Electronic energy transfer

A compact donor-acceptor molecular dyad has been synthesized by attaching an N,N-dimethylamino fragment to a naphthalic anhydride residue. The dyad shows fluorescence from an intramolecular charge-transfer state (i.e., charge-recombination fluorescence) in fluid solution, with the photophysical properties being strongly dependent on the solvent polarity. Similar emission is seen for single crystals of the target compound, molecules being aligned head-to-head, although time-resolved emission profiles display dualexponential kinetics. A second polymorph having the head-to-tail alignment also gives rise to two lifetimes, these differ somewhat from those of the first structure, which are assigned to bulk and surfacebound molecules. Growing the crystal in the presence of Rhodamine B localizes the dye around the surface. Excitation of the crystal is followed by sub-ps exciton migration along the aligned stacks, with occasional crossing to adjacent stacks and trapping at the surface. Rhodamine B present at very low levels acts as the acceptor for excitons entering the surface layer. Crystals embedded in a polyester resin form an artificial light-harvesting antenna able to sensitize an amorphous silicon solar cell.

A compact donor-acceptor molecular dyad has been synthesized by attaching an N,N-dimethylamino fragment to a naphthalic anhydride residue. The dyad shows fluorescence from an intramolecular charge-transfer state (i.e., charge-recombination fluorescence) in solution, with the photo-physical properties being strongly dependent on the solvent polarity. Similar emission is seen for single crystals of the target compound, the molecules being aligned head-to-head, although time-resolved emission profiles display dual-exponential kinetics. A second polymorph with the head-to-tail alignment also gives rise to two lifetimes that differ somewhat from those of the first structure, which are assigned to bulk and surface-bound molecules. Growing the crystal in the presence of Rhodamine B localizes the dye around the surface. Excitation of the crystal is followed by sub-ps exciton migration along the aligned stacks, with occasional crossing to adjacent stacks and trapping at the surface. Rhodamin...

We have combined electronic and vibrational spectroscopy in a cryogenic ion trap to produce highly resolved, conformer-selective spectra for the ground and excited states of a peptide containing two chromophores. These spectra permit us to determine the precise three-dimensional structure of the peptide and give insight into the migration of the electronic excitation from phenylalanine to tyrosine because changes in the excited-state infrared spectra are sensitive to localization of the electronic energy in each chromophore. The well-controlled experimental conditions make this result a stringent test for theoretical methods dealing with electronic energy transfer.

The process of intermolecular electronic excitation transfer (EET) in a monodimensional supramolecular arrangement of molecules in confined space has been modelled and investigated by means of first‐principles molecular dynamics simulations. The chosen model system consists of a wire of chlorine molecules hosted in the noncrossing channels of the zeolite bikitaite. The time evolution of the system in its first excited singlet state has been described by the restricted open shell Kohn–Sham formalism. Simulation results have highlighted that excitation, initially localized on a guest molecule, is transferred to an adjacent moiety in the molecular wire on the picosecond scale via a collision‐induced Dexter‐type short range EET. Analysis of the modifications of the electronic structure of the system brought about by EET has given insight into the microscopic details of the process.

A new and general procedure is described for a detailed analysis of time-resolved fluorescence depolarisation data in the presence of electronic energy migration. An isotropic ensemble of bifluorophoric molecules (D 1 -R-D 2 ) has been studied to demonstrate its utility. Intramolecular donor-donor energy migration occurs between the two donor groups (D), which are covalently connected to a rigid linker group (R). These groups undergo restricted reorientational motions with respect to the R group. The analysis of depolarisation data basically involves the search for best-fit parameters which describe the local reorienting motions, the interfluorophore D 1 -D 2 distance, as well as the mutual orientations of the donors. For this, the analysis is partly performed in the Fourier domain and the best-fit parameters are determined by using an approach based on a Genetic Algorithm. The energy migration process has been described by using Monte Carlo simulations and an extended Förster theory. It is found that this theory provides the least time-consuming computational method. Since one-photon and two-photon excited fluorescence experiments can be applied for energy migration studies, a general and unified theoretical formulation is given. To exemplify the developed quantitative approach the depolarisation of the fluorescence in the presence of electronic energy migration within a bis-(9-anthrylmethylphosphonate) bisteroid molecule has been studied by time-resolved two-photon excited fluorescence depolarisation experiments. To solely obtain information about local reorientations of the 9anthrylmethyl group, also the mono-(9-anthrylmethylphosphonate) bisteroid was studied, which enabled modelling of the ordering potential of the donor. Values of the two-photon absorption tensor components were obtained. To describe the discrepancy between the measured values of the initial anisotropy and fundamental anisotropy predicted by theory the distribution of absorption tensor caused by fast processes have been introduced. An angular parameter of absorption tensor was determined. Reasonable values of the distance between the 9-anthrylmethyl groups, as well as for their mutual orientation were obtained.

