Molecular Beam Source

As well as producing higher beam intensities, the supersonic nozzle sources have a further advantage. In the supersonic expansion through the Laval slit the gas is adiabatically cooled to very low temperatures of ca. 40°K [187] but increases its translational energy in the beam direction to the flow velocity of the gas, e.g. Mach number ca. 15 or peak velocity 70% higher than the most probable velocity of a 900°K oven [187]. Supersonic nozzles produce very narrow velocity distributions compared with the Maxwell—Boltzmann distribution obtainable from an effusive oven at the same temperature. 

The intermediate translational energy range of ca. 0.25—20 eV is conveniently covered by sputter-ion sources [202—207]. This technique 

Gersh and Bernstein [145] have predicted that this intermediate energy region should include thresholds for several interesting reactions 

Hebbing and Rothe [209] have measured the threshold for the reaction Cs + Br2 Cs + Br2 

Obviously, due to the many possible reaction channels which are open in this energy range, a simple surface ionization detector (see Section 

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To date, the IR-CRLAS studies have concentrated on water clusters (both FI2O and D2O), and methanol clusters. Most importantly, these studies have shown that it is in fact possible to carry out CRLAS in the IR. In one study, water cluster concentrations in the molecular beam source under a variety of expansion conditions were characterized [34]- hr a second study OD stretching bands in (020) clusters were measured [35]. These bands occur between 2300... 

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],... 

Figure 3.9. Transient C02 formation rates on Pd30 (a) and Pd8 (b) mass-selected clusters deposited on a MgO(lOO) film at different reaction temperatures [74]. In these experiments CO was dosed from the gas background while NO was dosed via a pulsed nozzle molecular beam source. The turnover frequencies (TOFs) calculated from the experiments displayed in (a) and (b) are displayed in the last panel (c). C02 formation starts at lower temperatures but reaches lower maximum rates on the larger cluster. (Figure provided by Professor Heiz and reproduced with permission from Elsevier, Copyright 2005).

The apparatus shown in Fig. 2 consists of three main components two rotatable molecular beam sources, laser ionization and TOF spectrometer. 

The ideal free radical molecular beam sources for photodissociation dynamics studies should have the following features. 

Figure 18. Illustration of the molecular beam-electron beam crossing region used in the electron impact ionization experiments. The molecular beam source, buffer, and hexapole chambers are similar to those shown in Figure 3.

Figure 20.2. Schematic outline of typical pump-probe-detect experiments with femtosecond pulses, a molecular beam source, and mass spectrometric detection of transient species. Computer control and data processing instruments, as well as various optical components, are not shown. The time separation Af between pump and probe pulses is dictated by the difference in optical path lengths. Ad, traversed by the two components of the original pulse.

The experimental setup used by our group is shown in Figure 7-1. This apparatus consists of a molecular beam source coupled to a chamber housing the quadrupole mass spectrometer. The continuous beam source consists of a Campargue-type nozzle, an expansion chamber, and a collimation chamber. The nozzle assembly itself is mounted on a micrometer and is fitted with a gas handling line which... 

Figure 2 Schematic view of the apparatus used in studies of the steric effects in gas-surface scattering. A detail of the crystal mount with die orientation rod at 1 cm in front of the surface is shown in die right hand corner. A detailed drawing of the hexapole state selector is given below the main figure. The voltage is applied to die six small rods indicated by an arrow. Key Q quadrupole mass spectrometer R Rempi detector M, crystal manipulator SI, beam source for state selected molecules H electric hexapole state selector C mechanical beam chopper V pulsed gas source S2, continuous molecular beam source. From Tenner et al. [34].

During operation of the molecular beam sources infrared measurements in reflection absorption mode (IRAS) are possible. The time resolution per IR spectrum is 38 ms that allows following the variation of coverage during a pulse of reactants (typical duration several seconds). However, to have an acceptable signal/noise ratio it is necessary to accumulate successively around thousand spectra, that needs to have a perfectly periodic beam modulation and a stationary kinetic phenomenon during the total duration of the measurements (typically 20 min at a resolution of 8 cm-1 for 30 spectra on a CO pulse of 1 s). 

