Energy transfer of highly vibrationally excited azulene: collisions between azulene and krypton

The energy-transfer dynamics between highly vibrationally excited azulene molecules and Kr atoms in a series of collision energies (i.e., relative translational energies 170, 410, and 780 cm(-1)) was studied using a crossed-beam apparatus along with time-sliced velocity map ion imaging techniques. "Hot" azulene (4.66 eV internal energy) was formed via the rapid internal conversion of azulene initially excited to the S4 state by 266-nm photons. The shapes of the collisional energy-transfer probability density functions were measured directly from the scattering results of highly vibrationally excited or hot azulene. At low enough collision energies an azulene-Kr complex was observed, resulting from small amounts of translational to vibrational-rotational (T-VR) energy transfer. T-VR energy transfer was found to be quite efficient. In some instances, nearly all of the translational energy is transferred to vibrational-rotational energy. On the other hand, only a small fraction of vibrational energy is converted to translational energy (V-T). The shapes of V-T energy-transfer probability density functions were best fit by multiexponential functions. We find that substantial amounts of energy are transferred in the backward scattering direction due to supercollisions at high collision energies. The probability for supercollisions, defined arbitrarily as the scattered azulene in the region 160 degrees 2000 cm(-1) is 1% and 0.3% of all other collisions at collision energies 410 and 780 cm(-1), respectively.     

Energy transfer of highly vibrationally excited azulene. III. Collisions between azulene and argon

The energy transfer dynamics between highly vibrationally excited azulene molecules (37 582 cm(-1) internal energy) and Ar atoms in a series of collision energies (200, 492, 747, and 983 cm(-1)) was studied using a crossed-beam apparatus along with time-sliced velocity map ion imaging techniques. The angular resolved collisional energy-transfer probability distribution functions were measured directly from the scattering results of highly vibrationally excited azulene. Direct T-VR energy transfer was found to be quite efficient. In some instances, nearly all of the translational energy is transferred to vibrational/rotational energy. On the other hand, only a small fraction of vibrational energy is converted to translational energy (V-T). Significant amount of energy transfer from vibration to translation was observed at large collision energies in backward and sideway directions. The ratios of total cross sections between T-VR and V-T increases as collision energy increases. Formation of azulene-argon complexes during the collision was observed at low enough collision energies. The complexes make only minor contributions to the measured translational to vibrational/rotational (T-VR) energy transfer.     

Energy transfer between polyatomic molecules II: Energy transfer quantities and probability density functions in benzene, toluene, p-xylene, and azulene collisions

Collisional energy transfer, CET, is of major importance in chemical, photochemical, and photophysical processes in the gas phase. In Paper I of this series (J. Phys. Chem. B 2005, 109, 8310) we have reported on the mechanism and quantities of CET between an excited benzene and cold benzene and Ar bath. In the present work, we report on CET between excited toluene, p-xylene, and azulene with cold benzene and Ar and on CET of excited benzene with cold toluene, p-xylene, and azulene. We compare our results with those of Paper I and report average vibrational, rotational, and translational energy quantities, , transferred in a single collision. We discuss the effect of internal rotation on CET and the identity of the gateway modes in CET and the relative role of vibrational, rotational, and translational energies in the CET process, all that as a function of temperature and excitation energy. Energy transfer probability density functions, P(E,E'), for the various systems are reported and the shape of the curves for various systems and initial conditions is discussed. The major findings for polyatomic-polyatomic collisions are: CET takes place mainly via vibration-to-vibration energy transfer assisted by overall rotations. Internal free rotors in the excited molecule hinder energy exchange while in the bath molecule they do not. Energy transfer at low temperatures and high temperatures is more efficient than that at intermediate temperatures. Low-frequency modes are the gateway modes for energy transfer. Vibrational temperatures affect energy transfer. The CET probability density function, P(E,E'), is convex at low temperatures and can be concave at high temperatures. A mechanism that explains the high values of and the convex shape of P(E,E') is that in addition to short impulsive collisions there are chattering collisions where energy is transferred in a sequence of short encounters during the lifetime of the collision complex. This also leads to the observed supercollision tail at the down wing of P(E,E'). Polyatomic-Ar collisions show mechanistic similarities to polyatomic-polyatomic collisions, but there are also many dissimilarities: internal rotations do not inhibit energy transfer, P(E,E') is concave at all temperatures, and there is no contribution of chattering collisions.     