Recent years have witnessed a growing interest of the scientific community for the use of ab initio and density functional theory methods in theoretical studies of molecules containing many atoms. However, the ''scaling wall'' of some of the most accurate of such methods is often an obstacle for their applicability to systems of real-life interest, e.g. in biochemistry and nanotechnologies, one bottleneck being the evaluation and storage of the two-electron repulsion integrals. We have explored the possibility to avoid the expensive evaluation of the full integral matrix by designing and implementing approximate approaches based on Cholesky decomposition techniques. The results show the general applicability of this approximation to any quantum chemical method with substantial computational savings compared to conventional implementations-not uncommonly of 1-2 orders of magnitude. At the same time, the loss of accuracy is minimal and can be systematically reduced at the price of sustainable additional costs. A major theoretical achievement of this study has been the reformulation of the Cholesky approximation in terms of solution to a ''density fitting'' variational problem. This has lead to an elegant formulation of the analytic derivatives of the Cholesky vectors, a long-standing lack of this technique. Moreover, this observation has paved the way to a new generation of accurate density fitting approximations free from biases and derived fully ab initio.

This paper proposes the description of a soft nuclear density and the associated soft nuclear Coulombic repulsion. The idea just consists into describing soft nuclear repulsion expressions substituting the usual point-like nuclear charge density by means of a Gaussian distribution. The theoretical structure and management of such an approach is discussed.

The photophysics of a donor-acceptor system in which efficient electronic energy transfer occurs is analysed and discussed by the combined use of steady-state and time-resolved spectroscopy and DFT/TD-DFT computations. The donor and acceptor units belong to the class of BODIPY chromophores, and are conveniently linked through a calixarene scaffold, which allows the control of the mutual orientation and distance between chromophores. Our results highlight that the energy transfer process occurs with multiexponential dynamics strongly influenced by the solvent. Although the conformation adopted by the system is very similar in all the analysed solvents, highly polar media favour fast and efficient energy transfer. On the contrary, in non-polar media, the concomitant occurrence of backward energy transfer

The stimulated 331.1-nm emission in the K(5D-4S) quadrupole transition has been studied after excitation by two tunable dye laser waves, one of which is in the K(4S-> 3D) quadrupole transition. A four-wave mixing mechanism has been shown to be responsible for the UV emission based on intensity, temperature, and spectral measurements. The measured conversion efficiency is compared with the calculated value.

Protein fluorescence decays are difficult to interpret and often involving several energy transfer processes among Trp residues or Trp-ligands. In this study, we simulate EET rates by computing MD-averaged electronic couplings V. Fluorescence decays have been observed for the HSA protein bound to the S and R enantiomers of FBP. So far, our results in the HSA-FBP system agree with the experimental hypothesis (stereoselective energy transfer) and strongly support the binding modes proposed for the R and S enantiomers in HSA.

The dissociative attachment process in low-energy collisions ͓Na 2 (vЉ, jЉ)ϩe Ϫ → Na ϩ Na Ϫ , with electron energies Ͻ1.0 eV͔ is studied as a function of vibrational excitation over a broad range of excited levels. We employ a crossed electron-molecule beams arrangement and two optical methods for preparing vibrationally excited states: Franck-Condon pumping ͑FCP͒ as well as the efficient and highly state-specific technique of stimulated Raman scattering with adiabatic passage ͑STIRAP͒. Both methods are capable of preparing vibrational levels with energies up to about 60% of the Na 2 X 1 ⌺ g ϩ binding energy (vЉр30). Analysis of the FCP data shows an increase of more than three orders of magnitude in the state-dependent dissociative attachment rate as a function of the vibrational level up to vЉϭ12. For vЉϾ12, which is close to the exoergic threshold, no further increase with vЉ is observed. The dissociative attachment rates measured for single rovibronic states using the STIRAP technique confirm the validity of the conclusions drawn from our analysis of the FCP measurements. The potential curve of the Na 2 Ϫ A 2 ⌺ g ϩ state, which is essential for the dissociative attachment process, has been determined from variational valence shell multireference configuration interaction calculations in its bound region but by imposing Feshbach projection in its resonance region. The crossing of this potential with that of the Na 2 (X) state is found between vЉϭ11 and vЉϭ12, in full agreement with the conclusions from analysis of the experimental data. The variation of the theoretical enhancement factors with the electron energy, calculated in the framework of the traditional resonance theory, are also reported. After convolution with the electron flux distribution, good agreement with the experimental enhancement factors is found.

