Azulene: polarized absorption in stretched polymers and magnetic circular dichroism

Linear dichroic and magnetic circular dichroic spectroscopy is used to characterize the excited states of azulene. Comparison with crystal spectra underlines the necessity of using two-parameter equations for evaluations of stretched sheet spectra despite some previous claims to the contrary. The MCD spectrum reveals strong vibronic interactions in the second transition.The results are interpreted on the basis of Pariser—Parr—Pople (PPP) type calculations including some with extensive configuration interaction (CI). Calculated signs of the first three B terms spectrum agree with experimental signs irrespective of the details of the PPP model, and even the numerical values are in an order-of-magnitude agreement. Only the calculation with extensive CI gives agreement for signs of higher B terms. It suggest the possible presence of two excited states, but the present experimental evidence for these is insufficient. One of these new states would be best described as a predominantly doubly excited state.The origin of the B terms is traced to contributions due to magnetic mixing of individual zero-order excited states and the results permit determination of the absolute sense of magnetic dipole transition moments between excited states with respect to the sense of the vector product of the electronic dipole transition moments from the ground state to these excited states. A comparison is performed with B terms calculated using finite perturbation technique and gauge-invariant orbitals.     

Calculation of doubly excited even strontium states

The perturbation theory with zero order model potential has been successfully used for calculating the spectral characteristics (energies, radiative lifetimes, autoionization widths) of highly excited Rydberg and autoionization states of rare earths. Here, the same method is applied for investigating the 4dns, 4dnd and 4dng doubly excited Rydberg states of Sr. The obtained energies of the states are in good agreement with the experimental data of other authors. The calculation aims to give a more complete and reliable information about the experimentally observed spectra, as well as to predict the location of more highly excited states.     

The microwave spectrum and large-amplitude vibrations of N-methylaniline

Microwave spectra of the normal and amine-deuterated isotopic species of N-methyl-aniline in the ground and some low-lying excited vibrational states have been observed. The inertial defect indicates that the dihedral angle of the N-CH₃, bond with respect to the ring plane is somewhat less than that for the N—H bonds in aniline. The position of the amino-hydrogen atom is very poorly determined by the isotopic substitution method because of large zero-point effects. The excited vibrational states are consistent with a double-minimum potential for the inversion of the HNCH₃ group and there is some evidence for a lower barrier than in aniline. The excited states of the HNCH₃ torsion indicate a barrier in the range 8 < V₂ < 25 kJ mol^?1 to the internal rotation of this group. No splitting of the ground-state lines attributable to the torsion of the methyl group has been observed, which implies a barrier of V₃ > 8 kJ mol^?1.     

Excited States of Weak Interacting Complexes of Formaldehyde and Alkali Metal Ions

The electronically excited states of formaldehyde and its complexes with alkali metal ions are investigated with the time-dependent density functional theory (TD DFT) method. Vertical transition energies for several singlet and triplet excited states, adiabatic transition energies for the first singlet and triplet excited states S₁ and T₁, the adiabatic geometries and vibrational frequencies of the ground state S₀ and the first singlet and triplet excited states S₁ and T₁ for formaldehyde and its complexes are calculated. Better agreement with the experiment than that of the CIS method is obtained for CH₂O at the TD DFT level. The nonlinear C=O?M⁺ interaction in the excited states S₁ and T₁ is weaker than the linear interaction in the ground state. In the S₀ and S₁ states, the C=O bond is elongated by cation complexation and its stretching frequency is red-shifted, but in the T₁ state the C=O bond is shortened and its frequency is blue-shifted.     