Energy transfer between polyatomic molecules. 3. Energy transfer quantities and probability density functions in self-collisions of benzene, toluene, p-xylene and azulene

This paper is the third and last in a series of papers that deal with collisional energy transfer, CET, between aromatic polyatomic molecules. Paper 1 of this series (J. Phys. Chem. B 2005, 109, 8310) reports on the mechanism and quantities of CET between an excited benzene and cold benzene and Ar bath. Paper 2 in the series (J. Phys. Chem., in press) discusses CET between excited toluene, p-xylene and azulene with cold benzene and Ar and CET between excited benzene colliding with cold toluene, p-xylene and azulene. The present work reports on CET in self-collisions of benzene, toluene, p-xylene and azulene. Two modes of excitation are considered, identical excitation energies and identical vibrational temperatures for all four molecules. It compares the present results with those of papers 1 and 2 and reports new findings on average vibrational, rotational, and translational energy, , transferred in a single collision. CET takes place mainly via vibration to vibration energy transfer. The effect of internal rotors on CET is discussed and CET quantities are reported as a function of temperature and excitation energy. It is found that the temperature dependence of CET quantities is unexpected, resembling a parabolic function. The density of vibrational states is reported and its effect on CET is discussed. Energy transfer probability density functions, P(E,E'), for various collision pairs are reported and it is shown that the shape of the curves is convex at low temperatures and can be concave at high temperatures. There is a large supercollision tail at the down wing of P(E,E'). The mechanisms of CET are short, impulsive collisions and long-lived chattering collisions where energy is transferred in a sequence of short internal encounters during the lifetime of the collision complex. The collision complex lifetimes as a function of temperature are reported. It is shown that dynamical effects control CET. A comparison is made with experimental results and it is shown that good agreement is obtained.     

Energy transfer in combustion diagnostics: experiment and modeling

Laser induced fluorescence (LIF) of OH (A 2 sigma +) is measured in several atmospheric-pressure flames using a short-pulse laser system (80 ps duration) in conjunction with an intensified streak camera. The two-dimensional signal-detection technique allows one to simultaneously monitor rotational and vibrational relaxation as well as electronic quenching. Rotationally-resolved LIF spectra affected by energy transfer are compared with the results of a rate-equation model and are found to be in reasonably good agreement. It is shown that a significant contribution of fluorescence detected by broad-band techniques is due to levels populated by vibrational energy transfer (VET). Implications for picosecond LIF techniques for the time-resolved, quench-free detection of OH are discussed. A detailed analysis is presented for fluorescence spectra originating from levels populated by VET after excitation of states in the OH (A 2 sigma +, v' = 2) level.     

Identification of the point defects in diamond as measured by Raman spectroscopy: comparison between experiment and computation

Raman spectroscopy and molecular dynamics (MD) simulations are used to identify the vibrational spectrum of simple point defects in diamond. Two local mode frequencies are found in ion irradiated diamond. The first (with an energy of 185 meV) is clearly identified as arising from the vacancy defect, whereas a mode at 202 meV is demonstrated to be due to the [1 0 0] split interstitial.     

Energy transfer rates for vibrationally excited gas-phase azulene in the electronic ground state

Gas-phase azulene molecules were prepared with 17200 cm^?1 vibrational energy in the S₀ state by laser excitation of the S₁ state and subsequent internal conversion. Rates of vibrational energy removal (for several collision partners) were determined from the decay of the CH-stretch fluorescence at 3.3 μm. A stepladder model indicates each azulene-azulene collision removes 3500 cm^?1 of vibrational energy.     

