Picosecond spectroscopy techniques

There is still a gap between our models of liquid-state reactions and the often bewildering complexity of real chemical systems. Progress in shortening the gap will probably come only from the application of a variety of methods to this problem. The full promise of picosecond spectroscopy techniques for studying the details of the dynamics of reactive events in liquids has yet to be realized. How deeply can these methods probe the dynamics Computer simulations, another source of experimental information in reacting systems, are only beginning to be exploited. "" The description by direct computer simulation of both primary and secondary recombination dynamics, for example, would yield a wealth of information that could be used to test theories. 

Time-resolved fluorescence spectroscopy is also a valuable tool in biological and medical research [10.143-147]. Since the lifetimes involved are normally short, picosecond spectroscopy techniques are frequently employed (Sect. 9.4). Examples of fluorescence decay curves for tissue recorded with delayed coincidence techniques employing a frequency-doubled picosecond dye laser are depicted in Fig. 10.42. The decay characteristics allow the discrimination between tumour and normal tissue, and atherosclerotic plaque and normal vessel wall, respectively. General surveys of the use of LIF for medical diagnostics can be found in [10.148,149]. 

To conclude our description of techniques, the use of nanosecond and picosecond spectroscopy which has been applied to excited state intramolecular proton transfer (ESIPT) will be mentioned briefly (Beens et al., 1965 Huppert et al., 1981 Hilinski and Rentzepis, 1983). A large number of inter-and intramolecular proton transfers have been studied using these methods (Ireland and Wyatt, 1976) but in the case of processes which are thought to involve simple proton transfer along an intramolecular hydrogen bond it is usually only possible to estimate a lower limit for the rate coefficient. 

With the photographic flash lamp the light pulse has a duration of several microseconds at best. The Q-switched pulsed laser provides pulses some thousand times faster, and the kinetic detection technique remains similar since photomultiplier tubes and oscilloscopes operate adequately on this time-scale. The situation is different with the spectrographic technique electronic delay units must be replaced by optical delay lines, a technique used mostly in picosecond spectroscopy. This is discussed in Chapter 8. 

The conventional flash photolysis setup to study photochemical reactions was drastically improved with the introduction of the pulsed laser in 1970 [17], Soon, nanosecond time resolution was achieved [13], However, the possibility to study processes faster than diffusion, happening in less than 10 10 s, was only attainable with picosecond spectroscopy. This technique has been applied since the 1980s as a routine method. There are reviews covering the special aspects of interest of their authors on this topic by Rentzepis [14a], Mataga [14b], Scaiano [18], and Peters [14c],... 

In order to appreciate more fully the Impact that video imaging systems have had on measurements In Picosecond Spectroscopy some data obtained with the above techniques will now be presented covering various areas of Interest In Physics, Chemistry,and Biology. 

In the weakly anharmonic molecular crystal the natural modes of vibration are collective, with each internal vibrational state of the molecules forming a band of elementary excitations called vibrons, in order to distinguish them from low-frequency lattice vibrations known as phonons. Unlike isolated impurities in matrices, vibrons may be studied by Raman spectroscopy, which has lead to the establishment of a large body of data. We will briefly attempt to summarize some of the salient experimental and theoretical results as an introduction to some new developments in this field, which have mainly been incited by picosecond coherent techniques. 

Experimental studies of ions association became feasible with the advent of relaxation techniques, picosecond spectroscopy, and ESR spectroscopy. Pioneering work in this field was done by Eigen, e.g.298), who studied the association of inorganic ions in aqueous solutions. Recent development of field-modulation technique335) allows for detailed investigations of ions association in low polarity media. This technique requires very low concentrations of ions in the investigated systems - a condition met by solutions involving low polarity solvents. Its application to systems such as tetrabutyl ammonium picrate in... 

The kinetics of the photoisomerization of bilirubin has been studied because of the relevance to phototherapy. The fluorescence of bilirubin increases on binding to human serum albumin. This and other primary photoprocesses have been investigated by picosecond spectroscopy. Karvaly has put forward a new photochemical mechanism for energy conversion in bacteriorhodopsin. An extensive review of the photophysics of light transduction in rhodopsin and bacteriorhodopsin has been made by Birge. The dynamics of cis-trans isomerization in rhodopsin has been analysed by INDO-CISD molecular orbital theory. Similar calculations on polyenes and cyanine dyes have also been reported. A new picosecond resonance Raman technique shows that a distorted... 

Automation In data acquisition and treatment can be aimed at a variety of objectives inherent in the above-mentioned factors. It should be pointed out that physical and physico-chemical kinetic factors play a decisive role in the reduction of human Intervention. The acquisition of data at a high rate imposed by the technique Itself (e.g. picosecond spectroscopy [19]) or by the system Investigated (e.g. meaurements of rates of reactions with half-lives of the order of a few milliseconds by the stopped-flow methodology [20,21]) demand the use of a computerized system without which application of the particular spectroscopic technique or method would not be feasible. On the other hand, the so-called microprocessor-controlled spectroscopy , widely commercialized at present, broadens the scope and facilitates the operator s work by eliminating various sources of error. 

