Total emission spectrum

Weller24 has estimated enthalpies of exciplex formation from the energy separation vg, — i>5 ax of the molecular 0"-0 and exciplex fluorescence maximum using the appropriate form of Eq. (27) with ER assumed to have the value found for pyrene despite the doubtful validity of this approximation the values listed for AHa in Table VI are sufficiently low to permit exciplex dissociation during its radiative lifetime and the total emission spectrum of these systems may be expected to vary with temperature in the manner described above for one-component systems. This has recently been confirmed by Knibbe, Rehm, and Weller30 who obtain the enthalpies and entropies of photoassociation of the donor-acceptor pairs listed in Table XI. From a detailed analysis of the fluorescence decay curves for the perylene-diethyl-aniline system in benzene, Ware and Richter34 find that... 

In both models, the two species M and M L have their own characteristic emission spectra, the intensities of which depend upon [L], In other words, the total emission spectrum is the weighted sum of the individual contributions, which have very different expressions whether one considers model 1 or 2 (see eqs. (25a) and (25b)). 

Therefore, the emitting species M and M L can in principle be identified from a decomposition of the total emission spectrum, and thus TRES experiments are mainly based on the evaluation of emission spectra rather than luminescence decays. However, a detailed analysis of the decays allows one to derive important information that cannot be obtained through the emission spectra, as will be explained below. In the frame of model 2, it is easily shown that the expressions of the relative contributions of the two species to the global emission spectrum contain only one unknown parameter, Afapp, while the equivalent expressions under the frame of model 1 are much more complex. This raises the question as to whether model 2 can be considered a reasonable approximation of the more complex scheme 1. This issue can be discussed qualitatively on the basis of three distinct cases of model 1, depending on the importance of photochemical reactions. 

Figure 21. The total emission spectrum of anthracene at finite temperatures,61 taking the electronically excited state to be in thermal equilibrium at three different temperatures [Eq. (46a)]. The calculation was made using Eq. (131), with a linewidth f0 = 2f = 10 cm-1.

A methanol solution containing poly(3) exhibits abroad absorption band with A,max at 422 nm. In aqueous solution, the absorption maximum red-shifts to 432 nm and the absorption coefficient also decreases. The emission properties are greatly dependent upon the nature of the solvent. In a methanol solution, poly(3) exhibits a narrow emission band with A.max = 475 nm and a vibronic band at A,max = 502 nm. In aqueous solution, the emission band becomes broader and the fluorescence quantum yield (Ofi =0.016) is lower than that in methanol ( fi = 0.045). Interestingly, the emission maximum is at the same position as in methanol (475 nm) with a shoulder at 502 nm. Such behaviour is different from other PPE-based CPE, which exhibit large red-shifts of emission band in a poor solvent. The lack of a spectral shift for poly(3) is likely to be due to the fact that the aggregated state of the polymer has a much lower quantum yield, and therefore its contribution to the total emission spectrum is small. 

A total emission spectrum shows both fluorescence and any phosphorescence that can be detected. 

T2 suffices to shed light on the interaction. A typical example of the latter case is illustrated in fig. 10 which shows a typical transfer process between an excited donor (D) atom and an unexcited acceptor (A). Figure 10 also shows the temporal change or dynamics which are expected in such transfer processes. For these measurements, the donor system is excited with a pulsed excitation and either the intensity of some luminescent feature or the total emission spectrum is monitored as a function of time. Time resolved methods have long been, of course, established in conventional spectroscopy and evidence of transfer and other nonradiative relaxation processes were demonstrated through the use of pulsed broadband sources. 

New types of detectors, such as OMA (optical multichannel analyser), SMA (spectrum multichannel analyser) and MCPD (multichannel photodiode array), are able to record the total emission spectrum by a single shot after flash excitation, so that acquisition of time-resolved emission spectra becomes much easier. One uses appropriate gating techniques, synchronized to the excitation flash, to control the time scale in the data acquisition. At the moment semiconductor based detectors are still less sensitive than photomultiplier (FMT) detectors. 

The total emissivity of soot , is obtained by integration over the wavelength spectrum, giving... 

Human-made sources cover a wide spectrum of chemical and physical activities and are the major contributors to urban air pollution. Air pollutants in the United States pour out from over 10 million vehicles, the refuse of over 250 million people, the generation of billions of kilowatts of electricity, and the production of innumerable products demanded by eveiyday living. Hundreds of millions of tons of air pollutants are generated annu ly in the United States alone. The five main classes of pollutants are particulates, sulfur dioxide, nitrogen oxides, volatile organic compounds, and carbon monoxide. Total emissions in the United States are summarized by source categoiy for the year 1993 in Table 25-10. 

Emissivity is strongly dependent on the surface quality. The emissivity of a rough surface is greater than that of a smooth surface, increasing the rate of absorption. Emissivity values are found in textbooks. Care must be taken when using these values, as they usually denote total emissivities. The emissivity is considered constant in the spectrum, and this may be a poor approximation. 

Theory. If two or more fluorophores with different emission lifetimes contribute to the same broad, unresolved emission spectrum, their separate emission spectra often can be resolved by the technique of phase-resolved fluorometry. In this method the excitation light is modulated sinusoidally, usually in the radio-frequency range, and the emission is analyzed with a phase sensitive detector. The emission appears as a sinusoidally modulated signal, shifted in phase from the excitation modulation and partially demodulated by an amount dependent on the lifetime of the fluorophore excited state (5, Chapter 4). The detector phase can be adjusted to be exactly out-of-phase with the emission from any one fluorophore, so that the contribution to the total spectrum from that fluorophore is suppressed. For a sample with two fluorophores, suppressing the emission from one fluorophore leaves a spectrum caused only by the other, which then can be directly recorded. With more than two flurophores the problem is more complicated but a number of techniques for deconvoluting the complex emission curve have been developed making use of several modulation frequencies and measurement phase angles (79). 

In Fig. 3 the total corrected emission spectrum of MBC in ethylcellulose filmsisshown(curve A)... 

Figure 3. Total emission and phosphorescence spectrum of MBC (II) at —190°C, corrected for the sensitivity of the detecting system (%).

The emission spectrum of the irradiated PET yarn, when excited by 342 nm energy is totally dominated by the 460 nm emission, which has been attributed to the presence of mono-hydroxy-terephthalate, with only a shoulder as evidence of the residual fluorescence from the terephthalate units (Figure 9). 

A study of the wavelength or frequency of radiation absorbed or emitted by an atom or a molecule will give information about its identity and this technique is known as qualitative spectroscopy. This information is usually reported as the wavelength of radiation involved and is most easily represented as an absorption or emission spectrum (Figure 2.2). Measurement of the total amount of radiation will give information about the number of absorbing or emitting atoms or molecules and is called quantitative spectroscopy. 

The addition of one or several neutrons to a target nucleus leads to the formation of an isotope of greater mass. This can be unstable and decompose by ff emission (see the following equation). The total radioactivity generated in a sample containing many elements - each one having a family of isotopes - leads to a complex emission spectrum. 

The total transient Stokes shift (v(O)-v(oo)) observed in our time resolved experiments of coumarin in bulk water was 820 cm"1. In the case of C343 adsorbed on Z1O2 it is 340 cm 1. From measurements of the time-zero spectrum, i.e. the emission spectrum of C343 before solvent relaxation, Maroncelli et al. estimated the Stokes shift from solvation to be 1953 cm 1 for C343 in water [8]. Thus the time resolution of our experiments allows to observe about 42% of the total solvation process. Especially the very initial part, containing the inertial response is missed. 

---