Energy Transfer Channel Research Articles

Vibrational spectroscopy of HOD in liquid D2O. VI. Intramolecular and intermolecular vibrational energy flow

In a previous theoretical study [J. Chem. Phys. 117, 5827 (2002)] we calculated the vibrational lifetimes of the three fundamentals of HOD in liquid D2O. In that calculation the D2O solvent was treated as rigid, not allowing for the possibility of intermolecular vibrational energy transfer as a relaxation mechanism. In this paper we use both flexible and rigid solvent models, enabling us to include the possibility of intermolecular vibrational energy transfer, and also to estimate branching ratios for vibrational and nonvibrational relaxation channels. Our theoretical value for the lifetime of the OH stretch decreases modestly from 2.7 ps (in the original calculation) to 2.3 ps, which should be compared to the experimental value of about 1 ps. The lifetime of the OD stretch decreases dramatically from 18 ps to 390 fs due to resonant energy transfer to the solvent stretch. Our lifetime value for the bend actually increases from 220 to 380 fs, not because of the vibrational energy transfer channel, but rather because we find that Fermi’s Golden Rule (used in the original calculation) breaks down for this very fast process. We have calculated all the state-to-state rate constants for the low-lying vibrational energy levels of HOD, which allows us to construct and solve the vibrational master equation. We find that after excitation of the OH stretch, population flows into the HOD bend states (and to a lesser extent the OD stretch of HOD) on the time scale of 1 ps, in agreement with recent infrared pump/Raman probe measurements of Dlott and co-workers. From our results we estimate that for each quantum of OH stretch excitation, 0.26 quanta of solvent stretch is excited by direct intermolecular energy transfer, and yet we find, surprisingly, that there is almost no direct excitation of solvent bend. On the other hand, we suggest that because of the intramolecular Fermi resonance in D2O, rapid intramolecular vibrational relaxation occurs from solvent stretch to solvent bend. This would account for the experimental observation of solvent stretch and bend excitations on the time scale of 1 ps after the OH stretch is excited.

Infrared action spectroscopy and time-resolved dynamics of the OD–CO reactant complex

The infrared action spectrum of the linear OD–CO reactant complex has been recorded in the OD overtone region near 1.9 μm using an infrared pump-ultraviolet probe technique. The pure overtone band of OD–CO (2νOD) is observed at 5148.6 cm−1 and combination bands involving the simultaneous excitation of OD stretch and D-atom bend are identified 160.0 and 191.2 cm−1 to higher energy. Band assignments and spectroscopic constants are derived from the rotationally resolved structure of the spectra. The change in the ground state rotational constant upon deuteration demonstrates that the H/D-atom of the hydroxyl radical points toward CO in the OH/D-CO complex. Direct time-domain measurements yield a lifetime of 37(4) ns for OD–CO (2νOD) prior to decay via inelastic scattering or chemical reaction. This is significantly longer than the laser-limited lifetime of ⩽5 ns observed for OH–CO (2νOH), and is attributed in part to the closing of a near-resonant vibration to vibration energy transfer channel upon deuteration. Vibrational predissociation of OD–CO (2νOD) proceeds by a vibration to rotation and/or translation mechanism that yields highly rotationally excited OD (v=1) fragments. Intermolecular D-atom bend excitation, which drives the structural transformation from the reactant complex to the transition state for reaction, results in a dramatic shortening of the lifetime to ⩽6 ns (laser-limited). Excitation of the D-atom bend also supplies sufficient energy to reopen the near-resonant vibrational energy transfer channel, resulting in minimal rotational excitation of the OD (v=1) fragments. Finally, a ground state binding energy for OD–CO of D0⩽456 cm−1 is established from the OD (v=1) product state distribution.

Unexpected Sensitization Efficiency of the Near-Infrared Nd3+, Er3+, and Yb3+Emission by Fluorescein Compared to Eosin and Erythrosin

