Flash Photolysis-Principles

The flash lamp teclmology first used to photolyse samples has since been superseded by successive generations of increasingly faster pulsed laser teclmologies, leading to a time resolution for optical perturbation metliods tliat now extends to femtoseconds. This time scale approaches tlie ultimate limit on time resolution (At) available to flash photolysis studies, tlie limit imposed by chemical bond energies (AA) tlirough tlie uncertainty principle, AAAt > 2/j. 

As with solution experiments, flash photolysis in the gas phase has produced evidence for the existence of intermediates but no information about their structure. In principle gas phase IR spectra can provide much more information, although the small rotational B value of gaseous carbonyls and low lying vibrational excited states preclude the observation of rotational fine structure. As described in Section II, time-resolved IR experiments in the gas phase do not suffer from problems of solvent absorption, but they do require very fast detection systems. This requirement arises because gas-kinetic reactions in the gas phase are usually one... 

Carbon Suboxide Photoiysis. In principle, carbon suboxide (1) can be used as a precursor to atomic carbon and two molecules of carbon monoxide as shown in Eq. 2. However, this reaction is endothermic by 141 kcal/mol and can only be realized in the vacuum ultraviolet (UV) at wavelengths that destroy most organic substrates. However, photolysis of 1 at 1470 A produces C atoms in a low-temperature matrix. The short wavelength flash photolysis of 1 coupled with atomic absorption has been used to measure the rate constants for various spin states of carbon with simple substrates. 

The principles of electronic spectroscopy have been discussed by Herzberg (1950) for diatomic molecules, and in a classic review by Sponer and Teller (1941) for the more general case of polyatomic structures. Recent developments are described in articles appearing regularly in the Annual Reviews of Physical Chemistry. Triatomic molecules and radicals have been intensively studied, the latter by the powerful method of flash photolysis (Herzberg, 1959). As triatomic structures have been comprehensively reviewed recently (Ramsay, 1962) we include in this article only those triatomic systems that are of particular interest in organic chemistry. Otherwise attention will be directed to molecules of four or more atoms, including all known representatives of the important chromophores. 

Time-resolved UV/vis absorption spectroscopy has been initiated by Norrish and Porter who developed flash photolysis in the late 1940s, opening the way to the detection of transient chemical species with time resolution of a few microseconds [30, 31]. The present state of art transient absorption techniques allow detection of chemical intermediates with less than 10 fs resolution. The techniques used depend on the explored time scale but the principle, which is illustrated in Fig. 7.14, is the same. 

In principle, the application of time-resolved techniques permits identification of intermediates by monitoring their progress to the stable products of reaction. In 1973, Lehman and Berry [25] reported the first application of time-resolved photochemical methods to the study of aryl azides. Using conventional flash photolysis, they irradiated 2-azidobiphenyl in cyclohexane solution. Time-resolved absorption spectroscopy revealed an intermediate assigned as the triplet nitrene primarily on the basis of the similarity of its spectrum to that measured by Reiser [18] in low-temperature experiments. Lehman [25] monitored the rate of carbazole formation and found it to occur by a kinetically first-order process with a lifetime of 460 /is at room temperature. These findings led them to conclude that photolysis of 2-azidobiphenyl at room temperature leads rapidly to the triplet nitrene, and that this species is the precursor to carbazole [25], However, this point of view clearly is at odds with Swenton s triplet sensitization experiments [23],... 

Experiments carried out at low temperature are complimented by flash photolysis studies performed at room temperature. At low temperature, particularly in rigid media, reactive intermediates are stabilized because the rates of their unimolecular reactions are slowed, and bimolecular reactions are prevented by inhibition of diffusion. As we have just seen, this increased stability enables the application of a variety of spectroscopic methods which can aid in the determination of the structure of the intermediates. Flash photolysis experiments permit the study of absolute reactivity. These experiments can be carried out in the very short time scale required to monitor progress of reactive intermediates to stable products. In principle, the dual approach should permit thorough characterization low temperature methods reveal structure, flash photolysis probes reactivity. In practice, and particularly for the case of the aryl azides, complications can arise when the... 

The principle of time-resolved gain spectroscopy was first applied to a molecular chemical laser by L. Henry and coworkers 119>. The HC1 laser from the flash photolysis of an H2/CI2 mixture was chosen for this study. Initial vibrational population figures have been obtained and rate constants derived for the vibrational deactivation, as given in Table 17. 

Although modem laser techniques can in principle achieve much narrower energy distributions, optical excitation is frequently not a viable method for the preparation of excited reactive species. Therefore chemical activation—often combined with (laser-) flash photolysis—still plays an important role in gas-phase kinetics, in particular of unstable species such as radicals [88]- Chemical activation also plays an important role in energy-transfer studies (see chapter A3.131. 

CiSj. Photodissociation of CS2 at wavelengths X > 185 nm could lead to formation of both SCF) and S( D). The principle absorption system 200 nm populates the Sj( yl ) electronic state, which lies below the lowest state (in contrast to all the other molecules in this group apart from CSe2), and in consequence it correlates with CS(AT S" ") + S( D) (see Fig. 3.5.10). Flash photolysis of CS at X > 185 nm produced vibrationally excited CS(2f 2 ) and S( P) as the only detectable transients in absorption, but the detection of S( D) would have been prevented through rapid quenching by the diluent gas, N2. At the pressures employed, S( D) atoms would decay within a few nanoseconds, since the rate coefficient for quenching by N2 is 1.5 X 10 cm molecule" s ... 

---