Flash-photolysis

2 Flash Photolysis. Time-resolved spectroscopy techniques are a powerful means of studying materials, giving information about the nature of the excitations, energy transfer, molecular motion, and molecular environment, information that is not available from steady-state measurements. It is 

The different techniques of flash photolysis are used to detect transient species, that is atoms, molecules and fragments of molecules which have very short lifetimes. These cannot be observed by usual experimental techniques which require rather long observation times. For example, the measurement of an absorption or fluorescence spectrum takes several seconds, and this is of course far too long in the case of transient species which exist only for fractions of a second. In some cases these transient species can be stabilized through inclusion in low-temperature rigid matrices, a process known as matrix isolation . 

The reactants are premixed and reaction initiated by an intense flash of radiation lasting 1(T4 to 10-6 s. This disrupts reactant molecules into atoms and radicals in very high concentrations. The high concentrations of the intermediates produced make the subsequent kinetic analysis very much easier, and many of these reactions are often not picked up in low intensity-low concentration photochemical reactions. 

The first intense flash is followed by a series of secondary low intensity flashes which enable the identities and concentrations of the species to be found at various times, leading ultimately to rate constants for the reactions. Spectroscopic methods can easily measure intervals of 10-6 s, and lasers can take this down to 10 15 s. 

The initial flash must be sufficiently intense to give the high concentrations of intermediates required, but the higher the intensity of the flash the longer is its duration. This means that very fast reactions occurring in a time less than or equal to the duration of the flash will be over by the time the flash is over. A compromise must be made, and intensities are used so that the flash lasts 10-4 to 10-6 s, and half lives down to 10 6 s are easy to follow. The secondary flashes are for analysis only and are of low intensity, so that it is easy to have a flash lasting only 10-6 s. 

Flash photolysis greatly increases the number of intermediate reactions able to be studied. These are generally radical-radical reactions, in contrast to the radical-molecule reactions which predominate in low intensity photolysis. 

Question. Calculate the path-length differences which correspond to time intervals of 10-6, 10-7, 10-8, 10 9, 10 10, 10 n and 10 12 s between a photolysis and analytical flash. The velocity of light = 2.998 x 1010 cm s 1. Comment on the values. 

In flash spectroscopy a second spectroscopic flash is fired a short time after the photolysis flash and the transient absorption spectrum is registered on a photographic plate (Fig. 13). Repeating the experiments with different delay times gives complete information about the wavelength and the time behavior of the intermediate absorptions. 

In kinetic spectroscopy a continuous liglit source is used. The transient signal at a fixed wavelength is detected with a photomultiplier and displayed on a storage oscilloscope. Repeating the kinetic experiments at several wavelength positions again allows us to determine transient absorption spectra. 

Vary fast reactions, both in gaseous and liquid phases, can be studied by this method. In flash photolysis technique, a light flash of very high intensity and very short duration ( 10 6 sec) is produced in the neighborhood of the reaction vessel. This produces atoms, free radicals and excited species in the reaction system. These species undergo further reactions which can be followed by spectroscopic means. The method is also known as kinetic spectroscopy. The first order rate constant as large as 105 sec-1 and second order rate constants as large as 1011 mol dm sec-1 can be measured by this technique. 

Transient species, existing for periods of time of the order of a microsecond (lO s) or a nanosecond (10 s), may be produced by photolysis using far-ultraviolet radiation. Electronic spectroscopy is one of the most sensitive methods for detecting such species, whether they are produced in the solid, liquid or gas phase, but a special technique, that of flash photolysis devised by Norrish and Porter in 1949, is necessary. 

Pulsed lasers (Chapter 9) may be used both for photolysis and as a source. Since the pulses can be extremely short, of the order of a few picoseconds or less, species with comparably short lifetimes, such as an atom or molecule in a short-lived excited electronic state, may be investigated. 

While the steady-state measurements made under continuous illumination yield ratios of rate constants, the absolute values of the rate constants can be determined from a study of the transient phenomena (such as the decay of fluorescence or phosphorescence) that appear after the interruption of the incident radiation. Flash photolysis is an important method for studying transients in a photochemical system. 

Wecan treat the decay of the phosphorescence similarly. Instead of setting J[Ti]/rft = 0 as in Eq. (34.73), we write 

Since the initial concentration of is the steady-state concentration under illumination, we may use Eq. (34.73) and set /cfscLSilo = [TJoAp, so that this equation becomes 

Ordinarily Tpis many orders of magnitude smaller than Tp. Typically, 10 s, while Tp 1 to 10 s. This means that Tp — Tp Tp. It also means that after a few multiples of Tp (afew microseconds) have elasped, the term in exp (— t/Tp) has decayed to a negligible value and we are left with a simple exponential decay for the phosphorescence  

The existence of the nonzero lifetime, Tp, introduces a very slight deformation in the curve of lem versus t at very short times but does not affect the rate of decay at longer times. 

