Free jet molecular beams

The first stage was the production of a pulsed free-jet molecular beam of helium containing 20% CO, which was then crossed with an electron beam to produce ionisation. The ions were produced close enough to the beam nozzle for cooling to occur downstream. Some 4 cm from the nozzle the beam entered a confocal Fabry-Perot cavity where it was exposed to millimetre wave radiation close to 120 GHz in frequency. Following microwave excitation, when on resonance, the beam was probed with a Nd YAG pumped dye laser beam with the frequency chosen to drive rovibronic components of the A 2 U-X 2 + band system. Figure 11.54 shows two recordings of a spin component of the lowest rotational transition the line shown in (a) is... 

Molecular-Beam Mass-Spectrometry. This procedure was carried out on equipment described by Evans and Milne (H). Pyrolysis of the oils (or fractions) was performed under controlled conditions and followed in real time by a free-jet, molecular beam MS. Pyrolysis products and fragmentation ions were detected. 

Characterization Tools for Pyrolysis Oils. It wasn t too many years ago that the only tools available to the scientist interested in pyrolysis oil composition were gas chromatography and thermogravi-metric analysis. The complexity of the pyrolysis oils demands high performance equipment, and a list of such equipment mentioned during the symposium would include proton and carbon nuclear magnetic resonance spectroscopy, free-jet molecular beam/mass spectrometry (16.25), diffuse reflectEuice Fourier transform infrared spectrometry ( ), photoelectron spectroscopy ( ), as well as procedures such as computerized multivariate analysis methods (32) - truly a display of the some of the most sophisticated analytical tools known to man, and a reflection of the difficulty of the oil composition problem. 

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. 

Miller D R 1988 Free jet sources Atomic and Molecular Beam Methods ed G Sooles (New York Oxford University Press)... 

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... 

D. R. Miller, Free jet sources, in Atomic and Molecular Beam Methods, G. Scoles, ed., University Press, Oxford, 1988, Chapter 2, pp. 14 53. 

Campargue R (1984) Progress in overexpanded supersonic jets and skimmed molecular beams in free zones of silence. J Phys Chem A 88 4466-4474... 

In conventional gas electron diffraction experiments, an effusive beam is used in which vibrational levels of molecules are thermally populated and the width of a peak in a radial distribution curve is determined by thermally averaged mean amplitudes. When a molecular beam or a free jet is used, mean amplitudes could become small, since the contribution from the vibrationally excited levels is reduced significantly. As a consequence, sharper peaks are expected in the radial distribution curve, and the spatial resolution of the snapshot could be improved. However, it seems that the observed peaks in the radial distribution curve are considerably broad even though a molecular beam is used. There could be some reasons to have such broadened peaks in the radial distribution curve. 

A detailed experimental investigation of the THP-HzO complex (with molecular beam Fourier transform (FT) microwave spectroscopy, free-jet millimeter-wave spectroscopy, and ab inito calculations) revealed that the lowest energy species has the water in an axial position. <1998CEJ1974>. Similar studies into the TFIP-FICl complex allowed both the axial and equatorial hydrogen-bond complexes to be characterized it was not possible to unequivocally determine which of the two is more stable <1999AGE1772>. 

The interface used today between the atmospheric-pressure plasma and the low-pressure mass spectrometer is based on a differentially pumped two-stage interface similar to those used for molecular beam techniques [89-91]. The key to successful development of ICP-MS instruments was the use of a relatively large ( l-mm-diameter) sampling orifice so that continuum flow was attained with an unrestricted expansion of the plasma to form a free jet. When small orifices were used, a cold boundary layer formed in front of the orifice, resulting in substantial cooling of the plasma, including extensive ion-electron recombination and molecular oxide formation. The smaller orifices were also susceptible to clogging. 

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). 

In order to produce supported samples for STM or FEM study, clusters formed on the centerline of the condensation reactor are extracted through a 1 mm diameter capillary into a vacuum chamber typically kept at 10-5 Torr. The resulting supersonic free jet flow is collimated to form a molecular beam of metal clusters, uncondensed metal atoms and inert gas atoms. 

Free jet expansion from the capillary produces a molecular beam in which the clusters are all traveling approximately at the sonic velocity of the gas in the capillary. Thus, cluster velocity ranges from 3 x 10 cm/s if Ar is used in the MECS to 1 X 1Q5 cm/s if He is used. 

The arrangement we used for interfacing the picosecond laser to the molecular beam (or free jet) is shown schematically in fig. 1. The laser is a synchronously pumped dye-laser system whose coherence width, time and pulse duration were characterized by the SHG autocorrelation technique. The pulse widths of these lasers are typically 1-2 ps, or 15 ps when a cavity dumper is used. For detection one of three techniques... 

The ground-state rotational spectrum of the triply H-bonded DME dimer with three C-H- - -O-C blue-shifting H-bonds has been studied by molecular beam Fourier transform microwave and free jet millimeter wave absorption spectroscopies in Refs. [108,109a] Av xp(C-H) = 13-21 cm-, ... 

Fig. 2.2 Schematic representation of the nested-pair of cones used for sampling and molecular-beam formation. Species entering the sampling cone are entrained in a free-jet expansion into the vacuum stage (5 X 10 Pa). Only those species having moments directed along the cone axis enter the skimmer cone. (Reproduced from [16], with permission.)...

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