Near-infrared emissive lanthanide complexes were synthesized with covalently attached sensitizers that absorb in the visible. This functionalization was designed such that the sensitizer is in close proximity to the lanthanide ion, which is a prerequisite for efficient energy transfer from the excited sensitizer to the lanthanide ion. The sensitizers used were fluorescein, eosin, and erythrosin, which were linked via a -alanine spacer to the polydentate chelate. The sensitizers were chosen because they absorb visible light and are structurally very similar, but the intrinsic intersystem crossing quantum yields of the sensitizers vary significantly, because of the presence of the heavy atoms (bromine in eosin and iodine in erythrosin). It was expected that an intrinsic high intersystem crossing would be beneficial in the sensitization process, because energy transfer occurs through the triplet state of sensitizers. However, because of the enhanced intersystem crossing of the sensitizers by the nearby heavy and paramagnetic lanthanide ions, these intrinsic differences were largely diminished. It was even found that fluorescein acts as a more efficient sensitizer for the NIR emission than eosin and erythrosin. The donating triplet state of fluorescein is higher in energy than that of eosin and erythrosin, resulting in less energy back transfer and therefore in a higher efficiency of sensitized emission. This and considerations of selection rules for energy transfer to the lanthanide ions made it possible to distinguish the 4F9/2 level of Nd3+ as the main acceptor channel for energy transfer. In the Er3+ complexes, the enhancement in intersystem crossing was lower in the eosin and erythrosin complexes than in the fluorescein complex, which was concluded from the remaining complex fluorescence. Furthermore, it is tentatively concluded that additional pathways other than those allowed in Dexter energy transfer play a role in the sensitization of Er3+. In the Yb3+ complexes, the higher efficiency of sensitization by fluorescein is due to the enhanced intersystem crossing that is larger in the fluorescein complex than in the eosin or erythrosin complex.

Vibrational lifetimes of hydrogen in silicon

Characterization of defect and impurity reactions, dissociation, and migration in semiconductors requires a detailed understanding of the rates and pathways of vibrational energy flow, of the energy transfer channels, and of the coupling mechanisms between local modes and the phonon bath of the host material. Significant progress in reaching this goal has been accomplished in recent landmark studies exploring the excitation and dynamics of vibrational states associated with hydrogen in silicon. The lifetime of the Si–H stretch mode is found to be extremely dependent on the local solid-state structure, ranging from picoseconds for interstitial-like hydrogen, hundreds of picoseconds for hydrogen–vacancy complexes, to several nanoseconds for hydrogen bonded to Si surfaces—over three orders of magnitude variation. Such large variations in lifetime (transition probability) are extraordinarily rare in solid-state science. The level of theoretical investigation into the vibrational lifetime of the Si–H oscillator is less advanced. This state of affairs is partly because of the difficulties in explicitly treating slow relaxation processes in complex systems, and partly because, as suggested by experiment, a highly anharmonic coupling mechanism is apparently responsible for the (multiphonon) relaxation process. Even more importantly, because of the high frequency of the Si–H stretching motion, a quantum mechanical treatment of the Si–H oscillator is required. A combination of Bloch–Redfield theory and molecular dynamics simulation seems promising in describing the relaxation process of the Si–H vibrational modes. It is the aim of this review article to present a comprehensive overview of the recent accomplishments, current understandings, and future directions in this emerging field of time-resolved vibrational spectroscopy of point defects in solids.

Channels of energy transfer to atomic nitrogen in excited argon–nitrogenmixtures

The vacuum ultraviolet (VUV) and ultraviolet (UV) emissions of gaseousmixtures of argon with a small amount of nitrogen were investigated in theafterglow of a pulsed transverse discharge. The time dependences of the intensityof the VUV argon continuum at 126.0 nm, the UV nitrogen molecular band at337.0 nm, and the VUV nitrogen atomic lines at 149.47 and 174.52 nm wereanalysed in the total pressure range 150–600 hPa and the range 0.125–1.5 hPa ofthe admixture concentrations. The results show that all these emissions have acommon source of excitation energy, namely the atomic metastable 3P2state of argon. However, atomic nitrogen also receives energy from theatomic 3P1 state of argon and from the excited Ar2* (1u)molecules.

A studyof atomic and molecular energy transfer channels in Kr-Xe gas mixtures excited with radio frequency discharges

Simultaneous observations of the vacuum ultraviolet (VUV) andvisible-near-IR emission from radio frequency discharges ingaseous Kr with small (<0.1%) Xe admixture concentrations were conducted in the pressure range 80-400 hPa.The spectra were used for investigation of energy transfer fromthe lowest excited states of both atomic krypton and kryptonmolecules to the ground state xenon atoms. The VUV emissionspectra allowed us to obtain a value of 3.9×107×PXe hPa-1 s-1 for the energy transferrate from the 1u state of Kr2* molecules and arate of 1.7×106×PXehPa-1 s-1 for the energy transfer rate from thesystem of the four lowest Kr excited states to Xe ground stateatoms. A value of 1.6×106×PXehPa-1 s-1 was independently obtained for thisatom-to-atom energy transfer channel based on our observationsof IR emission.

