Vibrational energy transfer in ozone by infrared-ultraviolet double resonance

Absorption transients at 254 nm have been observed in O₃-O₂ mixtures following laser irradiation at 9.64 μm. From analysis of these transients, we are able to determine vibrational relaxation rate constants (O₃-O₂ λ₁^?1/[O₂] = (2560±370) Torr^?1 S^?1, λ₂^?1/[O₂] = (640±50) Torr^?1 S^?1, and also a v₁-v₃ equilibration rate constant (O₃-O₃) of (1.5±1.0) × 10⁶ Torr^?1 S^?1.     

Ultrafast excited-state deactivation and energy transfer in guanine-cytosine DNA double helices

The DNA double helix poly(dGdC).poly(dGdC) is studied by fluorescence upconversion spectroscopy with femtosecond resolution. It is shown that the excited-state relaxation of the duplex is faster than that of the monomeric components dGMP and dCMP. This contrasts with the behavior of duplexes composed exclusively of adenine-thymine base pairs, for which an overall lengthening of the fluorescence lifetimes with respect to that of an equimolar mixture of dAMP and TMP was reported previously. Despite the difference in the excited-state deactivation rate between the two types of duplexes, the signature of ultrafast energy transfer is present in both of them. It is attested by the decrease of fluorescence anisotropy decay of the duplexes on the subpicosecond time scale, where molecular motions are inhibited, and is corroborated by the fact that their steady-state fluorescence spectra do not change with the excitation wavelength. Energy transfer involves excited states delocalized over at least two bases, whose existence is revealed by the UV absorption spectrum of the duplex, clearly different from that of an equimolar spectrum of dGMP and dCMP.     

Collective behavior of Franck-Condon excited states and energy transfer in DNA double helices

Absorption of UV radiation by DNA bases is known to induce carcinogenic mutations. The lesion distribution depends on the sequence around the hotspots, suggesting cooperativity between bases. Here we show that such cooperativity may intervene at the very first step of a cascade of events by formation of Franck-Condon states delocalized over several bases and subsequent energy transfer faster than 100 fs. Our study focuses on the double helix poly(dA).poly(dT), whose fluorescence, induced by femtosecond pulses at 267 nm, is probed by the upconversion technique and time-correlated single photon counting, over a large time domain (100 fs to 100 ns). The time-resolved fluorescence decays and fluorescence anisotropy decays are discussed in relation with the steady-state absorption and fluorescence spectra in the frame of exciton theory.     

Energy transfer dye laser: Confirmation of energy transfer by reabsorption effect

The laser and fluorescence behavior was studied for the mixed dye system containing various concentrations of 3,3′-diethylthiacarbocyanine iodide and rhodamine 6G. A new way of confirming the occurrence of energy transfer is presented on the basis of the reabsorption effect.     

Direct observation of rotational energy transfer in ammonia by time-resolved infrared—microwave double resonance

An N₂O laser is used to pump the ground vibrational state (8,7) inversion doublet of ¹⁴NH₃ while simultaneously monitoring other ground state doublets. Time-resolved rotational energy transfer signals are observed in accordance with known selection values. Absolute rates of rotational energy transfer processes are estimated.     

Double proton transfer behavior and one-electron oxidation effect in double H-bonded glycinamide-formic acid complex

The behavior of double proton transfer occurring in a representative glycinamide-formic acid complex has been investigated at the B3LYP/6-311 + + G( * *) level of theory. Thermodynamic and, especially, kinetic parameters, such as tautomeric energy, equilibrium constant, and barrier heights have been discussed, respectively. The relevant quantities involved in the double proton transfer process, such as geometrical changes, interaction energies, and intrinsic reaction coordinate calculations have also been studied. Computational results show that the participation of a formic acid molecule favors the proceeding of the proton transfer for glycinamide compared with that without mediate-assisted case. The double proton transfer process proceeds with a concerted mechanism rather than a stepwise one since no ion-pair complexes have been located during the proton transfer process. The calculated barrier heights are 11.48 and 0.85 kcal/mol for the forward and reverse directions, respectively. However, both of them have been reduced by 2.95 and 2.61 kcal/mol to 8.53 and -1.76 kcal/mol if further inclusion of zero-point vibrational energy corrections, where the negative barrier height implies that the reverse reaction should proceed with barrierless spontaneously, analogous to that occurring between glycinamide and formamide. Furthermore, solvent effects on the thermodynamic and kinetic processes have also been predicted qualitatively employing the isodensity surface polarized continuum model within the framework of the self-consistent reaction field theory. Additionally, the oxidation process for the double H-bonded glycinamide-formic acid complex has also been investigated. Contrary to that neutral form possessing a pair of two parallel intermolecular H bonds, only a single H bond with a comparable strength has been found in its ionized form. The vertical and adiabatic ionization potentials for the neutral complex have been determined to be about 9.40 and 8.69 eV, respectively, where ionization is mainly localized on the glycinamide fragment. Like that ionized glycinamide-formamide complex, the proton transfer in the ionized complex is characterized by a single-well potential, implying that the proton initially attached to amide N4 in the glycinamide fragment cannot be transferred to carbonyl O13 in the formic acid fragment at the geometry of the optimized complex.     

