Radiation imprisonment

It is called trivial because it does not require any energetic interaction between the donor and the acceptor. It is merely reabsorption of fluorescence radiation in accordance with Beer s Law and shows r 2 dependence on donor-acceptor distance. Although called trivial, it causes radiation imprisonment and can be important factor to be considered in fluorescence measurements. It may introduce error and distort emission spectrum by absorbing only that portion which overlaps its absorption spectrum. It is specially troublesome in studies on concentration quenching. 

A detailed treatment of radiation imprisonment would be beyond the scope of the present chapter. The situation would be simple if the entering radiation had exactly the same wavelength distribution as the emitted radiation and if all emitted radiation would traverse a distance to the walls exactly equal to that traversed by the incident radiation in reaching the point of absorption. Neither of these requirements is met in practice. In a general way one may say that if photons are half absorbed on the average from each point of emission and if the mean life r is defined by the equation... 

Apparently monochromatic resonance radiation of mercury which passes through mercury vapor at the saturated pressure at 25 °C is about half absorbed in four millimeters distance. Beer s law is not obeyed at all because the incident radiation cannot be considered to be actually monochromatic, and absorption coefficients of mercury vapor vary many times between zero and very high values in the very short space of one or two hundredths of an Angstrom unit. Moreover, absorption of mercury resonance radiation by mercury vapor is sufficiently great even at room temperature to make radiation imprisonment a very important phenomenon. If the reaction vessel has any dimension greater than a few millimeters the apparent mean life of Hg(63P ) may be several fold the true radiative life of 1.1 x 10"7 sec, reaction (27), because of multiple absorption and re-emission. 

Of these /, may be the most difficult to determine accurately, but Gunning and his coworkers have determined the quantum yield of hydrogen formation from several hydrocarbons (e.g. cyclohexane) with quite high precision so that, barring pressure broadening and uncontrolled variations in radiation imprisonment, data can be obtained for testing these various relationships. 

Pulsed excitation methods are the main source of rate data for the excited noble gas atoms in the parent gas and have also provided limited data on quenching of metastable states by foreign gases. Almost all the studies of the Pi excited states have used these techniques, but photolysis as a means of populating resonant states has not been widely applied,partly because of severe problems due to radiation imprisonment. ""... 

A further factor which affects the shape of a resonance line is the transport of resonance radiation through the. parent gas. Milne s early theory of self-absorption by the imprisonment of resonance radiation has been revised by Holstein (3) and Bichermau (4), taking into account the incoherent scattering of resonance photons. Furthermore, Walsh (" ) has extended the imprisonment lifetime calculation for cases where Doppler and collision broadening of the resonance line are simultaneously present, and in addition this author has examined the complications caused by the, hfs. The line shape, linewidth, and other properties of self-absorbed lines hare been discussed recently bv Tako (6), who summarizes the various effects of self-absorption as follows ... 

Krause et a/.123-125 have recently reported a series of measurements of the spin-orbit relaxation of the alkali metals in their first excited states (2P). The technique, for example for atomic caesium with AE = 554 cm-1, consists of irradiating the metal vapour with light from a monochromator to excite only one of the 2P states. The vapour pressure of the metal is controlled at 10-6 torr to avoid imprisonment of the resonance radiation. The components of the fluorescence light are measured with a photomultiplier by isolating the 2P - 2S lines with interference filters. In the presence of added gases which cause the transitions... 

The Ar(3P(, Pt) levels are 11.623 and 11.827 eV, respectively, above the ground (1S) level. The lifetimes are 8.4 and 2.0 nsec (33), respectively. The Ar(3P,1 Pj) states are formed by absorption of the Ar resonance lines at 1067 and 1048 A. In the 1 to 100 mtorr concentration range the lifetime of Ar(3P, P() atoms is of the order of 10 /tsec [Hurst et al. (494)], which is 1000 times as long as that of isolated atoms because of imprisonment of resonance radiation. If the ionization potential ofa molecule is below 11.6 eV, it is possible to increase the photoionization yield (sensitize) by adding Ar to the sample. The increase of the ionization yield is caused by collisional energy transfer between Ar(3P, Pi) atoms and the molecule before the excited atoms return to the ground state by resonance emission. Yoshida and Tanaka (1065) have found such an increase in the Ar propane, and Ar-ammonia mixtures when they are excited by an Ar resonance lamp. Boxall et al. (123) have measured quenching rate constants for Ar(3P,) atoms by N2) 02, NO, CO, and H2. They are on the order of the gas kinetic collision rate. 

Eq. (24.2) obviously holds as long as the absorption coefficient neither depends on c nor on x. However, this is not always the case and there can be deviations from the Lambert-Beer law. These deviations are most frequently caused by the pressure-induced line broadening and the imprisonment of radiation [311, 535]. 

In emission (fluorescence) spectroscopy two classic examples of density-dependent processes are the phenomena of excimer and exciplex formation and decay, and the imprisonment (trapping) of resonance radiation in atomic (e.g., Hg, rare gases) gases. 

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