Electrically active centers in Si:Er light-emitting layers grown by sublimation molecular-beam epitaxy

Electrically active centers in light-emitting Si:Er layers grown by sublimation molecular-beam epitaxy (SMBE) on single-crystal Si substrates have been investigated by admittance spectroscopy with temperature scanning and by DLTS. The total density of electrically active centers is defined by shallow donor centers with ionization energies of 0.016–0.045 eV. The effect of growth conditions and post-growth annealing on the composition and density of electrically active centers has been studied. Significant differences in composition of the electrically active centers with deep levels and in channels of energy transfer from the electron subsystem of a crystal to Er3+ ions between Si:Er layers grown by SMBE and ion implantation have been revealed.

Concentration dependences of heat conductivity coefficients of inert gases in narrow pores

Calculation of the transfer of molecules in porous systems requires self-consistent expressions describing the kinetic transfer coefficients for various concentrations and temperatures. The concentration dependences of heat conductivity and self-diffusion coefficients for fluids with different densities, ranging from rarefied gases to liquids, were considered in terms of a unified model. For monoatomic gases (argon), the model takes into account two energy transfer channels, namely, the vacancy mechanism and energy transfer through collisions of molecules. The former channel is characteristic of rarefied gases, while the latter is noted for condensed phases. The energy parameters of the model were determined on the basis of data on the heat conductivity coefficient in the bulk phase. The heat conductivity coefficient follows a linear temperature dependence for low density; in the medium and large density regions, these dependences follow a more complex pattern that changes depending on temperature. The influence of the interaction of atoms with the pore walls on the concentration dependences of the heat conductivity coefficients was investigated for different total amounts of the adsorbate. These coefficients depend appreciably on the distance to the pore wall and on the direction of heat transfer.

Dissipation of strong Langmuir turbulence in nonisothermal non-Maxwellian plasma

The regime of strong Langmuir turbulence characterized by the plasma nonisothermality and by the presence of an appreciable non-Maxwellian hot-electron component was experimentally studied. Turbulence was excited in the preliminary produced plasma by the relativistic electron beam. Thomson scattering of laser IR radiation served as the main diagnostic method. The spatial spectra of the Langmuir turbulence and of the attendant ion-sound turbulence were studied using Thomson collective scattering. Thomson incoherent scattering was used for studying the plasma electron distribution function and searching for the local dips of plasma density. Stark spectroscopy of turbulent microfields and the method of observation of plasma radiation at the double plasma frequency were also used. Based on the experimental data, the mechanism of Langmuir oscillation damping by plasma electrons was analyzed. The Langmuir wave conversion induced by the ion-sound turbulence is the most probable channel for energy transfer from the turbulence to plasma electrons, the low-frequency fluctuations being the direct consequence of the strong Langmuir turbulence.

Site-selective excitation of Er 3+ ions in oxygen-rich silicon

Photoluminescence excitation spectroscopy is used to study the location of Er 3+ ions in oxygen-rich silicon and its excitation mechanisms. We find that Er luminescence excited within the range of the 4I 13/2 excited manifold of Er 3+ consists of two features: a broad band independent of excitation wavelength, λ exc, and a fluorescence narrowed line, dependent on λ exc. These results show that Er 3+ ions are excited via two energy transfer channels: the first one is due to non-radiative transfer of energy from excitons to the 4I 13/2 state, whereas the second one, leading to line narrowing, is typically resonant, site selective excitation of Er ions in silica glass. The results show that excitable Er ions are located most probably within nano-precipitates of SiO 2− δ .

Gateway Modes for Collisional Energy Transfer between Benzene and Ar

Quasiclassical trajectory calculations were used to explore the mechanism of collisional energy transfer between a benzene molecule and an Ar bath gas. Using single mode excitation, it was found that there are two factors that affect energy transfer: the frequency and the type of molecular motion that is associated with the mode. We find that the lowest frequency, out-of-plane torsion mode, is the gateway mode for energy transfer. When the same frequency is assigned also to the in-plane mode, the out-of-plane mode is still more efficient. Only when the frequencies of the out-of-plane and the in-plane modes are switched, the in-plane mode becomes the gateway mode for energy transfer. Therefore, the lowest the frequency is always the gateway mode, and given two modes of identical frequencies, the molecular motions determine the efficiency of energy transfer. These results agree with quantum calculations by Clary et al. [Clary et al. Faraday Discuss. 1995, 102, 423]. It is found that translation/rotation is a major channel of energy transfer and that vibration/translation energy transfer is efficiently mediated by rotations.

Modified windmill porphyrin arrays: coupled light-harvesting and charge separattion, conformational relaxation in the S1 state, and S2-S2 energy transfer.