Rotational energy transfer cross sections in methane (13CD4) from infrared double resonance measurements

Rotational relaxation times have been measured for methane (¹³CD₄) in collisions with itself, He, Ne, Ar, Kr, Xe, and CH₂F₂, using the method of infrared double resonance. Collision efficiencies range from one-half to greater than gas-kinetic, and the measured relaxation times are longer in the vibrational ground state than in the υ₄ = 1 excited state.     

A double rainbow interpretation of rotational energy transfer in energetic NO/Ag(111) collisions

The mechanism of rotational energy transfer in energetic (E ? eV) NO/Ag(111) collisions is investigated in quantal rigid-rotor/static-surface calculations. Reasonable agreement with recent experimental results is obtained for the case of a strongly asymmetric interaction potential which leads to two well-separated rotational rainbows at low and at high E_rot, respectively.     

Reversible energy transfer

Analysis of the transient and steady-state kinetics of reversible energy transfer shows that while the interpretation of lifetime measurements is difficult unless the donor and acceptor lifetimes are appreciably different, quantum yield measurements are relatively easy to interpret.

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Zusammenfassung Die Analyse der Kinetik der Übergangszustände und der stationären Zustände der reversiblen Energieübertragung zeigt, daß im Gegensatz zu einer schwierigen Interpretation der Messungen der Lebensdauer — es sei denn die Lebensdauer von Donor und Acceptor sind wesentlich voneinander verschieden — die Messungen der Quantumausbeute verhältnismäßig einfach zu interpretieren sind.

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Résumé L'analyse de la cinétique de l'état transitoire et de l'état stationnaire du transfert réversible d'énergie montre que, si l'interprétation des mesures de durée de vie est difficile, à moins queles durées de vie du donneur et de l'accepteur soient très différentes, il est par contre relativement facile d'interpréter les mesures de rendement quantique.

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Dedicated to the memory of Professor K. H. Hansen.     

Intramolecular energy transfer in water

The evolution of the vibrational motion of a water molecule which has been excited locally is studied. An exact solution is obtained of a model Hamiltonian representing the non-harmonic behaviour of the molecule. Since this is obtained numerically it does not explain what is observed but it leads to a simplified model of the motion in which different aspects can be isolated and discussed. For many initial modes, especially those of higher energy, there is a large initial drop in the probability of finding the energy in the original local mode. This is due to dephasing. The factors involved in dephasing are discussed. A simple hypothesis is suggested which leads to a formula for this drop and this agrees substantially with the graphical evidence. The relation between the drop and the amplitude of the initial mode is also discussed.     

In-plane aromaticity in double group transfer reactions

The main features of double group transfer reactions have been studied under the density-functional theory framework. It is found that a wide range of structure-types and processes including type II-dyotropic reactions and the Meerwein-Ponndorf-Verley reduction take place via highly synchronous in-plane aromatic transition structures. Actually, the orbital topology of these saddle points is equivalent to that which corresponds to a D2h-symmetric aromatic molecule such as pyrazine.     

Long-range energy transfer in DNA

Experimental data on strand breakage in DNA following the direct deposition of ionizing energy seem to require an explanation in terms of long-range energy transfer within the DNA duplex. It is proposed here that the mechanism underlying such energy transfer might involve solitons or solitary waves. These act, at one and the same time, to provide a local environment within the DNA molecule capable of supporting delocalized electronic excitation or charge, and also enable the transportation of such regions along the molecule through the formation of a mobile “open state” in the duplex. Such a mechanism, if established, would have considerable implications for the mechanistic understanding of radiobiology.     

