Measurement of lifetimes

The effect of predissociation on spectral features depends on whether it is the initial or final state of the transition that is predissociated. Predissociation can be detected either by direct measurements of lifetimes (r), linewidths (T), or level shifts (5E), or indirectly by observation of fragments. Table 7.2 surveys the range of predissociation rates sampled by different methods. Erman (1979) has reviewed the experimental methods for characterizing predissociation phenomena. 

Consider first predissociation of the initial state of a transition (i.e., the upper state in emission or the lower state in absorption). 

If there is only a barely observable weakening of the lines in emission, the nonradiative lifetime has the same order of magnitude as the radiative lifetime. For an allowed transition, the usual order of magnitude for rr is 10-8 s thus, a weakening of the emission can be seen if predissociation corresponds to rnr around 10-8 s, as for example in the predissociation of the Nj C2E+ state (see Table 7.3). If the transition is weak or nominally forbidden, much weaker predissociations can be detected. In the case of the L B3no+—X1E+ transition (AS = 1), with a lifetime of 10 6s, weak predissociations (rnr = 10-6 — 10 7s) have been observed. This also depends on the sensitivity of the technique. Br-zozowski, et al, (1976) have detected a very small decrease in the measured 

The mechanism of a predissociation may be characterized by measurements of the lifetime, rv, of each vibrational level or, even better, tvj, of every rotational level, carefully extrapolated to zero pressure. When the two unknown rates that appear in Eq. (7.4.6) have similar magnitudes, it is necessary to partition the observed total decay rate into tt and rnr. If the radiative lifetime is known for a non-predissociated level of the same electronic state, rT can be calculated for predissociated levels assuming an //-independent value for the electronic transition moment. The nonradiative lifetime is then deduced by subtraction of 1 /tt from the experimental 1/t value as follows  

To obtain the exact variation with v and J of rnr, the dependence of the electronic transition moment on R must be taken into account in the calculation of Tr. Note that the lifetime of a state generally varies very little with v. 

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A useftil applicadon of time-dependent PL is the assessment of the quality of thin III-V semiconductor alloy layers and interfaces, such as those used in the fabri-cadon of diode lasers. For example, at room temperature, a diode laser made with high-quality materials may show a slow decay of the acdve region PL over several ns, whereas in low-quality materials nonradiative centers (e.g., oxygen) at die cladding interface can rapidly deplete the free-carder population, resulting in much shorter decay times. Measurements of lifetime are significandy less dependent on external condidons than is the PL intensity. 

Rather sophisticated applications of Mossbauer spectroscopy have been developed for measurements of lifetimes. Adler et al. [37] determined the relaxation times for LS -HS fluctuation in a SCO compound by analysing the line shape of the Mossbauer spectra using a relaxation theory proposed by Blume [38]. A delayed coincidence technique was used to construct a special Mossbauer spectrometer for time-differential measurements as discussed in Chap. 19. 

Of great interest is the measurement of lifetime and intensity in mixtures of D20 and H20. Figure 48 shows the lifetime and intensity ratios for such mixtures in which Eu(N03)3 is dissolved. The results show that rD/rH and /d//h are directly proportional to the percentage of D20 present in the solvent. Kropp and Windsor give the following explanation. 

Non-steady state method13" The quenching experiments have also been carried out from measurements of lifetime by single photon time correlation technique. 

Fig. 11.5 Measurement of lifetime of anthracene in solution by single photon time correlation technique. Fluorescence decay curve of 8 X10-4 M anthracene in cyclohexane in the absence (A) and presence (B) of 0.41 M CC14. Points experimental data Line best fitting single exponential decay convoluted with instrumental response function (C) Time scale 0.322 nsec per channel. (Ref. 13).

Secondly, the emission of a particular complex species is characteristic of that species and can be used to identify the species present. Particularly is this so if excited state lifetimes are measured, as these vary dramatically depending on the number of OH groups coordinated to the Eu3+ or Tb3+ ion. This is because multiple excitation of the OH stretching mode provides an alternative deexcitation route. Measurement of lifetimes thus can be used to determine the number of coordinated water molecules.218... 

It should be noted with respect to method 1 that in most Rydberg excitation experiments to date pulsed lasers have been used even to obtain the highest resolution spectra, and this naturally precludes the measurement of lifetimes longer than -10 ns. 

Fig. 4.4 Schematic diagram of a typical arrangment for laser induced fluorescence measurements of lifetimes. A pulsed laser beam (or beams) passes through a heated glass cell containing alkali vapor and the time and wavelength resolved fluorescence is detected...

This methodology has been applied in many areas, such as the measurement of lifetimes of excited nuclear states and nuclear magnetic moments, the investigation of electric and magnetic fields in atoms and crystals, in the analysis of special relativity, the equivalence principle, and also in other applications [50-57],... 

From the above general discussion, it is obvious that the proposed mechanisms of the Curtius rearrangement must be regarded as tentative. In particular, measurements of lifetimes and rates of intersystem crossing of intermediate nitrenes, coupled with spectroscopic data on the available excited states of acyl azides, should resolve the problem. 

Because the direct measurement of lifetimes is easily performed nowadays, it is also useful to transform the Stern-Volmer relation into the form... 

In addition to measurements of lifetime of these vibrational excited states, time-resolved nonlinear IR could also give precise information on the mechanisms of deexcitation of these states. It could thus be shown that relaxation of the first excited state of modes of water molecules in liquid water was mainly due to resonance interactions of these modes with excited bending modes (65). As a result of the analysis of ID IR spectra shown above, Fermi resonance with bending modes allows the energy of the first excited state of to be transferred to the overtone of the bending band. It offers a fast relaxation path toward vibrational levels of a lower energy. Time-resolved nonlinear IR spectroscopy shows that this process is the main relaxation mechanism of and is at the origin of an unexpected increase of the relaxation time when temperature increases (66, 67). 

H. Measurement of Lifetimes using Rapid Passage Solutions of the... 

The electrogenerated chemiluminescence (ECl) of five l-amino-3-anthryl-9-propane derivatives has been studied in tetrahydrofuran. Emission from intramolecular exciplexes in ECl spectra and weak emission from the locally excited anthracene moiety were observed. The influence of triplet state interaction in ECl emission is discussed. The chemiluminescent decomposition of three a-peroxy-lactones gives CO2 and the corresponding ketone in high yield. The chemiluminescent species produced has been investigated in some detail by measurements of lifetime, energy-transfer activation parameters, and photochemical reactions. 

This method has certain important limitations. First of all, one obtains only the overall relaxation time t and not the relaxation time of individual vibrational levels. In many cases there is a single low-frequency vibrational mode that dominates the relaxation if not, however, one will obtain only an average relaxation rate for several vibrational states. A second limitation is the restricted frequency range available, which allows measurement of lifetimes only if they lie in the nanosecond time domain. Thus slowly relaxing liquids (such as Oj or N2) or rapidly relaxing ones (such as many larger molecules) cannot be studied. 

In another report of the measurement of lifetimes of humic materials (18) a similar constancy of t for samples of apparently widely different sources was also noted. 

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