The architecture of windmill hexameric zinc(II) -porphyrin array 1 is attractive as a light-harvesting functional unit in view of its three-dimensionally extended geometry that is favorable for a large cross-section of incident light as well as for a suitable energy gradient from the peripheral porphyrins to the meso-meso-linked diporphyrin core. Three core-modified windmill porphyrin arrays 2-4 were prepared for the purpose of enhancing the intramolecular energy-transfer rate and coupling these arrays with a charge-separation functional unit. Bisphenylethynylation at the meso and meso' positions of the diporphyrin core indeed resulted in a remarkable enhancement in the intramolecular S1-S1 energy transfer in 2 with tau=2 approximately 3 ps, as revealed by femtosecond time-resolved transient absorption spectroscopy. The fluorescence lifetime of the S2 state of the peripheral porphyrin energy donor determined by the fluorescence up-conversion method was 68 fs, and thus considerably shorter than that of the reference monomer (150 fs), suggesting the presence of the intramolecular energy-transfer channel in the S2 state manifold. Such a rapid energy transfer can be understood in terms of large Coulombic interactions associated with the strong Soret transitions of the donor and acceptor. Picosecond time-resolved fluorescence spectra and transient absorption spectra revealed conformational relaxation of the S1 state of the diporphyrin core with tau = 25 ps. Upon photoexcitation of models 3 and 4, which bear a naphthalenetetracarboxylic diimide or a meso-nitrated free-base porphyrin attached to the modified diporphyrin core as an electron acceptor, a series of photochemical processes proceeded, such as the collection of the excitation energy at the diporphyrin core, the electron transfer from the S1 state of the diporphyrin to the electron acceptor, and the electron transfer from the peripheral porphyrins to the diporphyrin cation radical, which are coupled to provide a fully charge-separated state such as that in the natural photosynthetic reaction center. The overall quantum yield for the full charge separation is better in 4 than in 3 owing to the slower charge recombination associated with smaller reorganization energy of the porphyrin acceptor.

Ultrafast Energy-Transfer Dynamics between Block Copolymer and π-Conjugated Polymer Chains in Blended Polymeric Systems

The blending of polymers has been conventionally employed to increase the quantum efficiencies of polymeric electroluminescence (EL) devices via the energy transfer process, which was interpreted in terms of the Forster-type energy transfer based on dipole−dipole interactions. The detailed analysis of various time-resolved spectroscopic data in the blended polymer between MEH-PPV (poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]) and DSiPV (poly[1,3-propanedioxy-1,4-phenylene-1,2-ethylene(2,5-bis(trimethylsilyl)-1,4-phenylene)-1,2-ethylene-1,4-phenylene]) provides some evidence that the ultrafast energy-transfer channel via the shortest interchain distance between the adjacent constituent polymer chains should be considered to account for the dynamics of stimulated emission (SE) and photoinduced absorption (PA) in the blended polymeric systems. These interchain interactions are also responsible for the diminishment of SE of the blended polymer in the transient absorption spectra, because PA is en...

Optically Functional Zeolites: Evaluation of UV and VUV Stimulated Photoluminescence Properties of Ce3+- and Tb3+-doped Zeolite X

The optical properties of Tb3+/Ce3+ doped zeolites are elucidated with emphasis on ultraviolet (UV) and vacuum ultraviolet (VUV) excitation and luminescence. Ce3+ sensitized Tb3+ emission with quantum yields of 85 % may be obtained at 330 nm excitation. Low absorptivity at 254 nm due to low Ce3+ concentrations or low Ce3+/Tb3+ ratios, which are required for the suppression of UV components, restricts their applicability as phosphors for Hg-based discharges, e.g., in conventional fluorescent lamps. Near band edge excitation at 172 nm revealed an immediate quantum yield of 50 % enabled by a zeolite → Ce3+ (5d1) → Tb3+ (4f75d1) energy transfer channel, which may be exploited for the down-conversion of the Xe2 excimer emission.

Optically Functional Zeolites: Evaluation of UV and VUV Stimulated Photoluminescence Properties of Ce3+- and Tb3+-doped Zeolite X

The optical properties of Tb3+/Ce3+ doped zeolites are elucidated with emphasis on ultraviolet (UV) and vacuum ultraviolet (VUV) excitation and luminescence. Ce3+ sensitized Tb3+ emission with quantum yields of 85 % may be obtained at 330 nm excitation. Low absorptivity at 254 nm due to low Ce3+ concentrations or low Ce3+/Tb3+ ratios, which are required for the suppression of UV components, restricts their applicability as phosphors for Hg-based discharges, e.g., in conventional fluorescent lamps. Near band edge excitation at 172 nm revealed an immediate quantum yield of 50 % enabled by a zeolite Ce3+ (5d1) Tb3+ (4f75d1) energy transfer channel, which may be exploited for the down-conversion of the Xe2 excimer emission.

