Brute Force Orientation of Asymmetric Top Molecules

Abstract

The versatility of the brute force orientation of polar asymmetric top molecules in a molecular beam has been investigated. In symmetric top molecules the electric field only mixes free rotor basis functions with different J but equal K and M values, but in asymmetric top molecules the mixing includes K, because of the asymmetry, in addition to field-induced J-mixing. This distinction is important with respect to the orientation behavior. For asymmetric top molecules all Stark curves for different J-states and different K-values, but equal M, in the corresponding symmetric eigenbasis, feature avoided crossings. Dependent on the velocity with which the molecules pass through the orientation field, these avoided crossings will be traversed adiabatically or nonadiabatically. For near-symmetric top molecules, such as iodobenzene, the crossings will in general be nonadiabatic, and, as expected, the behavior is similar to that of the corresponding symmetric top. If the crossings are adiabatic, the orientation behavior can be drastically different from the behavior of the corresponding symmetric top molecule. A strong asymmetry need not always be prohibitive in attaining a perceptible degree of orientation, as is demonstrated by the case of water.

Orientation of dipole molecules and clusters upon adiabatic entry into an external field

“…Because of the additional K mixing, a second-order perturbation approach is intractable, and one has to resort to numerical diagonalization 37,38 or molecular dynamics simulations. 39 There are suggestions that because of the nascent chaotic character of asymmetric top motion [40][41][42] and/or because of the general action of avoided crossings in the coupled Stark ͑Zeeman͒-rotational level diagram, 12 the statistical Langevin-Debye behavior will be restored.…”

“…For several asymmetric tops the adiabatic-entry orientation was calculated exactly and found to be close to the Langevin-Debye equation. 37,38 Curiously, this is the case even for the water molecule ͑where the Stark-split rotational energy levels are well ordered and well separated͒ already at relatively low rotational temperatures. 43 …”

Monodromy in the spectrum of a rigid symmetric top molecule in an electric field

“…[1][2][3][4][5][6][7][8][9] This method, known as the brute force technique, was initially applied to linear and diatomic molecules and then extended to symmetric and asymmetric polar molecules. A molecule in a moderately strong electric field exhibits two different regimes: pendular motion about oriented states at low energies and almost free rotational motion at high energies.…”

Rotational Control of Asymmetric Molecules: Dipole- versus Polarizability-Driven Rotational Dynamics

“…Simulating the rotational dynamics of asymmetric top molecules is far more computationally demanding than linear or symmetric top molecules owing to the existence of three-coupled rotational degrees of freedom associated with the three different principal moments of inertia [8,9,10,11]. At ambient temperatures the large number of thermally occupied states makes exact calculation methods very costly in terms of computation time, and in many cases practically not feasible.…”