Laser apparatus

The double-beam phosphate-glass Nd laser apparatus used in these experiments has been described elsewhere (3,4). Experiments were of the pump-probe type done at a rate of one per minute using a 531 nm (8 ps FWHM, TEM, 1 mj ) pulse to initiate the photodissociation... 

Shafirovich et al. [71] have also investigated the migration of electrons photo-injected into DNA by two-photon excitation (335 nm) of pyrene derivatives covalently bound to ss and ds DNA. They find that in ds DNA the photoinjected electrons migrate to the acceptor methyl viologen (MV + Figure 6) which is boimd to the DNA within the c. 7-ns time resolution of the laser apparatus. From the dependence of the MV+ yield upon the MV + concentration, and the assumption of a random distribution of the pyrene donor and acceptor, they conclude that... 

Fig. 16. Nanosecond laser apparatus. F=filters PD=photodiode, PM photomultiplier.

FIGURE 1 Reactor pumped laser apparatus for use with a fast burst reactor (FBR). 

Figure 2b shows a laser pulse at about 1.8-/xm wavelength which is obtained with the apparatus shown in Fig. 1 when the CO laser mixture is replaced by a He/Ar mixture. The fact that lasing occurs over most of the excitation pulse indicates that laser pulse termination is not a property of the laser apparatus itself but results from differences between the two laser gas mixtures. 

Fast burst reactors (FBRs) have the advantage of pro-dncing high pump power in the RPL in a short pulse which does not overheat the laser medium. FBRs are also typically housed in laboratory facilities which allow convenient access to the volume arotmd the reactor for setup of laser apparatuses. However, FBRs have very low pulse repetition rates because of the need to cool a large compact mass of reactor fuel between pulses. 

Another relatively cotmnon research reactor, the pulsed TRIGA reactor, operates by pulsing from a very low initial power level using control rods. The pulsed TRIGA reactor can be pulsed more frequently because it typically is located at the bottom of a tank of water which acts as both coolant and shield. The neutron spectmm of a TRIGA reactor also has a much lower average energy, which reduces or eliminates the need for neutron-moderator material in the laser apparatus. 

Transient Infrared Absorption (TRISP) and laser-induced fluorescence. Because the CJ temperatures are only 2000-3000 K, most of the molecular products are in the ground electronic state. Emission spectroscopy looks selectively at only a few extraordinary molecules which are scarcely representative of most of the products. Infrared absorption, on the other hand is ideal for probing the vibrotational states of the ground state molecules, and the fast response time of TRISP makes it ideal for detonations. The technique has not been applied extensively and is difficult to implement, but our preliminary attempts have shown that we can do it with the proper laser apparatus. Broadband CARS is an alternative approach if the instrumental difficulties of TRISP cannot be overcome. 

Figure 1. Block diagram of the laser apparatus with spectrophotometric detection.

This binary system has been studied at four compositions under pressure. With the reservations concerning polymer concentrations, isobaric critical hnes can be constructed from the isopleths of Figure 12. The thus obtained critical hnes from 1 bar to 600 bar, respectively, show in Figure 13 on a T- plane(the could points at P=lbar were measured with laser apparatus at atmosphere). In Figure 13 the coexistence boundaries are described on a T-< plane at the indicated pressures (bar). The shape of the coexistence curve depends... 

Besides that, the laser apparatus provides a useful tool for the routine determination of particle size distribution of such materials. 

This work was supported by the Commission of the European Communities (contract no. ESD-013-DK(G) of the Solar Energy Programme), and by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organisation for the Advancement of Pure Research (ZWO). We are particularly indebted to Prof. D. von Wettstein and Dr. B. L. Miller for their valuable co-operation. The assistance of Arie van Hoek is gratefully acknowledged, who together with Dr. A.J.W.G. Visser developed the picosecond laser apparatus. 

These limitations have recently been eliminated using solid-state sources of femtosecond pulses. Most of the femtosecond dye laser teclmology that was in wide use in the late 1980s [11] has been rendered obsolete by tliree teclmical developments the self-mode-locked Ti-sapphire oscillator [23, 24, 25, 26 and 27], the chirped-pulse, solid-state amplifier (CPA) [28, 29, 30 and 31], and the non-collinearly pumped optical parametric amplifier (OPA) [32, 33 and 34]- Moreover, although a number of investigators still construct home-built systems with narrowly chosen capabilities, it is now possible to obtain versatile, nearly state-of-the-art apparatus of the type described below Ifom commercial sources. Just as home-built NMR spectrometers capable of multidimensional or solid-state spectroscopies were still being home built in the late 1970s and now are almost exclusively based on commercially prepared apparatus, it is reasonable to expect that ultrafast spectroscopy in the next decade will be conducted almost exclusively with apparatus ifom conmiercial sources based around entirely solid-state systems. 

An interferometric method was first used by Porter and Topp [1, 92] to perfonn a time-resolved absorption experiment with a -switched ruby laser in the 1960s. The nonlinear crystal in the autocorrelation apparatus shown in figure B2.T2 is replaced by an absorbing sample, and then tlie transmission of the variably delayed pulse of light is measured as a fiinction of the delay This approach is known today as a pump-probe experiment the first pulse to arrive at the sample transfers (pumps) molecules to an excited energy level and the delayed pulse probes the population (and, possibly, the coherence) so prepared as a fiinction of time. 

Figure B2.3.9. Schematic diagram of an apparatus for laser fluorescence detection of reaction products. The dye laser is syncln-onized to fire a short delay after the excimer laser pulse, which is used to generate one of the reagents photolytically.

Many optical studies have employed a quasi-static cell, through which the photolytic precursor of one of the reagents and the stable molecular reagent are slowly flowed. The reaction is then initiated by laser photolysis of the precursor, and the products are detected a short time after the photolysis event. To avoid collisional relaxation of the internal degrees of freedom of the product, the products must be detected in a shorter time when compared to the time between gas-kinetic collisions, that depends inversely upon the total pressure in the cell. In some cases, for example in case of the stable NO product from the H + NO2 reaction discussed in section B2.3.3.2. the products are not removed by collisions with the walls and may have long residence times in the apparatus. Study of such reactions are better carried out with pulsed introduction of the reagents into the cell or under crossed-beam conditions. 

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. 

Figure C3.3.4 shows a schematic diagram of an apparatus tliat can be used to study collisions of tlie type described above [5, 9,12,16]. Donor molecules in a 3 m long collision cell (a cylindrical tube) are excited along tlie axis of tlie cell by a short-pulse excimer laser (typically 25 ns pulse widtli operating at 248 mil), and batli molecules are probed along tliis same axis by an infrared diode laser (wavelengtli in tlie mid-infrared witli continuous light-output...

Schematic diagrams of modem experimental apparatus used for IR pump-probe by Payer and co-workers [50] and for IR-Raman experiments by Dlott and co-workers [39] are shown in figure C3.5.3. Ultrafast mid-IR pulse generation by optical parametric amplification (OPA) [71] will not discussed here. Single-colour IR pump-probe or vibrational echo experiments have been perfonned with OP As or free-electron lasers. Free-electron lasers use...

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