Lifetimes pulse method

The fluorescent lifetime of chlorophyll in vivo was first measured in 1957, independently by Brody and Rabinowitch (62) using pulse methods, and by Dmitrievskyand co-workers (63) using phase modulation methods. Because the measured quantum yield was lower than that predicted from the measured lifetime, it was concluded that much of the chlorophyll molecule was non-fluorescent, suggesting that energy transfer mechanisms were the means of moving absorbed energy to reactive parts of the molecule. 

Pulsed method. Using a pulsed or modulated excitation light source instead of constant illumination allows investigation of the time dependence of emission polarization. In the case of pulsed excitation, the measured quantity is the time decay of fluorescent emission polarized parallel and perpendicular to the excitation plane of polarization. Emitted light polarized parallel to the excitation plane decays faster than the excited state lifetime because the molecule is rotating its emission dipole away from the polarization plane of measurement. Emitted light polarized perpendicular to the excitation plane decays more slowly because the emission dipole moment is rotating towards the plane of measurement. 

In contrast to the pulse method of determining subcriticality, which because of its labor-consumption and limited neutron tube lifetime is only used at considerable medium variations, the stationary method is used for constant and non-stop control of subcriticality. 

From the brief description of proposed pulse and stationary methods for SRP subcriticality calculation it is quite obvious that they require the introduction of calculation constants, i.e. the specified values of delayed neutron lifetime and effective fraction for the pulse method and the neutron source for the stationary one. Thus, it follows that for these experimental methods to be realized the SRP s parameters that characterize multiplication properties have to be calculated with a good accruacy. 

Where pulse equipment is unavailable or such experiments are impractical, other approaches can be adopted. Species with lifetimes down to about 1 ms or so can be observed in steady-state concentrations provided that a high rate of formation of the transient can be achieved. However, high formation rates may necessitate the use of flow conditions to maintain concentrations of starting materials. Species with longer lifetimes often can be detected for considerable periods of time in static systems. Whereas steady-state methods cannot provide the direct observations of chemical transformations that are possible with pulse methods, in many situations they can provide the same information in a less direct manner. 

Picosecond and nanosecond pulse methods have been used to measure the time-resolved absorption spectra of 9-nitroanthracene, 9-benzoyl-10-nitroanthracene, and 9-cyano-10-nitroanthracene. The long build-up time for triplet-triplet absorptions (72—86 ps) suggests that these do not represent the lifetimes of the singlet states but are the rates of internal conversion within the triplet manifold and that indirect intersystem crossing Si(n,n ) T (n,n ) - Ti(n,n ) is the most important process for populating Tj. 

The phenomena discussed above can be studied using various techniques, and not all methods are equally suitable in a particular case. Two principally different kinds of methods for measuring fluorescence lifetimes exist, namely, pulse methods and modulation or phase-shift methods. Phase-shift methods, despite the fact that they have been known for a longer time, have not found widespread use during the last decade. However, important technical advances have been made in phase-shift methods which in fact have inspired many researchers to apply them more frequently. Nevertheless, pulse methods are still the most widely used today, in particular for high time resolution. If carried out properly both types of methods must and will give the same result. Details of the measuring problem will determine which method is more appropriate in a particular case. 

In contrast to pulse methods described above, the phase-shift technique usually employs a continuous light source whose intensity is modulated by various means at some frequency /. The fluorescence response of the system is then also modulated at that frequency, albeit with some phase delay 0 and a reduced modulation depth m, as compared to the exciting light. "" From either of these quantities the fluorescence lifetime can be extracted. For a single-exponential decay the relationship between lifetime t, the modulation frequency /, phase shift 0, and the modulation depth m are given by tan(0) = /t and m = (1 -t-... 

The photoconductive decay (PCD) lifetime measuring technique was one of the first lifetime characterization methods to be used. [70] As the name implies it uses optical excitation of e-h pairs. The carrier decay is monitored as a function of time following the termination of the optical pulse. Traditionally the sample is provided with contacts and the current is measured as a function of time. More recently, non-contacting techniques have been developed that make the method attractive because it is fast and non-destructive. 

In the proposed book there is an emphasis cm luminescence lifetime, which is a measure of the transition probability and non-radiative relaxation from the emitting level. Luminescence in minerals is observed over a time interval of nanoseconds to milliseconds. It is therefore a characteristic and a unique property and no two luminescence emissions will have exactly the same decay time. The best way for a combination of the spectral and temporal nature of the emission can be determined by time-resolved spectra. Such techniques can often separate overlapping features, which have different origins and therefore different luminescence lifetimes. The method involves recording the intensity in a specific time window at a given delay after the excitation pulse where both delay and gate width have to be carefully chosen. The added value of the method is the energetic selectivity of a laser beam, which enables to combine time-resolved spectroscopy with powerful individual excitation. 

At still shorter time scales other techniques can be used to detenuiue excited-state lifetimes, but perhaps not as precisely. Streak cameras can be used to measure faster changes in light intensity. Probably the most iisellil teclmiques are pump-probe methods where one intense laser pulse is used to excite a sample and a weaker pulse, delayed by a known amount of time, is used to probe changes in absorption or other properties caused by the excitation. At short time scales the delay is readily adjusted by varying the path length travelled by the beams, letting the speed of light set the delay. 

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. 

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