The reaction of excess F atoms with HN, in a halocarbon-coated flow reactor has been used to generate and study the quenching reactions of NF(a'A,v'=O) at 300 K. Fifteen reagents, selected to represent a variety of chemical interactions, were tested in order to provide a survey of quenching rate constants that could be compared to O,(a'A,) and NH(a'A). A range of rate constants was found which varied from-1.0 X IO-'] 6171, s-' for N(CH,), to-1.0 X (3117, s-I for N2, H2, and NO. The NF(a'A) molecule is much less reactive than NH(a'A), and it seems to more closely resemble O,(alA,). However, the quenching rate constants for NF(a'A) generally are larger than for 02(a1A) and show a greater variation from one reagent being a small component of the total self-quenching reaction. to another. The self-quenching rate constant for NF(alA) is (2.2 k 1.2) X 10-7 cm3 s-I with energy pooling to give NF(b'Z+)

Rare gas binary mixtures under electron bombardment are characterized by strong emissions that have been traced to radiative transitions of diatomic heteronuclear ions (e.g.,(ArXej*) .The kinetic processes governing heteronuclear ion formation and decay are not well understood,but the more Important of these processes should be elucidated by time resolved spectroscopy. In particular, since heteronuclear rare gas ions are known to be only weakly bound, their characteristic emissions are expected to be greatly affected by changes in temperature. Under this assumption,an experiment was devised to study the (ArXe) + 320nm emission under time resolved conditions for different temperatures and mixtures of the gas. The experiment necessitated the design and installation of a temperature variable gas cell. Although the apparatus allows one to vary temperature over a wider range,data,thus far, were taken between 24BK and 348K. In this range, preliminary time dependent spectra have been observed to vary significantly as a function of temperature. encouraging and instructive, making the work involved in this thesis considerably easier. Thanks are extended to Mr. M. G. Ourrett for continuing to guide and aid the author in the lab and for his collaboration in this work. The author is grateful to Mr. D. B. Vanderslice for his helpful assistance in the lab. Last, but not least, the author would like to express his eternal gratitude to his parents, whose love and understanding are an unending source of motivation.

A finite-dimensional pseudo-unitary framework is set up for describing the dynamics of free elementary particles in a purely relativistic quantum mechanical way. States of any individual particles or antiparticles are defined as suitably normalized vectors belonging to the two-complexdimensional spaces that occur in local orthogonal decompositions of isomorphic copies of Cartan's space. The corresponding dynamical variables thus show up as bounded pseudo-Hermitian operator restrictions that possess real discrete spectra. Any measurement processes have to be performed locally in orthocronous proper Lorentz frames, but typical observational correlations are expressed in terms of symbolic configurations which come from the covariant action on spaces of state vectors of the Poincaré subgroup of an adequate realization of SU (2, 2). The overall approach turns out to supply a supposedly natural description of the dynamics of free twofold systems in flat spacetime. One of the main outlooks devised here brings forward the possibility of carrying out methodically the construction of a background to a new relativistic theory of quantum information.

To extend the capabilities of the existing spectroscopic methods of investigation and diagnostics of gaseous media, we consider a macroscopic effect of an increase in the rate of spontaneous emission, which results in the transition of a considerable part of excited molecules to lower-lying optically accessible states with light pulse emission along the direction of propagation of the exciting laser radiation. As an application of the effect of amplified spontaneous emission, we propose the population with its help of optically inaccessible electronic states of atoms and molecules.

PHYSICAL REVIEW LETTERS 14 Jvr.v 1975 and Development Administration under contract with the Union Carbide Corporation. /Post-doctoral research appointment through the University of Kentucky and supported by Los Alamos Scientific Laboratory. 'For helium, see D.

Two-photon laser-assisted reactions yielding XeCl(B,C) in 5-15-torr Xe/C12 mixtures and XeCl(B,C) and XeI(B,C) in 5-15-torr Xe/IC1 mixtures have been observed. The order dependence, the time dependence, and the excitation wavelength dependence are consistent with transfer from the ground-state, nonreactive potential to an excited-state, reactive potential followed by chemical reaction. The reactive potential may be the ion pair potential, V(Xe+,ClL), or a potential involving electronically excited chlorine, V(Xe,C12**), or more likely a mixed potential having character of both states.