Energy transfer between benzophenone and biacetyl

Energy transfer from benzophenone to biacetyl in the gas phase was studied by measuring the intensity and decay time of phosphorescence and fluorescence of both compounds as a function of pressure and composition of the mixture. We have established that benzophenone transfers energy along two parallel channels: singlet→singlet with high efficiency, k_SS? 2 ×10⁸ torr^?1s^?1, and triplet→triplet with lower efficiency, k_TT?8 × 10⁵ torr^?1s^?1.     

Argon and krypton adsorption on templated mesoporous silicas: molecular simulation and experiment

In this work we report molecular simulation results for argon and krypton adsorption on atomistic models of templated mesoporous silica materials. These models add atomistic levels of detail to mesoscale representations of these porous materials, which were originally generated from lattice Monte Carlo simulations mimicking the synthesis process of templated mesoporous silicas. We generate our atomistic pore models by carving out of a silica block a ‘mathematically-smooth’ representation of the pores from lattice MC simulations. Following that procedure, we obtain a model material with mean mesopore and micropore diameters of 5.4 nm and 1.1 nm, respectively (model A). Two additional model materials were considered: one with no microporosity, and with mesopores similar to those of model A (model B), and a regular cylindrical pore (model C). Simulation results for Ar and Kr adsorption on these model materials at 77 K and 87 K shows that model A provides the best agreement with experimental data; however, our results suggest that fine-tuning the microporosity and/or the surface chemistry (i.e., by decreasing the density of OH groups at the pore surface) of model A can lead to better agreement with experiments. The filling of the mesopores in model materials A and B proceeded via a classical capillary condensation mechanism, where the pores fill at slightly different pressures. This observation contrasts with what was observed in our previous study (Coasne, et al. in Langmuir 22:194–202, 2006), where we considered atomistic silica mesopores with an important degree of surface roughness at length scales below 10 Å, for which we observed a quasi-continuous mesopore filling involving intermediate phases with liquid-like “bridges” and gas-like regions. These results suggest that pore surface roughness, and other morphological features such as constrictions, play an important role in the mechanism of adsorption and filling of the mesopores.     

The excess enthalpy for a mixture of krypton and xenon: experiment and theory

Measurements of the excess enthalpy of krypton and xenon mixtures at 163 K are reported. These results are found to disagree with the only other published result. This discrepancy is discussed. Conformal solution theory is used to provide an unbiased prediction of the excess enthalpy. We review published experimental and calculated excess thermodynamic properties of the K_r/Xe system at zero pressure and 161.38 K and 165 K.     

Energy transfer between samarium and europium in phosphate glasses

A study of energy transfer from samarium to europium in phosphate glasses was performed for a range of donor and acceptor concentrations corresponding to a donor-acceptor distance of 13–24 Å. The energy transfer probabilities were calculated. The mechanism of transfer was deduced by fitting the experimental decay curves to the theoretical curves obtained by Inokuti and Hiroyama. Theoretical transition probabilities based on Dexter's formula were calculated. It was inferred that the energy is transferred by a dipole-quadrupole mechanism which is assisted by phonons. It was possible to indicate the path by which the transfer takes place.     

Energy transfer of highly vibrationally excited azulene. II. Photodissociation of azulene-Kr van der Waals clusters at 248 and 266 nm

Photodissociation of azulene-Kr van der Waals clusters at 266 and 248 nm was studied using velocity map ion imaging techniques with the time-sliced modification. Scattered azulene molecules produced from the dissociation of clusters were detected by one-photon vacuum ultraviolet ionization. Energy transfer distribution functions were obtained from the measurement of recoil energy distributions. The distribution functions can be described approximately by multiexponential functions. Fragment angular distributions were found to be isotropic. The energy transfer properties show significantly different behavior from those of bimolecular collisions. No supercollisions were observed under the signal-to-noise ratios S/N=400 and 100 at 266 and 248 nm, respectively. Comparisons with the energy transfer of bimolecular collisions in thermal systems and the crossed-beam experiment within detection limit are made.     

