VUV beam

Figure 11.3 schematically shows the experimental setup. The approach consists of a pulsed nozzle, skimmer, and reflectron TOF mass analyzer. The 125 nm VUV beam was introduced along the jet axis and focus in the ionization volume. A doubled NdiYAG laser (532 nm) was used for desorption, with pulse energy densities of 10 to 100 mj /cm and a spot size of about 1 mm diameter. Vaporization was caused by substrate heating only. Optimal injection of the vapor into the jet expansion occurred when the desorption spot was about 1 mm in front of and 0.5 mm below the nozzle, and the desorption laser was fired near the peak of the jet gas pulse. The pulsed valve was operated with Ar or Xe at a backing pressure of 8 atmospheres. 

Kotationally cold OCS molecules are ejected from a pulsed supersonic jet. The molecular beam is dissociated by a pulsed polarized laser beam at 222 nm. The internal states of the CO fragments are monitored by a circular polarized tunable VUV beam aligned perpendicularly to the dissociation beam. The VUV beam monitors the various vibrational rotational states through the CO s X A ir transition. It is shown that the CO photofragments are produced in the vibrationless ground state with a highly excited rotational distribution sharply peaked at J = 56. 

The SU5 beamline [89] at the Super-ACO synchrotron (LURE, Paris ) employed an electromagnetic undulator to produce fully variable polarization in the VUV region [83, 90, 91]. This beamline was equipped with a gas filter for the suppression of unwanted higher order radiation [92] and had a VUV polarimeter [93] permanently installed just before the experimental chamber that could be rapidly lowered into the beam for polarization determinations. Full polarization analyses had been performed in commissioning, with 53 values ranging from 0.9 to 0.96 for rep and from 0.9 to 0.99 for Icp [93]. The remainder was determined... 

The apparatus consists of a pulsed molecular beam, a pulsed ultraviolet (UV) photolysis laser beam, a pulsed vacuum ultraviolet (VUV) probe laser beam, a mass spectrometer, and a two-dimensional ion detector. The schematic diagram is shown in Fig. 1. 

Fig. 1. Schematic diagram of the multimass ion imaging detection system. (1) Pulsed nozzle (2) skimmers (3) molecular beam (4) photolysis laser beam (5) VUV laser beam, which is perpendicular to the plane of this figure (6) ion extraction plate floated on V0 with pulsed voltage variable from 3000 to 4600 V (7) ion extraction plate with voltage Va (8) outer concentric cylindrical electrode (9) inner concentric cylindrical electrode (10) simulation ion trajectory of m/e = 16 (11) simulation ion trajectory of rri/e = 14 (12) simulation ion trajectory of m/e = 12 (13) 30 (im diameter tungsten wire (14) 8 x 10cm metal mesh with voltage V0] (15) sstack multichannel plates and phosphor screen. In the two-dimensional detector, the V-axis is the mass axis, and V-axis (perpendicular to the plane of this figure) is the velocity axis (16) CCD camera.

The molecular beam, photolysis laser beam, and VUV laser beam are perpendicular to each other. However, the crossing points of the photolysis laser beam and the VUV laser beam with the molecular beam are not the same. The crossing point of the photolysis laser beam with the molecular beam is adjustable. It is 3-10 cm upstream with respect to the crossing point of the VUV laser with the molecular beam. For short distances (<3cm) between these two crossing points, the direction of the UV laser beam is changed. It is in the plane formed by molecular beam and VUV laser beam, but the angle between the photolysis laser beam and the VUV laser beam is 15 degrees. 

One of the key components in the system is the mass spectrometer. Fragments are ionized by a VUV laser pulse between a pair of plane parallel-plate electrodes (6 x 14 cm). One of the plane electrodes has a slit of 1 x 10 cm, which is covered by a metal mesh. The slit is parallel to the VUV laser beam and is the entrance of the mass spectrometer. Ions are accelerated by a pulsed electric field present between the plane parallel-plate electrodes, and then pass through the slit before they enter the mass spectrometer. 

In this chapter we have discussed the successful implementation in our laboratory, for the first time, of the soft (i.e. low energy) electron-impact ionization method for product detection in crossed molecular beams reactive scattering experiments with mass spectrometric detection. Analogous to the approach of soft photoionization by tunable VUV synchrotron radiation,... 

Previous workers had used the molecular beam TOF technique (134) and the VUV flash photolysis LIF technique (135). Ling and Wilson (136) had suggested that either the A(2n) state of CN is produced in the original photolysis process or that I atoms were produced in the Pi/2 and 3/2 states. It had been previously shown (135), by collisional quenching studies, that the A state of CN was not produced. This earlier work has been reviewed by Baronvaski (137) but recently both he and others have done further work on this molecule using excimer laser sources in both static gases and pulsed molecular beams. 

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