Supersonic

The term detonation often employed to describe knocking is incorrect because the phenomenon can not be attributed to the propagation of a flame in the supersonic region, accompanied by a shock wave.. 

Fuel passing through certain hot zones of an aircraft can attain high temperatures moreover it is used to cool lubricants, hydraulic fluids, or air conditioning. It is therefore necessary to control the thermal stability of jet fuels, more particularly during supersonic flight where friction heat increases temperatures in the fuel tanks. 

Figure A3.5.5. Rate constants for the reaction of Ar with O2 as a fiinction of temperature. CRESU stands for the French translation of reaction kinetics at supersonic conditions, SIFT is selected ion flow tube, FA is flowing afterglow and HTFA is high temperature flowing afterglow.

Figure A3.5.7. Schematic diagram of a selected ion flow drift tube with supersonic expansion ion source.

As with most methods for studying ion-molecule kinetics and dynamics, numerous variations exist. For low-energy processes, the collision cell can be replaced with a molecular beam perpendicular to the ion beam [106]. This greatly reduces the thennal energy spread of the reactant neutral. Another approach for low energies is to use a merged beam [103]. In this system the supersonic expansion is aimed at the tluoat of the octopole, and the ions are passed tluough... 

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. 

Syage J A, Felker P M and Zewail A H 1984 Picosecond dynamics and photoisomerization of stilbene in supersonic beams. I. Spectra and mode assignments J. Chem. Phys. 81 4685-705... 

Nesbitt D J 1994 Fligh-resolution, direct infrared-laser absorption-spectroscopy in slit supersonic ]ets—intermolecular forces and unimolecular vibrational dynamics in clusters Ann. Rev. Phys. Chem. 45 367-99... 

THz spectrum of such clusters fomred in a pulsed planar supersonic expansion [65],... 

Chapman W B, Blackman B W, Nizkorodov S and Nesbitt D J 1998 Quantum-state resolved reactive scattering of F + H2 in supersonic jets Nascent HF(v,J) rovibrational distributions via IR laser direct absorption methods J. Chem. Rhys. 109 9306-17... 

Valentin J J, Coggiola M J and Lee Y T 1977 Supersonic atomic and molecular halogen nozzle beam source Rev. Sc/. Instrum 48 58-63... 

Gorry P A and Grice R 1979 Microwave discharge source for the production of supersonic atom and free radical beams J. Rhys. E Sc/. 12 857-60... 

Dietz T G, Duncan M A, Powers D E and Smalley R E 1981 Laser production of supersonic metal cluster beams J. Chem. Rhys. 74 6511-12... 

Kaiser R I and Suits A G 1995 A high-intensity, pulsed supersonic carbon course with C( P ) kinetic energies of 0.08-0.7 eV for crossed beam experiments Rev. Sc/. Instrum. 66 5405-11... 

Sinha M P, Schulz A and Zare R N 1973 Internal state distribution of alkali dimers in supersonic nozzle beams J. Chem. Phys. 58 549-56... 

The fonnation of clusters in the gas phase involves condensation of the vapour of the constituents, with the exception of the electrospray source [6], where ion-solvent clusters are produced directly from a liquid solution. For rare gas or molecular clusters, supersonic beams are used to initiate cluster fonnation. For nonvolatile materials, the vapours can be produced in one of several ways including laser vaporization, thennal evaporation and sputtering. 

Figure Cl. 1.1. Schematic of a typical laser vaporization supersonic metal cluster source using a pulsed laser and a pulsed helium carrier gas.

Rare-gas clusters can be produced easily using supersonic expansion. They are attractive to study theoretically because the interaction potentials are relatively simple and dominated by the van der Waals interactions. The Lennard-Jones pair potential describes the stmctures of the rare-gas clusters well and predicts magic clusters with icosahedral stmctures [139, 140]. The first five icosahedral clusters occur at 13, 55, 147, 309 and 561 atoms and are observed in experiments of Ar, Kr and Xe clusters [1411. Small helium clusters are difficult to produce because of the extremely weak interactions between helium atoms. Due to the large zero-point energy, bulk helium is a quantum fluid and does not solidify under standard pressure. Large helium clusters, which are liquid-like, have been produced and studied by Toennies and coworkers [142]. Recent experiments have provided evidence of... 

Molecular clusters are weakly bound aggregates of stable molecules. Such clusters can be produced easily using supersonic expansion, and have been extensively studied by both electronic and vibrational spectroscopy [146,... 

Smalley R E 1985 Supersonic cluster beams an alternative approach to surface science Comparison of Ab Initio Quantum Chemistry with Experiment for Small Molecules ed R J Bartlett (Boston Reldel)... 

Alford J M, Weiss F D, Laaksonen R T and Smalley R E 1986 Dissociative chemisorption of H2 on niobium cluster ions. A supersonic cluster beam FT-ICR experiment J. Rhys. Chem. 90 4480... 

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