The reaction of F atoms with HN 3 forms the products HF (+N 3) or HNF (+N 2). The product branching fraction for this reaction has been investigated in a room-temperature flow reactor using laser-induced fluorescence to monitor the concentration of HNF. A microwave discharge applied to a dilute flow of CF 4 in argon served as the F atom source. Using reactant concentrations of (0.7-3.0) × 10 12 and (1.7-20.0) × 10 12 molecules cm-3 for CF 4 and HN 3 , respectively, the rate constant for formation of HNF from HN 3 + F was determined to be (6.3 (3.5) × 10-12 cm 3 molecule-1 s-1. The average product branching fraction for formation of HNF was determined to be 0.03-0.01 +0.02. The quenching rate constant of electronically excited HNF by Ar was determined to be (3.0 (0.3) × 10-11 cm 3 molecule-1 s-l. In addition, secondary reactions between HNF and HN 3 , F, and H 2 S were examined. No reaction was observed to occur between HNF and HN 3. The reaction of HNF + F was observed to occur with an estimated rate constant of 2.0 × 10-10 cm 3 molecule-1 s-1. The rate constant for the reaction between HNF and H 2 S was found to be less than 1 × 10-12 cm 3 molecule-1 s-1 .

We present interactive quantum chemistry simulation at the atom superposition and electron delocalization molecular orbital (ASED-MO) level of theory. Our method is based on the divideand-conquer (D&C) approach, which we show is accurate and efficient for this non-self-consistent semiempirical theory. The method has a linear complexity in the number of atoms, scales well with the number of cores, and has a small prefactor. The time cost is completely controllable, as all steps are performed with direct algorithms, i.e., no iterative schemes are used. We discuss the errors induced by the D&C approach, first empirically on a few examples, and then via a theoretical study of two toy models that can be analytically solved for any number of atoms. Thanks to the precision and speed of the D&C approach, we are able to demonstrate interactive quantum chemistry simulations for systems up to a few hundred atoms on a current multicore desktop computer. When drawing and editing molecular systems, interactive simulations provide immediate, intuitive feedback on chemical structures. As the number of cores on personal computers increases, and larger and larger systems can be dealt with, we believe such interactive simulations-even at lower levels of theory-should thus prove most useful to effectively understand, design and prototype molecules, devices and materials.

An efficient computational algorithm to implement a local operator approach to partitioning electronic energy in general molecular systems is presented. This approach, which rigorously defines the electronic energy on any subsystem within a molecule, gives a precise meaning to the subsystem ground and excited electronic energies, which is crucial for investigating electronic energy transfer from first principles. We apply the technique to the 9−((1−naphthyl)−methyl)-anthracene (A1N) molecule by partitioning A1N into anthracenyl and CH2−naphthyl groups as subsystems, and examine their electronic energies and populations for several excited states using Configuration Interaction Singles method. The implemented approach shows a wide variety of different behaviors amongst these excited electronic states.

We extend the localized operator partitioning method (LOPM) [J. Nagesh, A.F. Izmaylov, and P. Brumer, J. Chem. Phys. 142, 084114 (2015)] to the time-dependent density functional theory (TD-DFT) framework to partition molecular electronic energies of excited states in a rigorous manner. A molecular fragment is defined as a collection of atoms using Stratman-Scuseria-Frisch atomic partitioning. A numerically efficient scheme for evaluating the fragment excitation energy is derived employing a resolution of the identity to preserve standard one-and two-electron integrals in the final expressions. The utility of this partitioning approach is demonstrated by examining several excited states of two bichromophoric compounds: 9−((1−naphthyl)−methyl)−anthracene and 4−((2−naphthyl)−methyl)−benzaldehyde. The LOPM is found to provide nontrivial insights into the nature of electronic energy localization that are not accessible using simple density difference analysis.

The localized operator partitioning method [Y. Khan and P. Brumer, J. Chem. Phys. 137, 194112 (2012)] rigorously defines the electronic energy on any subsystem within a molecule and gives a precise meaning to the subsystem ground and excited electronic energies, which is crucial for investigating electronic energy transfer from first principles. However, an efficient implementation of this approach has been hindered by complicated one- and two-electron integrals arising in its formulation. Using a resolution of the identity in the definition of partitioning, we reformulate the method in a computationally efficient manner that involves standard one- and two-electron integrals. We apply the developed algorithm to the 9 - ((1 - naphthyl) - methyl) - anthracene (A1N) molecule by partitioning A1N into anthracenyl and CH2 - naphthyl groups as subsystems and examine their electronic energies and populations for several excited states using configuration interaction singles method. The impl...