The generation of reactive species by a fast pre-reaction is a well established process in kinetics, particularly in flow systems, although this technique is now being used in molecular beam sources [see (f), below]. As atoms are commonly generated by thermal or microwave dissociation (see above), reactions are mainly used to produce radicals. However, some atoms may be produced by reaction as a more convenient alternative to direct vaporisation or dissociation. Scandium and yttrium atoms have been generated [25] by the reaction... 

The subject of molecular beam kinetics is very extensive and in this section, therefore, we will deal only briefly with the relevant aspects of the topic. Molecular beam sources are often thermal, operating as a flow system with a gas or a vapour from a heated oven. The velocity distribution of species in such beams is Maxwell—Boltzmann in form. For many experiments, this does not provide sufficient definition of initial translational energy and some form of velocity selection may be used [30], usually at the expense of beam intensity. 

Molecular beam sources for use at high collisional energies have also employed the methods of charge exchange [41], sputtering [42] and rotor beams [43]. Pulsed metal beams have been generated by the novel method of the evaporation of a metal film by a high power pulsed laser [44],... 

Figure 8.2. Simplified block diagram of the apparatus used to study the magnetic resonance spectrum of H2. The molecular beam source was cooled to liquid nitrogen temperature. M denotes fractionating pumps, whilst S and F are diffusion pumps. The appropriate molecular trajectories are not shown in this diagram the reader is referred back to figure 8.1.

Kaiser s studies employed a conventional spectrometer with A and B electric quadrupole fields, and by passing the HC1 gas through a microwave discharge situated prior to the molecular beam source, populations in the ratios 21 3 1 for the v = 0, I and 2 vibrational levels were obtained. An effusion source was operated at 170 K and line widths close to 1 kHz were obtained similar studies of DC1 were described, except that in this case the gas was preheated to 1440 K to produce increased vibrational excitation. Kaiser was able to observe spectra of H35C1 in J = 1, v = 0, 1,... 

In addition to the extensive data on the yl-doublet transitions of OH already described, Meerts and Dymanus [142] also measured similar spectra of the species OD, SH and SD. The SH and SD radicals were produced by reacting H atoms with either H2S or D2S in the molecular beam source. In all cases hyperfine components of the A -doublet transitions were measured for a number of rotational levels in both fine structure states. The theoretical analysis of the spectra was similar to that already described for OH, with the addition of deuterium quadrupole interactions in OD and SD. In table 8.26 we fist the A-doubling and nuclear hyperfine constants determined for the four species. Note that the parameters fisted above for OH differ slightly from those given subsequently by Brown, Kaise, Kerr and Milton [115]. 

Figure 1.2 Representation of a simple crossed-molecular-beam source [16]. The primary beam effusing from an oven source (A) is velocity selected (S) and then crosses the thermal beam issuing from a second source (B). This diagram shows the detector (D) positioned at the lab angle 0.

The essential elements.of the experiment are a) an effusive molecular beam source, b) inhomogeneous deflecting electric polefaces, c) surface ionization detector, capable of translation in order to obtain the deflected beam pattern. 1, 2, are the distance from the source to the front of the polefaces, the length of the polefaces and the distance from the back of the polefaces to the detector, respectively. A general review of deflection methods for determining polarizabilities is given by Miller and Bederson (8). 

Molecular Beams—Kinetics and Mechanism. —An outstanding example in this field is by Wachs and Madix who employed a modulated molecular beam source to study the reaction of HCOOH on Ni(llO). Products were examined individually as was the phase-lag in their appearance in their quadrupole MS detector. They could measure reaction rates for coverages of 10 to 1 monolayer and reaction times from 10 to 10 s over a temperature range of 400—800 K. The processes investigated were adsorption desorption of HCOOH, and HCOOH -> CO2, CO, H2, and H2O. Their findings on selectivity and rates for all possible reactions can be found in their paper. 

Binary Compounds.—Halides. According to the results of molecular beam source mass spectrometry of MF5 (M = Nb or Ta), the vapour phases of these halides contain dimers and trimers niobium fluoride vapour contains ca. 98 % Nb3pi5, less than 2% monomer, and only trace amounts of the dimer/... 

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