Beam sources seeded

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],... 

Using a seeded nozzle beam source of N02 rather than a thermal beam, it was concluded [260] that the energy disposal is insensitive to reagent rotational excitation, but that there might be a slight increase in rotational excitation of the OH product as the reagent translational energy is increased. 

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... 

Fig. 2.18. Side elevation of the molecular beam machine s two-chamber setup. Chi vacuum chamber to generate the molecular beam by adiabatic expansion. A seeded supersonic beam source is placed here. Further details are shown in Fig. 2.19. Ch2 here, the molecules interact with the laser pulses. Detection is performed by a quadrupole mass spectrometer (QMS) with a 90° ion deflector between the mass filter and secondary electron multiplier (SEM) (Fig. 2.20) or the Langmuir-Taylor detector (Fig. 2.22)...

The study of the reactions of excited species is becoming an increasingly important area of research in kinetics [49, 50]. The excitation may take the form of enhanced translational, rotational, vibrational or electronic energy. Reactions with translational excitation are most commonly studied under molecular beam conditions using seeded nozzle beams or other types of sources to provide the enhanced energy [51, 52]. Translationally hot atoms may also be generated by nuclear recoil [53] or photodissociation [ 54 ]. 

Photolytic Source In case of photochemically active molecules, supersonic beams of radicals can be generated via photodissociation of helium-seeded precursors (Fig. 11.4). 

In a seeded supersonic expansion source [35-37[, the material is heated in an oven and a mixture of gaseous material and a seed gas expands through a nozzle into the vacuum. This source produces a highly intense beam of small pure and mixed metal clusters, but it is limited to metals with a low boiling point (Li, Na, K, etc.). Laser vaporization sources [13,38[ are more widely used as they produce pure and mixed clusters of most elements and when operated at low frequency (1-10 Hz) they have been successfully used... 

The breakthrough experiment was carried out by Whitham et al. [39,40] in France. They used a Smalley-type laser vaporization source (Fig. 4) to provide a molecular beam of Ca atoms entrained in He or Ar gas. The second harmonic (532 nm) from a pulsed Nd YAG laser was focused (Fig. 4) on a rotating calcium rod. About 500 jus prior to this, a pulsed valve (left side of Fig. 4) is opened and the plume of vaporized metal is entrained in Ar or He gas. The carrier gas is seeded with a few percent of the oxidant such as H20. The plume of excited- and ground-state metal atoms are carried down a short channel and react with the oxidant. At the end of the channel, the product molecules such as CaOH expand into the vacuum chamber and cool. After a short expansion, the pressure has dropped so low that the molecules are effectively in a collisionless, ultracold (<10K) environment. 

At beam velocities greater than thermal, velocity selection is not normally used (except for sputter-ion sources) since the methods of supersonic beams, seeded supersonic beams and charge-exchange tend to produce very narrow velocity distributions (see Section 3.2.2). 

---