Diatomic trimers

The leading nonadditive term in the many-body expansion of a potential is the three-body interaction. Similarly like dimers, trimers (and larger clusters) can be selectively studied by molecular beam spectroscopy. A number of such trimers have been the subjects of investigations. Among them are the Rg2-diatom trimers mentioned above, with the most extensive data available for Ar2-HF [64]. Both empirical [29,30] and ab initio [33] nonadditive potentials have been obtained for this system. A large number of spectral data are available also for the water trimer [65,66]. An accurate three-body potential for water has recently been developed [34]. 

The last example of a sequential approach is from Sanov (excerpt 130). A series of increasingly complex experiments is proposed to study the photochemistry of 02, and OCS . Sanov begins with the easier diatomic anions (02 and which will serve as prototypes for subsequent experiments. Next, he will study a larger, polyatomic anion (OCS ) and its cluster ions, 0CS (H20)]j. In the future, he will study even larger dimers and trimers (OCS)n (n > 2) and their hydrated counterparts. 

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... 

Figure 7 Phytochelatin concentrations in the marine diatom Thalassiosira weissflogii as a function of log Cd ) (unchelated cadmium concentration). Chain lengths for each phytochelatin dimer (circles) trimer (triangles), and tetramer (squares) (after Ahner et al, 1994).

At large atom-diatom separations R, the interaction potential can be approximated by an effective potential, composed of a repulsive centrifugal term and the long-range interaction potential. For alkali-metal trimer systems, the atom-diatom long-range potential is a van der Waals interaction potential and behaves as The classical... 

As we have discussed earlier, with the two-body potentials of equations (1) and (2), it was necessary to invoke a special set of rules to allow only free atoms and diatomic molecules in the system, and not trimers, quadrumers, etc. These rules were arbitrary, and may not give a proper phenomenological description of the dissociation and the formation of diatomic molecules. It was of interest to introduce three-body interactions, following Murrell, et al. [6], as described in Section 2, to see what difference they would make in the dissociation and association processes. 

Most of the pump control experiments carried out so far have used diatomic molecules, because in such simple systems, with only one vibrational degree of freedom, the dynamics can be controlled relatively easily. In larger molecular systems with three or more vibrational degrees of freedom, the situation becomes much more complicated and it is an interesting question whether the concept of controlled molecular dynamics can still be realized. Here, it is shown that different vibrational modes of the sodium trimer can be selectively excited during an electronic excitation with ultrashort laser pulses. For this reason, it should in future be possible to control subsequent reactions. The relevant control parameter in these investigations is the duration of the pump pulse. 

Applications of relativistic quantum mechanical techniques to chemical problems have been numerous in the last several years. These techniques have been applied to both diatomics and polyatomics containing very heavy atoms. The author has written several reviews " which have dealt with the applications of these techniques to main group hydrides, halides and chalconides, main group dimers and trimers, and actinide and lanthanide containing molecules. In this section we shall give very selected applications to some very heavy molecules to demonstrate the power of these techniques. The readers are referred to these extensive reviews on the topic for further details. 

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