Alkali halide dimer

See Jordan, K. D. Kurtz, H. A. "Theory of Metal Atom-Water Interactions and Alkali Halide Dimers" ACS Symp. Series,... 

Such plots are shown In references Aa and Ab for a range of stoichiometries. These Include the alkali halides, alkali halide dimers, diatomic oxides, nitrides, and sulfides, and molecules of the type MX2. Good agreement with the model is obtained with many species. Including those which are non-lonlcal-ly bonded. 

Milne and Cubicciotti(14) have applied ionic models to calculate the heats of dimerization for gaseous alkali halide dimer molecules, and the results are as follows ... 

Electric Dipole Polarizabilities of Alkali Halide Dimers... 

In Figure 3 we show a typical deflection pattern for one of the alkali halide-dimer combinations (RbCl). 

In fact, the situation with regards to the excited states of the monomers Is not much better. The first values for polar-zabllltles of some of the alkali halide dimers were reported at this Symposium by Bederson and co-workers (18). Experimental values of the polarizabilities of the monomers are not available and would be difficult to determine by deflection studies due to contributions resulting from the permanent dipole moments. 

It appears unlikely that the cyclic dimers will have stable anions. Calculations (32) on cyclic (L1H)2 failed to give a stable anion. It Is likely that the quadrupole fields of the cyclic alkali halide dimers are also too small to bind an electron. (Be0)2, which has a much larger quadrupole field, has been shown (33) to have a stable anion. The dissociative attachment process ... 

The remainder of this section will be devoted largely to a summary of recent results from our group on the polarizabilities of LIF and (L1F)2- These represent, respectively, the first calculation of an alkali halide polarizability In which electron correlation effects have been included and the first calculation of a polarizability of an alkali halide dimer. The section concludes with a summary of recent theoretical results for the Ionization potentials for (LlF)jj, n 1-4. 

J., "Electric Dipole Polarizabilities of Alkali Halide Dimers", presented at 181st National ACS Meeting, Atlanta, 1981. 

The molecular structure and K-F bond distance were determined by Akishin and Rambidi (2), using electron diffraction with sector microphotoraetry. The bond angles are estimated by comparison with other alkali halide dimers. The vibrational frequencies are taken from Berkowitz (3 ) which were calculated based on an ionic model. The three principal moments of... 

White, et al., ( ) from their spectral data have assigned two observed frequencies and estimated the remaining frequencies using the ionic model calculations of Berkowitz (2) as a guide. They proposed a planar rhombic structure of symmetry, similar to the alkali halide dimers, for the previously undetected LigO molecule, and estimated the angle, 116 , and bond distance, 1.90... 

Fourier transform techniques have become very powerful, particularly when used with pulsed sources of the sample. They are ubiquitous in studies of van der Waals molecules, in which two or more entities are very weakly bound together in the gas phase, such as HCT Ar, but are also used to determine structures of many other compounds, of which the mixed alkali halide dimer LiNap2 is an example. In a typical experiment, the sample is introduced into the cell in a pulsed supersonic jet expansion, the rotational temperature being reduced to a few Kelvin. A pulse of microwave radiation then aligns the dipole moments of the molecules, so that the sample is polarized on a macroscopic scale. The subsequent decay of this polarization is recorded, and Fourier transformation of the free induction decay yields the spectrum [9]. 

There are monomers, dimers and sometimes trimers in the vapors of alkali halides. It can be shown that Eq. (22) is valid not only for the total pressure but also for the partial pressures of each species. 

Many investigations of salt vapors have demonstrated that alkali halide vapor molecules associate.8 Theoretical calculations concerning the relatively simple dimers have been very difficult and have required a large amount of input data. Calculations concerning the trimers are even more difficult and have never been attempted. The techniques of dimensional analysis can be of value in reducing the amount of input data necessary and also in making possible the calculation of relative quantities where the systems could not be treated otherwise. ... 

Fig. 3.4. Processes defending the survival probability of F centres in alkali halide crystals 1 -tunnelling recombination of close F, H defects, 2 - their annihilation, 3 - trapping of mobile H centre at impurity, 4 - formation of immobile dimer centre, 5 - H-centre leaves its geminate partner in random walks on a lattice.

A large body of experiments (e.g., [70, 73, 74]) deals with the kinetics of dimer centre accumulation in alkali halide crystals it was observed experimentally that n (F2) increases often as squared single-vacancy concentration, n2(F). In our formalism, n(F2) (iV)n(F). As it is well seen in Fig. 7.11, (N) turns out to be a linear function of 77(F) in a wide range of defect... 

Lastly, the present theory confirms the quadratic dependence between the concentration of dimer F2 centres (also called M centres [9, 12]) and concentration of single F centres - observed more than once experimentally in alkali halide crystals [70, 73, 75],... 

