Lasers experiments

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

Figure B2.5.18 compares this inter molecular selectivity with intra molecular or mode selectivity. In an IR plus UV, two-photon process, it is possible to break either of the two bonds selectively in the same ITOD molecule. Depending on whether the OFI or the OD stretching vibration is excited, the products are either IT -t OD or FIO + D [24]- hr large molecules, mirmnolecular selectivity competes with fast miramolecular (i.e. unimolecular) vibrational energy redistribution (IVR) processes, which destroy the selectivity. In laser experiments with D-difluorobutane [82], it was estimated that, in spite of frequency selective excitation of the...

An example of a fuel layer model is discussed in the next section. In a first approximation no detailed description of the release of volatiles need to be included. Later, the results of the tunable diode laser experiments can be implemented in such a fuel layer model. 

This number is based on the observation that in the reaction with SO], k(observed) for formation of optical density at 340 nm (where the OH adduct of benzonitrile absorbs) is proportional to [benzonitrile] up to the saturation limit ( a 20 mM, k(observed) = 5 x 10 s" ), as shown by 248 nm laser experiments... 

Time-resolved laser experiments make possible the detection of species with short lifetimes [1-6]. Usually, such experiments are carried out under normal conditions and a quick enough snapshot can capture the intermediate in action. A different philosophy is to prolong the lifetime of the reactive species by generating it in an inert environment at low temperature. Matrix isolation... 

The mechanism of the cycloaddition appears to be concerted for various reagents however, for several cases, radical cation cycloaddition-cycloreversions have a stepwise component. For example, CIDNP effects observed during the PET induced dimerization of spiro[2.4]heptadiene (97) identify a dimer radical cation with spin density only on two carbons of the dienophile fragment this intermediate must be a doubly linked radical cation ( 99 + 282,283 pulsed laser experiment at high concentrations of 97 supports a second dimer radical cation at high... 

Numerous examples of hydrogen abstraction by rcrt-butoxyl radicals have been reported. The unusual choice of melatonin is not accidental note that the radical has simply been identified as (melatonin) and seems to emphasize that the site of reaction (in fact, rather well established in this case) may not necessarily be derived from the laser experiment. The laser technique observes all sites and forms of reaction regardless of which species is monitored and how well characterized it may be (see below). The trace of Figure 18.7 shows the formation of the melatonin radical following hydrogen transfer to ferf-butoxyl. ... 

The electronic structure of fluorenes and the development of their linear and nonlinear optical structure-property relationships have been the subject of intense investigation [20-22,25,30,31]. Important parameters that determine optical properties of the molecules are the magnitude and alignment of the electronic transition dipole moments [30,31]. These parameters can be obtained from ESA and absorption anisotropy spectra [32,33] using the same pump-probe laser techniques described above (see Fig. 9). A comprehensive theoretical analysis of a two beam (piunp and probe) laser experiment was performed [34], where a general case of induced saturated absorption anisotropy was considered. From this work, measurement of the absorption anisotropy of molecules in an isotropic ensemble facilitates the determination of the angle between the So Si (pump) and Si S (probe) transitions. The excited state absorption anisotropy, rabs> is expressed as [13] ... 

Raman was not the only type of laser experiment based on scattering. Several experiments were developed that exploited detection and analysis of scattered light. These included the use of light scattering to study critical phenomena (36), to probe surfaces (37), and to monitor growth of nanoparticles... 

A fascinating category of experiments can be found in Table IV. These are the use of lasers to determine thermodynamic parameters. These include calorimetry (43), enthalpies of vaporization and vaporization rates (44, 45), and heat capacities (46). Other laser experiments that can be found in Table IV include the use of CW laser spectroscopy to determine the iodine binding-energy curve (47), the study of vibrational line profiles to determine intermolecular interactions (48), two photon ionization spectrometry (49), a study of optical activity and optical rotatory dispersion (50) and the development of several experiments using blue diode lasers (57). 

Table II. Laser Experiments - Fluorescence and Transient Absorption...

Table III. Laser Experiments Raman, Light Scattering and Interferometry...

Table IV. Laser Experiments Thermodynamic Measurements and Miscellaneous Topics...

As with terbium, a great deal of study has been made upon this ion in hopes of using it in a visible laser. Experiments along this line appear to have been very successful, and reports have been made on oscillation in crystals (757), glasses, and liquids (138). 

His laser experiments produced some very unusual results. The laser spikes first appeared at about 800 /zsec after cessation of the flash. This is quite a long time indeed, and Chang believes that this might be because the cascade processes from the highly excited to the 5D0 states are very slow. 

To a very large extent, most of the recent data on fluorescent decay times of the other trivalent ions (those beside terbium, neodymium, and europium) stems in some way from laser experiments. In this section some representative data on these are considered. 

R. de Vivie-Riedle and J. Manz Prof. Neumark s question about detecting the hole burning in the nuclear wavepacket of the electronic ground state is very stimulating. In this context, we have developed a scheme for detecting the hole in the wavepacket by a femtosecond chemistry laser experiment that involves two laser pulses Our explanation will be for the specific system K2, but more general applications for other systems are obvious ... 

Zewail and co-workers have reported femtosecond laser experiments on the photodissociation of mercury(II) iodide (Hgl2) [151] ... 

As pointed out hv Scoutland. "The general principles of ultrafast laser experiments are well known. All ultrafast experiments are variants on the pump-probe scheme, in which lime resolution is obtained by spatial delay of a probe pulse relative to the pump, or excitation, pulse (I ps = 3.0 nim)"... 

Figure 2.16 shows an example for such a biexponential decay measured with time correlated single-photon counting.78,75 Several picosecond laser experiments have explored this early time behavior of the equilibration... 

A spectrophotometric investigation of the ground-state absorption of benzophenone in pure SC CO2 and in the presence of 2-propanol was performed in order to determine both benzophenone solubility and the potential for solute/solute or co-solvent/solute aggregations. Linear Beer s law plots of absorbance versus concentration of benzophenone over a concentration range of 1-15 mM in SC OO2 insured that the ketone was fully disolved and in a non-associated form under the conditions of the laser experiments (- 10 mM). Analysis of the absorption spectrum of benzophenone in SC CO2 ranging from high to low pressure, resulted in absorption bands nearly... 

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