Radiative and nonradiative lifetimes in excited states of Ar,Kr and Xe atoms in a neon matrix

Synchrotron radiation with its intense continuum and its excellent time structure has been exploited for time resolved luminescence spectroscopy in the solid state. By selective excitation of n = 1, n′ = 2 exciton states of Xe, Kr and Ar atoms in a neon matrix we were able to identify the emitting states involved. Lifetimes within the cascade of radiative and radiationless relaxation between excited states as well as the radiative lifetimes for transitions to the ground state have been derived from the decay curves. Energy positions and radiative lifetimes of the emitting states correspond quite well with those of the free atoms. Radiative and radiationless relaxation processes take place within the manifold of excited states of the guest atoms. The rate constants for radiationless decay confirm an energy gap law. The order of the radiationless processes reaches in some cases extremely high values. Selection rules for spin and angular momentum are essential to understand the observed radiationless transition rates.     

One-Electron Pseudo-Potential Determination of Stable Isomers of the Li*Ar n Excited Clusters: Absorption Spectra

We present pseudo-potential calculations of geometrical structures of stable isomers of LiAr _n clusters with both an electronic ground state and excited states of the lithium atom. The Li atom is perturbed by argon atoms in LiAr _n clusters. Its electronic structure obtained as the eigenfunctions of a single-electron operator describing the electron in the field of a Li⁺Ar _n core, the Li⁺ and Ar atoms are replaced by pseudo-potentials. These pseudo-potentials include core-polarization operators to account for the polarization and correlation of the inert core with the valence Lithium electron [J Chem Phys 116, 1839 1]. The geometry optimization of the ground and excited states of LiAr _n (n = 1–12) clusters is carried out via the Basin-Hopping method of Wales et al. [J Phys Chem 101, 5111 2; J Chem Phys 285, 1368 3]. The geometries of the ground and ionic states of LiAr _n clusters were used to determine the energy of the high excited states of the neutral LiAr _n clusters. The variation of the excited state energies of LiAr _n clusters as a function of the number of argon atoms shows an approximate Rydberg character, corresponding to the picture of an excited electron surrounding an ionic cluster core, is already reached for the 3s state. The result of optical transitions calculations shows that the absorption spectral features are sensitive to isomer structure. It is clearly the case for transitions close to the 2p levels of Li which are distorted by the cluster environment.     

Calculation of ground- and excited-state potential energy curves for barium-rare gas complexes in a pseudopotential approach

Adiabatic potential curves for the ground state and several low-lying excited states of the barium atom interacting with Ne, Ar, Kr and Xe have been obtained from valence ab initio configuration-interaction calculations. Atomic cores are replaced by scalar-relativistic l-dependent pseudopotentials, while core-polarization potentials are used for describing correlation contributions of the rare-gas atoms and the Ba²⁺ cores. Implications of the resulting potential curves for the interpretation of experimental data are discussed, together with first applications of the curves for calculating absorption profiles of the (6s ²)¹S→(6p)¹P Ba transition. Received: 7 April 1998 / Accepted: 27 July 1998 / Published online: 12 October 1998     

Radiative lifetimes and depopulation cross-sections of excited rubidium2 S and2 D states

The radiative lifetimes of excited rubidiumn ² S (n=8?13) andn ² D (n=6?11) states have been measured using the time resolved laser induced fluorescence method. The cross-sections for depopulation of the excited states in collisions with the ground-state Rb atoms have been determined. The influence of the blackbody radiation on the measured values is discussed.     

Translational energy distribution of the excited hydrogen atom produced by controlled electron impact on HCl

The translational energy distribution of an atom can be calculated by differentiating the Doppler line shape of its emission line taken at a high optical resolution. The Balmer-β line of the excited hydrogen atom (n = 4) produced by electron impact on HCl has been measured at a high resolution (0.033Å) and at two angles (55° and 90°) with respect to the electron beam. The translation energy distribution depends on the electron energies and has almost two groups of components: ≈ 5 eV (fast) and ≈ eV (slow). Anisotropy is imporant for the slow component. The excitation function shows the corresponding structures. It is concluded that Rydberg states converging to the ²Π state of HCl⁺ produce the fast component and Rydberg states converging to the repulsive HCl⁺ states which cross the ²Σ⁺ state produce the slow component.     

