Pump-probe Flash Techniques

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.

The double-pump flash photolysis technique has been used for two different applications the photogeneration of products different from those obtained with the pump and probe flash photolysis technique anda mapping of the potential surfaces of electronic excited states positioned above the lowest electronic excited state. For example, the flash irradiation of the metallophthalocyanine, M(pc)X (M = Rh(III), A1 (III) and X = C1 or Br) in the presence of Co(bpy)j+ produces the cyclic process described in Equations 6.60-6.62.28... 

Pump-probe technique A flash photolysis technique in which the light beam (probe) used for spectral analysis is generated from a portion of the excitation (pump) beam. A time delay in the latter allows the obtention of kinetic data. 

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. 

The best way to monitor this transient absorption change is the pump-probe technique (flash photolysis), where a short laser is used to excite the material and a second pulse (or lamp) is used to probe the change in the absorption spectrum. This technique has been used for decades in the field of photochemistry, but without being generally recognized as a third-order nonlinear optical process. 

In particular, in relation to the laser-flash photolysis and pump-probe techniques developed after 1960, the essence of this race is to generate ever shorter laser pulses for pumping a sample and well-controlled pulse delays for probing the sample s time evolution, where the lengths of the pulses define... 

We have previously pointed out that, under the appropriate conditions, the sigmoidally shaped fluorescence induction curves should also be observed when the PS II reaction centers are partially closed by short, pulsed light flashes and when the fluorescence yield is measured with a weak probe light flash delivered at some time 6t (30 - 100 ps) after the variable - intensity pump flash (3). This follows from the assumption that under either steady-state or flash-excitation conditions, the fraction of closed reaction centers q should depend simply on the number of photons absorbed by PS II In both cases. However, using pump flashes of less than 1 /is in duration, the fluorescence induction curves measured by the pump-probe technique have been shown to be exponential in shape [3.4]. Similar obsenrations have been made by Mauzerall and his co-workers [5.6] who concluded that the probability of escape of an exciton from a PS II unit with a closed reaction center to a unit with an open one. is less than 0.25 and that the apparent optical cross-section of PS II with open and closed traps is constant to within + 10 % [7]. The exponentiaiity of the pump-probe fluorescence Induction curves implies that the variable fluorescence Fy = (F[l ] - Fo)/(Fmax " Fq) is proportional to q under these conditions, where 1 represents the fiuence of the pump flash expressed in units of incident photons/cm. ... 

Typical controls obtained with either the pump-probe technique (the pump, or P>. flash was the dye laser), or steady-state illumination (He-Ne laser) are shown in Fig. 1 (top and bottom, respectively). 

Flash techniques were originally developed for the study of gas reactions [2,g] but were soon applied to solutions [2,h]. By the mid-60s, apparatus with a time-resolution of a few microseconds, using gas flash-lamps, had come into common use. With such equipment it was possible to identify transient species in solution from their spectra, and to determine their rates of decay and other processes. Excited states became recognised as distinct chemical species. The first study in which the spectra of the initial excited state, of the products and of some radical intermediates, were all detected in solution, and the kinetics investigated, was published in 1958 [2,k], Nanosecond pulses became available after the invention of the laser in 1960, but were not applied in flash photolysis until the problem of synchronising the analysing ( probe ) flash with the initiating ( pump ) flash was solved... 

Pump-probe techniques use similar pulses to excite the sample to create the chemistry and to observe the reaction (see figure 5). The observation pulse is delayed so that time can be varied (see figure). The technique can be used both for absorption and emission (up conversion). The time resolution is then only limited by the width of the pulse that does the excitation. Multiple experiments must be done because each experiment will only measure one time. The approach can be used both for flash photolysis and for pulse radiolysis. 

FIGURE 10 Schematic diagram illustrating layout of femtosecond flash photolysis technique. The pump and probe pulses are separated in time by adjusting the light path of the latter. Beams of molecules in the sample tube are excited or dissociated by the pump pulse, and the fragments monitored by the probe pulse on its path to the detector. Reproduced with permission from Scientific American (see Bibliography). 

The nanosecond techniques described above make possible a way of determining rates of reaction, known as the pump-and-probe technique [19], which is of exceptional interest for picosecond and sub-picosecond work. The optical absorbance of the sample is determined at a series of times after the initiating pump flash, by sending a weaker picosecond... 

Besides the successful adaptation of the pump-and-probe technique to ultrafast flashes, some remarkable new experimental arrangements were developed. These include the fol-... 

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