Molecular beam accelerator

Figure 5.9 shows the time evolution of the radii of selected 2D spherulites from Fig. 5.8. We observe that the process is non-linear and accelerated, (fR/df > 0. It is also interesting to notice that, at a given time, the radial growth velocity Ur = dR/dt (slope) is nearly the same for all spherulites, which implies that it depends on the deposition time and certainly not on the radius of the spherulites. In the case discussed here the thickness of the film is increasing with time because of continuous exposure to the molecular beam. The non-linearity is more pronounced at the beginning of the experiment (roughly between 250 and 350 s) and the velocity nearly tends towards an asymptotic value, so that 2D spherulites that are formed last show almost linear growth.

Mass spectrometers Molecular beam apparatus Ion sources Particle accelerators Electron microscopes Electron diffraction apparatus Vacuum spectographs Low-temperature research Production of thin films Surface physics Plasma research Nuclear fusion apparatus Space simulation Material research Preparations for electron microscopy... 

A CH4 pyrolysis mechanism appears to be consistent with our observation that preheating improves partial oxidation selectivity. First, higher feed temperatures increase the adiabatic surface temperature and consequently decrease the surface coverage of O adatoms, thus decreasing reactions lOa-d. Second, high surface temperatures also increase the rate of H atom recombination and desorption of H2, reaction 9b. Third, methane adsorption on Pt and Rh is known to be an activated process. From molecular beam experiments which examined methane chemisorption on Pt and Rh (79-27), it is known that CH4 must overcome an activation energy barrier for chemisorption to occur. Thus, the rate of reaction 9a is accelerated exponentially by hi er temperatures, which is consistent with the data in Figure 1. 

We now turn to the spin-rotation constants, noting first that the proton constant was determined in earlier molecular beam magnetic resonance studies by Code, Khosla, Ozier, Ramsey and Yi [106]. First there is a relativistic acceleration correction ch (acc) to the spin rotation constant,... 

The technique consists of introducing a gas into the collision cell as a molecular beam directed perpendicularly to the ion beam accelerated by a potential of a few kilovolts towards an outlet where a pump removes it from the instrument so as to preserve a vacuum compatible with the use of electromagnetic analysers. This collision cell is located at a focal point to avoid losses. It is better to float the collision cell electrically to distinguish the metastable from the CID products. 

Molecules that are ionized by electron impact in the ion source are accelerated, sent through a conventional 90° magnetic sector analyzer, postaccelerated by a few thousand volts, and arrive at the electron multiplier detector. The output of the electron multiplier detector consists of pulses of about lO- coulomb per ion. The pulses are amplified and sent through a gated amplifier and an electronic switch which is synchronized with the beam chopper so that one of the ion counters records ions only when the beam chopper is open, the other only when the beam chopper is closed. The difference between the two ion counts represents the ion intensity contributed by the molecular beam, while the square root of the sum of the two ion counts is approximately equal to the standard deviation of the measurement and serves as a useful indicator of the quality of the data being obtained. 

A continuous-wave green laser beam (argon ion laser, all lines) with a maximum power of up to 28 W is focused to the beam width of only 4 fim. As shown in Fig. 1, the vertically aligned laser beam runs orthogonal to the molecular beam. All molecules that pass the laser beam at or very close to the focus are heated to an internal temperature above 3000 K and ionize. The positive fullerene ions are then accelerated towards an electrode at 10 kV where they induce the emission of electrons. The electrons in turn are again multiplied and the charge pulses are subsequently counted. The overall molecule detection... 

Two recent papers [1,2] provide updated views of advances in the production of intense and continuous beams of aligned molecules. In [1], it was demonstrated that in the prototypical case of a seeded supersonic expansion of a molecular beam of benzene, besides acceleration and cooling, orientation of the molecular plane also occurs because of the anisotropy of the intermolecular forces which govern collisions. This work is reviewed in Sec.2.1. Previous studies on the collisional alignment of the rotational... 

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