Radiative continuous absorption

Badger RM, Wright AC, Whitlock RF (1965) Absolute intensities of the discrete and continuous absorption bands of oxygen gas at 1.26 and 1.065 p and the radiative lifetime of the Ag state of oxygen. J Chem Phys 43 4345-4350... 

Because water of depths below about 2 m does not absorb much solar radiation direcdy, the radiation is absorbed and converted to heat primarily in the basin floor, which thus should have high radiative absorptance in the solar radiation spectmm. It is also noteworthy that if the stUl is designed to have low heat losses to the ambient, and if the ambient temperature drops, distillation will continue for some time even in the absence of solar energy input, because the saline water may remain warmer than the condensing glass surface and thus continue evaporating. 

Let N(j,Ni,N2, and Nj, be the equilibrium population densities of the states 0, 1,2, and 3, respectively (reached under continuous wave excitation intensity Iq), and let N = NQ + Ni+N2 + N3he the total density of optical absorbing centers. The up-converted luminescence intensity ho (corresponding to the transition 2 0) depends on both N2 and on the radiative emission probability of level 2, A2. This magnitude, which is dehned below, is proportional to the cross section a20 (called the emission cross section and equal to the absorption cross section ao2, as shown in Chapter 5). Thus we can write... 

In Chapter 3 we considered briefly the photoexcitation of Rydberg atoms, paying particular attention to the continuity of cross sections at the ionization limit. In this chapter we consider optical excitation in more detail. While the general behavior is similar in H and the alkali atoms, there are striking differences in the optical absorption cross sections and in the radiative decay rates. These differences can be traced to the variation in the radial matrix elements produced by nonzero quantum defects. The radiative properties of H are well known, and the radiative properties of alkali atoms can be calculated using quantum defect theory. 

In this method, two pulsed lasers are used, both usually in the nanosecond regime. One (the burn laser) is operated at high power, and is scanned across the absorption spectrum. It excites molecules (or clusters) from the particular vibrational level (usually the i = 0 level) to an electronically excited state. The upper state relaxes (radiatively or otherwise) back to the ground state, but not necessarily to i = 0. Thus, depletion in the population of this species is achieved. A second, low-power laser (the probe laser) is fired after a suitable time delay (to allow complete decay of the emission induced by the pump laser). It is tuned to one of the excitation spectrum vibronic bands of the system, and the fluorescence induced by it (the signal ) is continuously monitored. Whenever the frequency of the bum laser corresponds to excitation of the species giving rise to the absorption of the probe laser, the signal is reduced. This reduction appears as a hole that is burned in the spectrum—hence the name of the method. If a different species is excited (another molecule or a different vibrational level) no change in fluorescence intensity is incurred. 

Donor acceptor charge transfer complex based photoreceptors continue to be described in the literature and studied using modern spectroscopic techniques but none has been commercialized. For example, the photoconducting charge transfer complex between poly(V-epoxypropylcarbazole) and TNF has been studied with transient absorption and time-resolved fluorescence. On the basis of Monte Carlo simulations, the results were interpreted in terms of a heterogeneity of charge transfer complexes with different radiative probabilities and a distribution of initial charge pair separation distances [30c]. 

In the absorption process, the electrons are excited across the band gap, from the valence band to the conduction band, by providing an appropriate energy greater than the band gap of the material. These excited electrons must decay back to the valence band by radiative or non-radiative thermal processes. Non-radiative processes are the predominant routes to de-excitation when the electron-phonon coupling is strong. Phonon coupling provides quasi-continuous states which the... 

The model (9.73)—(9.75) was presented as an initial value problem We were interested in the rate at which a system in state 0) decays into the continua L and R and have used the steady-state analysis as a trick. The same approach can be more directly applied to genuine steady state processes such as energy resolved (also referred to as continuous wave ) absorption and scattering. Consider, for example, the absorption lineshape problem defined by Fig. 9.4. We may identify state 0) as the photon-dressed ground state, state 1) as a zero-photon excited state and the continua R and L with the radiative and nonradiative decay channels, respectively. The interactions Fyo and correspond to radiative (e.g. dipole) coupling elements between the zero photon excited state 11 and the ground state (or other lower molecular states) dressed by one photon. The radiative quantum yield is given by the flux ratio Yr = Jq r/(Jq r Jq l) = Tis/(Fijj -F F1/,). 

Note that in such spectroscopic or scattering processes the pumping state 0) represents a particular state of energy Eq out of a continuous manifold. In most cases this state belongs to one of the manifolds R and L. For example, in the absorption lineshape problem this photon-dressed ground state is one particular state of the radiative (R) continuum of such states. 

Energy bundles are followed through their histories after emission from each surface until absorption at a boundary. Because of the assumption of radiative equilibrium, any bundles absorbed within a medium volume element must be balanced by an emission from that element this is simulated by simply reemitting an absorbed bundle in a new direction and continuing the history until final absorption at a boundary. The medium temperature distribution is computed by equating the emission from the element to the absorption. The flow chart for this case is shown in Fig. 7.20. 

Since the total number of nuclei N = N, + Np,itia evident from equation (1) that (N — N ), and hence the intensity of the n.m.r. signal, is proportional to N. Typically, the excess population in the -state is 1 10 . Application of the appropriate radiofrequency field induces both upward and downward transitions, but the former predominate. The observed signal indicating energy absorption would quickly be saturated (iVp = iV ) but for a non-radiative process, spin-lattice relaxation, by which the Boltzmann distribution can be continuously re-established. Spin-lattice relaxation times, that is the time required for a collection of nuclei to return to the Boltzmann distribution after perturbation, varies according to the type of nucleus (e.g., etc.) and, for a... 

Activation of the vibrational energy of ions can also be induced by the absorption of IR radiations. A popular type of IR radiation source is far-IR laser. In fact, many molecules have a broad IR absorption band. The most widely used IR source is a continuous wave (c.w.) CO2 laser, with the wavelength of 10.6 pm. This wavelength corresponds to an energy of 0.3 eV per laser photon. Because decomposition of a chemical bond requires >1 eV, laser excitation has to extended over hundreds of milliseconds to allow ions to absorb multiple IR photons. This method is known as infrared multiphoton dissociation (IRMPD). Another type of similar technique is black-body infrared radiative dissociation... 

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