Molecular beams state selection

Undoubtedly, the technique most suited to tackle polyatomic multichannel reactions is the crossed molecular beam (CMB) scattering technique with mass spectrometric detection and time-of-flight (TOF) analysis. This technique, based on universal electron-impact (El) ionization coupled with a quadrupole mass filter for mass selection, has been central in the investigation of the dynamics of bimolecular reactions during the past 35 years.1,9-11 El ionization affords, in principle, a universal detection method for all possible reaction products of even a complex reaction exhibiting multiple reaction pathways. Although the technique is not usually able to provide state-resolved information, especially on a polyatomic... 

It is now possible to design the experiments using molecular beams and laser techniques such that the initial vibrational, rotational, translational or electronic states of the reagent are selected or final states of products are specified. In contrast to the measurement of overall rate constants in a bulk kinetics experiment, state-to-state differential and integral cross sections can be measured for different initial states of reactants and final states of products in these sophisticated experiments. Molecular beam studies have become more common, lasers have been used to excite the reagent molecules and it has become possible to detect the product molecules by laser-induced fluorescence . These experimental studies have put forward a dramatic change in experimental study of chemical reactions at the molecular level and has culminated in what is now called state-to-state chemistry. 

Sometimes the atoms (or molecules) in molecular beams are put into selected electronic, vibrational and rotational states. The initial state selection can be made with lasers. A laser beam of appropriate frequency is shined onto a molecular beam and the molecule goes onto an appropriate excited state. The efficiency of selection depends upon the absorption coefficient. We can attain sufficient absorption to get highly vibrationally excited molecule with the laser. A spin forbidden transition can also be achieved by using a laser. 

Supermolecular spectra could perhaps be studied with state-selection using adequate molecular beam techniques. That would not be easy, however, because of the smallness of the dipole moments induced by in-termolecular interactions. For the purpose of this book, we will mostly deal with bulk spectra, or interaction-induced absorption of pure and mixed gases. A great variety of excellent measurements of such spectra exists for a broad range of temperatures, while state-selected supermolecular absorption beam data are virtually non-existent at this time. Furthermore, important applications in astrophysics, etc., are concerned precisely with the optical bulk properties of real gases and mixtures. 

In preliminary experiments using the new apparatus we have produced rovibrationally state-selected Hj, CO+, NJ, NH3, and NO+ and have begun to study simple bimolecular reactions, for example, H + H2 - H3 + H at low collision eneigy (<0.5 eV) [29], For studying the H2/H2 reaction, where the molecular ions react with the species that itself is the precursor neutral molecule, the beam consists of neat H2 only, backing pressure of 2 bars. However, for other reactions such as CO+ + H2, the ion precursor and reactant gases are coexpanded from the same nozzle. The translational temperature has been measured (see Section IV.B) to be of order 2 K, and the rotational temperatures, determined from REMPI spectra, are typically in the range 5-20 K (except H2). 

Many factors and considerations are germane to future research in this area. On the technical side, achieving cluster size selection stands as one of the most important and sought-after goals. It would be most desirable to achieve this while maintaining sufficiently high densities for studies of photoinitiated reactions to be carried out with product state resolution and/or ultrafast time resolution. The two methods that, in our opinion, are most viable are molecular beam deflection, as pioneered by Buck (1994) and coworkers, and laser-based double-resonance methods. Less direct approaches are deemed inferior. 

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].

Finally, case (iv) leads to the observation of enhanced stimulated absorption again molecular beam techniques which select the lower (a) state can be used. It is interesting to note that all four population cases are observed in different interstellar gas clouds. 

Figure 8.1. Principles of magnetic state selection and molecular beam magnetic resonance.

As the name suggests, electric resonance experiments make use of electric fields to achieve molecular state selection. Figure 8.25 shows a schematic diagram of a molecular beam electric resonance instrument, which we will discuss in more detail when we describe experiments on the CsF molecule. In contrast to the magnetic resonance apparatus discussed earlier, the A, B and C fields in figure 8.25 are all electric fields. In... 

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