Computation of general multipole moment expansions for N atoms by MAPLE

A program written in the MAPLE symbolic computational language was written to compute analytic and numerical expressions for the long-range multipole moment expansion of the electrostatic (Coulombic) perturbation operator for a system of N atoms in arbitrary orientations in space. These expressions are written in terms of the multipole moment operators on individual interacting atoms. The program was tested on a system consisting of a ground-state hydrogen atom interacting with two protons in three different configurations, and the results were found to be in agreement with manual calculations. This approach can produce, with relative ease, the long-range multipole expansion for systems with large numbers of interacting atoms or ions. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 62 : 343–351, 1997     

Ultraviolet Photodissociation Dynamics of m-Bromofluorobenzene at around 240 nm?

The photodissociation dynamics of m-bromofluorobenzene has been experimentally investigated at around 240 nm using the DC-slice velocity map imaging technique. The kinetic energy release spectra and the recoiling angular distributions of fragmented Br(²P_3/2) and Br(²P_1/2) atoms from photodissociation of m-bromofluorobenzene have been measured at different photolysis wavelengths around 240 nm. The experimental results indicate that two dissociation pathways via (pre-)dissociation of the two low-lying ¹ππ^* excited states dominate the production process of the ground state Br(²P_3/2) atoms. Because of the weak spin-orbit coupling effect among the low-lying triplet and singlet states, the spin-orbit excited Br(²P_1/2) atoms are mainly produced via singlet-triplet state coupling in the dissociation step. The similarity between the present results and that recently reported for o-bromofluorobenzene indicates that the substitution position of the fluorine atom does not significantly affect the UV photodissociation dynamics of bromofluorobenzenes.     

A detailed consideration of resonance radiation trapping in the argon inductively coupled plasma

The extent and duration of trapping of argon resonance radiation (106.7 and 104.8 nm) in the ICP was calculated using a model incorporating line shape contributions from both Doppler and pressure broadening. The trap was found to be at the pressure-broadened limit, giving an escape factor of 7.9 × 10^?4 when excitation of the 4s states in the plasma annulus is assumed. Theoretical apparent radiative lifetimes τ_app for argon ³P₁ and ¹P₁ resonance states are calculated to be 8 and 1.9μs respectively. The quartet of 4s states, rapidly mixed by electron collisions, are presumed to share an overall apparent radiative lifetime τ_app = 1.6μs for the purpose of plasma modeling. Effects of this radiation trapping on the argon 4s atom density and on electron-ion recombination are discussed.     

Structures and binding energies of the (dibenzoylmethanato)boron difluoride complexes with aromatic hydrocarbons in the ground and excited states. Density functional theory calculations

Structures of the (dibenzoylmethanato)boron difluoride molecule (DBMBF2) and its complexes with a series of aromatic hydrocarbons (benzene; toluene; o-, m-, and p-xylenes, naphthalene; anthracene; and pyrene) in the ground and the first singlet excited states have been calculated. The calculations have been performed by the density functional theory (DFT) and time-dependent density functional theory (TDDFT) for the ground and excited states, respectively, with the empirical dispersion correction. It has been shown that the complexes in the ground and excited states have similar stacking structures and are characterized by short contacts between the F atom of DBMBF2 and H atoms of the hydrocarbon molecule, which decrease on transition from the ground to the excited state. The calculated binding energies in the complexes in the excited state are two to three times higher than those in the ground state. The charge transfer in the ground state of the complexes is insignificant and directed from DBMBF2 to the ligand, while in the excited state it is 0.6–0.8 e and directed from the ligand to DBMBF2.     

Level-crossing experiments by two-photon excitation

Complete orientation of the T17² P _3/2 (F=2) state by two-photon excitation allows Hanle-effect measurement in direct fluorescence as well as in the optical radiation cascade. The experiment provides estimations either of both the lifetimes of the involved excited states, or, if one of the lifetimes is reliably known, two independant determinations of the other. The measurements are reported on the background of other nonlinear Hanle-effect and level-crossing experiments.     