Since these values are of the same order of magnitude, Hammond assumed that the quenching was diffusion controlled with kg — 2 x 10 liters/mole-sec in (3.21). With this value for kg, the rates of the other processes can be determined and are shown in Table 3.1. As we shall see, these values agree well with those obtained by Linschitz using the technique of flash photolysis. 

The technique of flash photolysis was originally developed by Norrish and Porter as a method for studying reactive species such as triplets and radicals with relatively short lifetimes (t 1 x 10 sec). The beauty of this technique is that it involves the direct observation of the species of interest. The principal problem, however, is to determine the identity of the species causing the new electronic absorption. For th r efforts in the development of this technique Norrish and Porter, along with Eigen, received the Nobel Prize in chemistry in 1961. 

The absorption bands measured by the flash spectrographic method are often assigned by (a) comparison with known singlet-singlet absorption spectra, (b) comparison of the lifetime of the species responsible for the absorption with the phosphorescence lifetime, (c) comparison with calculated energies and intensities of the various possible absorptions by semi-empirical molecular orbital methods, and (d) comparison with published triplet absorption spectra and decay kinetics of model compounds. 

Under these conditions the time constant (or time for the flash intensity to decay to l/e of the initial value) is given by 

For a S-D resistance and capacitor charged to 2 F, the time constant would 

When a suitable wavelength has been found at which to study the reaction kinet- 

Recommended reviews are mentioned in the second paragraph below. 

Since its invention by Norrish and Porter in 1949,184 flash photolysis is the most important tool to produce transient intermediates in sufficient concentration for time-resolved spectroscopic detection and for the identification of elementary reaction steps (see Section 5.1). The term photolysis strictly implies the light-induced breaking of chemical bonds (the Greek expression lysis means dissolution or decomposition). However, since its inception, the term flash photolysis has been used to describe the technique of excitation by short pulses of light, irrespective of the processes that follow. 

Transient intermediates are most commonly observed by their absorption (transient absorption spectroscopy see ref. 185 for a compilation of absorption spectra of transient species). Various other methods for creating detectable amounts of reactive intermediates such as stopped flow, pulse radiolysis, temperature or pressure jump have been invented and novel, more informative, techniques for the detection and identification of reactive intermediates have been added, in particular EPR, IR and Raman spectroscopy (Section 3.8), mass spectrometry, electron microscopy and X-ray diffraction. The technique used for detection need not be fast, provided that the time of signal creation can be determined accurately (see Section 3.7.3). For example, the separation of ions in a mass spectrometer (time of flight) or electrons in an electron microscope may require microseconds or longer. Nevertheless, femtosecond time resolution has been achieved,186 187 because the ions or electrons are formed by a pulse of femtosecond duration (1 fs = 10 15 s). Several reports with recommended procedures for nanosecond flash photolysis,137,188-191 ultrafast electron diffraction and microscopy,192 crystallography193 and pump probe absorption spectroscopy194,195 are available and a general treatise on ultrafast intense laser chemistry is in preparation by IUPAC. 

Beware of artefacts This cautionary remark should always be kept in mind when carrying out flash photolysis. Stray light and fluorescence, acoustic shock waves and inhomogeneous transient distributions produced in the sample by focused laser pulses, extraneous electronic pulses from flash lamps or Q-switches, signal echoes and the like often distort the transient waveforms or spectra. 

Fast kinetic traces. A. A single kinetic trace fits the data well, shown as a line through the data. 

Two kinetic terms, a fast and slow component, are required to fit the data both are shown as lines. 

Femtochemistiy Direct Characterization of Transition States, Part I 

Throughout the 20th century it was a cornerstone of mechanistic analysis that we could never see a transition state. By definition, a transition state is not a stable point on a potential energy surface. Its lifetime is comparable to or less than that of a vibration, so how could we ever hope to see a transition state Well, it turns out that vibrational times are on the order of ps (10 s) convince yourself of this by converting a typical IR stretch (3000-1000 cm ) to a time. What if we could do transient spectroscopy on a time scale that is faster than ps, namely in the femtosecond (10 s) time domain What would we see  

Starting in the late 1980s, fs lasers became available, and it became possible to watch a reaction evolve on this time scale. What emerges is very much like a real-time movie of a chemical reaction, in which the transition state is just one frame of the film. Using pump-probe techniques with delays on the order of fs, we can watch as the 

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Figure Bl.16.15. TREPR spectmm after laser flash photolysis of 0.005 M DMPA (5) in toluene, (a) 0.7 ps, 203 K, RE power 10 mW O, lines CH (8), spacing 22.8 G , benzoyl (6), remaining lines due to (7). (b) 2.54 ps, 298 K, RE power 2 mW to avoid nutations, lines of 7 only. Reprinted from [42].

Work by Koga et aJ [62] demonstrates how the polarization iiiechanism can change upon alteration of the chemical enviromnent. Upon laser flash photolysis, excited xanthone abstracts a proton from an alcohol... 

Closs G L and Miller R J 1979 Laser flash photolysis with NMR detection. Microsecond time-resolved CIDNP separation of geminate and random-phase polarization J. Am. Chem. Soc. 101 1639—41... 

