Nozzle source, supersonic

Scattering studies with metastable atoms are in many cases easier (and less expensive) than experiments with ground-state atoms, The discussion that follows is mainly concerned with helium, as most of the information is available for this atom. Figure 2 shows a skeletal setup of the experiment. A helium beam from a supersonic nozzle source is excited by electron impact to its two metastable states. The singlet state can be quenched by the 2g radiation from a helium-gas discharge lamp ... 

FIG. 5. Schematic overview of the set-up with the laser vaporization source and the nozzle for supersonic expansion used in the production of clusters at Goteborg. The figure shows the laser vaporization source with the target material, the laser beam for evaporation, the small volume where a plasma of atoms and ions exist and the region where the clusters are formed in the expansion through the nozzle. 

As well as producing higher beam intensities, the supersonic nozzle sources have a further advantage. In the supersonic expansion through the Laval slit the gas is adiabatically cooled to very low temperatures of ca. 40°K [187] but increases its translational energy in the beam direction to the flow velocity of the gas, e.g. Mach number ca. 15 or peak velocity 70% higher than the most probable velocity of a 900°K oven [187]. Supersonic nozzles produce very narrow velocity distributions compared with the Maxwell—Boltzmann distribution obtainable from an effusive oven at the same temperature. 

Q A beam of helium atoms, originating from a supersonic nozzle source held at a temperature of 300 K, strikes a close-packed nickel surface perpendicularly. The kinetic energy of the helium atoms is equal to 2.5 kpT],where is the Boltzmann constant. Given that the spacing between the rows of nickel atoms is 216 pm, calculate the angle to the perpendicular at which the six first-order diffraction peaks will be observed. 

Calculate the wavelength of a supersonic beam of argon leaving a nozzle source at a temperature of 300 K (kinetic energy of argon atoms = 2.5 g nozzie)- diffraction likely to be observed when this beam is incident on a close-packed nickel surface ... 

A variety of means has been developed recently for the preparation of molecular beams containing sufficient concentrations of free radicals for spectroscopic analysis by MPI-MS or LIF detection. For larger species, it is particularly important to prepare them in such a manner that they can also be cooled to low temperatures by supersonic expansion to relieve the considerable spectral congestion typical of hot radical sources. To accomplish this, several devices have been developed which combine radical production with supersonic nozzle sources. 

FIGURE 3.1 Schematic of seeded supersonic nozzle source (top) and gas aggregation source (bottom). Reprinted with permission from Ref. [17]. American Physical Society. 

Generation of the Molecular/Cluster Beam. In the first chamber the molecules and clusters are produced in a seeded supersonic nozzle source. It is perhaps the most intense molecular/cluster beam source available. In this source, alkali metal is vaporized in a hot oven as sketched in Fig. 2.19. The alkali... 

In the study of complexes, a Fabry-Perot cavity is employed as a sample cell along with a pulsed supersonic nozzle source. The weakly bonded complex is produced at very low temperatures (a few degrees Kelvin), by expanding (adiabatically) as a short pulse a mixture of the reactive species in an inert buffer gas. Weak complex formation occurs undoubtedly via three-body collisions, for example,... 

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... 

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],... 

Figure 1. Schematic illustration of the laser-vaporization supersonic cluster source. Just before the peak of an intense He pulse from the nozzle (at left), a weakly focused laser pulse strikes from the rotating metal rod. The hot metal vapor sputtered from the surface is swept down the condensation channel in dense He, where cluster formation occurs through nucleation. The gas pulse expands into vacuum, with a skinned portion to serve as a collimated cluster bean. The deflection magnet is used to measure magnetic properties, while the final chaiber at right is for measurement of the cluster distribution by laser photoionization time-of-flight mass spectroscopy.

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... 

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