Nozzle beam techniques

The ability to cool (and eventually liquefy) gases by adiabatic expansion underlies industrial gas liquefaction processes. Adiabatic cooling of gaseous nozzle-jet expansions is also an important technique in modem molecular beam and mass spectrometric research. Thermodynamicist John Fenn, winner of the 2002 Nobel Prize in Chemistry, pioneered many of the techniques of adiabatic nozzle-beam cooling. 

Gillies [237], They used the technique of laser ablation to form nozzle beams of the molecules, injected into a Fourier transform microwave spectrometer. In both cases the ground electronic state is 4X, an example of which we met in chapter 9, the excited a state of the CH radical. However in that example the coupling was close to case (b), whereas VO and NbO provide examples of a case (a) 4 XV state. The = 1/2 and 3/2 states are split by second-order spin-orbit coupling the splitting is... 

Two particularly interesting chemiluminescent reactions have been discovered using beams of alkali dimer molecules (M2). These species are formed in high yield in an alkali metal nozzle beam source and they can be separated from the remaining atoms by means of an inhomogeneous magnetic field. This technique has made it possible to study reactions of these diatomic species by molecular beam methods for the first time [364] and as a result it has been demonstrated conclusively that reactions of the type... 

Desorbed polyatomic molecules feature broad electronic absorption bands, and hence low spectral selectivity in multiphoton excitation and ionization. However, the use of the supersonic-molecular-beam technique radically changes the situation (Levy 1980). With this technique, the molecules are deeply cooled by injecting them into a carrier gas that is then allowed to expand into a vacuum through a small hole (a nozzle). The size of the hole should be large compared with the mean free path of the expanded... 

Although the present volume, like the first in this series on fast reactions, is devoted to elementary reactions in the gas phase, the emphasis has shifted. The main theme in Volume 1 was the experimental measurement of thermal rate constants. Here, we are concerned with more detailed information about elementary reactions, and its interpretation. With the availability of nozzle beam sources, lasers for selective excitation, and sensitive spectroscopic techniques for identifying molecular states, the experimental reaction dynami-cist can now control the energies of chemical reagents and observe the states of reaction products to an extent only dreamt of two decades ago. Already one can foresee the day when state-to-state probabilities, like those for spectroscopic transitions, will become available for chemical reactions. 

The AeroSizer, manufactured by Amherst Process Instmments Inc. (Hadley, Massachusetts), is equipped with a special device called the AeroDisperser for ensuring efficient dispersal of the powders to be inspected. The disperser and the measurement instmment are shown schematically in Figure 13. The aerosol particles to be characterized are sucked into the inspection zone which operates at a partial vacuum. As the air leaves the nozzle at near sonic velocities, the particles in the stream are accelerated across an inspection zone where they cross two laser beams. The time of flight between the two laser beams is used to deduce the size of the particles. The instmment is caUbrated with latex particles of known size. A stream of clean air confines the aerosol stream to the measurement zone. This technique is known as hydrodynamic focusing. A computer correlation estabUshes which peak in the second laser inspection matches the initiation of action from the first laser beam. The equipment can measure particles at a rate of 10,000/s. The output from the AeroSizer can either be displayed as a number count or a volume percentage count. 

Pyrolysis method involves thermal decomposition of suitable precursors to produce free radicals. Pyrolysis sources based on continuous molecular beam nozzles are well developed (for example, methyl6 8 and benzyl9). Recently, Chen and co-workers have pioneered a flash pyrolysis/supersonic jet technique to produce free radical beams (Fig. I).10 In this radical... 

A technique which is not a laser method but which is most useful when combined with laser spectroscopy (LA/LIF) is that of supersonic molecular beams (27). If a molecule can be coaxed into the gas phase, it can be expanded through a supersonic nozzle at fairly high flux into a supersonic beam. The apparatus for this is fairly simple, in molecular beam terms. The result of the supersonic expansion is to cool the molecules rotationally to a few degrees Kelvin and vibrationally to a few tens of degrees, eliminating almost all thermal population of vibrational and rotational states and enormously simplifying the LA/LIF spectra that are observed. It is then possible, even for complex molecules, to make reliable vibronic assignments and infer structural parameters of the unperturbed molecule therefrom. Molecules as complex as metal phthalocyanines have been examined by this technique. 

One issue that is particularly interesting for activated dissociation is the importance of translational vs. vibrational activation since this relates to the topology of the barrier location on the PES (see Section 2.3.1). In analyzing experiments, it has been traditional to define the vibrational efficacy rjv as in eq. (2.5). This analysis, however, assumes that A(v) is the same for all v and this may not be universally true. In this case, describing vibrational efficacy is more complicated. Very recently, experiments for CH4 dissociation on transition metals even combine supersonic nozzle molecular beams with laser state preparation techniques to probe the reactivity of specifically prepared vibration rotation states [115-118] (see Section 4.3.1.3). 

Molecular beams are very important tools for characterizing intermolecular and intramolecular reactions. In fact, the 1988 Nobel Prize in Chemistry was awarded to Yuan Lee, Dudley Herschbach, and John Polanyi for studies which were mostly made possible by this technique. A particularly useful variant is the supersonic molecular beam, which in the simplest case pushes a high-pressure mixture of helium and trace amounts of some larger guest molecule through a nozzle. When the helium atoms enter the... 

The method of choice for the generation of vdW clusters utilizes supersonic expansions. In this technique, the species to be clustered are allowed to expand from a high pressure to a low pressure region through a molecular beam nozzle. The basic principles of adiabatic expansion have been the focus of a number of reviews (Hagena 1974, 1987 Scoles 1988) and only the pertinent aspects will be described here. 

CoO-coated Co cluster and oxide-coated Fe cluster assemblies were prepared by a plasma-gas-aggregation cluster-beam-deposition technique [37-39]. For preparation of CoO-coated Co cluster assembly, oxygen gas was introduced through a nozzle near the skimmer into the deposition chamber. The Co clusters with CoO shells were formed before deposition onto the substrate [37], Figure 8 shows a TEM image of the clusters produced at oxygen gas flow rate R(02) = 1 seem. Clusters are almost monodispersed, with the mean diameter of about 13 nm. Electron diffraction pattern indicated the coexistence of Co and CoO phases. The cluster assemblies were formed on a polyimide film with a thickness of about 100 nm. 

With free jet expansion techniques, we have produced clusters of aqueous nitric acid (3 ), hydrochloric acid, sulfuric acid (4, pure acetic acid ( 5), and sulfur dioxide (6). For analogy to buffering, the formation of clusters containing ammonia have also been examined. These have included ammonia with aqueous nitric acid (7 ), hydrogen sulfide (7J), and sulfur dioxide (8). The basic experiment involves expansion of vapor through a nozzle, collima-tion of the jet with a skimmer to form a well-directed molecular beam, and detection of clusters via electron impact ionization and quadrupole mass spectrometry. Some variations include the introduction of a reactive gas into vacuum near the expansion as described elsewhere (4, 8) and the implementation of an electrostatic quadrupolar field to examine the polarity of the neutral clusters. The electric deflection technique is described by Klemperer and coworkers (9). 

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