Molecular Clusters:  Structure and Dynamics of Weakly Bound Systems

Abstract

Weakly bound atomic and molecular clusters can be viewed as finite-size “pieces” of the condensed phases of matter. From both the experimental and theoretical points of view, the finite size of these clusters is of great advantage in simplifying the system, permitting molecular level characterization of solvent effects on the solute spectroscopy and chemical reaction dynamics. In addition, the ability to vary the number of atoms or molecules in the cluster enables us to observe how its various structural and dynamical properties evolve with size, from those typical of isolated gas phase molecules toward the bulk limit. By far the most detailed information has been obtained for binary complexes, where the issues of interest are the determination of intermolecular potential energy surfaces and understanding the vibrational dynamics of these complexes. Considerable progress has also been made in the characterization of higher-order complexes, which provide information on many-body interactions and more realistically approximate solvated systems. Since the breadth of this field precludes a complete review, we attempt here only to give some examples that illustrate the important issues and convey some of the excitement.

Resonances in rotationally inelastic scattering of OH(X2Π) with helium and neon

“…Since non-bonding interactions are relatively weak, the magnitudes of the cross sections are mostly sensitive to the repulsive part of the PES, except at very low collision energies. An alternative, spectroscopic approach to gaining information on PES's is the determination of the energies of the bound levels of van der Waals complexes of the collision partners [2,3]. The energies of the bound levels are mainly sensitive to the attractive part of the PES's.…”

Formation of coherent rotational wavepackets in small molecule-helium clusters using impulsive alignment

“…For such weakly bound clusters bending vibrations, internal rotations or other types of internal motion occur on similar time scales and therefore it is difficult, or often impossible to separate the wavefunction into distinct rotational and vibrational parts. Hence, a conventional rigid rotor-type Hamiltonian fails to describe their rotational level structure 66 . While pure rotational spectroscopy provides insight into entangled internal motions of molecular complexes and clusters, the experimental data reported in the literature is comparatively sparse.…”

Revisiting the Complexity of Finding Globally Minimum Energy Configurations in Atomic Clusters

“…In fact, even the addition of a single atom can result in an entirely different structure. Only by successively adding more and more atoms will a crystal-like structure eventually be produced and some knowledge of the condensed phase attributes be determined [1].…”