Detection system picosecond lasers

Ultrafast TRCD has also been measured in chemical systems by incoriDorating a PEM into the probe beam optics of a picosecond laser pump-probe absorjDtion apparatus [35]. The PEM resonant frequency is very low (1 kHz) in these experiments, compared with the characteristic frequencies of ultrafast processes and so does not interfere with the detection of ultrafast CD changes. 

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. 

Laser flash photolysis experiments48,51 are based on the formation of an excited state by a laser pulse. Time resolutions as short as picoseconds have been achieved, but with respect to studies on the dynamics of supramolecular systems most studies used systems with nanosecond resolution. Laser irradiation is orthogonal to the monitoring beam used to measure the absorption of the sample before and after the laser pulse, leading to measurements of absorbance differences (AA) vs. time. Most laser flash photolysis systems are suitable to measure lifetimes up to hundreds of microseconds. Longer lifetimes are in general not accessible because of instabilities in the lamp of the monitoring beam and the fact that the detection system has been optimized for nanosecond experiments. 

As described above, recent advances in accelerator technology have enabled the production of very short electron pulses for the study of radiation-induced reaction kinetics. Typically, digitizer-based optical absorbance or conductivity methods are used to follow reactions by pulse radiolysis (Chap. 4). However, the time resolution afforded by picosecond accelerators exceeds the capability of real-time detection systems based on photodetectors (photomultiplier tubes, photodiodes, biplanar phototubes, etc.) and high-bandwidth oscilloscopes (Fig. 8). Faster experiments use streak cameras or various methods that use optical delay to encode high temporal resolution, taking advantage of the picosecond-synchronized laser beams that are available in photocathode accelerator installations. 

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... 

The presence of a fast relaxing intermediate electron acceptor was first discovered when picosecond spectroscopy was applied to the study of photosynthetic systems. A laser flash-induced signal was detected when the primary quinone acceptor was prereduced [18,19], the lifetime of which was so short as to be observed only in the submicrosecond time range. The fast disappearance of the signal can be understood if it is considered that the rate of charge recombination in is very fast... 

The fluorescence detection system employed is shown in Figure 29.3. Both the IR and visible light ( 3 ps, 15 cm-1) generated by the picosecond laser system were introduced into a home-made laser fluorescence microscope [29, 30]. For the measurement of both solutions and fluorescent beads, both beams were adjusted onto a co-linear path by a beam-combiner and focused into the sample by an objective... 

Photocathode-based picosecond electron accelerators are conceptually simpler than pre-bunched thermionic systems, although they require reasonably powerful, multicomponent femtosecond or picosecond laser systems to drive the photocathode. In addition, the availability of synchronized laser pulses allows the development of advanced detection capabilities with unprecedented time resolution. The combination of ease of use and powerful detection methods has stimulated strong interest in photocathode gun systems. Since the installation ofthe first photocathode electron gun pulse radiolysis system at BNL [5,13], four additional photocathode-based facilities have become operational and two more are in progress. The operational centers include the ELYSE facility at the Universite de Paris-Sud XI in Orsay, France [7,8], NERL in Tokai-Mura, Japan [9,10], Osaka University [11,12], and Waseda University in Tokyo [13]. Facilities under development are located at the Technical University of Delft, the Netherlands, and the BARC in Mumbai, India. 

Picosecond spectroscopy enables one to observe ultrafast events in great detail as a reaction evolves. Most picosecond laser systems currently rely on optical multichannel detectors (OMCDs) as a means by which spectra of transient species and states are recorded and their formation and decay kinetics measured. In this paper, we describe some early optical detection methods used to obtain picosecond spectroscopic data. Also we present examples of the application of picosecond absorption and emission spectroscopy to such mechanistic problems as the photodissociation of haloaromatic compounds, the visual transduction process, and inter-molecular photoinitiated electron transfer. 