On vibrational cooling upon photodissociation of carbonmonoxymyoglobin and its microscopic mechanism from the viewpoint of vibrational modes of heme

We report some results via molecular dynamics simulations for the photodissociation process of carbonmonoxymyoglobin in vacuo. The vibrational temperature of heme shows a biphasic decay (time constants, 0.84 ps (66%) and 18.7 ps (34%), are in good agreement with the experimental ones). The vibrational frequencies of the active center were obtained by time-resolved Fourier transform of velocity autocorrelation functions. Then, the vibrational modes of propionate groups of heme would couple with those of a water molecule. Since these groups are exposed to the water solvent, they should play an important role for the channel of fast energy transfer from the heme protein to the solvents.

Phonon-Mediated Intraband Relaxation of Image-State Electrons in Adsorbate Overlayers: N2/Xe/Cu(111)

Using time-resolved two-photon photoemission spectroscopy, we have studied the dynamics of image potential states in an adsorbate heterolayer system on Cu(111), consisting of 1 monolayer (ML) N2 on top of 1−2 ML Xe. The image potential states show long lifetimes of up to 1.6 ps, which is attributed to the negative electron affinity of the N2 layer that acts as a barrier between the image-state wave functions and the metal substrate. The most striking dynamical feature is an intraband relaxation within the two-dimensional band of the first image state, resulting in a pronounced dependence of the rise and decay times of the electron population on the wave vector parallel to the surface. As underlying mechanism of the intraband relaxation, the interaction with the Cu electron system can be excluded on the basis of theoretical estimates concerning the effect of e−e scattering and the long-range interaction envisaged as “image charge friction”. On the other hand, a threshold for the intraband decay of (6 ± 2) meV parallel kinetic energy indicates that the process is linked to the excitation of low-energy phonons. The most likely channel for energy transfer from image-state electrons to nuclear degrees of freedom is the excitation of frustrated rotations (librations) in the N2 layer. This is supported by theoretical estimates for freely rotating N2 molecules.

Methylation effects on the collisional quenching of vibrationally excited benzene derivatives by unexcited parent molecules

The collisional quenching of vibrationally excited methyl substituted benzenes and deuterated isomers, produced from Nd–YAG 266 nm absorption, by unexcited parent molecules was studied by measuring the time-resolved C–H (or C–D) infrared fluorescence at ∼3.3 μm (or ∼4.4 μm). The collisional quenching efficiency is enhanced almost linearly as the number of methyl groups increases. The deuterium effect of methyl substituted benzene series is not much significant compared to that of azabenzenes and benzene having no methyl group. The results provide a detailed energy transfer mechanism that the vibration–rotation/translation energy transfer channel via low frequency vibrational modes such as methyl internal rotation plays an important role in the quenching process of vibrationally excited methyl substituted benzenes by parent bath molecules.

TeV gamma-ray emission from gamma-ray bursts and ultra high energy cosmic rays

Some recent experiments detecting very high energy (VHE) gamma-rays above 10-20 TeV, independently reported VHE bursts for some bright gamma-ray bursts (GRBs), although further confirmation is necessary. If these signals are truly from GRBs, these GRBs must emit a much larger amount of energy as VHE gamma-rays than in the ordinary photon energy range of GRBs (keV-MeV). We show that such extreme phenomena can be reasonably explained by synchrotron radiation of protons accelerated to eV. Protons must carry a much larger amount of energy than electrons, suggesting that energy transfer from protons into electrons in shocked matter is very inefficient. The total energy of GRBs becomes as large as ergs and hence a strong beaming of GRB emission is highly likely. There may also be TeV emission in afterglow phase from external shocks, and proton synchrotron in this phase gives a quantitative explanation for the famous long duration GeV emission from GRB 940217. Most TeV gamma-rays are absorbed in intergalactic space by interactions with the infrared and microwave background radiations, and finally form gamma-ray background in GeV range. This process may explain the extragalactic background radiation observed in the EGRET range. We also discuss absorption of TeV gamma-rays in the shocked region of GRBs by the process, which may provide a new channel for energy transfer from protons into electrons (and positrons). This process should be important for the energetics of GRBs.

Interaction between an Alfvén wave and a particle undergoing acceleration along a magnetic field

The energy exchange between a plasma particle and an Alfvén wave propagating along a magnetic field is considered. It is shown that, in the presence of an accelerating force parallel to the background magnetic field, there exists a new channel for nonresonant energy transfer between the wave and the particle.