We have investigated a new scheme for excitation of the 5s, J = 2 metastable level of Kr (5s[3/2] J =2) which can be readily extended to other rare gases. In the scheme, an ultraviolet (UV) lamp is used to create a population of Kr atoms in the 5s[3/2] J =1 level in a gas cell. The excited atoms are then pumped to the 5p[3/2] J =2 level, using 819 nm light from a Ti:sapphire laser, from which they decay to the metastable state with a branching ratio of 77%. We made two striking observations: (1) the laser power required to saturate the second step decreases markedly as a function of gas cell pressure, and (2) the UV photon flux is converted with very high efficiency (≈10%) to metastable atom flux. A Monte Carlo study of the scattering of UV photons in the cell reproduces the trends observed. The understanding achieved points to the design of a higher flux source of metastable atoms.

The time-dependent density functional based tight-binding (TD-DFTB) approach is generalized to account for fractional occupations. In addition, an on-site correction leads to marked qualitative and quantitative improvements over the original method. Especially, the known failure of TD-DFTB for the description of σ → π * and n → π * excitations is overcome. Benchmark calculations on a large set of organic molecules also indicate a better description of triplet states. The accuracy of the revised TD-DFTB method is found to be similar to first principles TD-DFT calculations at a highly reduced computational cost. As a side issue, we also discuss the generalization of the TD-DFTB method to spin-polarized systems. In contrast to an earlier study [Trani et al., JCTC 7 3304 (2011)], we obtain a formalism that is fully consistent with the use of local exchange-correlation functionals in the ground state DFTB method.

Some aspects of the energy transfer processes between poly(9,9-dihexylfluorene-2,2-dyil) and MEH-PPV in solutions of good and poor solvents and in solid state blends are addressed. The resonant non-radiative energy transfer in solution is discussed in terms of the Förster dynamic model and using steady-state and dynamical photoluminescence we observed that it is a very inefficient process, almost independently of the solvent. Also in solution the intensity decrease was discussed in terms of trivial energy transfer process. On the other hand, in solid state the resonant non-radiative energy transfer process is more relevant and strongly dependent on the MEH-PPV concentration in the blends. In this case, Förster approach may be relevant but it is not good enough to correlate the quenching efficiency with the blend composition. Trivial energy transfer process together with resonant energy transfer process play an important role for systems with donor higher concentration.

Two-photon excitation of Xe atoms in a static gas cell was used to prepare the Xe(7p[3/2] 2 , 7p[5/2] 2 , and 6p′[3/2] 2) states in mixtures of Xe with rare gases, H 2 (D 2), CO, Cl 2 , HCl, CCl 2 F 2 , CCl 4 , SF 6 , NF 3 , CF 4 , CH 4 , CH 3 F, and N 2 O at 300 K. The total quenching rate constants were measured, and product fluorescence spectra were used to assign reaction pathways. The total quenching constants for these Xe* Rydberg states by molecular reagents are very large, corresponding to cross sections of 200-1500 Å 2. Qualitative models are introduced to discuss the quenching processes, which are mainly reactive quenching and excitation transfer. For reagents with very large electron affinities, ion-pair formation may be important. A correlation of the quenching cross section with the dipole moment of the reagent was observed. These three Xe* states have nearly the same energy (∼11 eV), but the Xe(7p) states nominally have the Xe + (2 P 3/2) ion-core, whereas the Xe(6p′) states have the Xe + (2 P 1/2) core. The XeCl(D) and XeF(D) states also have the Xe + (2 P 1/2) core, and the fraction of XeCl(D) and XeF(D), relative to XeCl(B,C) or XeF(B,C), formed by the reactions of Xe-(6p′[3/2] 2) atoms with halogen-containing reagents can be used to measure the degree of conservation of the Xe + (2 P 1/2) core. The Xe(7p) and Xe(6p′) reactions actually gave nearly the same B and D distributions, with XeCl(B) and XeF(B) being favored, and the Xe + (2 P 1/2) ion-core state is not conserved during the reactions of Xe(6p′) atoms. Two-photon excitation of the Xe(7p,6p′) states leads to intense amplified spontaneous emission (ASE) unless special feature are incorporated into the experimental design. This ASE emission can cause complications because other Xe* states are generated during the laser pulse. The measurements reported here are free from ASE, but some consequences of ASE for experiments with Xe(6p′ and 7p) excitation are summarized. The relative two-photon cross sections of the Xe(6p, 6p′, 7p) states also are discussed.

