Structure Distribution of Gaseous Ions in Strong Electrostatic Fields

In drift tube experiments, where ions move in gases under the action of an electrostatic field, collision excitation is implemented for the study of the energy partitioning in the molecular degrees of freedom and the corresponding relaxation rates when the excitation is turned off. In the case of flexible ions, the vibration modes related to metastable molecular structures have been activated in ion mobility spectrometry and their population has been probed with respect to the field strength and the gas temperature. Here, we study the angular vibrational excitation and relaxation of such systems by examining the motion of molecular ions with one bending mode at strong fields using a nonequilibrium molecular dynamics simulation method. The relatively stable structures are introduced through the use of an intramolecular angular potential with minima at the position of the most stable conformations. We calculate the first few moments of the velocity and angular velocity distribution functions as well as the distribution of the conformers, and find that they follow unified curves when plotted with respect to the relative ion-atom collision energy. At high field strengths, the angular vibration is excited and a portion of the ions interchanges conformations continuously in time with the populations of the molecular structures to attain limiting values. In addition, orientational alignment, with the perpendicular angular momentum being greater than the one parallel to the field, is observed. Our observations, although based on a specific system, must be rather general for the case of large flexible molecular ions.