Pseudo-Rephasing and Pseudo-Free-Induction-Decay Mechanism in Two-Color Three-Pulse Photon Echo of a Binary System

ABSTRACT: We investigate the two-color three-pulse photon echo peak shift in a (left−right) binary system, where each component consists of a heterodimer. On the basis of the model, we find that the effect of the excitonic asymmetry between two components leads to an additional factor in the peak shift. A pseudo-rephasing and pseudo-freeinduction-decay mechanism is proposed to explain the resultant negative peak shift, when the differences between the two left/right components have the opposite sign. In such a case, estimates of the electronic coupling strength via two-and one-color peak shift experiments lead to an underestimate of the coupling magnitude

Photon Echo Studies on Coherence Dynamics in the Photosynthetic Bacterial Reaction Center

Photosynthetic light harvesting begins with collecting ambient photons via excitation of pigment molecules. The sophisticated design of the pigment-protein complexes is based on the cooperative network among structural motifs of pigment molecules and amino acid residues in proteins to achieve the remarkable efficiency and speed in light harvesting.The insight from nature's photosynthetic nano-machinery can be translated to an optimal material design. The principle behind such inherent excitation energy transfer dynamics were revealed with the advance of the femtosecond laser technique. One of the examples is the quantum mechanical coherence excitation energy transfer observed by nonlinear photon echo technique.This dissertation presents the investigation of the bacterial reaction center complexes using electronic coherence photon echo spectroscopy. The study focused on elucidating the origin of the coherence signals and identifying quantum mechanical components relevant to the excitation energy transfer. Experimental variables affecting the coherence signals were explored for the excitation energy transfer of photosynthetic bacterial reaction center. The result showed that the vibrational coherence mimics the behavior of the electronic coherence, and the lifetime is near 2 ps. A mutant bacterial reaction center was also examined to confirm the vibrational contribution. A new polarization control experiment indicates that majority of the observed vibrational coherence is from the electronic ground state

Advances in Internet congestion control

In this survey, we first review the concept of congestion control with a focus on the Transmission Control Protocol/Internet Protocol (TCP/IP). We describe many recently proposed algorithms to combat congestion and improve performance, particularly active queue management (AQM) algorithms such as random early detection (RED) and its variants. We then survey control-theoretic analysis and design of TCP congestion control with an AQM scheme. In addition, we discuss three problems associated with AQM proposals: parameter setting, the insensitivity to the input traffic load variation, and the mismatch between macroscopic and microscopic behavior of queue length dynamics. As alternatives to AQM algorithms, we also survey architectural approaches such as modification of source or network algorithms, and economic approaches including pricing or optimization of allocated resources. Finally, we list many open issues that persist in the design, operation, and control of the Internet. Internet congestion occurs when the aggregate demand fo

Role of electronic-vibrational mixing in enhancing vibrational coherences in the ground electronic states of photosynthetic bacterial reaction center.

We describe polarization controlled two-color coherence photon echo studies of the reaction center complex from a purple bacterium Rhodobacter sphaeroides. Long-lived oscillatory signals that persist up to 2 ps are observed in neutral, oxidized, and mutant (lacking the special pair) reaction centers, for both (0°,0°,0°,0°) and (45°,-45°,90°,0°) polarization sequences. We show that the long-lived signals arise via vibronic coupling of the bacteriopheophytin (H) and accessory bacteriochlorophyll (B) pigments that leads to vibrational wavepackets in the B ground electronic state. Fourier analysis of the data suggests that the 685 cm(-1) mode of B may play a key role in the H to B energy transfer