Phase-shift technique

At the early stages of development, the lack of a convenient and economic excitation modulation scheme has limited the use of such a phase shift technique in fluorescence thermometry. Now with the wide availability of cheap and easily modu-... 

An ingenious interferometric phase-shift technique for measuring short fluorescent lifetimes has been described by Carbone and Longaker (86), and is shown in detail in Fig. 18. 

An important development in the phase-shift technique is the use of a radiofrequency synthesizer as the driver for the Pockels cell modulator. In this way, the excitation beam can be modulated at any frequency between 1 and 200 MHz [137-139]. This approach allows use of cw lasers such as the He-Cd laser and even mode-locked lasers [139] as the excitation source. If d and M are measured at six to ten suitably spaced frequencies, least-squares curve-fitting techniques can be employed to obtain lifetimes with greatly enhanced precision. Typical data obtained by this multifrequency technique make measurement of decay times as short as 10 ps possible. Gratton and coworkers have developed other curve-fitting procedures to analyze data obtained on a multifrequency phase-shift fluorimeter. These experiments include the construction of time-resolved spectra [140], measurements of ro-... 

BOj (v = 40, J = 77). Lifetimes are measured by a phase-shift technique (the excitation radiation is amplitude modulated at 100 kHz) and the arrow points in the direction of increasing lifetime. Note that the difference vanishes at if = 0 and reverses between if > 0 and... 

For fluorescent compounds and for times in the range of a tenth of a nanosecond to a hundred microseconds, two very successful techniques have been used. One is the phase-shift technique. In this method the fluorescence is excited by light whose intensity is modulated sinusoidally at a frequency/ chosen so its period is not too different from the expected lifetime. The fluorescent light is then also modulated at the same frequency but with a time delay. If the fluorescence decays exponentially, its phase is shifted by an angle A( ) which is related to the mean life, x, of the excited state. The relationship is... 

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-... 

One advantage of the phase-shift technique is its ability to enhance selectively a particular lifetime component in a mixture of fluorophores directly, that is, without any need for data analysis. This provides a means for rather fast and direct data acquisition. A requirement is that the lifetimes be separated substantially. This is not so easily possible with the SPT technique, although some gating techniques have been used that achieve a simil2ir goal. An exact sep2iration of components is possible both in the SPT and in the phase-shift technique only after data analysis thus, the two techniques do not differ much in that respect. 

The laboratories of Lakowicz and Gratton in particular have developed phase-shift techniques and associated data analysis methods to considerable power in many respects its capabilities are now quite similar to those of the SPT technique. Nevertheless, the SPT technique is more widely applied, and research groups accustomed to working with pulsed lasers generally still prefer the SPT technique over the phase-shift technique. Apart from the fact that pulse techniques are more intuitive, this situation is also related to the fact that some instrumentation available in laboratories that use other pulsed spectroscopies can be also used for SPT, but only to a lesser extent for the phase-shift technique. At present commercial instrumentation is available for both kinds of techniques, and it is no longer necessary, as was the case until a couple of years ago, that researchers buy and assemble independent parts of the apparatus. Rather, complete and integrated instruments, including data analysis software, are now available commercially for both techniques. It thus becomes a practical choice to decide which instrumentation is more suitable for the particular task at hand. 

The phase-shifting technique is easy and useful to separate two electrode surface processes, whether faradaic or non-faradaic. Assume that has two components ... 

The second term now has only a real component. This means that the imaginary part of R ac solely contains the spectral information of the process ARaci in the first term in the right hand side of Eq. (40). This method is called the phase shifting technique [87, 88]. 

Fig. 2.26 Schematic representation of the principle of the phase-shifting technique by rotation of the complex plane coordinate.

An example of the use of the phase shifting technique is demonstrated in Fig. 2.27 [87]. The redox process of adsorbed cytochrome Cs, whose signal is much weaker than that of the simultaneously occurring solution redox process of methyl viologen, is successftiUy extracted at an Ag electrode. 

Using the results of the selfconsistent atomic-like calculation with the boundary conditions of the solid, we apply the phase-shift technique for the construction of pseudopotentials. By varying the screened pseudopotential, this technique minimizes a functional assembled from a set of conditions to be satisfied by the pseudopotential. The common condition requiring the continuous augmentation of the pseudo-wave-function at the cut-off radius Rc is transformed into a condition for the generalized phase-shift that depends on the energy monotonously, which ensures the numerical stability and gives the name to the technique. 

The images resulting from ESPI are then digitally processed. In this work, temporal phase shift techniques were used to assess the phase maps corresponding to each interferometric pattern. The minor mounted on a piezoelectric transducer (PZT) is used to... 

---