Polyatomic monomers

The described overall procedure and the final form of the potential are applicable to any complex of an atom and a linear molecule. They also may be generalized to more complex systems, composed of polyatomic monomers, as exemplified in the next Section by the water dimer. 

Complexes formed from atoms and linear polyatomic molecules are very similar to atom-diatom systems the coupled equations are identical, and the same angular momentum coupling schemes apply. The only added degree of complexity is that perpendicular transitions of the polyatomic monomer are possible, and these introduce an extra quantum number (/ or k) for the monomer vibrational angular momentum. Such states are analogous to those arising from A > 0 states of a symmetric top monomer, as discussed below. 

An atomistic approach, which has relevance to the current work, is the previously discussed normal-mode method. In the normal-mode method the constituent monomer units in the cluster are assumed to interact with a reasonable model potential in a fixed structure. From the assumed structure and model potential a normal-mode analysis is jjerformed to determine a vibrational partition function. Rotational and translational partition functions are then included classically. The normal-mode method treats the cluster as a polyatomic molecule and is most appropriate at very low temperatures where anharmonic contributions to the intermolecular forces can be ignored. As we shall show by numerical example, as the temperature is increased, the... 

The most abundant species in the equilibrium vapor over condensed metals are generally atoms as shown by Knudsen effusion mass spectrometry. The relative abundance of the homonuclear diatomic molecules was determined to be between 10 and 10 percent if they are detectable. Exceptions are bismuth and antimony. The dimer partial pressure over liquid bismuth at temperatures below 1000 K exceeds that of the monomer [83, 164, 165]. The tetramer is the most abundant species over liquid antimony [85]. Other polyatomic homonuclear species have so far been observed under equilibrium conditions for some of the alkali metals, as well as the group Ib, IVb, and Vb metals. Particularly large polymers up to 667 and Sn7 were detected for germanium [84,166,167] and tin [168],... 

Polar molecule a molecule that has a permanent dipole moment. (4.1) Polyatomic ion an ion containing a number of atoms. (2.6) Polyelectronic atom an atom with more than one electron. (7.9) Polymer a large, usually chainlike molecule built from many small molecules (monomers). (22.5)... 

The heat capacity as a function of cluster size was analyzed in a similar manner for several other polyatomic clusters (Wei et al., 1990, 1991a) and the results show that C [H+(NH3)J = 6(n - 1) (Tzeng et al., 1991), while C [H+(CH30CH3) = 6(n - 1) -I- 2n (Wei et al., 1991b). If only the van der Waals modes contributed, the heat capacities would be 6(n — 1) for both clusters because there are n — 1 monomers attached to the protonated core ions. However, in the case of the protonated ether clusters, just as in the mixed ammonia-acetonitrile, there are evidently two low energy ether vibrations which contribute to raise the heat capacity of this cluster. On the other... 

Polyatomic hydrides are a special case in that they have quite large rotational constants. It is to be expected that most Van der Waals complexes involving non-hydride monomers will be much closer to the near-rigid limit, and in some cases their spectra may be best interpreted in terms of conventional near-rigid quantum numbers, although considerable care will be needed in handling tunnelling motions. 

Once again, the body-fixed coupled equations are very similar in form to those for atom-diatom systems the only differences are that the potential matrix elements and the monomer energies takes a more complicated form. Helicity decoupling calculations may be performed in exactly the same way as for atom-diatom and atom-polyatom systems. 

For triatomic molecules many coordinate systems have been used to represent the vibrational motions, see ref. 12 for example. For polyatomic clusters it is possible to imagine a large number of possible coordinate systems and a similar proliferation of Hamiltonians. In this context it should be noted that the derivation and application of Hamiltonians in arbitrary coordinates is far from simple. The choice of an objectively inferior coordinate system for technical reasons is thus a common occur-ance. For polyatomic Van der Waals dimers however,so-called scattering coordinates based upon the interaction coordinate of the two monomers and associated angles of orientation would appear a natural choice. A general, body-fixed Hamiltonian for these coordinates has already been derived... 

The products of UV laser ablation range from atoms and diatoms to small polyatomic molecules and small fragments of the polymer [304, 2031, 2040, 2042, 2045, 2046, 2308]. The product of laser ablation is also wavelength dependent. From poly(methyl methacrylate) of initial molecular weight M = 10 at 193 nm, the products are oxides of carbon, methyl methacrylate monomer and low molecular weight (M = 1500) fragments of the polj er. At 248 nm, the principal product is a low molecular weight fraction (M = 2500) of the polymer. From polyimide, the products are oxides of carbon, benzene, HCN and elemental carbon. 

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