Spontaneous emission phosphorescent

Spontaneous emission of radiation from an electronically and/or vibrationally excited species not in thermal equilibrium with its environment is called luminescence when the emission is accompanied by the formation of a molecular entity of the same spin multiplicity it is called fluorescence (F), whereas luminescence involving change in spin multiplicity (typically from triplet to singlet) is called phosphorescence (Ph) [1],... 

The units of Au are s. This spontaneous emission, which is independent of the radiation density, is called fluorescence or phosphorescence, according to some arbitrary time division (e.g., phosphorescence if relaxation time is... 

In addition to absorption and stimulated emission, a third process, spontaneous emission, is required in the theory of radiation. In this process, an excited species may lose energy in the absence of a radiation field to reach a lower energy state. Spontaneous emission is a random process, and the rate of loss of excited species by spontaneous emission (from a statistically large number of excited species) is kinetically first-order. A first-order rate constant may therefore be used to describe the intensity of spontaneous emission this constant is the Einstein A factor, Ami, which corresponds for the spontaneous process to the second-order B constant of the induced processes. The rate of spontaneous emission is equal to Aminm, and intensities of spontaneous emission can be used to calculate nm if Am is known. Most of the emission phenomena with which we are concerned in photochemistry—fluorescence, phosphorescence, and chemiluminescence—are spontaneous, and the descriptive adjective will be dropped henceforth. Where emission is stimulated, the fact will be stated. 

If the radiative transition occurs from the first excited triplet to the singlet ground state (Ti So, a spin-forbidden transition AS 0), the phenomenon is called phosphorescence. The spontaneous emission may persist for long periods, even hours, but the lifetimes usually last milliseconds to seconds. 

Similarly the parent problem for fluorescence of phosphorescence emission in dilute gases or solution is the influence of coupling to foreign molecules on spontaneous emission [189]. The emission rate for an isolated molecule A is given by the Einstein A coefficient,... 

The term luminescence denotes the spontaneous emission of radiation from an electronically excited species and comprises fluorescence and phosphorescence, and also chemiluminescence (Section 5.6). 

In addition to the spontaneous emission of excited molecules, fluorescence and phosphorescence (Section 2.1.1), the interaction of electromagnetic radiation with excited molecules gives rise to stimulated emission, the microscopic counterpart of (stimulated) absorption. Albert Einstein derived the existence of a close relationship between the rates of absorption and emission in 1917, before the advent of quantum mechanics (see Special Topic 2.1). 

Ln-L distance, energy transfer occurs as long as the higher vibrational levels of the triplet state are populated, that is the transfer stops when the lowest vibrational level is reached and triplet state phosphorescence takes over. On the other hand, if the Ln-L expansion is small, transfer is feasible as long as the triplet state is populated. If the rate constant of the transfer is large with respect to both radiative and nonradiative deactivation of T, the transfer then becomes very efficient ( jsens 1, eqs. (11)). In order to compare the efficiency of chromophores to sensitize Ln - luminescence, both the overall and intrinsic quantum yields have to be determined experimentally. If general procedures are well known for both solutions (Chauvin et al., 2004) and solid state samples (de Mello et al., 1997), measurement of Q is not always easy in view of the very small absorption coefficients of the f-f transitions. This quantity can in principle be estimated differently, from eq. (7), if the radiative lifetime is known. The latter is related to Einstein s expression for the rate of spontaneous emission A from an initial state I J) characterized by a / quantum number to a final state J ) ... 

Phosphorescence Spontaneous emission of light by a molecule from an electronically excited state that has a different total electronic spin from the ground state. 

Both states can exhibit spontaneous emission. If the origin is a singlet state, emission is called fluorescence-, from the triplet state phosphorescence [2,7]. The transition ends at different vibrational levels of the state Sq. The different possible emissive transitions are included in Fig. 2 and drawn as a fluorescence spectrum (F) at the right in the diagram. The thermal equilibration within the excited vibrational states causes fluorescence spectra shifted to long wavelengths compared to the absorption spectra. In most cases they form a kind of mirror image of the absorption spectrum. 

Spontaneous emission from higher excited singlet (S2, S3, etc.) states is rare, since the system quickly finds the lowest excited singlet state by nonradiative transitions (internal conversion. Kasha s rule). Usually, emission spectra are formed either from the lowest excited singlet state of a molecule (fluorescence) or from the lowest triplet state (phosphorescence). In some other cases, low-lying CT states, singlets or triplets, are deexcited radiatively. 

PiJip)) is called the Lorentzian lineshape function. Its fwhm is equal to y, and is inversely proportional to the lifetime t = Ijy. It approaches zero as o) + oo, and maximizes at co == coq (Fig. 8.1). Physically, y itself will have several components in any real absorption line, arising from spontaneous emission (fluorescence or phosphorescence), nonradiative excited-state decay (intersystem crossing, internal conversion, photochemistry), collisional deactivation, etc. ... 

The energy of the absorbed radiation corresponds to the energy of a transition from ground to an excited state. Decay of an excited state back to the ground state may take place by a radiative or non-radiative process. The spontaneous emission of radiation from an electronically excited species is called luminescence and this term covers both fluorescence and phosphorescence. A discussion of these phenomena requires an understanding of the electronic states of multi-electron systems, and we return to emission spectra in Section 20.8. 

The presence of in this relation implies that spontaneous emission can be largely ignored at the relatively low frequencies of vibrational transitions but may be important for transitions in the visible and ultraviolet regions. We shall see later (Sections 12.9 and 12.10) that spontaneous emission accounts for the phenomena of fluorescence and phosphorescence. Stimulated emission underlies the functioning of lasers ( laser is an acronym formed from light ampliflcation by the stimulated emission of radiation ). 

In polymers undergoing autoxidation, chemiluminescence is the weak, spontaneous emission of light dependent on sample heating. Fluorescence is radiation caused be a transition in an atom from absorption of light. Fluorescence is characterized differently than phosphorescence in that the transidon is from a higher energy level to the ground state. 

The luminescence of an excited state generally decays spontaneously along one or more separate pathways light emission (fluorescence or phosphorescence) and non-radiative decay. The collective rate constant is designated k° (lifetime r°). The excited state may also react with another entity in the solution. Such a species is called a quencher, Q. Each quencher has a characteristic bimolecular rate constant kq. The scheme and rate law are... 

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