Transition probability for the line 2203.53 A of Pb II and transition probabilities for 39 lines of Pb I have been determined from measurements of emission line intensities in an optically thin laser-produced plasma of Pb and Sn in an atmosphere of Ar. Lines for which transition probabilities have not been previously published are included. In order to establish this homogeneity and the existence of local thermodynamic equilibrium, plasma has been studied. A temperature of 11060 K and an electron density of 10 16 cm-3 were found. The results were compared with the available experimental and theoretical data given by other authors.

A ligand controlled catalytic system for the aerobic oxidation of 1° amines to nitriles and imines has been developed where the varying π-acidic feature of BIAN versus phen in the frameworks of ruthenium catalysts facilitates switchable selectivity.

Mexico 87545 (U.S.A.) Lifetime data at 25% and 393.5 nm excitation have been obtained for the decay of SOZ(~B~) molecules in the presence of high pressures of CHII, CaH6, and C3Hs. At high pressures the physical quenching channel saturates and a kinetic model which incorporates this effect has been applied to the 332,

Ltfettmes of electronically excited IJTg molecules have been determrned at room temperature from 0.01-80 ton. An estimate for the collision free lifetime ~40 ps for IJFb excited at 393 nm was determmed. The Stern-Volmer plots for IJF6 selfquenching revealed two regions of curvature. * Work performed under the auspices of the US_ Energy Resexch and Development Administration.

The lifetimes of the 6p Dz&z, 6p Pzzz, 6p D7&2, 6p D&&2, and 6p F7&& levels in Xe u have been measured with a beam-laser technique. A detailed spectroscopic investigation of the fluorescence channels of the excited states has been performed to confirm the identifications. As a consequence of some reassignments experimental lifetime values can now unambiguously be ascribed to these levels.

We discuss the theory of quenching of Rydberg states by atoms with small electron affinities and perform calculations for quenching of s and d states of Ne by Ca. There are two contributions to the quenching cross section: the ion-covalent coupling contribution, which is due to the formation of an intermediate ionic state, and the impulse contribution. The first is very sensitive to the electron affinity of the projectile and may be used for studies of weakly bound anions. The ion-covalent coupling contribution is dominant for the process of quenching of Rydberg states with large quantum defects (e.g. s states) at relatively low principal quantum number n, whereas the impulse contribution dominates the process of quenching of states with small quantum defects (e.g. d states of Ne). We present calculations of quenching of s states by Yb atoms with a hypothetical electron affinity of 2 meV and show that experimental studies of quenching of Rydberg states by Yb might help in determining the electron affinity of Yb even if it is negative. In the latter case the quenching cross section as a function of n should exhibit oscillations (Borodin V M and

We extend the localized operator partitioning method (LOPM) [J. Nagesh, A.F. Izmaylov, and P. Brumer, J. Chem. Phys. 142, 084114 (2015)] to the time-dependent density functional theory (TD-DFT) framework to partition molecular electronic energies of excited states in a rigorous manner. A molecular fragment is defined as a collection of atoms using Stratman-Scuseria-Frisch atomic partitioning. A numerically efficient scheme for evaluating the fragment excitation energy is derived employing a resolution of the identity to preserve standard one-and two-electron integrals in the final expressions. The utility of this partitioning approach is demonstrated by examining several excited states of two bichromophoric compounds: 9−((1−naphthyl)−methyl)−anthracene and 4−((2−naphthyl)−methyl)−benzaldehyde. The LOPM is found to provide nontrivial insights into the nature of electronic energy localization that are not accessible using simple density difference analysis.

Synopsis We report our recent experimental study regarding H 2 and D 2 continuum radiation (a 3 Σ g + → b 3 Σ u +) in the spectral range of 200-650 nm, excited by electron impact at near-threshold impact energies. In contrast to earlier studies, present results indicate that H 2 and D 2 continuum radiation is detectable in the spectral range above 500 nm. To supplement this, we present excitation-emission photon efficiency curves (PEC) of D 2 measured at 650 nm, with precautions taken to rule out other possible signal sources.

