Pulsed valve, molecular beam experiment

A Newly Designed Pulsed Valve for Molecular Beam Experiments... 

Fig. 17.10. Experimental scheme of the relevant elements for performing pulsed molecular beam experiments show/ing the UHV chamber with the pulsed piezo-electric valve, the variable leak valve and the absolutely calibrated mass spectrometer. The cluster...

Fig. 1.52. Typical experimental setup for a pulsed molecular beam experiment for studying the catalytic properties of size-selected clusters on surfaces. It mainly consists of a pulsed valve for the generation of a pulsed molecular beam and a differentially pumped, absolutely calibrated quadrupole mass spectrometer. The length of the valve extension tube is adjusted to obtain a beam profile of similar dimensions as the sample under investigation. A typical time profile is also shown. It can be adjusted up to continuous operation. The pulse-to-pulse stability is better than 1%...

The setup used for crossed beam experiments is basically the same apparatus used in the H2O photodissociation studies but slightly modified. In the crossed beam study of the 0(1D) + H2 — OH + H reaction and the H + HD(D2) — H2(HD) + D reaction, two parallel molecular beams (H2 and O2) were generated with similar pulsed valves. The 0(1D) atom beam was produced by the 157 photodissociation of the O2 molecule through the Schumann-Runge band. The 0(1D) beam was then crossed at 90° with the... 

A systematic view of the relevant elements is depicted in Figure 17.10. The deposited clusters can be exposed to different reactant gases by two kinds of valves. First, they can be exposed isotropically to e.g. O2 by a commercial, ultra-high vacuum (UHV) compatible, variable leak valve. Second, reactant molecules (e.g. CO) can be introduced via a pulsed molecular beam produced by a piezo-electric driven, pulsed valve. This pulsed valve has a high pulse-to-pulse stability (time profile), and allows the study of catalytic processes on supported clusters at relatively high pressures (up to 10 mbar). Furthermore, a stainless steel tube is attached to the pulsed nozzle in order to collimate the molecular beam and to expose the reactant molecules to the substrate only. The pulse duration at the position of the sample can, in principle, be varied from 1 ms up to continuous operation. For the experiments described below a constant pulse duration of about 100 ms was used. The repetition rate of the pulsed valve can be up to 100 Hz. The experiments were carried out at 0.1 Hz the 10 s interlude allows the reactant gas to be pumped completely. 

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. 

I- 1 REMPI of S( P2,i,o> D2)IC H S. The experimental setup and procedures used to measure the electronic S( P2.i,o, 2) state distribution formed in the 193-nm photodissociation of organosulfur species have been described in Section II.A [58-60]. In this experiment, a pulsed molecular beam of neat thiophene is produced by supersonic expansion through a pulsed valve (nozzle diameter = 0.5 mm, temperature 298 K, stagnation pressure = 90 Torr). 

As mentioned earlier, a well-developed method for producing supersonic molecular beams of highly refractory metals employs a pulsed laser to ablate the material from a solid target into a channel attached to a pulsed high-pressure valve (see Hopkins et al. (1983)). In the supersonic expansion formed on leaving the channel, a variety of metal atoms, van der Waals clusters and molecules can be produced, allowing for their study in beam experiments. Figure 24.3... 

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