Supersonic beam apparatus

Figure 7. Schematic diagram of the supersonic beam apparatus which combines laser-induced fluorescence spectroscopy with time-of-flight mass spectrometry. Reproduced with permission from Ref [92a].

Several instniments have been developed for measuring kinetics at temperatures below that of liquid nitrogen [81]. Liquid helium cooled drift tubes and ion traps have been employed, but this apparatus is of limited use since most gases freeze at temperatures below about 80 K. Molecules can be maintained in the gas phase at low temperatures in a free jet expansion. The CRESU apparatus (acronym for the French translation of reaction kinetics at supersonic conditions) uses a Laval nozzle expansion to obtain temperatures of 8-160 K. The merged ion beam and molecular beam apparatus are described above. These teclmiques have provided important infonnation on reactions pertinent to interstellar-cloud chemistry as well as the temperature dependence of reactions in a regime not otherwise accessible. In particular, infonnation on ion-molecule collision rates as a ftmction of temperature has proven valuable m refining theoretical calculations. 

A technique which is not a laser method but which is most useful when combined with laser spectroscopy (LA/LIF) is that of supersonic molecular beams (27). If a molecule can be coaxed into the gas phase, it can be expanded through a supersonic nozzle at fairly high flux into a supersonic beam. The apparatus for this is fairly simple, in molecular beam terms. The result of the supersonic expansion is to cool the molecules rotationally to a few degrees Kelvin and vibrationally to a few tens of degrees, eliminating almost all thermal population of vibrational and rotational states and enormously simplifying the LA/LIF spectra that are observed. It is then possible, even for complex molecules, to make reliable vibronic assignments and infer structural parameters of the unperturbed molecule therefrom. Molecules as complex as metal phthalocyanines have been examined by this technique. 

In 1967, Yuan T. Lee (1936- ) joined Herschhach as a researcher after completing his Ph.D. at Berkeley. Lee designed and built an apparatus with supersonic beam nozzles and an electron bombardment ionizer that functioned as a universal detector. Supersonic beams propel species in the same direction at nearly the same speed and allow very few collisions. For this reason, chlorine atoms, much more reactive than potassium atoms, could be employed in the new and even more sensitive apparatus ... 

Figure 24.1 shows a schematic layout of a crossed-beam apparatus in which the Na (FCH3) (n = 1 to 5) clusters were produced by the pick-up technique. For this, a hot effusive beam of Na atoms is crossed with a pulsed, cold supersonic beam of FCH3. 

The details of the crossed-beam apparatus used in our experiment can be found in many earlier publications [17,18]. Briefly, the alkali dimer source consisted of a resistively heated molybdenum oven and nozzle assembly, with the temperatures of the nozzle and the oven being controlled independently by different heating elements. Sodium vapour carried by an inert gas, which was either He or Ne, expanded out of the 0.2 mm diameter nozzle to form a supersonic beam of Na/Na2/inert gas mixture. The Na2 concentration was about 5% molar fraction of the total sodium in the beam when He was used as carrier gas. The beam quality dropped severely when we seeded Na2 in Ne so the dimer intensity became much weaker. No substantial amount of trimers or larger clusters was detected under our experimental conditions. The Na2 beam was crossed at 90 by a neat oxygen supersonic beam in the main collision chamber under single collision conditions. The O2 source nozzle was heated to 473 K to prevent cluster formation. Both sources were doubly differentially pumped. The beams were skimmed and collimated to 2 FWHM in the collision chamber. Under these conditions, the collision energies for the reaction could be varied from 8 kcal/mol to 23 kcal/mol. 

The Molecular Beam Machine and the Detection System. The second component of the real-time MPI experiments is a molecular supersonic beam machine [116] with a quadrupole mass spectrometer (QMS), allowing the detection of ionized molecules and clusters with high sensitivity. A side elevation is shown in Fig. 2.17. The production of the molecular beam and the interaction of the laser pulse trains with the molecular beam are performed in a differentially pumped vacuum apparatus consisting of two separate chambers, which are briefly described in the following two paragraphs. A more detailed sketch of the two-chamber system is presented in Fig. 2.18. The production sub-chamber (oven chamber) is pumped by a 3000 /s oil diffusion pump (Balzers) with a baffle at the flange to the oven chamber to allow a pressure in the chamber of less then 10 mbar. During the experiments the pressure is typically 5 x 10 to 3 x 10 bar. In the second chamber a maximum pressure of 10 mbar is established by a 2200 /s turbomolecular pump (Balzers). 

Figure 7.3 Apparatus for flash studies of fast reactions in molecular beams (1). Schematic drawing of a molecular-beam flash apparatus. The pump and probe pulses (see text. Section 4.2.4.3) are produced by a tunable dye laser, a beam-splitter, and a delay line, not shown in the figure (see Figure 7.4). The two pulses are recombined by the beam-splitter BS and sent coaxially into the molecular-beam apparatus. A supersonic jet of (e.g.) argon gas is generated by expanding the gas through a nozzle into a vacuum chamber (not shown), for time-of-flight measurements. This apparams was used in smdies of the dissociation of iodine molecules and subsequent recombination (see below. Section 7.3.4.1 and Ref. [17]). Monitoring was by laser-induced fluorescence (LIF).

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],... 

The LC-MS with a supersonic molecular beam (LC-SMB-MS) apparatus, which is schematically shown in Figure 8.13, is based on a modified homemade GC-MS with an SMB system that was previously described [76]. The heart and soul of this system is the soft thermal vaporization nozzle (STVN) chamber. The STVN accepts the liquid flow from the LC or the flow injection liquid... 

The experimental apparatus, as shown in Figure 11-1, was a standard molecular beam machine with a heated pulsed valve for vaporization of the non-volatile species and for supersonic cooling. Samples of 1-methyluracil, 1,3-dimethyluracil and thymine were purchased from Aldrich Co. and used without further purification. The sample 1,3-dimethylthymine was synthesized from thymine following a literature procedure [33], and its purity was checked by nuclear magnetic resonance (NMR) and infrared absorption (IR) spectroscopy. The heating temperatures varied for different samples 130°C for DMU, 150°C for MU, 180°C for DMT, and 220°C for thymine. No indication of thermal decomposition was observed at these... 

Figure 11-1. Experimental apparatus. The sample is supersonically cooled and intercepted by counter-propagating laser beams. Both fluorescence and ion signals can be observed...

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