Emission spectra of the radical cations of 1,3-dichlorobenzene, 1,4-dichlorobenzene and 1,3,5-trichlorobenzene in the gas phase

Emission spectra of the radical cations of 1,3-dichlorobenzene, 1,4-dichlorobenzene-h₄ (and -d₄), and 1,3,5-trichlorobenzene, excited in the gas phase by controlled electron impact, are presented. The band systems, which lie in the 500–750 nm wavelength region, are attributed to the B?(π^?1) → X?(π^?) electronic transition of the cations on the basis of photoelectron spectroscopic data. The NeI excited photoelectron spectra and the ionisation energies of chloro-,o-,m-,p-dichloro- and 1,3,5-trichlorobenzene have been obtained. The information acquired from the emission and photoelectron spectra is discussed and compiled to deduce the symmetry of the B? states. Emission, with quantum yield > 10^?5, could not be detected with electronically excited radical cations of chloro-,o-dichloro-, 1,2,4- and 1,2,3-trichloro- and tetrachloro-benzenes. This is attributed to the nature of the B? states, which arise by σ^?1 ionisation processes. The lifetimes of the zeroth and some vibrationally excited levels of the B?(π^?1) states were also measured and found to be 22 ± 2 ns for 1,3,5-trichlorobenzene cation and < 6 ns for 1,3- and 1,4-dichlorobenzene cations. The lifetimes of the latter two electronically excited cations are estimated to be two orders of magnitude shorter than 6 ns from the measurement of the relative emission intensities of the B? → B? band systems of the three title cations.     

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.     

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.     

Semi-empirical calculations of π-electron affinities for some conjugated organic molecules

Energy levels have been calculated for some conjugated systems containing C, N, and O atoms using a semi-empiricalmethod based upon a variableβ-γ modification of the Pariser-Parr-Pople approximation to the Hartree-Pock equation. Koopmans' theorem is used to relate the calculated energy of the lowest vacant molecular orbital, ?_LVMO, to the adiabatic electron affinity of a molecule. The approach is identical to that used previously by Kunii and Kuroda [13]. An excellent correlation is found between electron affinities deduced from recent beam experiments and ?_LVMO. This relationship is used to predict electron affinities for over 100 other organic molecules. In addition, excited state energies for negative ions are calculated, and good agreement is found with the available experimental data. Bound excited states are also predicted for some organics which contain the =C(CN)₂ substructure. The additive contribution of group substitutions to the electron affinity is discussed for the case of CN substitutions to ethylene, benzene, and naphthalene.     

Electronic structure of pyrazine: a study of the geometries in the lowest excited singlet π *← n and π*←π states

Electronic, excitation energies, charge distributions and geometries of pyrazine in the lowest excited singlet π^*←π and π^*←n states have been studied by the VE—PPP, CNDO/2 and CNDO/s-CI molecular orbital methods. Study of the change of geometry in the π^*←n excitation requires localization of the density matrices in the ground and excited states, and with the help of these σ-bond orders are defined. Charge distributions and bond orders in the lowest excited singlet π^*←π and π^*←n states are compared. Whereas in the lowest singlet π*←π excitation the pyrazine ring expands uniformly, in the case of the π^*←n excitation C-C bonds contract whereas C-N bonds elongate. The predictions of theory are in agreement with experimental results, showing that the method used can be employed to obtain reliably the trends of geometry changes following a π^*←π excitation of a molecule before a more complete theoretical or experimental study is performed.     

Semiempirical molecular orbital studies of phthalocyanines

Theπ andσ lone pair electron system of the phthalocyanine molecule has been studied by a semiempirical SCF-MO method. Electronic transitions of bothπ-π ^* andn-π ^* types have been considered. The excited states have been calculated by means of the method of superposition of configurations where all singly excited states are included. Assignments for the electronic spectrum of phthalocyanine could be made in good agreement with experiment. The position of the lowest electronically allowedn-π ^* transition is predicted to be found in the region of the strong Soret band.     

Transient behavior of a nonequilibrium plasma formed by merged beams

A low density, low temperature plasma formed by two merged beams of electrons and ions at near zero relative velocity is studied by solving a set of time-dependent rate equations. In particular, we investigate the role played by the radiative recombination (RR) and three-body recombination (TBR) on the population of excited states during the initial stage of plasma rearrangement and relaxation. It is found in the case of hydrogenic plasmas that low-lying states are filled predominantly by RR, while high Rydberg states are populated mainly by TBR. As the plasma further relaxes, the collisional excitation and de-excitation, and radiative decays become important, filling the median excited states. The dependence of the TBR and RR rates on ionic chargeZ is discussed to extend the result to plasmas of highZ ions.     

