Picosecond beam technique

Alkylanilines are particularly attractive to study by the picosecond-beam technique for a number of reasons. First, the stimulating (time-integrated) experiments of Smalley and co-workers10e have revealed spectral broadenings for some of the species at particular energies, which broadenings, when analyzed by means of a kinetic model similar to the one discussed in Section II A, led them and others to the conclusion that very rapid IVR occurs in the molecules at these energies. In order to unequivocally establish that this is... 

The development of the picosecond-jet technique is presented. The applications of the technique to the studies of coherence (quantum beats), photodissociation, isomerization and partial solvation of molecules in supersonic-jet beams are detailed with emphasis on the role of intramolecular energy redistribution. Experimental evidence for intramolecular threshold effect for rates as a function of excess molecular energy is given and explained using simple theory for the redistribution of energy among certain modes. Comparison with R.R.K.M. calculation is also made to assess the nature of the statistical behaviour of the energy redistribution. 

Our own interest has focused on the development of the picosecond-jet technique to study the time-resolved dynamics following the picosecond excitation of isolated molecules in supersonic jet beams. This technique was applied to a number... 

The extensive data available on the catalytic reaction of CO at Pt surfaces supports mechanism (3). The residence time t of CO on Pt is of the order of milh-seconds, as measured by using molecular beam techniques. This value is many orders of magnitude higher than that of one would expect if an ER mechanism (either of type (1) or (2)) were responsible for the oxidation reaction. Clearly, if this had been the case, then residence time values of a few picoseconds would have been measured (as mentioned earlier, direct ER mechanisms usually occur within a single collision event). 

A second way to overcome the high reactivity of carbenes and so permit their direct observation is to conduct an experiment on a very short timescale. In the past five years this approach has been applied to a number of aromatic carbenes. These experiments rely on the rapid photochemical generation of the carbene with a short pulse of light (the pump beam), and the detection of the optical absorption (or emission) of the carbene with a probe beam. These pump-probe experiments can be performed on timescales ranging from picoseconds to milliseconds. They provide an important opportunity absent from the low temperature experiments, namely, the capability of studying chemical reactions of the carbene under normal conditions. Before proceeding to discuss the application of these techniques to aromatic carbenes, a few details illuminating the nature of the data obtained and the limitations of the experiment need to be introduced. 

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. 

Three different approaches have been employed to observe radiation-induced kinetics on the picosecond timescale. The first, pulse-probe detection, is covered in this section. Section 3.3 discusses a related technique that uses temporally-dispersed probe beams to record a kinetic trace in a single shot. Einally, Sec. 3.4... 

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... 

This technique utilizes a pulse pump-probe experiment and monitors the absorption of a weak probe beam in the presence of a strong pump beam. Fig. 8 depicts the experimental set-up for a two-beam pump-probe experiment, which includes homodyne and heterodyne Kerr gate measurements and polarization-controlled transient absorption measurement. Generally, the input beam is produced from an amplified pulse laser system with 1 KHz repetition rate, which can produce picosecond or femtosecond pulses. This pumping light beam is divided into two beams by a beam-splitter with an intensity ratio of 30 1 therefore, the one with the stronger intensity will act as the pump and the weaker one will be the probe. The position of the sample is where these two beams focus and overlap spatially. The time delay between the pulses from these two beams is controlled by a retroreflec-... 

Acridine has been studied by means of the echelon technique.(24,25) The changes in absorption (AA) were measured with a double beam picosecond spectrometer similar to the one shown in Figure 3. The difference absorption spectrum at one selected delay time after excitation was calculated according to... 

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