Dynamics and kinetics of molecular high Rydberg states in the presence of an electrical field: An experimental and classical computational study
Authors: Rabani Eran , Levine R. D. , Mühlpfordt Annette , Even U. 
Published: July 2002

The effect of an electrical field on the dynamics and decay kinetics of a high Rydberg electron coupled to a core is discussed with special reference to simulations using classical dynamics and to experiment. The emphasis is on the evolution of the system within the range of Rydberg states that can be detected by delayed pulsed ionization spectroscopy (which is n≳90 for both the experiment and the computations). The Hamiltonian used in the computations is that of a diatomic ionic core about which the electron revolves. The primary coupling is due to the anisotropic part of the potential which can induce energy and angular momentum exchange between the orbital motion of the electron and the rotation of the ion. The role of the field is to modulate this coupling due to the oscillation of the orbital angular momentum l of the electron. In the region of interest, this oscillation reduces the frequency with which the electron gets near to the core and thereby slows down the decay caused by the coupling to the core. In the kinetic decay curves this is seen as a stretching of the time axis. For lower Rydberg states, where the oscillation of l is slower, the precession of the orbit, due to the central but not Coulombic part of the potential of the core, prevents the oscillation of l and the decay is not slowed down. Examination of individual trajectories demonstrates that the stretching of the time axis due to the oscillatory motion of the electron angular momentum in the presence of the field is as expected on the basis of theoretical considerations.

The relation of this time stretch to the concept of the dilution effect is discussed, with special reference to the coherence width of our laser and to other details of the excitation process. A limit on the principal quantum number below which the time stretch effect will be absent is demonstrated by the computations. The trajectories show both up and down processes in which the electron escapes from the detection window by either a gain or a loss of enough energy. Either process occurs in a diffusive like fashion of many smaller steps, except for a fraction of trajectories where prompt ionization occurs. The results for ensembles of trajectories are examined in terms of the decay kinetics. It is found that after a short induction period, which can be identified with the sampling time of the available phase space, the kinetics of the decay depend only on the initial energy of the electron and on the magnitude of the field, but not on the other details of the excitation process. The computed kinetics of the up and down channels are shown to represent competing decay modes. A possible intramolecular mechanism for long time stability based on the sojourn in intermediate Rydberg states is discussed. The available experimental evidence does not suffice to rule out nor to substantiate this mechanism, and additional tests are proposed. The theoretical expectations are discussed in relation to observed time resolved decay kinetics of high Rydberg states of BBC (bisbenzenechromium) and of DABCO (1,4-diazabicyclo[2.2.2]octane).

The experimental setup allows for the imposition of a weak (0.1–1.5 V/cm) electrical field in the excitation region. The role of the amplitude of the time delayed field, used to detect the surviving Rydberg states by ionization, is also examined. The observed decay kinetics are as previously reported for cold aromatic molecules: Most of the decay is on the sub-μs time scale with a minor (∼10%) longer time component. The decay rate of the faster component increases with the magnitude of the field. Many features in such an experiment, including the absolute time scales, are similar to those found in the classical trajectory computations, suggesting that the Hamiltonian used correctly describes the physics of the faster decay kinetics of the high Rydberg states.