The objective of this contract is the study of state-to-state, electronic energy transfer reactions following two-photon laser excitation. We have chosen to study reactions of Xe 5pnp because of their relevance to the XeCl excimer laser. We are studying deactivation reactions in collisions with heavy atoms such as Ar, Kr, and Xe and reactive collisions with chlorides. The reactants are excited by multiphoton laser absorption. Product channels are observed by their fluorescence, or by laser induced fluorescence using a second color laser. Reaction rates are measured by observing the time dependent decay of signals from reactant and product channels. In addition we measure interaction potentials of the reactants by laser spectroscopy where the laser induced fluorescence or ionization is measured as a function of laser wavelength (excitation spectra) or by measuring fluorescence spectra at fixed laser frequencies with monochromators. The spectra are obtained in the form of either lines...

A flowing-afterglow technique is described for measuring the absolute yield of a radiative product state from ion-electron recombination. The technique is applied to CO 2 ϩ ϩe Ϫ dissociative recombination. The measured yield of CO(a 3 ⌸)ϩO(3 P) is 0.29Ϯ0.10. This includes cascade from higher triplet states of CO. The vibrational distribution in CO(a 3 ⌸,vϭ0-3) is approximately Boltzmann, with an effective temperature of 4200Ϯ300 K. The measured rate constant for quenching of CO(a) by CO 2 is (1.0Ϯ0.2)ϫ10 Ϫ11 cm 3 s Ϫ1 , somewhat lower than previous measurements.

We have measured radiative lifetimes in the 5 snp 1P 1 (n=5-29) and 5 snf 1F 3 (n=4-11) Rydberg series in neutral strontium. The measurements were performed with single-step laser excitation starting from the ground state or from a metastable state populated by collisions. The decay photons were detected using delayed coincidence technique or a transient recorder. The presence of configuration interaction in the 5snp 1p~ series can be observed around n = 8. The perturbation in the 5 s nf 1F 3 series is not reflected in the behaviour of the lifetime values.

The collisional behaviour is presented of electronically excited germanium atoms, Ge(4p2('D,)), 0.883 eV above the 4pa(aP,) ground state, in the presence of a range of simple molecules. We have shown that the 'Dz atom, which is optically metastable, may be produced photochemically in the pulsed mode and monitored by timeresolved atomic resonance absorption spectroscopy in the UV. The electronically excited germanium atoms are normally generated from the low-wavelength pulsed irradiation (A>160 nm) of Ge(CH& in the presence of excess helium gas and added reactant gases in a slow flow system, kinetically equivalent to a static system. This photochemical precursor yields higher densities of Ge(4'Da) for kinetic studies than GeCl, which is also used. The electronically excited atom was monitored photoelectrically in the repetitive mode by time-resolved atomic resonance absorption spectroscopy in the UV with direct computer interfacing for data capture and analysis. Ge(4rDJ was monitored using the resonance transition at A=241.737 nm (Ge(4d('D,)) + (4pa('D,))). The following absolute second-order rate constants {ka, errors 20) are reported for the collisional removal of Ge(4'DJ with various collision partners (R): R kR (cm" motecule-l s-l) (300 K) N2 02 co NO HCI b&G CO, CHH, CF, SF, (5.9~o.3)x10-'2 (2.5 *o.l)xlo-'~ (1.8;t;0.1)x10-" (9.0*0.5)x10-'~ (2.6~O.l)xlO-" (2.1*0.1)x lo-" (7.0+ 0.4) x lo-l3 (5.7* 0.3) x lo-r' (4.3*0.2)x 1o-'z (1.0*0.1)X10-'* The resulting rate data are compared with analogous collisional data for Ge(4p2('S,)) (E-2.029 eV) reported from studies in the single-shot mode. In all cases, collisional removal of the 'D2 state is significantly faster than the 'S, state. The rate data are also considered in the context of Group IV atoms in the low-lying np'('D,, 'So) metastable states, in terms of the nature of the potential surfaces involved on collision, on the basis of the weak spin-orbit coupling approximation and (I, 0) coupling.

The absorption oscillator strength of the xenon 147-nm resonance transition is measured to be 0.264+0.016. This value is from direct absorption measurements with equivalent widths from =1 to =10 cm. This f-value measurement is compared to others in the literature and is used in Monte Carlo simulations of trapped decay rates. The simulations include an angle-dependent partial frequency redistribution. The simulation results are compared to trapped decay rates in the literature.

Collision-based spectroscopy of a Xe qϩ (q ϭ 6, 7) beam with a target T (T ϭ Na, Ar) has been performed in the 35-800-nm wavelength region. Twenty-five new Xe VI lines and twenty-two new Xe VII lines were classified, twelve new energy levels of Xe VI and nine new energy levels of Xe VII were established, and one energy level of Xe VI was revised from the classified transitions. The analysis was supported by Hartree-Fock calculations.

