Picosecond Flash Techniques

These discussions provide an explanation for the fact that fluorescence emission is normally observed from the zero vibrational level of the first excited state of a molecule (Kasha s rule). The photochemical behaviour of polyatomic molecules is almost always decided by the chemical properties of their first excited state. Azulenes and substituted azulenes are some important exceptions to this rule observed so far. The fluorescence from azulene originates from S2 state and is the mirror image of S2 S0 transition in absorption. It appears that in this molecule, S1 - S0 absorption energy is lost in a time less than the fluorescence lifetime, whereas certain restrictions are imposed for S2 -> S0 nonradiative transitions. In azulene, the energy gap AE, between S2 and St is large compared with that between S2 and S0. The small value of AE facilitates radiationless conversion from 5, but that from S2 cannot compete with fluorescence emission. Recently, more sensitive measurement techniques such as picosecond flash fluorimetry have led to the observation of S - - S0 fluorescence also. The emission is extremely weak. Higher energy states of some other molecules have been observed to emit very weak fluorescence. The effect is controlled by the relative rate constants of the photophysical processes. 

In an entirely different experimental approach the unsymmetrical mixed-valence ion shown in equation (76) was subjected to laser flash photolysis.100 Excitation was carried out into the MLCT absorption band of the Ru11 -> 7t (pz) chromophore. Following excitation, one of the deactivation channels leads to the unstable mixed-valence isomer and its subsequent relaxation to the final, stable oxidation state distribution was observed directly using picosecond laser techniques. 

Photodissociation of CO ( flash ) from the reduced enzyme after mixing with O2 ( flow ) in the dark has been the main method of initiating the reaction. Some concern has been voiced as to the possibility that this technique might introduce artifacts due to CO, but results using a rapid O2 mixing system without CO have coirfirmed the applicabihty of the flow/flash technique. Yet, it has been shown by infrared spectroscopy that upon photolysis, the heme 03-bound CO is first transferred in less than a picosecond to the nearby Cub, to which it remains bound for about a microsecond at room temperature before diffusing out of the enzyme, as also verified by EXAFS data. ... 

It is clear from the first report of the application of picosecond flash photolysis to the study of porphyrin photophysics that this technique will become increasingly important.349 Thus it has been found possible to record the absorption spectra of both the singlet state (r 500 ps) and triplet states of (OEP)SnCl2. Further, could be measured, and the value differed from that found by previous workers. For (OEP)Pd both singlet and triplet states could be observed, whereas only one species was found for the paramagnetic (OEP)Cu and (TPP)Cu complexes. Nanosecond laser flash photolysis has been used in the study of the transients formed from Cu11 and Pbn porphyrins.360 No observable intermediates were detected for the Agn, Nin, and Co111 derivatives. Triplet-triplet absorption spectra of a series of porphyrins and their Zn complexes have been recorded.361... 

Figure 7.1 Dissociation of ICN in the gas phase by ultrafast flash technique. Transients showing change of signal during the first picosecond after excitation of ICN in the gas phase by ultrafast flashes at a series of pump-pulse energies (wavelength range ca. 3890 to 3905 A). See text. From Ref. [2,e].

Chapter 6 described the current techniques employed in time-resolved fluorescence spectrocopy. The time resolution of these techniques ranges from a few picoseconds (streak cameras) to a few hundreds of picoseconds (single-photon timing with flash lamp excitation). The time resolution can be greatly improved by using the fluorescence up-conversion technique. 

As better and better methods for following fast reactions with precision were introduced and exploited, characteristic reaction times faster than a second— times measured in milhseconds (ms, 10 s), or microseconds (ps, 10 s), or nanoseconds (ns, 10 s) and then in picoseconds (ps, 10 s)—were measured through stopped-flow techniques (Chance, 1940), flash photolysis (Norrish and Porter, 1949), temperature-jump and related relaxation methods (Eigen, 1954), and then... 

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. 

More recently, powerful time-resolving techniques began to evolve. Nanosecond [13] and picosecond [14] flash absorption and emission spectroscopy made it possible to obtain UV spectra of transient species with very short lifetimes. 

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],... 

The current detailed understanding of photo-induced electron transfer processes has been advanced dramatically by the development of modern spectroscopic methods. For example, the application of time-resolved optical spectroscopy has developed from modest beginnings (flash-phyotolysis with millisecond resolution) [108,109] to the current state of the art, where laser spectroscopy with nanosecond resolution [110-113] must be considered routine, and where picosecond [114-116] or even femtosecond resolution [117] is no longer uncommon. Other spectroscopic techniques that have been applied to the study of electron transfer processes include time-resolved Raman spectroscopy [118], (time resolved) electron spin... 

Ultrafast techniques have really come into their own since the development over the past decade of reliable, relatively inexpensive femtosecond lasers, but in the 1980 s, ultrafast still meant picoseconds, and the chosen paper typifies work in this time-domain. The whole subject of course owes its inspiration to George Porter, who with Norrish pioneered flash photolysis, first in the milli-, then micro-, and ultimately nano- and picosecond time-domains. 

Nanosecond and picosecond laser-flash photolysis techniques have been used by different groups to elucidate the various intermediate stages involved in the photo-induced reactions of amines. The overall mechanism involving the electron-transfer process in a fluid medium is illustrated in Scheme 13. The dynamics of the process involve the formation of an encoimter complex between the excited-state molecule and the ground-state molecule [117, 140, 141]. The encounter complex can be described as an intermoleculai ensemble of excited- and ground-state molecules, separated by a small distance (ca 7 A) and surroimded by solvent molecules. During... 

Future progress may, however, be accelerated by recent and forthcoming developments. Recent flash photolysis studies of metal-carbonyls and metal-metal bonded systems should provide a foundation for more extensive development of this area. Applications of picosecond and nanosecond flash systems to a wide variety of problems is expected. Such techniques should be... 

---