Instrumentation ultrafast

Since there are a large number of different experimental laser and detection systems that can be used for time-resolved resonance Raman experiments, we shall only focus our attention here on two common types of methods that are typically used to investigate chemical reactions. We shall first describe typical nanosecond TR spectroscopy instrumentation that can obtain spectra of intermediates from several nanoseconds to millisecond time scales by employing electronic control of the pnmp and probe laser systems to vary the time-delay between the pnmp and probe pnlses. We then describe typical ultrafast TR spectroscopy instrumentation that can be used to examine intermediates from the picosecond to several nanosecond time scales by controlling the optical path length difference between the pump and probe laser pulses. In some reaction systems, it is useful to utilize both types of laser systems to study the chemical reaction and intermediates of interest from the picosecond to the microsecond or millisecond time-scales. 

Although very detailed, fundamental information is available from ultrafast TRIR methods, significant expertise in femtosecond/picosecond spectroscopy is required to conduct such experiments. TRIR spectroscopy on the nanosecond or slower timescale is a more straightforward experiment. Here, mainly two alternatives exist step-scan FTIR spectroscopy and conventional pump-probe dispersive TRIR spectroscopy, each with their own strengths and weaknesses. Commercial instruments for each of these approaches are currently available. 

Today, ultrafast pulsed-laser techniques, high-speed computers, and other sophisticated instrumentation make it possible to measure the time evolutions of reactants, intermediates, transition structures, and products following an abrupt photoactivation of a starting material. Detailed theoretical calculations, experienced judgments based on the literature, and newly accessible femtosecond-domain experimental data providing observed intensities of chemical species versus time can provide insights on the atomic-scale events responsible for overall reaction outcomes. 

The accuracy achieved through ab initio quantum mechanics and the capabilities of simulations to analyze structural elements and dynamical processes in every detail and separately from each other have not only made the simulations a valuable and sometimes indispensable basis for the interpretation of experimental studies of systems in solution, but also opened the access to hitherto unavailable data for solution processes, in particular those occurring on the picosecond and subpicosecond timescale. The possibility to visualize such ultrafast reaction dynamics appears another great advantage of simulations, as such visualizations let us keep in mind that chemistry is mostly determined by systems in continuous motion rather than by the static pictures we are used to from figures and textbooks. It can be stated, therefore, that modern simulation techniques have made computational chemistry not only a universal instrument of investigation, but in some aspects also a frontrunner in research. At least for solution chemistry this seems to be recognizable from the few examples presented here, as many of the data would not have been accessible with contemporary experimental methods. 

The dynamics of the interfacial electron-transfer between Dye 2 and TiOz were examined precisely by laser-induced ultrafast transient absorption spectroscopy. Durrant et al.38) employed subpicosecond transient absorption spectroscopy to study the rate of electron injection following optical excitation of Dye 2 adsorbed onto the surface of nanocrystalline Ti02 films. Detailed analysis indicates that the injection is at least biphasic, with ca. 50% occurring in <150 fsec (instrument response limited) and 50% in 1.2 0.2 psec. 

These methods work best at nanomolar chemical concentrations so that the focal volume contains typically 1 to 100 molecules on average. Because the method is so sensitive, it is susceptible to perturbation by background fluorescence and instrumentation fluctuations. These problems have become quite tractable during the last decade, such that FCS now supports more than 100 publications per year. A current challenging application is analysis of protein folding kinetics, protein structure fluctuations, and ultrafast chemical kinetics by new methods yet to be published. 

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. 

In this book, we have made an effort to provide an overall view of the emerging trends in radiation chemistry authored by experts in the field. The introductory chapter covers the history of radiation chemistry, underlining its achievements and issues that need to be addressed in future research. By renewing its research directions and capabilities in recent years, radiation chemistry research is poised to thrive because of its critical importance to today s upcoming technologies. Detailed accounts of fast and ultrafast pulse radiolysis instrumentation development and recent advances on ultrafast... 

In terms of mass spectrometry instrumentation, the currently available instruments such as time-of-flight (TOF) analyzers and hybrid quadrupole-TOF analyzers are able to acquire complete mass spectra at rates compatible with fast CE separations. As CE or ultrafast chromatography replaces conventional, slow HPEC applications, TOF-based mass spectrometers will be needed to replace the less efficient scanning types of instruments such as quadrupoles and ion traps for most high-throughput applications. FTICR mass spectrometry remains unsurpassed in terms of resolution and mass accuracy for both MS and MS-MS applications. However, the throughput of FTICR mass spectrometric... 

In this section, two operational instruments based on the principles discussed in the previous sections are described. One is useful for monitoring processes with reaction times in the 10 s to >10 s range—a so-called suprananosecond kinetic spectrometer. The other—an ultrafast kinetic spectrometer—is employed for reactions occurring in the range <10" s to >10 s. Both these spectrometers are in use in these authors laboratory for investigating photoinduced electron-transfer reactions, inter alia. The use of these homegrown instruments to illustrate this chapter is not... 

The majority of ultrafast instruments have employed electronic absorption spectrometry for determining the ultrafast dynamic properties of the systems of interest. However, in recent years some investigators have successfully turned their attention... 

In a series of papers, the group of Volmer [130-132] studied the analysis of azaspiracid biotoxins. Ultrafast and/or high-resolution LC of azaspiracids on monohthic LC columns was evaluated [130]. Chromatograms of five azaspiracids on a 100-mm and a 700-mm monolithic column are shown in Figure 14.11. Fragmentation of azaspiracids in MS-MS on ion-trap and triple-quadrupole instruments was studied as well [131]. The interpretation was confirmed using accurate-mass data from a Q-TOF instrument. Validation of a quantitative method for AZA-1 was also reported [132]. The LOQ was 5 and 50 pg/ml extract using a triple-quadrapole in SRM mode and an ion-trap instrument, respectively. 

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