Fringe-field effects on the time evolution of pendular states

Abstract

The interaction of the permanent electric dipole moment of a polar molecule with a strong static electric field can create oriented rotational wave packets, which are termed pendular states. Molecules traveling in a molecular beam experience a fringing field due to the edges of the conducting plates which create the electric field as a slowly varying time-dependent perturbation. Since Stark energies of some asymmetric-top molecules present numerous avoided crossings as a function of field strength, the time evolution of rotational wave functions for molecules entering the field is complex. We have studied, by solving the time-dependent Schro ¨dinger equation, the dynamics caused by selected avoided crossings, and the time evolution of the orientation corresponding to 24 rotational wave functions, for the near-prolate asymmetric-top iodobenzene. Implications for the population distribution of molecules in the electric field and for experimental schemes to decelerate molecules are discussed. PACS number~s!: 33.55.Be, 33.80.2b, 03.65.Ge, 33.90.1h The study of the interaction of molecules with strong electric fields is becoming a topic of great interest in chemical physics. The possibility of orienting polar molecules by combining cooled molecular beams with strong static electric fields was studied by Loesch and Remscheid @1# and Friedrich and Herschbach @2#. Also, ensembles of nonpolar molecules can be aligned ~but not oriented! by using intense pulsed lasers, which interact with the molecular polarizability, inducing a dipole moment. The induced dipole can further interact with the laser field, giving rise to pendular states @3#. These states are field-induced coherent superpositions of field-free rotor states. For high enough field strengths, molecules, in these hybrid states, librate about the field direction instead of executing free rotational motion. The time evolution of pendular states created by nonresonant lasers was recently studied for linear molecules @4#, to determine the character ~adiabatic versus diabatic! of the evolution from field-free eigenstates to aligned states. A variety of behaviors was found depending on the duration and intensity of the laser pulse and the rotational constant of the molecule.