Alkali dimers

Sinha M P, Schulz A and Zare R N 1973 Internal state distribution of alkali dimers in supersonic nozzle beams J. Chem. Phys. 58 549-56... 

As we have seen, the basis set requirements for CC and CV correlation axe very stringent. There is therefore considerable attraction in methods that treat these effects semi empirically. One approach is to treat CV correlation effects by an effective operator. The core polarization potential used by Muller et al. for CV correlation in the alkali atoms and alkali dimers is one such approach [101]. This method has been used successfully for other atoms, such as copper [98]. 

One of the main goals of the crossed-beam experiment is to measure the internal energy AEvlh rol transferred to the molecule. In principle, this is possible in either of two ways. First, the scattered molecules could be detected and their product-state population analyzed. Infrared emission or absorption techniques may be considered, similar to those used in cell experiments.13 21 Although such studies would lead to the most detailed results (at least for polar molecules), under crossed-beam conditions they are impossible for intensity reasons, even if the possibility of measuring differential cross sections is renounced and the molecules in the scattering volume itself are detected. Detection via electronic molecular transitions may be invisaged. Unfortunately, the availability of tunable lasers limits this possibility to some exotic molecules such as alkali dimers. The future development of UV lasers could improve the situation. Hyper-Raman... 

In order to give an idea of these effects, let us consider in greater detail a concrete case of a hydrogen-like molecule, in particular the test molecule Na2 (see Fig. 4.25). In order to evaluate g1 from (4.60) we will consider that the structure of an alkali dimer is completely hydrogenlike, in other words, that only the valence electron participates in the formation of the molecule, and one may assume, following [87], that Z is close to unity. Thus, gj is always positive and does not, in such approximations, depend on v, J and on the electronic term. For alkali dimers the values of g1, as... 

Table 4.2. Measured Lande factor g.j -values for diamagnetic electronic states of alkali dimers and for the tellurium dimer...

Let us first discuss a system which is traditional for optical pumping in the Kastler sense [106, 224, 226], namely an optically oriented alkali atom A (see Fig. 1.1) in a noble gas X buffer surrounding. It is important to take into account the fact that in alkali atoms, owing to hyperfine interaction, nuclear spins are also oriented. However, in a mixture of alkali vapor with a noble gas alkali dimers A2 which are in the 1SJ electronic ground state are always present. There exist two basic collisional mechanisms which lead to orientation transfer from the optically oriented (spin-polarized) atom A to the dimer A2 (a) creation and destruction of molecules in triple collisions A + A + X <—> A2 + X (6) exchange atom-dimer reaction... 

From the shape of the resonances the rotational magnetic moment values were determined for the simplest diatomics H2, D2, HD and later on for the alkali dimers Li2, Na2, Cs2 (see [87]), and others (see also [155, 294]). The drawbacks of the method were connected with the complexity of the set-up and the lack of selection over vibrational and rotational states. 

Sinha, M.P., Caldwell, C.D. and Zare, R.N. (1974). Alignment of molecules in gaseous transport Alkali dimers in supersonic nozzle beams, J. Chem. Phys., 61, 491-503. 

Weber, H.G., Glass, H.-J., Huber, R., Kompitsas, M., Schmidt, G. and zu Putlitz, G. (1974). Optical pumping method for studying nuclear-spin polarization in alkali dimers, Z. Physik, 268, 91-95. 

Despite an extensive study of the reactions of alkali dimers (see the reviews of Grice [208] and Herm [216]), there have been few direct measurements of energy disposal in these systems. The chemiluminescent reactions of oxygen and halogen atoms with alkali dimers have already been described [Sects. 3.1.7(c) and 3.1.8(b)], as have the alkali atom-alkali dimer exchange reactions [Sect. 3.1.2(b)], In this section we describe the chemiluminescent reactions of alkali diatomic molecules with halogen molecules. 

Two particularly interesting chemiluminescent reactions have been discovered using beams of alkali dimer molecules (M2). These species are formed in high yield in an alkali metal nozzle beam source and they can be separated from the remaining atoms by means of an inhomogeneous magnetic field. This technique has made it possible to study reactions of these diatomic species by molecular beam methods for the first time [364] and as a result it has been demonstrated conclusively that reactions of the type... 

The reactions of alkali dimers with halogen molecules also exhibit a multiplicity of reaction pathways with production of electronically excited atoms, electronically excited molecules, and ions all apparently possible [364, 367-370], In these reactions, however, the dominant pathway, with a reactive cross section >150 A2, leads to products in their electronic ground... 

Alkali Dimers.—Harris and Jones118 have also calculated binding energy curves for the ground state of the alkali dimers Lia—Fr2, using the density functional... 

The binding energy curve E °(r) in equation (183) was then found by HJ by evaluating Ev(r) using the frozen core density determined above. The energy curve depends on Rc and can be calculated only for r>2Rc. The usefulness of the procedure lies in the large cancellation of the effects of core renormalization in the molecule and atoms, so that c (r) is much less dependent on Rc than its two components separately. In some cases, such as Cua, HJ find that this error cancellation is almost complete, and the cancellation is substantial even in the heavier alkali dimers, which have very extended cores. 

Regarding the charge density, the alkali dimers behave very differently from the s-p bonds in the first row molecules B2-F2. In particular, in these cases the molecular density at the nuclei is less than in the isolated atoms. 

Table 16 Estimates of binding energy and equilibrium separation of alkali dimers,...

Figure 14 Results of Harris and Jones113 for the ground state x2+ of the alkali dimers.

It has been noted by Wellegehausen (28,29) that one can obtain good laser emission in the A eJ-X eT and B IIy-X Eg systems of Li2 and Na2, only weak laser action in these systems of K2 but, so far, no laser emission has been observed in Rb2 and Cs2. The results presented here, together with unpublished calculations on excited states of LI2 (21) suggest that self absorption of any laser emission should become a more probable event the heavier the alkali dimer. Thus, it appears that Wellegehausen s observations might be explained by such self absorptions. 

The most reliable values for the bond energies of the majority of these molecules have been obtained by using Knudsen effusion mass spectrometry ( ). For some of them, notably the alkali dimers, and Group IIA and Group IIB dimers, as well as for some of their intermetallic diatomic molecules, optical spectroscopic methods have provided the best or only determination of dissociation energies ( 3, ). 

Finally we note that similar scaling procedures to that shown here for the alkali hydrides are discussed extensively for the alkali dimers in this volume (43). 

By optical excitation with argon and krypton laser lines, continuous laser oscillation on A -> X and B -> X transitions of Li Na2 and K molecules can be achieved dimer lasers show such interesting features as multiline emission, extremely low threshold pump intensities and forward-backward amplification asymmetry. Basic principles, operating conditions and applications of these lasers will be discussed. The dimer lasers operate between bound electronic states, resulting in the emission of discrete lines. To achieve tunable laser oscillation, continuous emission bands from bound-free transitions have to be considered. Some possibilities for alkali dimers are outlined and recent spectroscopic investigations on UV excited diffuse bands are reported. 

We report on saturated absorption experiments in Na2> realized with a tunable and stabilized argon laser. These experiments provide both spectroscopic and physical results, which help in understanding the behavior of optically pumped alkali dimer lasers. We briefly describe a new double resonance experiment which enables us to study the gain line-shapes of the dimer laser and to demonstrate the backward-forward gain competition. 

The thermal functions were calculated using a direct summation technique analogous to the alkali dimers and diatomic halogens (12). As in the diatomic fluorine case, the values of G(v), and were directly input into the calculation. Values... 

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