Figure 7.15 shows the joint correlation function of similar defects in the region of concentration saturation as calculated from the results of the simulation. We see that the fraction of the close Frenkel defects (of the type of dimer F2-centres) exceeds by approximately threefold the value expected in the Poisson distribution, which agrees well with the analytical theory presented in Section 7.1 for the annihilation mechanism (see also [31, 111]) and with actual experiments for alkali halide crystals [13]. 

Several alkali halides have been studied at elevated temperatures394 and dimeric species forming four-membered rings have been found to predominate. There are indications of rather large amplitude vibrations in these dimers. It should be pointed out that monomeric species are also present in addition to the predominating dimers and several of these have been indentified by microwave spectroscopy44 ... 

Surface ionisation on a hot wire or ribbon is restricted to substances with low ionisation potentials and has been mainly used for detecting alkali atoms, dimers and halides and a few other similar species [30]. There is evidence that the efficiency of surface ionisation is dependent on the degree of internal excitation [104]. 

The calculated stability of Li H (13) agrees well with our experimental results, which place tRe dimerization energy of LiH close to that of LiBr given above. This supports the statement that the LiH dimer is similar to the alkali halide(9). 

Photoionization and Photoelectron Spectroscopy of Alkali Halide Monomers, Dimers, and Trimers... 

The ultraviolet photoelectron spectra of diatomic alkali halide molecules are reviewed and interpreted. Data for lithium halide dimers, 112X2> are presented and it is shown that the dimers have significantly larger ionization thresholds than the corresponding monomers. Some historical controversies regarding the presence of dimers and their ionization energies are clarified. Photoionization mass spectrometry is used to determine the adiabatic ionization potential of lithium chloride trimer, in order to probe the trend of I.P. with cluster size. The predictions of Hartree-Fock, Xa and ionic model calculations on this point are presented. 

It is now a well-recognized fact that the saturated vapors of alkali halides contain associated species (dimers, trimers, etc.) as well as the diatomic molecules. The first evidence of this behavior was an electron-Impact mass spectrometrlc study by Ionov (] ). He reported the observation of K2I and Na2l" , which he attributed to the process... 

We had previously used this technique for studying several alkali halides, including Nal, (23) the cesium halides (2A) and the rubidium halides. (25) The apparatus consisted of a one-meter normal incidence monochromator and a magnetic mass spectrometer. In these earlier studies, ionic species attributable to dimer and monomer were studied, but the intensity of trimer was too weak for measurement. With our more recent interest in trimer, we once again turned to the lithium halides. This time, our apparatus consisted of a three-meter normal incidence monochromator and a quadrupole mass spectrometer. (7 )... 

This manuscript summarizes the most significant current experimental information regarding the photoionization behavior of alkali halide monomers, dimers and trimers. Some data exist on higher ionization potentials, (30) and ionic model calculations have also been applied to these higher levels. (19, 30)... 

Previous experimenters have found dimerization in all alkali halides at vapor pressures corresponding to source temperatures appropriate for effusive beams ( 5, 6, T). The percent dimerization increases as molecular weight decreases towards lighter alkali halides, with lithium fluoride possessing the largest fractional dimerization. 

We now briefly discuss the deflection patterns to be expected when alkali halide monomers and dimers pass between the polefaces of an inhomogeneous electric field (a detailed analysis of the deflection pattern due to molecules possessing strong electric dipole moments will be presented elsewhere). The geometry of the experiment is shown in Figure 1. 

These led to matrix studies of the structure of ion pairs and triple ions, such as the thorough studies by Devlin and coworkers on matrix isolated alkali nitrate (21), chlorate (22) and perchlorate ion pairs (23 ). For relatively simple salts, such as the alkali halides, investigations were conducted into the structure of the dimeric salt species (6, 7, ), which is present in a gas phase equilibrium with the monomeric salt species. These dimers have been found to be very strongly bound in a cyclic structure. 

Only occasionally have salt molecules been vaporized for use as a reactant toward another species in matrix isolation studies. Devlin (24,25,26) conducted extensive experiments in which salt molecules were vaporized and condensed into argon matrices containing from 1% to 90% H2O or NH3, to study the effects of stepwise solvation of the salt molecule, as a model for solution studies. Margrave (27 ) and Snelson (28) each have used salt molecules as reactants, but most commonly toward another salt molecule to form a mixed salt dimer. The work described below, which was initiated at the University of Virginia and has been continued at the University of Cincinnati, employs alkali halide salt molecules as reactants toward a variety of species, including both Lewis acids and Lewis bases. The initial intent was to react a salt molecule such as NaCl with HCl in an excess of argon to bring... 

---