Fluorescence from a single vibronic level of SO(B 3 Σ-, V′ e 3) prepared by a pulse excitation has been detected and thorough kinetic analyses for the photochemical processes have been made. Deconvolution analysis by the integrated-profiles method of the time-resolved fluorescence profiles recorded at various buffer gas (Ar and N 2) pressures has given the deactivation rates of initially prepared vibrational levels. Componential analysis of the dispersed fluorescence measured at different buffer gas pressures has provided the rate coefficients for level-to-level vibrational relaxation. It has been found that not only the single-quantum relaxation (∆V) 1) but also the multiquantum relaxation (∆V) 2 and 3) occurs by collisions with Ar and N 2. The efficiency of quenching is strongly dependent on the vibrational levels and correlates with the energies of the lowest nonfluorescent rotational levels. Candidates for the electronic states governing the quenching process have been discussed based on the kinetic and spectroscopic data.

The chemiluminescence produced by the Ba + Cl* reaction was recorded as a function of He and Nz pressure. A modified Stern-Volmer treatment of competitive electronic quenching ol'BaCI* and BaClz emission yielded upper limits to the half pressures p 1,2(He) < 9.0 r 3 mtorr and pllZ(N2) < 1.1 f 0.2 mtorr for quenching of BaCI; by: helium and nitrogen, respectively. A lower limit of the BaClr radiative lifetime is plxed at :R > 100 cr.

We present results of our investigations on radiation trapping in sodium vapour excited to 3.P level by weak laser pulses. The evolution of the fluorescence signal as well as the evolution of excited atoms distribution were studied using the Monte Carlo method. Some predictions of these investigations were checked experimentally.

Abstract: The absolute quenching cross sections for Hg(³P₁) with a number of quenching gases have been obtained in a steady-state photolytic experiment. The intensity of 2537-Å, emission has been followed as a function of absorbed intensity, and also pressure of the quenching gas. Low Hg concentrations are maintained in order to avoid the complication of radiation imprisonment. Quenching rate constants are determined from modified Stern-Volmer plots. The results are compared to cross sections from Zemansky type experiments on a relative basis and in several cases to the absolute results of Yarwood, Strausz, and Gunning who applied Holstein theory to Zemansky type experimental results. Comparison is also made to other low opacity studies and, where possible, to the relative cross sections determined from chemical methods. Generally, good agreement is obtained. NO provides the only gross disagreement in these comparisons of the nine molecules studied.

The DЈ (2 3 P 2 ) 7 AЈ(2 3 ⌸) transition in ClF is studied in emission, in fluorescence, and through OODR excitation experiments. The emission spectrum is excited in a flowing Tesla discharge of ClF in half an atmosphere of Ar and is photographed at high resolution. The excitation and fluorescence spectra are obtained by pumping through perturbed levels of the B (0 ϩ 3 ⌸) valence state. The emission data yield a detailed characterization of vЈ levels 0 and 1 and v Љ levels 3-8. The excitation spectra extend the vЈ range to vЈ ϭ 16 and put both states on an absolute energy scale. The low-resolution fluorescence spectra span v Љ ϭ 1-17, the upper limit of which lies within ϳ20 cm Ϫ1 of dissociation. Key spectroscopic parameters (cm Ϫ1 ) from the study, valid for 35 Cl 19 F, are: TЈ e ϭ 55 253, Ј e ϭ 365.22, e xЈ e ϭ 2.005, T Љ e ϭ 18 257, Љ e ϭ 363.53, e x Љ e ϭ 8.30, BЈ e ϭ 0.21790, aЈ e ϭ 0.00175, B Љ e ϭ 0.33412, a Љ e ϭ 0.00631, RЈ e ϭ 2.5069 Å, R Љ e ϭ 2.024 Å. A near-dissociation fit of the DЈ 3 AЈ emission and fluorescence data locates the first dissociation limit at 21 495(5) cm Ϫ1 , which is lower than the accepted value, but by just one standard error.

We show that amplified spontaneous emission (ASE) can significantly influence population transfer between electronically excited molecular states. Such transfer can lead to anomalous conclusions being drawn from collisional energy transfer studies. We illustrate this point by considering two apparently conflicting studies involving E0g+(3P) to D0u+(3P) collisional transfer in I 2 and show that both sets of results can be reproduced when the ASE process is correctly identified and controlled.