby fluorescence integrated over the entire band from 620 to 770 nm is independent of temperature (to 5%) in the region from-196 to 240°C, for the emission detected here is only the A-line part of the total fluorescence emission. 

Figure 11.22. The ruby fluorescence intensity recorded in the experiment.

Figure 11. Schematic representation of a laser heating experiment in the DAC. The IR laser beam is directed onto the absorbing sample immersed in a compression medium acting also as thermal insulator. The thermal emission of the sample is employed for the temperature measurement, while the local pressure is obtained by the ruby fluorescence technique (see next section).

The linear calibration of the ruby fluorescence emission, initially based on the equation of state of sodium cloride [71, 89, 96, 103, 237], is reliable up to 30 GPa for a quasi-hydrostatic environment. The quasi-hydrostatic calibration was extended up to 80 GPa [157], and a slight nonlinearity of the calibration curve at high pressure was found. Calibration of the ruby scale against primary... 

Figure 14. (Upper panel) The ruby fluorescence spectrum measured in quasi-hydrostatic conditions at 7.7 GPa. (Lower panel) The empirical law describing ruby Ri line shift with pressure [96] is also reported.

Calibration of the pressure is best accomplished using the Ruby scale. Generally this is done using a metal gasket between the two diamond windows surrounding the sample in which a liquid (such as Nujol or Teflon oil) is added to produce hydrostatic pressure. The technique measures the pressure dependence of the sharp Ruby Ri fluorescence transition at 692.8 nm, although the R2 band at 694.2 nm can also be used. The Ruby fluorescence is induced by the blue excitation of the Ar+ (488.0 nm) or the He- C d (441.6nm) lasers. 

In order to obtain a direct and more accurate pressure determination, various internal pressure calibrants (e.g. quartz and ruby chips) are generally used. Internal calibrants, however, could not be used in the high temperature hydrothermal experiments due to interactions with the chemical system. In such cases, one of the more prevalent phases of the chemical system was calibrated as pressure indicator. For fluid-rich systems (methane-water), the pressures were also determined using the known phase equilibria of the methane hydrate decomposition and using shifts in the ruby fluorescence peak [8]. 

In our experiments, we typically used Ri ruby fluorescence [11] to measure the pressure according to the relation,... 

Since diamond is transparent to visible light, the easiest method of measuring the pressure is with the ruby-fluorescence (Al203 Cr ) scale. The fluorescence associated with the Rj and R2 transitions of the ion around 14400 cm at ambient pressure shifts by —7.57 cm GPa under pressure, and pressure variations of —100 GPa can be measured with a modest spectrometer. Excitation-argon-laser powers of —10 mW are sufficient to obtain measurable signals from ruby chips with a volume of —1000 (cm. A chromium content in the ruby of anywhere between 500 and 5000 p.p.m. 

Fig. 1.36 The radial pressure distribution in the gasket of a bevelled anvil system, measured by ruby fluorescence . (Reprinted with permission from Rev. Sci. Instr., 56, 1422, (1985).)...

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