Picosecond laser techniques

Diphenylmethylene was the first carbene to be studied using fast, time-resolved spectroscopic methods (Closs and Rabinow, 1976). Since then both nanosecond and picosecond laser techniques have been used to probe this intermediate (Eisenthal et al., 1980, 1984 Hadel et al., 1984a,b Griller et al., 1984b Langan et al., 1984 Sitzmann et al., 1984). The results of these experiments are essentially undisputed, but the interpretation of them still remains somewhat controversial. 

By the late 1960s the development of mode locking (Chapter 1) allowed the study of picosecond laser techniques. Excited-state processes carried out in the picosecond domain allow such processes as intersystem crossing, energy transfer, electron transfer and many pho-toinduced unimolecular reactions to be investigated. 

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. 

On the fundamental side, the research on photocatalysis has focused on several topics, including a) the primary processes involved in the production and trapping of photogenerated electrons and holes, using pulsed femtosecond or picosecond laser techniques, b) measurements on the kinetics of the photodecomposition processes on longer time scales, and c) measurements on the kinetics on small size scales. For the first topic, the reader is referred to several recent publications.69-7 This work is of great practical importance, because it helps to point out the critical factors involved in the photocatalytic materials themselves. 

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. ... 

The LIF technique is extremely versatile. The determination of absolute intermediate species concentrations, however, needs either an independent calibration or knowledge of the fluorescence quantum yield, i.e., the ratio of radiative events (detectable fluorescence light) over the sum of all decay processes from the excited quantum state—including predissociation, col-lisional quenching, and energy transfer. This fraction may be quite small (some tenths of a percent, e.g., for the detection of the OH radical in a flame at ambient pressure) and will depend on the local flame composition, pressure, and temperature as well as on the excited electronic state and ro-vibronic level. Short-pulse techniques with picosecond lasers enable direct determination of the quantum yield [14] and permit study of the relevant energy transfer processes [17-20]. 

The primary photochemical reaction for nitromethane in the gas phase is well supported by experiments to be the dissociation of the C—N bond (equation 98). The picosecond laser-induced fluorescence technique has shown that the ground state NO2 radical is formed in <5 ps with a quantum yield of 0.7 in 264-nm photolysis of nitromethane at low pressure120. The quantum yield of NO2 varies little with wavelength, but the small yields of the excited state NO2 radical increase significantly at 238 nm. In a crossed laser-molecular beam study of nitromethane, it was found that excitation of nitromethane at 266 nm did not yield dissociation products under collision-free conditions121. 

These selective transitions (1), (7), and (9) may be achieved by proper optimization of the parameters eo and w, as described elsewhere [13, 18, 21]. Extensions to IR femtosecond/picosecond laser-pulse-induced dissociation or predissociation have been derived in Ref. 16, using either the direct or the indirect solutions of the Schrodinger equation (2) the latter requires extensions of the expansion (5) from bound to continuum states [16,31]. (The consistent derivation in Ref. 16 is based on S. Fliigge in Ref. 31). The same techniques can also be used for IR femtosecond/picosecond laser-pulse-induced isomerization as well as for more complex systems that are two dimensional, three dimensional, and so on, at the expense of increasing numerical efforts due to the higher dimensionality grid representations of the wavepackets f/(t) or the corresponding expansions (5) (see, e.g., Refs. 18, 20, and 21). 

In summary, we have combined state of the art optical multichannel analyzer techniques with well established low repetition rate picosecond laser technology to construct an instrument capable of measuring transient spectra with unprecedented reliability. It is, in its present form, a powerful tool for the investigation of ultrafast processes in biological, chemical, and physical systems. We foresee straightforward extension of the technique to the use of fourth harmonic excitation (at 265 nm) and also a future capability to study gaseous as well as condensed phase samples over a more extended spectral range. 

Therefore very narrow excitation pulse widths are necessary, for example, to measure sub-nanosecond relaxation times. A number of methods for generating picosecond laser pulses have been devised and several reviews of these techniques are available [10, 11]. 

With the advent of picosecond and subsequently femosecond laser techniques, it became possible to study increasingly fast chemical reactions, as well as related rapid solvent relaxation processes. In 1940, the famous Dutch physicist, Kramers [40], published an article on frictional effects on chemical reaction rates. Although the article was occasionally cited in chemical kinetic texts, it was largely ignored by chemists until about 1980. This neglect was perhaps due mostly to the absence or sparsity of experimental data to test the theory. Even computer simulation experiments for testing the theory were absent for most of the intervening period. 

A major breakthrough in the measurement of VER occurred in 1972. Laubereau et al. (32) used picosecond laser pulses to pump molecular vibrations via stimulated Raman scattering (SRS) and time-delayed incoherent anti-Stokes probing to study VER of C-H groups in ethanol and methanol ( " -3000 cm-1). Alfano and Shapiro (33) used the same technique to monitor both the decay of the initially excited (parent) C-H stretch excitation and the appearance and subsequent decay of a daughter vibration,... 

Kurita et al. 2 Takeda et al. 3 and Parthenopoulos and Rentzepis11 used picosecond laser photolysis techniques to study the photochromic processes of furyl fulgide. They found that the excited states of furyl fulgide and its colored form were singlet states and had n,n characteristics. Takeda et al.13 reported from theoretical studies that the values of the oscillator strength and the radiation lifetime (/r) were... 

The arrangement we used for interfacing the picosecond laser to the molecular beam (or free jet) is shown schematically in fig. 1. The laser is a synchronously pumped dye-laser system whose coherence width, time and pulse duration were characterized by the SHG autocorrelation technique. The pulse widths of these lasers are typically 1-2 ps, or 15 ps when a cavity dumper is used. For detection one of three techniques... 

Last year we extended the application of the picosecond-jet technique to the study of the dynamics of isolated molecules in various stages of solvation with various solvents (water, alcohol, etc). - The idea was to study this controlled solvation and its dependence on the energy redistribution. Also we wanted to examine the photodissociation of these different solvated species or complexes following selective pumping by the picosecond laser. The systems we studied in some detail are azine-solvent complexes made in the jet with He or Ar as the carrier gas. 

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... 

A simplified view of the early processes in electron solvation is given in Figure 7. Initially, electron pulse radiolysis was the main tool for the experimental study of the formation and dynamics of electrons in liquids (Chapter 2), first in the nanosecond time range in viscous alcohols [23], later in the picosecond time range [24,25]. Subsequently, laser techniques have achieved better time resolution than pulse radiolysis and femtosecond pump-probe laser experiments have led to observations of the electron solvation on the sub-picosecond to picosecond time scales. The pioneering studies of Migus et al. [26] in water showed that the solvation process is complete in a few hundreds of femtoseconds and hinted at the existence of short-lived precursors of the solvated electron, absorbing in the infrared spectral domain (Fig. 8). The electron solvation process could thus be depicted by sequential stepwise relaxation cascades, each of the successive considered species or... 

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