Time-dependent emission spectra

Figure 10 Reconstructed time-dependent emission spectra of WS2 nanoparticles in octane. The times (in picoseconds) corresponding to each spectrum are also given. (From Ref. 68.)...

Time-dependent emission spectra from Wurtz-synthesized PMPS (Mw = 1.1 X 104), poly(methylpropylsilane) (PMPrS, Mw = 4.1 X 104) and... 

FIGURE 24. Time-dependent emission spectra of PMPS (a) 100 s irradiation and (b) 1000 s irradiation. (Reprinted from Ref. 19.)... 

Early simulation studies on solvation dynamics mostly focused on the behavior of small atomic or diatomic solutes in small molecule solvents [141,142], One of the most popular solvation probes is coumarin Cl53 (its scheme is given in Figure 11-16). Due to the fact that transition from ground to excited state in Cl53 is associated with a large molecular dipole moment increase, this has a measurable effect on polar solvents and leads to time-dependent emission spectra, shown in Figure 11-16. 

Switching from the imaging to the speetrally resolved mode of the Kerr-gated microscope reveals the time dependent emission spectrum of the nanobelts (Figure 7.5). In this case, for the sake of simplieity we placed only two nanobelts in the view of the microseope. The lumineseence intensity shown at the top... 

Under our experimental conditions, multifrequency data are collected at six equally spaced emission wavelengths spanning the entire emission spectrum. For this particular case, the time-dependent emission center of gravity is then given by (27) ... 

The spectra in Figure 5 have not been corrected for the wavelength dependent sensitivity of our apparatus in the "gated" mode. We discuss this subject in detail below. This correction, which should be identical for all time values, is, however, small as judged by a comparison of the non-time-resolved emission spectrum (Figure 6a) to the late time-gated spectrum which should be responsible for more than 99% of the non-time-resolved fluoresence (Figure 6b). 

Other Sulphides. The phase systems As283-Na28, Ag-Bi-8, ° and Ga-8b-8 have been studied. The thermal dissociation of C82 in C82-Ar gas mixtures has been investigated for gas pressures between 10 and 40 Torr and temperatures up to 2400 The time-resolved emission spectrum of gaseous C82, excited by an N2 laser, and its temperature dependence have been observed. 

Time-resolved emission spectra Although there have been several attempts to simplify the characterisation of the SR process, the determination of time-resolved emission spectra (TRES) is certainly the most general and most precise way to quantitatively describe the solvent response. The time-resolved emission spectra are usually determined by spectral reconstruction [96, 97, 106]. The time-resolved emission spectrum at a given time t is calculated from the wavelength dependent time-resolved decays by relative normalization to the steady-state spectrum [107]. By fitting the TRES at different times t by the empirical log-normal function, the emission maximum frequencies i (t) (or 2(t) see Fig. 6.26) and the total Stokes-shift Ac (or A2) are usually derived [106]. Since c(t) contains both information about the polarity (Ac) and the viscosity of the reported environment, the spectral shift c(t) may be normalized to the total shift Ac. The resulting correlation functions C(t) (Eq. (7)) describe the time course of the solvent response and allow for comparison of the SR-kinetic and, thus, of relative micro-viscosities, reported from environments of different polarities [96, 97, 106, 108, 109, 116, 117, 122]... 

For monitoring the relaxation process, we used two parameters of the time-resolved emission spectrum (a) the full width in the half-maximum (fwhm) at the time t after excitation, 5(t), and (b) the correlation function, C(f), corresponding to the normalized time-dependent Stokes shift [130] ... 

At high excitation fluence (2.4 mJ/cm2) the emission spectrum collapses into a narrow line located at 2.53 eV (see Fig. 8-2 a, dashed line), slightly blue-shifted with respect to the low intensity emission. The process strongly depends on the shape of the excitation area. For a rectangular spot, line narrowing occurs at 55 pj/cm2, i.e. 45 times a lower fluence than for a small circular spot, while the... 

Eckstein, J. W., et al. (1990). A time-dependent bacterial bioluminescence emission spectrum in an in vitro single turnover system energy transfer alone cannot account for the yellow emission of Vibrio fischeri Y-l. Proc. Natl. Acad. Sci. USA 87 1466-1470. 

Initial position of instant spectrum of fluorescence and character of spectral shifts in time depend on the excitation frequency, i.e., inhomogeneous broadening is of dynamic nature as a degree of broadening is maximal at the initial instants of time and decreases with time of emission registration (demonstration in panel d of Fig. 5). 

Different lanthanide metals also produce different emission spectrums and different intensities of luminescence at their emission maximums. Therefore, the relative sensitivity of time-resolved fluorescence also is dependent on the particular lanthanide element complexed in the chelate. The most popular metals along with the order of brightness for lanthanide chelate fluorescence are europium(III) > terbium(III) > samarium(III) > dysprosium(III). For instance, Huhtinen et al. (2005) found that lanthanide chelate nanoparticles used in the detection of human prostate antigen produced relative signals for detection using europium, terbium, samarium, and dysprosium of approximately 1.0 0.67 0.16 0.01, respectively. The emission... 

An alternative way to calculate the SLE spectrum is to expand the molecular density matrix to second order in the field and compute the time-dependent photon emission rate. The resulting expression is [23]... 

Unfortunately, the procedure just described to determine C(t) can consume many hours of spectrometer time, since several transients must be acquired and processed. Recently, an alternative timesaving procedure for measuring C(t) was developed [23,31], The procedure, which is approximate, requires a single emission transient and certain photophysical data on the probe. It is based on a simple photodynamic model, in which it is assumed that the spectrum of the probe is a simple function of a single solvent parameter, X, denoted as the solvent polarization. During the solvation process, X is time-dependent, such that C(t) = [X(t) — A (oo))]/ [X(0) - X(oo)]. 

One of the most spectacular observations in time-resolved emission spectroscopy is the rise and decay of molecular and excimer (or exciplex) spectra, illustrated in Figure 7.35(b). The structured molecular emission decreases immediately while the excimer emission increases up to a time of tens of ns, depending on the concentration. At longer times only the broad red-shifted excimer spectrum is observed. In Figure 7.35(b) the steady-state spectrum is shown in white this represents of course the integration of all the instantaneous spectra which can be obtained only through time-resolved spectroscopy. 

Comparison of emission spectra between 2100 A and 6500A has shown only small differences in relative concns of excited species between low-pressure diffusion flames and explns, whereas during explns peak intensities may be as much as 100 times greater. The time dependence of the free-radical emission during expln indicates the formation sequence to be OH, CH, C2, and evidence for the forbidden CO Cameron bands has been obtained. Similarly the ultraviolet absorption spectrum of the OH radical in acetylene— H2—02 detonations has been measured in conjunction with the associated rarefaction waves (Ref 7). Analysis of the absorption spectrum has indicated average rotational temps greater than 3000°K during the initial 310 microseconds... 

Fig. la c. Illustration of the time dependent theory of emission spectroscopy for one-dimensional harmonic potential energy surfaces, a schematic view of the emission transition, b time dependence of the overlap < (f> t) >, e calculated emission spectrum... 

The time-dependent theoretical treatment of the electronic absorption spectrum is very similar to that of the emission spectrum because the same two... 

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