Statistics – Computation
Scientific paper
Mar 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003aps..marg33001m&link_type=abstract
American Physical Society, Annual APS March Meeting 2003, March 3-7, 2003, , abstract #G33.001
Statistics
Computation
Scientific paper
Several types of dipolar molecules have been synthesized and mounted on flat surfaces of solids and mercury. Each molecule contains an axle about which the dipolar rotor can rotate. In azimuthal rotors, the axle is normal to the surface, and in altitudinal rotors, it is parallel to the surface. The rotor-carrying surfaces have been characterized by standard tools: grazing incidence spectroscopy, ellipsometry, Auger spectroscopy, scanning microscopy, and in the case of mercury, also Langmuir isotherm and electrochemistry. The response of the mounted rotors to electric field has been studied by dielectric measurements as a function of temperature. The resulting data have been compared with the results of molecular dynamics simulations performed using the Universal Force Field of Rappe and Goddard. For a rotating electric field, these can be summarized in the form of log-log plots of electric field strength versus frequency. Different regions in this "rotor phase diagram" correspond to different types of response by the rotor to the driving field: synchronous rotation, asynchronous rotation, random driven motion, random thermal motion, and hindered motion. In order for the rotor to act like an electrical motor, at low frequencies the driving force has to overcome the larger of the two resisting factors, random thermal motion or intrinsic potential barrier to rotation, and at high frequencies, it needs to overcome friction (intramolecular energy redistribution from the driven rotational mode into other degrees of freedom). A simple "shaking washboard" model fits the computational results well and permits the outcome of a large number of numerical simulations to be summarized in values of a frequency-dependent friction constant.
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