Lifetime Measurements with Lasers

Measurements of lifetimes of excited atomic or molecular levels are of great interest for many problems in atomic, molecular, or astrophysics, as can be seen from the following three examples  

In a gas cell at the temperature T the mean relative velocity between collision partners A and B with masses Ma, Mb is 

Using the thermodynamic equation of state p = N kT, we can replace the density Ab in (6.54) by the pressure p and obtain the Stern-Vollmer equation  

It represents a straight line when 1/r is plotted versus p (Fig. 6.86). The slope tan O = bak yields the total quenching cross section ak and the intersect with the axis p = 0 gives the radiative lifetime (p = 0), 

In the following subsections we will discuss some experimental methods of lifetime measurements [794,795]. Nowadays lasers are generally used for the selective population of excited levels. In this case, the induced emission, which contributes to the depletion of the excited level, has to be taken into account if the exciting laser is not switched off during the fluorescence detection. The rate equation for the time-dependent population density of the level k), which gives the effective lifetime is then 

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Since lifetime measurements represent important contributions to the determination of absolute transition probabilities, several experimental techniques were developed before the invention of lasers. For a recent review, see [2.13]. The introduction of pulsed or mode-locked lasers has greatly enhanced the experimental capabilities. Because of their high intensity and the short pulse duration, the sensitivity and the time resolution have been greatly improved and lifetimes in the picosecond range can now be measured accurately. Some commonly used techniques of lifetime measurements with lasers will be discussed in Sect.11.2. 

The quantum yield and the lifetime are the more common properties measured in photodynamic experiments. Both properties are in reality only defined for excitation of a single state, but, since the MEs are so closely spaced in pyrazine, it is unavoidable that experiments have been reported for quantum yields and lifetimes measured with broad lasers, encompassing more than one ME, and in this section we will pay some attention to this problem. The treatment of... 

NaLS) by copper(II) yields assemblies in which Cu2+ ions constitute the counter ion atmosphere of the micelle (Fig. 4.8). These may be photoreduced to the monovalent state by suitable donor molecules incorporated in the micellar interior. An illustrative example is that where D = N,N -dimethyl 5,11-dihydroindolo 3,3-6 carbazole(DI). When dissolved in NaLS micelles, DI displays an intense fluorescence and the fluorescence lifetime measured by laser techniques is 144 ns. Introduction of Cu2+ as counterion atmosphere induces a 300 fold decrease in the fluorescence yield and lifetime of DI. The detailed laser analysis of this system showed that in Cu(LS) micelles there is an extremely rapid electron transfer from the excited singlet to the Cu2+ ions. This process occurs in less than a nanosecond and hence can compete efficiently with fluorescence and intersystem crossing165. This astonishing result must be attributed to a pronounced micellar enhancement of the rate of the transfer reaction. It is, of course, a consequence of the fact that within such a functional surfactant unit regions with extremely high local concentrations of Cu2+ prevail. (Theoretical estimates predict the counterion concentration in the micellar Stem layer to be between 3 and 6 M). 

Normal pulsed lasers, chopped cw laser radiation, or mode-locked dye lasers can be used for the generation of the light pulses. The output powers of the cw and mode locked lasers are rather low and they can be applied only in the visible or, after frequency doubling, in the UV spectral region [35]. They can be used for lifetime measurements with high accuracy [36] Only radiation from pulsed lasers can be transformed down to the VUV spectral region by using nonlinear effects. 

Fluorescence Lifetimes and Picosecond Dynamics. The fluorescence lifetime (ts) of fran.s -stilbenes at room temperature is rather short. A value of approximately lOOps may be estimated from radiative rate constant ( r) using ts = f//rr [295]. Direct lifetime measurements with picosecond laser pulses confirm this finding (Table 14). Important progress in understanding of the excited state dynamics was achieved in several laboratories [314-375], especially by Yoshihara [314 319], Hochstrasser [262, 320 335], Fleming [343-348], Troe [352 355], Zewail [361-364] and their co-workers. Saltiel and Sun [28] extensively discussed the literature con-... 

Fig.9.28. Lifetime measurement with the beam-laser technique using Doppler tuning [9.117]...

Fig.11.12a,b. Lifetime measurement with the phase shift method, (a) Experimen-tal arrangement and modulation of laser light and fluorescence, (b) analogous electrical circuit... 

Luminescence lifetime spectroscopy. In addition to the nanosecond lifetime measurements that are now rather routine, lifetime measurements on a femtosecond time scale are being attained with the intensity correlation method (124), which is an indirect technique for investigating the dynamics of excited states in the time frame of the laser pulse itself. The sample is excited with two laser pulse trains of equal amplitude and frequencies nl and n2 and the time-integrated luminescence at the difference frequency (nl - n2 ) is measured as a function of the relative pulse delay. Hochstrasser (125) has measured inertial motions of rotating molecules in condensed phases on time scales shorter than the collision time, allowing insight into relaxation processes following molecular collisions. 

The subsequent development of laser diode sources at low cost, and improved electronic detection, coupled with new probe fabrication techniques have now opened up this field to higher-temperature measurement. This has resulted in an alexandrite fluorescence lifetime based fiber optic thermometer system,(38) with a visible laser diode as the excitation source which has achieved a measurement repeatability of l°C over the region from room temperature to 700°C, using the lifetime measurement technique. 

The fact that several lasers can generate very short light pulses (down to 10 sec duration) with high peak powers (up to 10 watts) which can be used to investigate short term transitions (e. g. lifetime measurements, flash photolysis, etc.). 

Fig. 6. Experimental arrangement for lifetime measurements by the phase-shift method, using laser excitation. The laser beam is amplitude-modulated by a Pockel cell with analysing Nicol prism and a small part of the beam is reflected by a beam splitter B into a water cell, causing Rayleigh scattering. This Rayleigh-scattered light and the fluorescence light from the absorption cell are both focused onto the multiplier cathode PMl, where the difference in their modulation phases is detected. (From Baumgartner, G., Demtroder, W., Stock, M., ref. 122)).

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