STUDIES OF ELECTRONIC PROPERTIES OF MEDIUM AND LARGE MOLECULES ORIENTED IN A STRONG UNIFORM ELECTRIC FIELD

Abstract

Polarization spectroscopy of oriented gas phase medium and large molecules achieved via a uniform DC electric field provides a means to determine the direction of transition dipoles. In this article, the theoretical background of this orientation method, its characterization, and its application in studies of electronic transitions, will be presented. Mature gas phase spectroscopic methods have been developed for studies of small molecules, but studies of medium to large sized species are faced with special challenges. These challenges arise from differences between large and small molecules: large systems typically exhibit fast internal conversion, slow dissociation, and low translational energy release upon dissociation. Thus conventional gas phase spectroscopic techniques are not applicable to derive the direction of the transition dipole. DC induced orientation offers a solution to this problem. It is ideal for studies of systems with small rotational constants and large permanent dipoles, even when a detailed knowledge of the molecular structure, such as the direction of the permanent dipole in the molecular frame, is unknown. The degree of orientation can be calculated using the linear variation method, given the rotational temperature and the size of the permanent dipole. The associated experimental observables can be used to confirm the effect of orientation, or to determine the direction of a transition dipole. These observables include the ratio of excitation probabilities under different polarization directions and spectroscopic features. In some cases, the direction and size of the permanent dipole of the excited electronic state can also be determined. Examples of this type of polarization spectroscopy are presented for asymmetric tops such as diazines, acetelye-HF clusters, nitroaromatics and butyl nitrite. Illustrations of pendular states and its application in linear and diatomic molecules are also briefed. Applications of this method for studies of large molecules and potential pitfalls will be discussed.