Spin uncoupling in ethylene activation by palladium and platinum atoms

Spin‐dependent effects in complex formation reactions of the ethylene molecule with palladium and platinum atoms were studied by electron correlation calculations with account of spin–orbit coupling. Simple correlation diagrams illustrating spin‐uncoupling mechanisms were obtained, showing that the low spin state of the transition‐metal atom or the transition‐metal atom complex is always more reactive than are the high spin states because of the involvement of the triplet excited molecule in the chemical activation. Spin–orbit coupling calculations of the reaction between a platinum atom and ethylene explain the high‐spin Pt(³D) reactivity as due to an effective spin flip at the stage of the weak triplet complex formation. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 581–596, 1999     

Recombination radiation from organic solids

The recombination radiation from organic solids, defined as the light emission following the fusion of oppositely charged carriers into an electrically neutral state, is discussed as a phenomenon underlying the function of organic light-emitting diodes (LEDs). Its intensity and spectral range depend on the population and nature of the emissive states, which differ, in general, from those created using light. These differences are pointed out and shown to be a result of the reverse pathways of the mutual transformation of localized molecular excitons and coulombically-correlated charge-pair excited states formed either by photoexcitation or electron-hole recombination. Spectral features of the radiation produced by the recombination of statistically independent charge carriers are discussed in terms of two molecules-based excited states like excimers or electromers in single-component materials and exciplexes or electroplexes in multicomponent materials. Consequences for optical and electrical characteristics of organic LEDs are discussed and illustrated by examples. Progress in the fundamental and applied research may be expected based on properties of recombination-produced electronic excited states.     

State and velocity distributions of Cl atoms produced in the photodissociation of ICI at 237 nm

Photofragment spectroscopy of ICI molecules photodissociated at 237 nm is studied by 2 + 1 resonance-enhanced multi-photon ionization and time of flight techniques. Doppler profiles of the chlorine atom fragments in two spin—orbit states show that chlorine atoms in the ground state, ²P_3/2, are produced from a perpendicular dissociative transition, and chlorine atoms in the excited state, ²P , arise from a parallel transition. The possible electronically excited states leading to dissociation in both the perpendicular and parallel cases are considered.     

Degenerate symmetry-adapted perturbation theory of weak interactions between closed- and open-shell monomers: application to Rydberg states of helium hydride

Symmetry-adapted perturbation theory is extended to the (quasi) degenerate, open-shell case. The new formalism is tested in calculations of the interaction energies for a helium atom in the ground state interacting with an excited hydrogen atom. It is shown that the method gives satisfactory results if the coupling with higher Rydberg states of the dimer is small, as is the case for the A²Σ⁺,B²Π,E²Π,3²Π, and 1²Δ states of HeH. For the C²Σ⁺ state convergence of the method is very slow, but it can be improved by including the n=3 states in the model space. Received: 3 June 1998 / Accepted: 9 September 1998 / Published online: 7 December 1998     

Ab Initio and DFT Studies on the Structure,Infrared Spectrum,and Thermodynamic Properties of Hoechst 33258

Ab initio and DFT(B3LYP) levels of theory, with basis sets of 3–21G, 6–31G*, and LanL2DZ, have been applied to the specific DNA binder Hoechst 33258. All calculations lead to a structure with two benzimidazole groups and one phenyl group being coplanar. Atoms H₃₃ and H₃₇ bear large positive charges, and the distance between the two atoms is about 5 Å, which allows the molecule interacting with the negative sites of neighboring bases in the DNA helix and thus facilitates Hoechst 33258 to gravitate on AT‐rich regions in the minor groove of DNA, owing to the deeper negative potential in such sequences. The most stable conformer corresponds to the dihedral angles of ca. 0° for ?_{1‐2‐14‐15} and ?_{27‐26‐5‐4}, and 180° for ?_{20‐19‐11‐12}. This fact provides clear evidence that Hoechst 33258 has an arc‐like shape with coplanar aromatic rings. Both the HOMO and the LUMO are made up of the P_z orbitals of the non‐hydrogen atoms in two benzimidazole groups and one phenyl group. On going from the ground state to the first singlet excited state, the lengths of the single bonds between the aromatic rings decrease, and the aromatic rings are more conjugated with each other in the excited state. The heat of formation (ΔH_f) of Hoechst 33258, evaluated from the isodesmic reaction, is 406.32 kJ/mol at the B3LYP/6–31G* level.     

