Crossed laser-molecular beam studies

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. 

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. ... 

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 ... 

It is now possible to design the experiments using molecular beams and laser techniques such that the initial vibrational, rotational, translational or electronic states of the reagent are selected or final states of products are specified. In contrast to the measurement of overall rate constants in a bulk kinetics experiment, state-to-state differential and integral cross sections can be measured for different initial states of reactants and final states of products in these sophisticated experiments. Molecular beam studies have become more common, lasers have been used to excite the reagent molecules and it has become possible to detect the product molecules by laser-induced fluorescence . These experimental studies have put forward a dramatic change in experimental study of chemical reactions at the molecular level and has culminated in what is now called state-to-state chemistry. 

The more exoergic reaction Ba + NzO has a smaller reaction cross section ( 90 A2 or 27 A2) [347, 351] and crossed-molecular beams studies [349] show that the BaO product is backward-scattered with a large amount of internal excitation ((Fr) < 0.20). Laser-fluorescence measurements [348] of the BaO(X Z+) product for the reaction in the presence of an argon buffer gas, find population of vibrational states up to v = 32. The relative populations have a characteristic temperature of 600 K for v = 0—4 and 3600 K for v = 5—32 with evidence of non-thermal population of v — 13—16. This study also observes population of A n and a 3II states of BaO with v = 0—4. A molecular beam study of Ba + N20 with laser-induced fluorescence detection indicates that the BaO( X) product is formed with a very high rotational temperature. 

In light of previous experimental and theoretical work on the F f H2 reaction, it can be seen why an experisient of this complexity is necessary in order to observe dynamic resonances in this reaction. The energetics for this reaction and its isotopic variants are displayed in Figure 1. Chemical laser (11) and infrared chemiluminescence (12) studies have shown that the HF product vibrational distribution is hi ly inverted, with most of the population in v=2 and v°°3. A previous crossed molecular beam study of the F + D2 reaction showed predominantly back-scattered DF product (13). These observations were combined with the temperature dependence of the rate constants from an early kinetics experiment (14) in the derivation of the semiempirical Muckerman 5 (M5) potential energy surface (15) using classical trajectory methods. Although an ab initio surface has been calculated (16), H5 has been the most widely used surface for the F H2 reaction over the last several years. 

A common experimental arrangement for the study of molecular Rydberg states is depicted in Fig. 5.31. The output beams of two pulsed narrow-band dye lasers, pumped by the same excimer laser, are superimposed and cross the molecular beam perpendicularly. The fluorescence emitted from the intermediate level (u, J ) or from the Rydberg levels (u, 7 ) can be monitored by a photomultiplier. The ions produced by autoionization (or for levels slightly below IP by field ionization) are extracted by an electric field and are accelerated onto an ion multiplier or channel plate. This allows the detection of single ions. In order to avoid electric Stark shifts of the Rydberg levels during their excitation, the extraction field is switched on only after the end of the laser pulse. Experimental details and more infor-... 

F + H2 reaction first attracted attention due to the application of chemical laser. This is the first reaction which has product vibrational state resolved measurements. Using chemical laser [20] and infrared light emitting [21, 22], researchers found that the population of the product HF vibrational states is highly inverted. Crossed molecular beam studies of this system are the main work of Yuan Tseh Lee s Nobel Prize in Chemistry in 1986 [12, 13]. In this chapter, studies on resonance phenomenon in the F + H2 reaction are mainly described. In Sect. 3.1, studies on resonance in the F + H2 reaction are reviewed crossed molecular beam studies in the F( P2/3) -I- H2 HF - - H reaction are introduced in Sects. 3.2 and 3.3 discusses the studies of the F( P2/3) -1- HD -> HF + H reaction, and the last section is a summary. 

Crossed molecular beam studies using imaging detection of products have been restricted to the determination of non-state-selective differential cross-sections, in which an atomic product is probed using (1 + 1) REMPI, or in which a molecular product is probed by universal photoionization. This is because molecular beams are normally skimmed and typical product densities per quantum are only of the order lO cm this approaches the sensitivity limit of state-selective REMPI. In addition, with the exception of (1 + 1) REMPI, the probe laser must be focused, thus creating a very small interaction volume that yields extremely low count rates. The reaction F + CU4 (see Liu (2001)) was the first to be studied by (2 + 1) REMPI in a crossed molecular beam experiment to measure state-selected differential cross-sections. The first study of a neutral bimolecular reaction using a crossed-beam arrangement and 1 + 1 REMPI detection of an atomic product was carried out for... 

A common experimental arrangement for the study of molecular Rydberg states is depicted in Fig. 10.28. The output beams of two pulsed narrow-band dye lasers, pumped by the same excimer laser, are superimposed and cross the molecular beam perpendicularly. The fluorescence emitted from the interme-... 

In this contribution we report the results of a crossed molecular beam study of the dependence of internal IF product state distributions on the collision energy in the reaction F + CF3I -> IF + CF3. The laser-induced fluorescence product state analysis of this process at a collision energy of 3.2 kJ mol" in conjunction with the analogous hydrogenated system F + CF I has been reported earlier [7]. In the present work the analysis of the internal product energy distribution of the F + CF3I... 

Laser photolysis of a precursor may also be used to generate a reagent. In a crossed-beam study of the D + FI2 reaction [24], a hypertliennal beam of deuterium atoms (0.5 to 1 eV translational energy) was prepared by 248 mn photolysis of DI. This preparation method has been widely used for the preparation of molecular free radicals, both in beams and in experiments in a cell, with laser detection of the products. Laser photolysis as a method to prepare reagents in experiments in which the products are optically detected is fiirtlier discussed below. 

Hancock and MacRobert used single-mode CO2 laser pulses, tailored by electro-optic crystal switching to give uniform intensities and well-defined temporal characteristics, to measure IRMPE cross-sections in SFg at 944.2 cm as separate and quantitative functions of intensity and fluence. In the study of Luijks et molecular beams of SFg were irradiated by a... 

On the experimental side, the coupling of crossed molecular beam techniciues with sophisticated detection techniques (Laser Induced Fluorescence, CARS or REMPI spectroscopy, Rydberg tagging photoionisation using synchrotron radiation or U lasers) has improved considerably the detailed study of chemicid reactivity. It is now possible to prepare reactants in a well defined state and to analyze the reaction products at a fixed scattering angle for a. collision at a well defined kinetic energy [1]. 

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