Molecular beam sampling detection

Figure 4.2. Experimental set-up for kinetic studies of SiHj reactions by excimer laser photolysis in a tubular slow-flow reactor with time-resolved detection by molecular beam sampling, near-threshold ionization electron impact mass spectrometry...

Detection of fluorine atoms was by means of mass spectrometry with molecular beam sampling. The value of kf was reported to be 10 exp (—7Q0jT) cm mol s in the range 300 to 400 K. 

Special interfaces are used for highly specialized tasks. For example, Sloane and Rat-cliffe [207] devised an instrument for molecular beam sampling of transient combustion phenomena by TRMS. It was possible to detect various flame components with good spatial and temporal resolution [207]. Reaction kinetics and the chemistry of low-pressure flames was studied with aid of a photoionization method [208]. 

Blauwens et al (1977) investigated low-pressure hydrocarbon/02/N2 flames using molecular beam sampling and mass spectrometric detection of NO, O, OH, H, and other species. Temperature was inferred from the measured concentrations and an equilibrium constant assuming partial... 

Molecular beams provide the answer. We first met molecular beams in Box 4.1, where we saw how a velocity selector is constructed. A molecular beam consists of a stream of molecules moving in the same direction with the same speed. A beam may be directed at a gaseous sample or into the path of a second beam, consisting of molecules of a second reactant. The molecules may react when the beams collide the experimenters can then detect the products of the collision and the direction at which the products emerge from the collision. They also use spectroscopic techniques to determine the vibrational and rotational excitation of the products. 

Neutral species are best sampled using molecular beam methods in which the neutral beam is modulated between the sampling orifice and the ionization chamber of the mass spectrometer and only the modulated component of the mass spectrometer output is recorded. This approach enables all neutral species, including radicals to be detected with a comparable sensitivity. If modulation techniques are not used, the sensitivity for detecting condensible or reactive species is much less than for non-condensible, non-reactive neutral molecules because of the much larger effective pumping speed for the former in the mass spectrometer chamber. However, the ease of installation of non-line-of-sight non-modulated... 

Figure 1 Schematic diagram of a molecular beam machine. A gas at elevated pressure emanates from an orifice. The gas stream is collimated in three differential pumping stages to reduce the gas load on the target chamber. Only the part of the gas stream that reaches the target though all collimators is indicated as shaded. In the UHV target chamber a crystal is positioned in the beam path. The pressure and the target chamber and particles reflected or desorbed from the sample surface are detected by particle detectors. Inert beam flags can be moved into the beam to determine the beam intensity and the sticking coefficient. From Kleyn [22],...

Figure 2. Molecular beam mass spectrometer. Key A, nozzle-skimmer chamber B, middle chamber C, mass spectrometer chamber D, sampling nozzles E, skimmers F, choppers G, gate valve assembly H, mass spectrometer and I, nylon flanges permitting electrical biasing for detection of ions from coal burner.

A few years ago, a powerful version of molecular optical spectroscopy with supersonic beams and jets was developed by Smalley, Wharton and Levy . Supersonic expansion of molecules in an inert carrier gas yields an ideal spectroscopic sample. As a result of the expansion, the translational temperature of the carrier gas decreases to extremely low values (below O.I K). The flow is collisionless so that even extremely unstable species survive. Special attention was paid to fluorescence excitation spectroscopy but the technique is by no means limited to this type of spectroscopy. (Because of fundamental difficulties, however, direct measurement of light absorption in molecular beams is not easy.) Cooled molecules in the beam are electronically excited with a tunable dye laser. The emitted fluorescence is detected and plotted against the wavenumber of the exciting radiation. The obtained fluorescence excitation spectrum is generally very similar to the corresponding absorption spectrum. The technique was used for analysis of the spectra of interesting vdW molecules He. .. NOj, He... Ij, X. .. tetrazine and Xj. .. tetrazine (X = He, Ar, H ) complexes . 

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