Rotational predissociation of extremely weakly bound atom-molecule complexes produced by Feshbach resonance association

We study the rotational predissociation of atom-molecule complexes with very small binding energy. Such complexes can be produced by Feshbach resonance association of ultracold molecules with ultracold atoms. Numerical calculations of the predissociation lifetimes based on the computation of the energy dependence of the scattering matrix elements become inaccurate when the binding energy is smaller than the energy width of the predissociating state. We derive expressions that represent accurately the predissociation lifetimes in terms of the real and imaginary parts of the scattering length and effective range for molecules in an excited rotational state. Our results show that the predissociation lifetimes are the longest when the binding energy is positive, i.e., when the predissociating state is just above the excited state threshold.     

Study of the degradation of polyurethane—Part 5: Photo-decomposition of ethyl N-phenylcarbamate, methylene bis (ethyl N-phenylcarbamate) and polyurethane in solution

In order to elucidate the photo-decomposition mechanism of polyurethane based on polyester diol-diphenylmethane-p,p′-diisocyanate, the effects of triplet quenchers, piperylene and oxygen on the photo-decomposition of the polymer, methylene bis (ethyl N-phenylcarbamate) (MEPC) and ethyl N-phenylcarbamate (EPC) were examined in solution. Energy levels and lifetimes of the excited states of these compounds were also determined.Piperylene and oxygen did not affect the photo-decomposition of the samples examined. The results imply that the photo-decomposition of the polymer starts from the excited singlet state. The energy levels and lifetimes for the photo-decomposition of the polymer were as follows: the excited singlet state (S₁): 98·6 kcal/mol (3·2 nsec): the excited triplet state (T₁): 76·7 kcal/mole (2·9 sec).     

Photophysics and photochemistry of sterically strained α-haloanthraquinones: The bromo and/or chloro substituted compounds

The steric hindrance between the oxygen and halogen atoms results in the structural deformation of α-haloanthraquinones and their lowest excited triplet (T₁) states are of mixed nπ ^*-ππ ^* or ππ ^* character with unusually short lifetimes. Moreover, the rates of hydrogen-atom abstraction from ethanol by the T₁ states decrease with their increasing ππ ^* character, and the proximity of the halogen atom to the hydroxy group causes the photochemical intramolecular elimination of hydrogen halide from the initial photoproducts (α-haloanthrahydroquinones) yielding α-haloanthraquinones (or anthraquinone) with one less halogen atom than the original molecule; the final product is anthrahydroquinone. The remarkably large structural deformation of 1,8-dihaloanthrasemiquinone radicals which gives rise to the simultaneous formation of 1,8-dihaloanthrahydroquinones and the original anthraquinones. Of particular interest is observation of the absorption band(s) attributable to the second excited triplet (T₂) states of 1,8-dihaloanthraquinones. However, the electron transfer from triethylamine (TEA) to these T₂ states generating the radical anions is observed only in acetonitrile, while that to the T₁ states generating their exciplexes with TEA is observed not only in acetonitrile but also in toluene and ethanol.     

Calculations of D‐wave bound states and resonance states of the screened helium atom using correlated exponential wave functions

We have investigated the effects of screened Coulomb (Yukawa) potentials on the bound ^1,3D states and the doubly excited ^1,3 D^e resonance states of helium atom using highly correlated exponential basis functions. The Density of resonance states are calculated using stabilization method. Highly correlated exponential basis functions are used to consider the correlation effect between the charged particles. A total of 18 resonances (nine each for ¹ D^e and ³ D^e states) below the n = 2 He ⁺ threshold has been calculated. For each spin states, this includes four members in the 2pnp series, three members in the 2snd series, and two members in 2pnf series. The resonance energies and widths for various screening parameters ranging from infinity to a small value for these ^1,3 D^e resonance states are reported along with the bound‐excited 1s3d ^1,3 D state energies. Overall behavior of the spectral profile of 1s3d ¹D state of helium atom due to electron‐electron and electron‐nucleus screening are also presented. Accurate resonance energies and widths are also reported for He in vacuum. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010