Miller R J and Closs G L 1981 Application of Fourier transform-NMR spectroscopy to submicrosecond time-resolved detection in laser flash photolysis experiments Rev. Sc/. Instrum. 52 1876-85... 

Porter G 1995 Flash photolysis into the femtosecond—a race against time Femtosecond Chemistry ed J Manz and L Woste (New York VCH) pp 3-13... 

Porter G and Topp M R 1968 Nanosecond flash photolysis and the absorption spectra of excited singlet states Nature 220 1228-9... 

Many experimental methods may be distinguished by whether and how they achieve time resolution—directly or indirectly. Indirect methods avoid the requirement for fast detection methods, either by detemiining relative rates from product yields or by transfonuing from the time axis to another coordinate, for example the distance or flow rate in flow tubes. Direct methods include (laser-) flash photolysis [27], pulse radiolysis [28]... 

The time resolution of these methods is detennined by the time it takes to mitiate the reaction, for example the mixing time in flow tubes or the laser pulse width in flash photolysis, and by the time resolution of the detection. Relatively... 

B2.5.4 FLASH PHOTOLYSIS WITH FLASH LAMPS AND LASERS... 

One of the most important teclmiques for the study of gas-phase reactions is flash photolysis [8, ]. A reaction is initiated by absorption of an intense light pulse, originally generated from flash lamps (duration a=lp.s). Nowadays these have frequently been replaced by pulsed laser sources, with the shortest pulses of the order of a few femtoseconds [22, 64]. 

B2.5.4.2 LASER FLASH PHOTOLYSIS AND PUMP-PROBE TECHNIQUES... 

Figure B2.5.8. Schematic representation of laser-flash photolysis using the pump-probe technique. The beam splitter BS splits the pulse coming from the laser into a pump and a probe pulse. The pump pulse initiates a reaction in the sample, while the probe beam is diverted by several mirrors M tluough a variable delay line.

This technique with very high frequency resolution was used to study the population of different hyperfme structure levels of the iodine atom produced by the IR-laser-flash photolysis of organic iodides tluough multiphoton excitation ... 

Figure B2.5.11. Schematic set-up of laser-flash photolysis for detecting reaction products with uncertainty-limited energy and time resolution. The excitation CO2 laser pulse LP (broken line) enters the cell from the left, the tunable cw laser beam CW-L (frill line) from the right. A filter cell FZ protects the detector D, which detennines the time-dependent absorbance, from scattered CO2 laser light. The pyroelectric detector PY measures the energy of the CO2 laser pulse and the photon drag detector PD its temporal profile. A complete description can be found in [109].

The conmron flash-lamp photolysis and often also laser-flash photolysis are based on photochemical processes that are initiated by the absorption of a photon, hv. The intensity of laser pulses can reach GW cm or even TW cm, where multiphoton processes become important. Figure B2.5.13 simnnarizes the different mechanisms of multiphoton excitation [75, 76, 112], The direct multiphoton absorption of mechanism (i) requires an odd number of photons to reach an excited atomic or molecular level in the case of strict electric dipole and parity selection rules [117],... 

Although modem laser teelmiques ean in prineiple aehieve mueh narrower energy distributions, optieal exeitation is frequently not a viable method for the preparation of exeited reaetive speeies. Therefore ehemieal aetivation—often eombined with (laser-) flash photolysis—still plays an important role in gas-phase kmeties, in partieular of unstable speeies sueh as radieals [ ]. Chemieal aetivation also plays an important role in energy-transfer studies (see chapter A3.13). 

A recent study of the vibrational-to-vibrational (V-V) energy transfer between highly-excited oxygen molecules and ozone combines laser-flash photolysis and chemical activation with detection by time-resolved LIF [ ]. Partial laser-flash photolysis at 532 mn of pure ozone in the Chappuis band produces translationally-... 

Guldi D M 1997 Capped fullerenes stabilization of water-soluble fullerene monomers as studied by flash photolysis and pulse radiolysis J. Phys. Chem. A 101 3895-900... 

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. 

How does one monitor a chemical reaction tliat occurs on a time scale faster tlian milliseconds The two approaches introduced above, relaxation spectroscopy and flash photolysis, are typically used for fast kinetic studies. Relaxation metliods may be applied to reactions in which finite amounts of botli reactants and products are present at final equilibrium. The time course of relaxation is monitored after application of a rapid perturbation to tire equilibrium mixture. An important feature of relaxation approaches to kinetic studies is that tire changes are always observed as first order kinetics (as long as tire perturbation is relatively small). This linearization of tire observed kinetics means... 

Lewis J W, Yee G G and Kliger D S 1987 Implementation of an optical multichannel analyzer controller for nanosecond flash photolysis measurements Rev. Sol. Instrum. 58 939-44... 

Figure 3.22 Principal components of an early flash photolysis absorption experiment...

Flash-age fixation Flash charges Flashing membranes Flash memories Flash photolysis Flash point... 

Fig. 3. Schematic of apparatuses for flash photolysis, (a) A simple instmment, and (b) a more sophisticated one utilising longitudinal excitation.

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