Agitated vessel particle-liquid mass transfer: A comparison between theories and data

Mass transfer from suspended solids in agitated vessels has been explained by both the Kolmogoroff and the terminal velocity-slip velocity theories.It is shown that data plots using the dimensionless groupings obtained from the Kolmogoroff theory are unsatisfactory, because of the swamping effect in the groups of the enormous range of particle size as compared to the ranges of the other variables. These dimensionless groupings are misleading, since they suggest equal power input leads to equal mass transfer coefficients. Experimental evidence shows, that different impeller-vessel configurations enable complete suspension to be achieved at significantly different power inputs, whilst the mass transfer coefficient remains the same. This observation complies with the terminal velocity-slip velocity theory.However, though the fundamental concepts underlying the two theories are different, the resultant numerical values of the mass transfer coefficients are approximately the same.     

Energy transfer in collisions between two vibrating molecules

We study vibrational energy transfer in inelastic collinear collisions between two diatomic molecules. The system is represented by two linearly driven parametric oscillators with a bilinear, time-dependent residual coupling between them. We account for the time evolution of the linearly driven parametric oscillators with an operator algebra, and use perturbation theory and basis expansions to include the residual coupling. Results are presented for H₂FH and N₂CO. Direct two-quantum transitions are found to be important even for low relative collision energies.     

Elastic constants of uniaxial nematic liquid crystals: A comparison between theory and experiment

Using the unified molecular theory developed in our earlier paper (1992, Phys. Rev. A, 45, 974) we study in detail the influence of molecular interactions on the fundamental elastic properties of uniaxial nematic liquid crystals composed of molecules of cylindrical symmetry. The expressions for the elastic moduli associated with 'splay', 'twist' and 'bend' modes of deformations are written in terms of order parameters characterizing the nature and amount of ordering in the phase and the structural parameters which involve the generalized spherical harmonic coefficients of the direct pair correlation function of an effective isotropic liquid. Numerical calculations are done for a model system, the molecules of which have prolate ellipsoid of revolution symmetry and interact via a pair potential having both repulsive and attractive parts. The repulsive interaction is represented by a repulsion between hard ellipsoids of revolution. The attractive potential is represented by the dispersion and electrostatic interactions. Results for the elastic constants are reported for a range of molecular length-width ratio, temperature, density and molecular parameters and are compared with the experimental values of p-azoxyanisole (PAA) and 4'-n-octyloxy-4-cyanobiphenyl (8OCB). It is found that the inclusion of electrostatic interactions reduces the values of the ratios K₂/K₁ and K₃/K₁. The absolute values of the elastic constants and their ratios are in good agreement with the experimental and computer simulation values. The temperature dependence of the elastic constants and their ratios is studied. It is observed that the twist elastic constant has a weak temperature dependence but a pronounced influence is observed on the bend moduli. We also observed a pronounced increase in the values of the twist and bend elastic constants on approaching the nematic-smectic A transition temperature.     

Energy transport in peptide helices: a comparison between high- and low-energy excitations

