Supersonic apparatus

Several instniments have been developed for measuring kinetics at temperatures below that of liquid nitrogen [81]. Liquid helium cooled drift tubes and ion traps have been employed, but this apparatus is of limited use since most gases freeze at temperatures below about 80 K. Molecules can be maintained in the gas phase at low temperatures in a free jet expansion. The CRESU apparatus (acronym for the French translation of reaction kinetics at supersonic conditions) uses a Laval nozzle expansion to obtain temperatures of 8-160 K. The merged ion beam and molecular beam apparatus are described above. These teclmiques have provided important infonnation on reactions pertinent to interstellar-cloud chemistry as well as the temperature dependence of reactions in a regime not otherwise accessible. In particular, infonnation on ion-molecule collision rates as a ftmction of temperature has proven valuable m refining theoretical calculations. 

The spectrometer is fitted with a skimmed c.w. supersonic molecular beam source. Many chiral species of interest are of low volatility, so a heated nozzle-reservoir assembly is used to generate, in a small chamber behind a 70-pm pinhole, a sample vapor pressure that is then seeded in a He carrier gas as it expands through the nozzle [103], Further details of this apparatus are given elsewhere [36, 102, 104],... 

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. 

The LC-MS with a supersonic molecular beam (LC-SMB-MS) apparatus, which is schematically shown in Figure 8.13, is based on a modified homemade GC-MS with an SMB system that was previously described [76]. The heart and soul of this system is the soft thermal vaporization nozzle (STVN) chamber. The STVN accepts the liquid flow from the LC or the flow injection liquid... 

FIGURE 8.12 The supersonic LC-EI-MS apparatus. (Reproduced from Amirav, A. et al., Rapid Commun. Mass Spectrom., 15, 811, 2001. Copyright 2001. With permission from Elsevier.)... 

In the lower part of the Fig A, the dotted lines show the experimental apparatus designed for combustion in "supersonic flow. In order to make supersonic flow possible, there must be a jet constriction L between the chamber in which the gas is at rest and the pipe in which it is flowing (Ref 15, pp 120-21)... 

The experimental apparatus, as shown in Figure 11-1, was a standard molecular beam machine with a heated pulsed valve for vaporization of the non-volatile species and for supersonic cooling. Samples of 1-methyluracil, 1,3-dimethyluracil and thymine were purchased from Aldrich Co. and used without further purification. The sample 1,3-dimethylthymine was synthesized from thymine following a literature procedure [33], and its purity was checked by nuclear magnetic resonance (NMR) and infrared absorption (IR) spectroscopy. The heating temperatures varied for different samples 130°C for DMU, 150°C for MU, 180°C for DMT, and 220°C for thymine. No indication of thermal decomposition was observed at these... 

Figure 11-1. Experimental apparatus. The sample is supersonically cooled and intercepted by counter-propagating laser beams. Both fluorescence and ion signals can be observed...

Rowe and co-workers are developing a so-called diffusion technique to extend the temperature and pressure range. The technique will use the conversion of the initial kinetic energy (per unit volume) of the jet into a pressure increase downstream of the mass spectrometer, when the flow is brought from a supersonic to a subsonic regime through suitably shaped tubing. Also, it has been shown that the use of pulsed Laval nozzles reduces the appreciable amounts of gas that are consumed in the continuous flow CRESU apparatus [55]. 

The apparatus used in the studies which are the primary focus of our discussion is shown schematically in Figure 1. The details of the apparatus are discussed elsewhere (1.39). Briefly, the output of an argon ion pumped dye laser is brought into a suitably equipped vacuum chamber. Here, the laser beam intersects a supersonically expanded metal beam produced through use of an appropriate oven system. The oven depicted in the figure is a... 

Figure 6.19. Schematics of apparati first used to synthesize fullerenes. Illustrated are (a) the Smalley/ Curl supersonic laser evaporation system, and (b) the Huffman/Kratschmer electric arc apparatus.

Berthelot s latent heat apparatus, 307 binary liquid mixtures, supersonic velocities in, 66... 

FIb.1. Schematic diagram of the apparatus. A continuous supersonic Hg/Ar expansion produces the clusters. They can be ionised by electron or i oton impact. An electric pulse between the plates IH) and PI accelerates ions into the time-of-fiight mass spearometer. The first time focus is at the slit of the mass selector, the second is at the detector II. The mass selector can be used to select one mass only, say Hgjo, which can subsequently decay in the free-flight region into Hg, + Hg due to internal excitation. The reflector can separate parent (n = 20) and daughter (r — 19) cluster ions. 

Figure 7. Schematic diagram of the supersonic beam apparatus which combines laser-induced fluorescence spectroscopy with time-of-flight mass spectrometry. Reproduced with permission from Ref [92a].

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