Charge density flow as a driving force of distortion in excited protonated azaaromatics

The INDO method was used to calculate electronic charge densities in the ground and lowest excited singlet and triplet states of neutral and protonated 2,3- and 1,4-diaza derivatives of naphthalene and phenanthrene, and benzo (a) phenazine. Experiniental changes of pK_a upon excitation can be correlated with the values of electron density flow into the heterocy- clic ring. Electron-density increase turns out to be a major factor which causes distortion in some excited protonated species.     

Charge transfer in 1.4 MeV/u Ni q+→Kr collisions,q=16−22

Cross sections for the electron transfer from neutral Kr atoms to highly charged Ni ^q+ projectiles were measured at 1.4 MeV/u. Estimates on the contributions of single and double capture into the Ni-L shell and into excited states are extracted from measured cross sections for the Ni-L x-ray emission for incoming projectileL vacancies (q≧19). The role of metastable states produced in the capture process is analysed. We find that most of the capture directly populates the Ni-L shell, and the capture into excited states seems to be dominated by two-electron capture. The electrons involved in double transfer processes seem to be strongly correlated. An increase in the low-probability non-correlated double transfer with increasing number of incomingL-vacancies is indicated by the data.     

Coupling between autoionizing states in atomic strontium

I present a theory and calculations of transitions between autoionizing states in Sr. Such transitions are found to become significant at photon fluxes above 10²⁹ photons/cm² s and tend to leave the ion in excited states. Examples of transitions to the excited ionic states 4d and 5p are presented. Their dependence on laser intensity is found to be quite intricate.     

Rydberg and valence-shell character as functions of internuclear distance in some excited states of CH,NH, H2, and N2

SCF MO computations have been carried out on several excited states of CH and NH in which the excited MO 4σ is a Rydberg or near-Rydberg MO at internuclear distances R near that (R_e) of equilibrium in the ground state, but becomes an antibonding valence-shell MO as R increases toward dissociation. For the lowest ³Π_g state of H₂ and the lowest ³Π_g and ³Π_u states of N₂ the extent of 3dπ Rydberg character in the excited MO as a function of R for some R values ? R_e is evaluated by SCF MO computations.     

Pseudomolecular atoms: geometry of two-electron intrashell excited states

Supermultiplet patterns indicating pronounced collective motion of electrons are prominent among intrashell excited states of helium or alkaline earth atoms. Many aspects of these patterns have previously been discussed in terms of an empirical rovibrator model, e-core-e, analogous to alinear triatomic molecule. However, this has appeared incompatible with a dimensional scaling treatment that predicts pseudomolecular features and relates properties of some excited states to the ground state, but corresponds to abent equilibrium geometry. We examine both the full two-electron Hamiltonian, including angular momentum, and a prototype model for angular correlation which restricts the two particles to the surface of a sphere. By virtue of the boundary conditions imposed by the Jacobian volume element, we find that alinear equilibrium geometry is not allowed. If the Hamiltonian is transformed to reduce the Jacobian to unity, an infinite barrier appears in the effective potential when the electrons are 180° apart and equidistant from the nucleus. We find the supermultiplet patterns, including some “antimolecular” features, are consistent with a floppy butbent geometry, anasymmetric rotor model. The most probable interelectron angle ? _m varies markedly with the principal quantum number and also with the space-quantization of the angular momentum with respect to body-fixed axes.     

Ab initio study of the ground and lower-lying excited electronic states of NiX2 and FeX2 (X=F,Cl, Br,I) molecules

The ground and lower-lying excited electronic states of FeX₂ and NiX₂ (X=F, Cl, Br, I) molecules are systematically investigated by ab initio method at the complete active space self-consistent field (CASSCF) and multiconfigurational quasi-degenerate second-order perturbation (MCQDPT2) levels of theory. It is concluded that the dynamic electron correlation has to be taken into account in the prediction of the properties for such kind of molecules. The equilibrium bond lengths r_e(M–X), force constants and harmonic vibrational frequencies are calculated for the ground and lower-lying excited electronic states. The spin-orbit coupling (SOC) effects are analysed.