Lifetimes under pressure

The measurement of a lifetime is much more accurate and reliable than the measurement of the absolute luminescence intensity under pressure. Therefore, the majority of studies on f-element compounds reported on pressure-dependent lifetimes only. The compounds studied so far can be found in table 1. In most cases a decrease of the lifetime under pressure has been observed. The following sections will treat the changes in lifetimes or intensities according to the mechanisms responsible for the observed variations. 

The removal of dirt particles, etc. is particular important in pipe extrusion pipe lifetimes under pressure are increased when particular care is taken to purify the polymer. 

To consider gas molecules as isolated from interactions with their neighbors is often a useless approximation. When the gas has finite pressure, the molecules do in fact collide. When natural and collision broadening effects are combined, the line shape that results is also a lorentzian, but with a modified half-width at half maximum (HWHM). Twice the reciprocal of the mean time between collisions must be added to the sum of the natural lifetime reciprocals to obtain the new half-width. We may summarize by writing the probability per unit frequency of a transition at a frequency v for the combined natural and collision broadening of spectral lines for a gas under pressure ... 

In the next section the rare-earth compounds that have been studied by optical means under pressure so far will be reviewed. Then, after a brief introduction of the most commonly used high pressure device, the diamond anvil cell, sect. 4 presents a discussion of the pressure-induced changes of the crystal-field levels and their interpretation. In sects. 5 and 6 some aspects of the dynamical effects under pressure are discussed. These include lifetime and intensity measurements, the influence due to excited configurations and charge transfer bands, and the electron-phonon coupling. 

The optical studies performed on most samples of table 1 were aimed at different aspects of the f-electron properties. A considerable amount of the work was concerned with the energy level shifts under pressure. From these shifts, variations of free-ion parameters, crystal-field parameters or crystal-field strengths with pressure have been deduced. Other studies concentrated on changes in lifetimes or intensities, the efficiency of energy transfer between rare earths or rare earths and other impurities or on electron-phonon coupling effects under pressure. The various aspects investigated under high pressure will be presented within the next sections. 

In a second step, Shen and Bray (1998a) studied the changes of the 5Do and 5Di lifetimes of SrFCl Sm2+ and CaFCl Sm2+ under pressure. In both systems they observed an exponential decrease as shown in fig. 13 for the case of the 5Do lifetime at room temperature. According to their analysis of the temperature effects, the measured lifetime of the 5Do 7Fo transition represents an almost pure radiative lifetime. A strong decrease under pressure therefore indicates an increase in the radiative rate Wj. This in turn was attributed to an increased elec-... 

Under pressure, Gleason et al. (1993) observed a strong decrease of the 5D3 lifetime, which was attributed to a nonradiative transfer to the excited 4f75d1 configuration. This configuration... 

To get further evidence about which model is more appropriate, Webster and Drickamer (1980a) have measured the luminescence efficiency of La202S Eu3+ and Y202S Eu3+ and the lifetimes of the lanthanum compound under pressures up to 12 GPa. Intensities and life-... 

Similar conclusions were drawn by Blanzat et al. (1984) who studied TbxLai xP50i4, EuxLai xP50i4 and mixed single crystals TbxEui xP50i4 under pressure. By selective excitation of the 5D4 multiplet of Tb3+ the luminescence of Eu3+ could be observed due to an efficient Tb3+ -> Eu3+ energy transfer. Under pressure the lifetime of the Eu3+ luminescence decreased which was interpreted by a weak back-transfer from Eu3+ to Tb3+ due to the increased overlapping of the 5D4 multiplet of Tb3+ with the 5D2 multiplet of Eu3+. 

Many companies that do industrial research on catalysis choose not to make their own catalysts. Catalyst preparation and marketing is a specialty chemical, high technical service business. Manufacturers are under pressure to make their catalysts more active, more selective, and with a greater cost performance and lifetime than those of their competitors. Development of a new catalyst or process historically has taken many years (5-10) which is a disadvantage in project economics. The overall catalyst business is expanding and catalyst life is finite. The challenge is to make a cost-effective product with a sufficiently high rate of return on investment. 

The basic design and operation of the MDI has changed little over its lifetime. Aerosols are generated from a formulation of drug (0.1-1% w/w) either suspended or in solution in the liquefied propellant. The formulation is held under pressure in a canister. 

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