Coherent Pathways Research Articles

Photoinduced electron-transfer processes along molecular wires based on phenylenevinylene oligomers: a quantum-chemical insight.

Quantum-chemical techniques are applied to model the mechanisms of photoinduced charge transfer from a pi-electron donating group (tetracene, D) to a pi-electron-acceptor moiety (pyromellitimide, A) separated by a bridge of increasing size (p-phenylenevinylene oligomers, B). Correlated Hartree-Fock semiempirical approaches are exploited to calculate the four main parameters controlling the transfer rate (k(RP)) in the framework of Marcus-Jortner-Levich's formalism: (i) the electronic coupling between the initial and final states; (ii) and (iii) the internal and external reorganization energy terms; and (iv) the variation of the free Gibbs energy. The charge transfer is shown to proceed in these compounds through two competing mechanisms, coherent (superexchange) versus incoherent (bridge-mediated) pathways. While superexchange is the dominant mechanism for short bridges, incoherent transfer through hopping along the phenylene vinylene segment takes over in longer chains (for ca. three phenylenevinylene repeat units). The influence of the chemical structure of the pi-conjugated phenylenevinylene bridge on the electronic properties and the rate of charge transfer is also investigated.

Multiple phase control in Mg through the continuum

We present results from perturbative calculations on a scheme of coherent control in Mg. The scheme involves the excitation of Mg in the vicinity of an autoionizing resonance lying above the first two ionization thresholds by a two-color field composed of a fundamental frequency and its third harmonic, whose relative phase can be continuously controlled. Four as well as two-photon coherent excitation pathways, involving appropriate combinations of the two frequencies, proceed through the atomic continuum simultaneously contributing to the excitation, while three-photon ionization by the fundamental is also energetically possible. As a result, some of the excitation processes involve above-threshold ionization (ATI). The calculated ionization yields for the various groups of photoelectrons reveal significant modulation as the relative phase of the field component frequencies is varied. The modulation patterns for the different photoelectron groups are mutually shifted due to the multiphoton matrix element phases associated both with the ATI process and the multichannel nature of the final state. The effect is particularly pronounced when the autoionizing state is in near four-photon resonance with the fundamental frequency. The role of an autoionizing state as a final state as well as that of the continuum as an intermediate state in coherent control processes is discussed.

Singly vibrationally enhanced infrared four wave mixing spectroscopy

We describe a new nonlinear, nonparametric process that provides a direct measure of vibrational enhancement in four wave mixing. This process has 14 coherent pathways that create the final coherence. In the limit of no pure dephasing and a single vibrational resonance, the 14 pathways combine into a single process that is vibrationally enhanced. The frequency and concentration dependence expected for this process matches that observed in experiments with the methyl- and methylene C–H stretch vibrations in hexane samples. The third-order nonlinear susceptibility for the C–H vibrations was measured by the interferometric method of Levenson and Bloembergen. The vibrational nonlinearity was observed as a difference between the nonresonant electronic contributions and the contribution that depended on resonance with the vibrational absorption transition. The measurements show that the vibrational enhancement is 50 times larger than the nonresonant electronic contribution and it shows that spectroscopic methods based on resonant nuclear nonlinear polarizations are observable above the electronic nonlinear polarization background.

Rate expressions for excitation transfer. IV. Energy migration and superexchange phenomena

General microscopic mechanisms of electronic excitation (energy) transfer (EET) in multichromophoric assemblies are investigated. Aspects of superexchange-mediated EET and energy migration (EM) and their contribution to the efficiency of donor-to-trap energy transport processes in macromolecules are discussed from a quantum mechanical viewpoint. The possibility of superexchange pathways for EM via higher excited states of the intermediate chromophores is introduced. The role of quasicoherent EM pathways, and how they are manifested in the quantum mechanical rate expression, is investigated and the significance of contributions to the rate arising through quantum mechanical interference between pathways is elucidated. The theory indicates conditions under which coherent EM pathways may significantly increase the efficiency of energy transport and trapping and the applications to natural and synthetic light-harvesting systems are outlined.

Tunneling versus sequential long-range electron transfer: Analogy with pump–probe spectroscopy

The interplay between the sequential and the superexchange (tunneling) mechanisms for electron transfer in condensed phases is studied by formulating the problem using the density matrix. The sequential mechanism proceeds via populations of intermediate electronic states (diagonal density matrix elements) whereas the superexchange proceeds through coherences (off diagonal density matrix elements). The present formulation establishes a complete formal analogy between these mechanisms and the incoherent and the coherent pathways in nonlinear optical measurements, in particular, pump–probe spectroscopy.

Four-wave mixing with dynamic Stark effects in flames

Four-wave mixing is studied in a Na-seeded flame in the regime of strong saturation of the Na D-line transitions. The output frequency differs from that of the input lasers so that spectral discrimination can distinguish the mixing signal. Scans of the D2 resonance with another laser set at the D1-line resonance reveals two resonances corresponding to the D2 resonance and the frequency difference of the D levels. The mixing efficiency at the D2 resonance increases with the detuning of the D1 resonance. The frequency difference resonance is weak because of interference between multiple coherent pathways.

Interference Effects between Different Optical Harmonics

It is shown that destructive interference between two coherent excitation pathways from different optical harmonics, a fundamental and a third harmonic, inhibits resonance-enhanced multiphoton ionization. Once the role of the third harmonic is realized, the interference follows naturally from simple considerations of Fermi's golden rule and Maxwell's equations.

Collisionally induced coherent signals and collisional redistribution

It is demonstrated that, in the presence of collisions and for nonresonant lasers, it is energetically possible to access resonantly excited states and induce a coherence between the states. This situation cannot arise in the absence of collisions if the starting state does not decay. This explains why, when collisions are introduced, a new signal can be generated in a wave-mixing type of experiment. The induced generation of a coherent signal is intimately related to the process of collisional redistribution. It is shown that all the induced coherent signals that have been reported up to now involve putting real populations in the excited states. This fact has not always been fully appreciated. No resonance between unpopulated states can exist. The time dependence of these new coherent signals is briefly discussed. In the case of a Raman resonance between equally populated states, it is pointed out that new coherent pathways of equal importance should be considered for explaining the experimental results. A new pressure-induced signal in four-wave mixing is also discussed. It is called collisionally triggered two-photon quasiresonant coherent signal.

Pressure-induced degenerate frequency resonance in four-wave light mixing

Phase-matched four-wave mixing in sodium vapor with a helium buffer gas is carried out in a noncoplanar geometry. The directions of the four light beams remain nondegenerate even if all frequencies are equal. A sharp zerodifference frequency resonance in the intensity of the parametrically generated fourth beam by three incident beams is observed that is proportional to the square of the pressure of the buffer gas. A theoretical interpretation of this effect caused by elimination of destructive interference between two coherent pathways is presented.