Since the field of picosecond spectroscopy has been reviewed extensively in recent years, we will confine this section to a brief discussion of experimental studies of vibrational relaxation in liquids. Picosecond techniques allow the study of relaxation in larger molecules than can be studied by other methods. 

The picosecond kinetics of tetracene dianions have been studied using a new extension of picosecond spectroscopy methods.100 The rise times of the Stokes-shifted fluorescence from rhodamine B, rhodamine 6G, and erythrosine dissolved in water have been investigated using picosecond techniques. Figure 4 schematically indicates the situation following excitation. The best fit to the data corresponds to a relaxation time within the vibronic manifold of S of <1 ps.101 Although these fast spectroscopic techniques provide direct means of examining the behaviour of short-lived species, indirect methods are more convenient and often quite successful. Such is the case for the determination of rf from calculated radiative rate constants and measured Of values for a series of cyanine dyes.102... 

W. Zinth, M.C. Nuss, W. Kaiser, A picosecond Raman technique with resolution four times better than obtained by spontaneous Raman spectroscopy, in Picosecond Phenomena III,... 

The development of picosecond laser techniques has led to a renewed interest in the spectroscopy and kinetics of aromatic molecules which may undergo excited state intramolecular proton transfer (ESIPT, fig. 1). The ESIPT reaction is evidenced by a large Stokes shift for fluorescence from the proton-transferred molecule. ... 

W. Zinth, M.C. Nuss, W. Kaiser A picosecond Raman technique with resolution four times better than obtained by spontaneous Raman spectroscopy . In Picosecond Phenomena HI, ed. by K.B. Eisenthal, R.M. Hochstrasser, W. Kaiser, A. Laubereau, Springer Ser. Chem. Phys., Vol.38 (Springer, Berlin, Heidelberg 1982) p.279... 

Light is an ideal source of energy to drive artificial molecular machines. First, it can be used to monitor the state of the machine using various spectroscopy techniques. Second, the response is very fast, even on nano- or picosecond scales. Third, photo-driven processes in such systems are usually reversible, resulting in photo-driven molecular machines having an autonomous operation that is very important for the construction of molecular devices. And last, photo-driven molecnlar machines are clean and convenient compared with chemically driven machines and... 

In sub-picosecond photoexdtation spectroscopy (or time-resolved excited spectroscopy) of the conjugated polymers, one can study the excited-state spectra (absorption, emission, and stimulated emission) and the time decay of the excited-state spectral features. For nonlinear excitations in conjugated polymers, this spectroscopy technique is useful, since their photogeneration processes usually occur in the sub-nanosecond time domain. 

In this section attention is focused on the vibrational aspects of the phenomena that can be probed at low temperatures by simple pump and probe" techniques, even without using very fast (femtosecond or picosecond) spectroscopy. The concepts developed for trans-PA can also be used for understanding of the photophysics of other polyconjugated materials. The main interest is to show that, from the viewpoint of infrared spectroscopy, photoexcitation corresponds to some kind of (clean) photodoping that introduces a displacement of charges (charge carrier) within domains of the chain, thus giving rise to photoinduced infrared (PIIR) spectra. It is then expected that DIIR and PIIR spectra may show common features, as was indeed found in photoexcited PA and in many other systems [41]. 

Optoelectronic detection systems such as fast photodiodes and sampling oscilloscopes have reached a time resolution of lO" s. However, this is still not sufficient to resolve many fast transient events on a picosecond time scale. In picosecond spectroscopy, therefore, new techniques had to be invented to measure durations and profiles of picosecond pulses and to probe ultrafast relaxation processes. 

The first laser Raman spectra were inherently time-resolved (although no dynamical processes were actually studied) by virtue of the pulsed excitation source (ruby laser) and the simultaneous detection of all Raman frequencies by photographic spectroscopy. The advent of the scanning double monochromator, while a great advance for c.w. spectroscopy, spelled the temporary end of time resolution in Raman spectroscopy. The time-resolved techniques began to be revitalized in 1968 when Bridoux and Delhaye (16) adapted television detectors (analogous to, but faster, more convenient, and more sensitive than, photographic film) to Raman spectroscopy. The advent of the resonance Raman effect provided the sensitivity required to detect the Raman spectra of intrinsically dilute, short-lived chemical species. The development of time-resolved resonance Raman (TR ) techniques (17) in our laboratories and by others (18) has led to the routine TR observation of nanosecond-lived transients (19) and isolated observations of picosecond-timescale events by TR (20-22). A specific example of a TR study will be discussed in a later section. 

Picosecond pedestal, 143 Pin-hole camera, 128 Plasma channels, 112, 147, 148 Plasma defocusing, 84, 91 Plasma frequency, 166 Plasma index of refraction, 147 Plasma mirror (PM) technique, 194 Plasma wakefield acceleration, 172 Plasma wavelength, 166 Plasma-induced effects, 83 Polarization, 97 Polarization control, 87 Ponderomotive force, 170 Population inversions, 19 Post-irradiation spectroscopy, 156 Pre-pulse, 143 Propagation, 81 Protein, 102 Pump depletion, 151... 

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