Energy transport in a short helical peptide in chloroform solution is studied by time-resolved femtosecond spectroscopy and accompanying nonequilibrium molecular dynamics (MD) simulations. In particular, the heat transport after excitation of an azobenzene chromophore attached to one terminus of the helix with 3 eV (UV) photons is compared to the excitation of a peptide C=O oscillator with 0.2 eV (IR) photons. The heat in the helix is detected at various distances from the heat source as a function of time by employing vibrational pump-probe spectroscopy. As a result, the carbonyl oscillators at different positions along the helix act as local thermometers. The experiments show that heat transport through the peptide after excitation with low-energy photons is at least 4 times faster than after UV excitation. On the other hand, the heat transport obtained by nonequilibrium MD simulations is largely insensitive to the kind of excitation. The calculations agree well with the experimental results for the low-frequency case; however, they give a factor of 5 too fast energy transport for the high-energy case. Employing instantaneous normal mode calculations of the MD trajectories, a simple harmonic model of heat transport is adopted, which shows that the heat diffusivity decreases significantly at temperatures initially reached by high-energy excitation. This finding suggests that the photoinduced energy gets trapped, if it is deposited in high amounts. The various competing mechanisms, such as vibrational T(1) relaxation, resonant transfer between excitonic states, cascading down relaxation, and low-frequency mode transfer, are discussed in detail.     

Time-dependent photoionization of azulene: competition between ionization and relaxation in highly excited states

Pump-probe photoionization has been used to map the relaxation processes taking place from highly vibrationally excited levels of the S(2) state of azulene, populated directly or via internal conversion from the S(4) state. Photoelectron spectra obtained by 1+2(') two-color time-resolved photoelectron imaging are invariant (apart from in intensity) to the pump-probe time delay and to the pump wavelength. This reveals a photoionization process which is driven by an unstable electronic state (e.g., doubly excited state) lying below the ionization potential. This state is postulated to be populated by a probe transition from S(2) and to rapidly relax via an Auger-like process onto highly vibrationally excited Rydberg states. This accounts for the time invariance of the photoelectron spectrum. The intensity of the photoelectron spectrum is proportional to the population in S(2). An exponential energy gap law is used to describe the internal conversion rate from S(2) to S(0). The vibronic coupling strength is found to be larger than 60+/-5 microeV.     

Energy transfer between conjugated polyelectrolytes in layer-by-layer assembled films

We present a study of Fo?rster resonance energy transfer (FRET) between two emissive conjugated polyelectrolytes (CPEs) in layer-by-layer (LbL) self-assembled films as a means of examining their organization and architecture. The two CPEs are a carboxylic acid functionalized polyfluorene (PFl-CO(2)) and thienylene linked poly(phenylene ethynylene) (PPE-Th-CO(2)). The PFl-CO(2) presents a maximum emission at 418 nm, while the PPE-Th-CO(2) has an absorption λ(max) centered at 431 nm, in sufficient proximity for effective FRET. Several LbL films have been constructed using varied concentrations of the deposition solutions and identity of the buffer layers separating the two emissive layers, using a system of either weak polyelectrolytes, poly(allylamine hydrochloride) (PAH)/poly(sodium methacrylate) (PMA), or strong polyelectrolytes, poly(diallylammonium chloride) (PDDA)/poly(styrene sulfonate) sodium (PSS). The efficiency of FRET has been monitored using fluorescence spectroscopy. Initially, the fluorescence of the PFl-CO(2) (E(g) ～ 3.0 eV), which emits at 420 nm, is quenched by the lower band gap PPE-Th-CO(2) (E(g) ～ 2.5 eV). For films using the PAH/PMA system as buffer bilayers and deposited from 1 mM solutions, the PFl-CO(2) fluorescence is progressively recovered as the number of intervening buffer bilayers is increased. Ellipsometry measurements indicate that energy transfer between the two emissive layers is efficient to a distance of ca. 7 nm.     

Energy transfer between dyes on glass surfaces and ions in glasses

A method for radiative and non-radiative energy transfer between flourescent organic dyes incorporated in a thin film deposited on a glass and inorganic ions in the bulk or surface of the glass is proposed.     

Energy transfer between coronene and rhodamine 6G in PMMA matrices

The fluorescence lifetime of coronene and the rate of dipole-dipole energy transfer from coronene to rhodamine 6G in PMMA matrices were found to be temperature dependent. For both these photophysical processes an activation energy of about 500 cm^?1 is obtained. The energy transfer results can be analyzed in terms of a model involving thermally activated energy transfer from excited states of coronene.