Crossed lasers

A description of a crossed laser beam technique for particle sizing and its application to shock tube experiments was presented by Waterson and Chou [340]. 

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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. 

Figure 1 Formation of a holographic grating. Crossed laser beams create a sinusoidal intensity pattern recorded in the film through photochemical processes.

I.r. laser spectroscopy and quadrupole mass spectrometry were used by Fischer et al. to study vibrational predissociation of clusters of C2H4, and CsHg, but-l-ene, cis- and trans-but-2-ene, and isobutene. They obtained spectra in the range 2900—3200 cm and for C2H4 clusters predissociation was observed to result from excitation near the v-i, and vg fundamentals and the i 2 + V12 combination band. The vibrational bands were observed to have Lorentzian lineshapes with IWHM of ca. 5 cm. A homogeneous broadening mechanism was assumed and the widths were used to calculate excited-state lifetimes. Valentini and co-workers studied the predissociation of C2H4 clusters at 950 cm in a crossed laser/molecular beam apparatus. 

Studies of the photodissociation dynamics of chlorinated benzene derivatives have been reviewed. Photodissociation of chlorobenzene at 266 nm has been investigated by the crossed laser-molecular beam technique, and a hot molecule mechanism is considered probable. Similar studies have been carried out for bromobenzene and p-bromotoluene, which show that for each of these molecules the dissociation is fast and the transition dipole moment is almost perpendicular to the C-Br bond. In deoxygenated aqueous solutions, 254 nm photolysis of chlorobenzene yields phenol and chloride ions as the main products, along with benzene, phenylphenols and biphenyl.lodo-benzene adsorbed on sapphire(OOOl) at 110 K undergoes C-I bond cleavage when irradiated at 193 nm. ... 

In most laser studies of optical isotope shifts the crossed, laser-atomic beam method is used to avoid Doppler-broadening. In some cases short-lived radioactive isotopes are detected directly on-line, in others, by irradiating a target which is subsequently heated to form the source of an atomic beam. Figure 7 shows a spectrum obtained some years ago in Oxford on the resonance line of barium at 554 nm S] ... 

We have described several examples of the direct observation of triplet biradicals. However, the fleeting nature of singlet biradicals has made it very difficult to obtain direct information on their structure and reactivity. With the advent of femtosecond lasers, however, this has become possible. For example, Zewail and co-workers directly characterized simple tetramethylene and trimethylene biradicals. The approach involved a molecular beam of either cyclopentanone or cyclobutanone with crossed laser beams. Photolysis led to extrusion of CO and formation of the biradical. For tetramethylene, a clear biradical structure is seen, with a lifetime of 700 fs. However, for... 

FIGURE 1. Crossed laser and molecular beam apparatus for LIF-studies. In addition, the experimental setup for polarization measurements is given [9]. 

Current research that focuses on low temperatures uses crossed laser beams to slow down gas atoms (the phrase "optical molasses" is a good analogy) so much that their "temperature" is close to absolute zero, (a) If atoms of Rb are moving at 1 cm/s, what is the approximate "temperature" of the Rb gas (You can use any definition of "average temperature"for this estimate.) (b) How relevant is the word "temperature"to systems like the one described Develop arguments for and against the applicability of the term to gas atoms trapped in optical molasses. 

M. F. Vernon, D. J. Krajnovich, H. S. Kwok, J. M. Lisy, Y. R. Shen, and Y. T. Lee,/. Chem. Phys., 77,47 (1982). Infrared Vibrational Predissociation Spectroscopy of Water Clusters by the Crossed Laser-Molecular Beam Technique. 

Lee, Y. T. and Y. R. Shen (1980). Studies with crossed lasers and molecular beams. Phys. Today 33, 52. 

Fig. 10.1.2. Visualization of a seed particle flying through an interference pattern created by two crossing laser beams...

Waschewsky et al. (1994) used crossed laser-molecular beam experiments to monitor the competition between photodissociation pathways in chloroacetone. Their studies proved that C—C bond fission, process (II), competes with C—Cl bond fission, process (1), in C1CH2C(0)CH3 photolysis at 308 nm, although the latter process is favored. Several kJ of translational energy were evident in the translation of the initial products of the two competing dissociation pathways. The authors point out that this may indicate that, for both channels, dissociation proceeds via a reaction coordinate that has a significant exit barrier (barrier to reverse reaction), so fragments exert a repulsive force on each other as they separate. The pathway to dissociation probably does not involve... 

The photodecomposition of l-bromo-2-propanone was reviewed recently by Calvert et al. (2008). The photodecomposition has been studied at 308 nm in crossed laser molecular beam experiments by Waschewsky et al. (1994) and Kash et al. (1994) and in broadband photolysis (300-400 nm) and laser beam photolysis at 308 and 351 mn by Burkholder et al. (2002). The photodecomposition follows the pattern of 1-chloro-2-propanone. The following primary processes should be considered ... 

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