Radiation trapping

The preceding calculation of the thermal energy balance of a planet neglected any absorption of radiation by molecules within the atmosphere. Radiation trapping in the infrared by molecules such as CO2 and H20 provides an additional mechanism for raising the surface temperature - the greenhouse effect. The local temperature of a planet can then be enhanced over its black body temperature by the atmosphere. 

Water vapour makes a sizeable contribution, and probably the largest, to radiation trapping and as the temperature increases the water vapour concentration increases. Temperature rises as a result of increased water vapour concentration and hence a mechanism for a positive feedback in the greenhouse effect that might lead to a runaway greenhouse effect. When the vapour pressure for water reaches saturation, condensation occurs and water rains out of the atmosphere this is what happens on Earth and Mars. On Venus, however, the water vapour pressure never saturates and no precipitation occurs and the global warming continues to increase. Thus Venus suffers from extreme temperatures produced by both its proximity to the Sun and the presence of water vapour and carbon dioxide in its atmosphere. 

It appears at this time that one of the most important mechanisms involved in the luminescence of rare earth ions is energy exchange between them. One may clearly differentiate between two distinct mechanisms (a) radiative exchange and (b) nonradiative exchange. In the radiative mechanism, a photon emitted by ion A is captured by ion B. Since the photon has left the A system, the capture of it by B cannot decrease the lifetime of A. However, f the photon is shuttled back and forth between similar or dissimilar ions, the fluorescent lifetime could well be increased by radiation trapping. This is an interesting phenomenon and warrants further discussion. 

The first report of radiation trapping in a solid was given by Varsanyi and co-workers (42) for 0.05 per cent ruby. Chromium is admittedly not a rare earth, but the results they obtained are so interesting that they deserve further mention. They clearly indicate a potential error in measurement to fee expected for rare-earth systems. 

We also had to overcome radiation trapping effects by monitoring the Na(3p) — Na(4s) emission off line-center. The results shown in Figs. 2 and 4 are obtained by subtracting the contribution of these processes from the observed Na(3p) signal. To do so we calibrated the contribution from the Na(3d) — ... 

The latter method has the advantage of low alkali-atom densities, thus avoiding radiation trapping and chemical reactions and allows selection of the initial kinetic energy of the A atom—subject, however, to some discussion about the velocity distribution and its relaxation before quenching. The excited atoms will loose their excitation energy be either spontaneous emission... 

The next crucial observation came from a thermodynamic study of the radiation which is emitted through an aperture in the wall of a heated and otherwise closed oven. Once more, it was the intensity distribution of the radiation emitted at different wavelengths that defied analysis. Presented in graphical form the observed distribution is Figure 2.5 The intensity dis- as shown in the Figure 2.5. The distribu-tribution of radiation trapped tion predicted by the laws of thermodynam-in a closed cavity. ics is shown as the Raleigh-Jeans curve. It... 

Arrowsmith et al used the crossed beam reaction F+Na— NaF+Na (3 P) to study radiative transfer and electronic energy transfer (E — E, V) in the Na (3 P)-1-NajCX S ) system. Previous studies of the Na2 system have utilized high-pressure cells or heat pipes in which radiation trapping is strong and Na + Na2 collisional energy transfer dominates. Time-resolved emission, following pulsed dye-laser excitation, has been used by Husain and his coworkers in a systematic survey of the excited-state behaviour of Mg(3 Pj), Ca(4 P,), and Sr(5 Pj). Dye-laser excitation of Mg vapour at 457.1 nm resulted in the observation of slow spontaneous emission from Mg(3 P,) which... 

The application of eqn (2) assumes that I v" is linearly proportional to This is valid in the limit of no radiation trapping and no optical pumping of the molecular sample. Under our experimental conditions, the former poses no problem, whereas the latter requires consideration because of the high laser intensity used. Indeed, we observe a departure from linearity, which decreases as the laser intensity is reduced. Data, such as that shown in fig. 3-6, are taken with the laser intensity attenuated by typically fourfold, at which condition the deviation from linearity is 15 %. To simplify data analysis, the same laser intensity (to within 20 %) is used for all scans, and only linear corrections to are made for variation in laser intensity. 

The radiation trapping between building walls and streets stores the energy inside the urban canopy. Consequently, when buildings are high and streets are narrow the decrease of the night temperatures is low. 

The four 4s levels play a key role in analytical glow discharges (e.g. for Penning ionization of the sputtered atoms) and they cannot easily be depopulated by radiative decay (due to forbidden transitions for the metastable levels, and to radiation trapping for the resonant levels). Therefore some additional loss processes are incorporated for these levels, in order to describe them with more accuracy ... 

Table 6.3 Estimated changes in trace tropospheric gas concentrations and their contributions to radiation trapping in preindustrial times compared to 1985 values (after Dickinson Cicerone 1986)...

Table 1 summarizes the techniques used to measure noble gases. By far the most important is mass spectrometry. Mass spectrometers in space are used, e.g., for solar wind and solar energetic particle measurements or atmospheric analyses on Venus, Moon, Mars and Jupiter, while mass spectrometers in the laboratory allow us to analyze extraterrestrial samples available on Earth, i.e., lunar samples, meteorites, interplanetary dust or solar corpuscular radiation trapped by foils exposed in space. Of course. 

All lines to the measuring vessel have radiation traps in the form of a tee placed in the guard vessel. 

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