Population Transfer Dynamics Research Articles

Construction of kinetic domains in energy trapping processes and application to a photosynthetic light harvesting complex

We develop a formulation allowing construction of various kinetic domains from a master equation for population transfer dynamics and enabling calculation of the time scales of transition between such domains. In this way, a complicated network of energy transfer steps can be simplified to a series of sequential kinetic transitions over the kinetic domains. We illustrate the advantage of this formulation by applying it to energy trapping in a photosynthetic light harvesting system and as a result we obtain a clear picture of energy transfer dynamics and the various rate determining steps in Photosystem I.

Simultaneous control of time-dependent population transfer dynamics and wave-packet quantum interferences inLi2by shaped ultrafast pulses

Ultrafast pulse shapes are used to control simultaneously the optimal population transfer coefficients and rotational wave-packet quantum interferences in the $E{}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ state of ${\mathrm{Li}}_{2}$ $({\ensuremath{\nu}}_{E}=9,$ ${J}_{E}=27$ and 29). By dividing the spectral bandwidth of the ultrafast pulses into multiple ``control domains'' centered on each resonant wavelength, the population transfer coefficients can be manipulated independently of the wave-packet interferences to maximize the ${\mathrm{Li}}_{2}$ photoionization yield at arbitrary short pump-probe time delays. To investigate the population transfer coefficients with and without wave-packet interferences, respectively, the pump polarization is set to be either parallel to or at the magic angle (\ensuremath{\sim}55\ifmmode^\circ\else\textdegree\fi{}) relative to the probe polarization. A comparison is made between phases that are symmetric and antisymmetric about the resonances. The effects of resonant and nonresonant frequencies are separately established and quantified. It is estimated that up to 90% of the possible nonresonant Rabi oscillations can be brought into phase simultaneously for each rovibrational state in the wave packet, while at the same time a constant phase offset added to one of the control domains establishes the phase of the wave-packet interference.

Dragline attachment pattern in the neotropical social spider anelosimus eximius (Araneae: Theridiidae)

The regularity of spiders orb webs has been considered a fascinating natural wonder by generations of naturalists. Although the majority of spider species build irregular three-dimensional networks, those who have evolved the two- dimensional spiral snare have drawn most of the attention (Vollrath, 1988). Even studies conducted on irregular webs are often explicitly devoted to the search of similarities with orb webs, resulting from behavioral preadaptations to this specific design (Eberhard, 1987, 1992). So the question of the distance between attachment points in an environ- ment that does not feature intrinsinc regulating properties, like a three-dimen- sional web or vegetal structure, remains widely open. In the overall context of swarming in the cooperative spider Anelosimus eximius (Furey et al, in prepa- ration), our focus was on amplification mechanisms involved in group cohesion and coordination of several individuals' activities. From these observations, it appears that dragline attachment pattern has a direct influence on network design, and consequently on the population transfer's dynamics, which is confirmed by theoretical projections about the efficiency of silk-mediated exploratory recruit- ment (Saffre et al, 1997, in press).

A generalized approach to the control of the evolution of a molecular system

The theory of active control of molecular motion by use of shaped laser pulses is developed emphasizing the role of interference and using thermodynamic analogies. Attention is focused on the control of the dynamics in a system with n states coupled by radiation, and the phase relations which generate particular control schemes are derived. Among the new results reported is an optimal control scheme which constrains the value of the phase. The n-state model can be considered to represent a molecule with n electronic potential energy surfaces and an arbitrary number of degrees of freedom or as the skeleton spectrum of system where each level in the spectrum can be associated with a specific set of quantum numbers for all of the degrees of freedom. We show how the control of the dynamics of an n-state molecule can be represented in terms of the control of the dynamics of a precisely defined surrogate fewer state system. This reduction is illustrated by use of a surrogate two state system to describe the dynamics of population transfer in a three state system.