Picosecond spectroscopy provides a means of studying ultrafast events which occur in physical, chemical, and biological processes. Several types of laser systems are currently available which possess time resolution ranging from less than one picosecond to several picoseconds. These systems can be used to observe transient states and species involved in a reaction and to measure their formation and decay kinetics by means of picosecond absorption, emission and Raman spectroscopy. Technological advances in lasers and optical detection systems have permitted an increasing number of photochemical reactions to be studied in. greater detail than was previously possible. Several recent reviews (1-4) have been written which describe these picosecond laser systems and several applications of them... 

Using a mode-locked Nd + YAG laser system to generate picosecond sample excitation pulses and picosecond probing continuum pulses in their double beam spectrometer, Spalink et. al. (30) were able to measure difference absorption spectra of irradiated samples of 11-cis-rhodopsin and 9-cis-rhodopsin at selected times after excitation by means of a PAR OMA-2 optical multichannel detection system. The difference absorption spectral data were obtained over the entire spectral range from 410 nm to 650 nm at one time with an OMCD as opposed to the... 

FIGURE 4 - Schematic diagram of the optics and detection system employed in picosecond diffuse reflectance laser flash photolysis... 

The picosecond laser system was essentially as described in (2), except the detection apparatus consisted of a pair of monochromators couple to diode arrays. 

The picosecond laser system was essentially as described in (4), except the detection apparatus consisted of a pair of monochromators coupled to diode arrays. Laser excitation was in the 600-nm band. Low-temperature measurements were made on samples in polyvinyl alcohol films (PVA). ... 

The limiting time resolution of a laser T-Jump arrangement seems to be between 10 and lOO picoseconds. It is dictated by the optical damage threshold (1) of the sample, the state of the art of the very short laser pulse technique handling energies of more than 200 mJ (7), as well as the sensitivity and bandwidth of suitable detection systems. 

Fig. 2.2. Experimental setup (as used for the investigations on NasB). An argon ion laser (ps mode-locked fs all lines, visible) pumps either a femtosecond laser system (a) (OPO synchronously pumped optical parametric oscillator SHG second-harmonic generator) or a picosecond laser system (b) (taken from [178]). The pulse duration and spectral width of the laser pulses are measured by an autocorrelator (A) and a spectrometer (S) respectively. A Michelson arrangement allows the probe pulses to be delayed At) with respect to the pump pulses. A quadrupole mass filter (QMS) enables the selection of the ensemble of investigated molecules ionized by a pump probe cycle. A secondary electron multiplier (SEM) detects the intensity I of the ions as a function of the delay time At. A Langmuir-Taylor detector (LTD) measures the total intensity /o of the cluster beam. The ratio I/Iq as a function of the delay time At is called the real-time spectrum...

The time resolution of the detection system is limited by the pulse width of thb excitation source and is usually around a nanosecond although with very high powered picosecond lasers, much better time resolution is possible. The compounds that can be studied are those which have a strong absorption at a wavelength near a strong laser line and which don t fluoresce. 

The commercially available laser source is a mode-locked argon-ion laser synchronously pumping a cavity-dumped dye laser. This laser system produces tunable light pulses, each pulse with a time duration of about 10 picoseconds, and with pulse repetition rates up to 80 million laser pulses/second. The laser pulses are used to excite the sample under study and the resulting sample fluorescence is spectrally dispersed through a monochromator and detected by a fast photomultiplier tube (or in some cases a streak camera (h.)) ... 

The subpicosecond pulse radiolysis [74,77] detects the optical absorption of short-lived intermediates in the time region of subpicoseconds by using a so-called stroboscopic technique as described in Sec. 10.2.2 ( History of Picosecond and Subpicosecosecond Pulse Radiolysis ). The short-lived intermediates produced in a sample by an electron pulse are detected by measuring the optical absorption using a very short probe light (a femtosecond laser in our system). The time profile of the optical absorption can be obtained by changing the delay between the electron pulse and the probe light. 

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