Supersonic expansion continued

The ability to detect discrete rovibronic spectral features attributed to transitions of two distinct conformers of the ground-state Rg XY complexes and to monitor changing populations as the expansion conditions are manipulated offered an opportunity to evaluate the concept of a thermodynamic equilibrium between the conformers within a supersonic expansion. Since continued changes in the relative intensities of the T-shaped and linear features was observed up to at least Z = 41 [41], the populations of the conformers of the He - lCl and He Br2 complexes are not kinetically trapped within a narrow region close to the nozzle orifice. We implemented a simple thermodynamic model that uses the ratios of the peak intensities of the conformer bands with changing temperature in the expansion to obtain experimental estimates of the relative binding energies of these complexes [39, 41]. 

The instrument in my laboratory uses laser desorption ionization with a Nd YAG laser and a TOF-MS. The particles are drawn into the instrument on a continuous basis and undergo a supersonic expansion when they pass through the inlet nozzle. During the expansion, the particles pick up different speeds that are a function of their size. They then pass through two scattering lasers. The time it takes the particle to travel between the two lasers can be correlated with particle size, allowing the particle size to be determined precisely. Knowing the particle speed and position, it is possible to time its arrival at the center of the spectrometer with a Nd YAG laser pulse (266 nm). The pulse is able to desorb ionized species from the particle, which can then be analyzed by the spectrometer. 

An example of an alternative use of FT technology in the UV/VIS is our work on the X2Z" B2Z+ emission spectrum of jet-cooled CN [21], These experiments were made possible by the development of the corona-excited supersonic expansion source by Engelking [20]. The Engelking source creates radicals in a continuous discharge, followed by immediate cooling in the expansion. A high number density of rotationally and translationally cold radicals in excited electronic and vibrational states is produced. As a result, excited vibronic states of reactive species can be studied with a minimum of rotational congestion. 

Two recent papers [1,2] provide updated views of advances in the produetion of intense and continuous beams of aligned molecules. In [1], it was demonstrated that in the prototypical case of a seeded supersonic expansion of a moleeular beam of benzene, besides acceleration and eooling, orientation of the molecular plane also oeeurs beeause of the anisotropy of the intermolecular forees which govern eollisions. This work is reviewed in Sec.2.1. Previous studies on the eollisional alignment of the rotational... 

Figure 3.2. Different methods of gas sampling during thin-film deposition using a continuous gas stream (supersonic expansion, on the left) and effusion (on the right) the broken line indicates the different sampling volumes with collision-free transfer into the analysis chamber.

Quack, M., Schmitt, U., and Suhm, M. A., Evidence for the (HF) complex in the HF stretching FTIR absorption spectra of pulsed and continuous supersonic jet expansions of hydrogen fluoride, Chem. Phys. Lett. 208, 446-452 (1993). 

FIb.1. Schematic diagram of the apparatus. A continuous supersonic Hg/Ar expansion produces the clusters. They can be ionised by electron or i oton impact. An electric pulse between the plates IH) and PI accelerates ions into the time-of-fiight mass spearometer. The first time focus is at the slit of the mass selector, the second is at the detector II. The mass selector can be used to select one mass only, say Hgjo, which can subsequently decay in the free-flight region into Hg, + Hg due to internal excitation. The reflector can separate parent (n = 20) and daughter (r — 19) cluster ions. 

If we allow for a further expansion at the nozzle exit, i.e. choose riim < li the theoretical model predicts a continuously increasing, supersonic velocity for pressure ratios bounded by... 

The motive nozzle is shaped like a Laval nozzle. This means there is an enlargement of the diameter after the smallest cross section. This is necessary to achieve velocities higher than sonic speed. For steam an expansion pressure ratio of only Pi/Po = is sufficient to just achieve sonic velocity (critical pressure ratio). At higher expansion ratios (supercritical pressure ratios), the exact critical pressure and sonic speed is achieved in the smallest cross section. In these cases in the divergent part of the motive nozzle, a supersonic velocity results from a continuing expansion. Owing to the blocking of the velocity to the sonic speed in the smallest cross section, the mass flow rate of such a supersonic nozzle only depends on the state of the motive media in front of the nozzle and of course on the diameter d.. Here the mass flow rate is proportional to the motive pressurep. ... 

Finally, a low temperature chemical reactor can be obtained using collimated uniform supersonic flows generated by the isentropic expansion of a buffer gas through an axisymmetric convergent-divergent Laval nozzle. Uniform supersonic flows were initially developed in a continuous flow version for the study of ion-molecule reactions. Later, the methodology was adapted to the study of reactions between neutral species in a continuous flow version and also in a pulsed flow version.This technique known as CRESU will be described in detail in Section 2.3. 

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