Collisional energy transfer in the intermediate states used for optical-optical double resonance excitation of ion-pair states in I2

The optical-optical double resonance spectra of I(2) and I(2)-Xe mixtures at room temperature reported in the literature using a fixed-wavelength, broad band pump laser have now been recorded using a tuneable, narrow band source. We show that during the time of the overlapped laser pulses ( approximately 10 ns) and with 10-20 Torr of Xe there is widespread collisional energy transfer in the intermediate state and that this phenomenon offers an alternative explanation for the broad bands in the excitation spectrum, assigned to XeI(2) complexes by the authors of the earlier study (M. E. Akopyan, I. Y. Novikova, S. A. Poretsky and A. M. Pravilov, Chem. Phys., 2005, 310, 287). Dispersed emission bands, previously attributed to direct fluorescence from the ion-pair state(s) of the complexes, are re-assigned to emission from ion-pair states of the parent I(2) that are populated by collisional energy transfer out of the initially excited state.     

Diffusion effects in reversible energy transfer

Back energy transfer reduces the apparent quenching constant, which is an important parameter in the interpretation of energy transfer data. This determination of kinetic results may be erroneous when possible diffusion effects and non-uniform configurational distributions are not taken into account.

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Triplet-triplet energy transfer in Langmuir-Blodgett films

The temperature effect on the efficiency of the triplet energy transfer between different molecules included in molecular layers by the Langmuir-Blodgett (LB) procedure was studied. The efficiency of the triplet energy transfer from the LB film of the donor to the LB film of the acceptor is determined by the homogeneous broadening of the energy donor levels.     

Multiphonon energy transfer in atom-surface scattering

The probability density P(ω,q) of transferring the energy ω and the parallel momentum q to the surface is determined by the maximum entropy subject to constraints procedure. The two important constraints are identified as the recoil energy and the mean parallel energy transfer. Having determined P(ω,q) one can evaluate all observable quantities, e.g., the triple differential reflection probability d³σ/dE_tdΩ or the trapping probability. A three-parameter model for d³σ/dE_tdΩ is derived by assuming a parameterized form for the recoil energy. This model may be regarded as an extension of the hard-cube model, because it reduces to the latter if the third parameter, the speed of sound c, is set to infinity. The comparison of the predicted velocity and angular distributions with recent experiments of Hurst et al. is excellent considering the simplicity of the model.     

Vibration-vibration energy transfer in gaseous HBr

The vibration-vibration energy transfer of HBr gas initially excited to the first vibrational state has been observed. Collisional pumping to the V = 2 level is measured by monitoring the fluorescence of the 2−1 transition. The rate constant for the process: HBr(V=0) + HBr(V=2) → 2HBr(V=1) is found to be 1.4×10⁵ sec⁻¹ torr⁻¹.     

Fluorescence energy transfer methods in bioanalysis

Energy transfer phenomena, in which excited fluorophores transfer energy to neighbouring chromophores, are well characterised in photochemistry and have found a wide range of applications in analytical biochemistry. The transfer of energy from a donor to an acceptor group is only significant over distances of a few nm, so it can be used as a spectroscopic ruler and as a means of detecting molecular interactions and conformational changes. Such methods usually retain the great sensitivity and sample handling flexibility of conventional fluorescence techniques. As a result many assays involving enzymes, antibodies and nucleotides utilise energy transfer measurement principles. This article outlines these principles for the main types of energy transfer, and summarises some of their most important areas of application.     

Phonon-mediated path-interference in electronic energy transfer

We present a formalism to quantify the contribution of path-interference in phonon-mediated electronic energy transfer. The transfer rate between two molecules is computed by considering the quantum mechanical amplitudes associated with pathways connecting the initial and final sites. This includes contributions from classical pathways, but also terms arising from interference of different pathways. We treat the vibrational modes coupled to the molecules as a non-Markovian harmonic oscillator bath, and investigate the correction to transfer rates due to the lowest-order interference contribution. We show that depending on the structure of the harmonic bath, the correction due to path-interference may have a dominant vibrational or electronic character, and can make a notable contribution to the transfer rate in the steady state.     

Quantum effects in intramolecular energy transfer

Anharmonic potentials which are presumed to be classically quasi-ergodic, but which have symmetries leading to degenerate quantum states, fail in the quantum case to transfer energy equivalently among rigorously equivalent phase space locations. This is shown using simple group theory and is illustrated for the case of the Henon-Heiles potential