1B2u State Research Articles

High-resolution ultraviolet laser spectroscopy on jet-cooled benzene molecules: Ground and excited electronic state polarizabilities determined from static Stark effect measurements

Static electric polarizabilities of the S0(1A1g) ground electronic and the vibronically excited 61 S1(1B2u) state of benzene were measured by applying ultraviolet (UV) laser Stark spectroscopy. The experimental setup consisted of a frequency doubled tunable narrow-band continuous wave (cw) laser system operating at 259 nm, a molecular beam apparatus, and a capacitor capable of generating strong static and homogeneous electric fields up to 250 kV/cm. Experimental linewidths of less than 30 MHz were achieved for the rotational transitions, and Stark displacements of typically several 10 MHz could be detected. Stark patterns of 16 rotational lines were recorded and analyzed under different field strengths. Validity of second-order perturbation theory was confirmed by the observed effects, and the diagonal components of the polarizability tensors could be adjusted by a weighted least-squares technique. Results were applied to calculating Stark spectra and comparing them to experimental data. Good agreement could be found, except for an obscure singular perturbation within one rotational line. The new polarizabilities were compared to both experimental and theoretical results in the literature. Ground state data are in general agreement, whereas no comparable experimental work addressing the excited electronic state exists. Finally, polarizabilities of the beginning five polyacenes were compiled and compared with the benzene data.

Optical spin polarization in the di-〈001〉-split interstitial (R1) centre in diamond

Abstract Irradiating diamond with electrons or neutrons produces the electron paramagnetic resonance (EPR) R1 centre which has been shown to be a di-〈001〉-split interstitial. We report that on cooling below a certain threshold temperature and illuminating with unpolarized light of energy greater than 1.7(1) eV, spin polarization within the S=1 multiplet of the ground state of the R1 centre is observed. The threshold temperature to observe this effect depends on the concentration of the defect in the diamond. For a type IIa stone irradiated to a dose of 7×1017 2 MeV e−cm−2 with an R1 concentration of 0.056(15) ppm the temperature is 67(2) K. The results of this study are consistent with a spin-selective, orientation-dependent optical transition, which changes the populations of the spin manifold and results in emissive EPR transitions. The spin polarization arises by selective excitation/repopulation of the 3Bu ground state of R1 mediated by spin–orbit coupling with a higher 1Bu state. Optical activity in the EPR system has yielded information about the electronic energy levels, and provides a bridge between optical and magnetic resonance spectroscopy. This system might have use as a light-pumped microwave or millimetre-wave maser material.

The ethylene 1 1B1u V state revisited

We describe a general procedure to resolve the problem of artifical valence/Rydberg mixing encountered in ab initio CI calculations on the V (1 1B1u) state of ethylene. Davidson and McMurchie realized that the key to this problem are orbitals which adequately represent the V state. A two-step procedure is proposed, in which the first step focuses on generating appropriate molecular orbitals and the second step aims to describe the electron correlation quantitatively. A series of the currently most extensive MCSCF, MR-CISD, and MR-AQCC calculations for basis sets up to quadruple zeta quality and up to 80 million configurations are presented. Size extensivity corrections turn out to be crucial for highly accurate excitation energies. Our best estimate for the N–V state excitation energy of 7.7 eV lies between the experimental absorption maximum of 7.66 eV and a vibrationally corrected value of 7.8 eV. Hence, we do not find it necessary to refer to nonadiabatic effects in order to achieve agreement with the experimental data. The V state is characterized by its spatial extent, measured through the expectation value 〈x2〉, where x is the out-of-plane direction. With 16.5–17.0a02 it has a strong valence character, as compared to ≈90a02 for the 2 1B1u Rydberg state and 11.7a02 for the ground state.

Vertical spectrum of ethene: uncontracted versus contracted correlation methods and the role of the adapted molecular orbitals

Ethylene vertical excitation energies (VEE) below 9.5 eV have been calculated with the complete active-space singles and doubles configuration interaction self-consistent size-consistent dressing (SC) 2CAS–SDCI. The mean of the absolute deviation from experiment for the calculated VEE are 0.14, 0.05 and 0.03 eV for three different molecular orbital (MO) sets tried. The results show that an uncontracted method and a realistic MO set avoid the difficulties present in the calculation of the strongly mixed Rydberg-valence 1B 1u states.

Theoretical study on the electronic structures of various F centers in MgO crystals

Ab initio SCF-MO calculations were performed on low-lying electronic states of various F centers in MgO crystals by using embedded cluster models. Because a simple cluster model embedded with point charges alone gave poor results, we generated environmental potentials (EPs) in a spectral representation scheme for Mg 2+ and O 2− ions and used them instead of point charges around the clusters under consideration. Using embedded cluster models with the EPs and point charges, we calculated the low-lying electronic states of various F centers in MgO. The calculated order in energy was found to be: the 1A 1 state of the surface F center < the 1E state of the surface F center < the 1B 3u state of the bulk M center (F aggregates) < the 1T 1u state of the bulk F center. The energy separation of the 1E , 1B 3u , and 1T 1u states from the lowest state were 1.22, 2.46 and 4.05 eV, respectively, which explains the observed bands.

Identification of excitons in conjugated polymers: A density-matrix renormalization-group study

This work addresses the question of whether low-lying excitations in conjugated polymers are comprised of free charge-carriers or excitons. States are characterised as bound or unbound according to the scaling of the average particle-hole separation with system size. We critically examine other criteria commonly used to characterise states. The polymer is described by an extended Hubbard model with alternating transfer integrals. The model is solved by exact diagonalisation and the density matrix renormalisation group (DMRG) method. We demonstrate that the DMRG accurately determines excitation energies, transition dipole moments and particle-hole separations of a number of dipole forbidden (Ag) and dipole allowed (Bu) states. Within a parameter regime considered reasonable for polymers such as polyacetylene, it is found that the charge gap, often used to define the exciton binding energy, is not a good criterion by which to decide whether a state is bound or unbound. The essential non-linear optical state mAg is found to mark the onset of unbound excitations in the Ag symmetry sector. In the Bu symmetry sector, on the other hand, it is found that all low lying states are unbound and that there is no well defined nBu state. That is, the 1Bu state marks the onset of unbound excitations in this sector.

Solvent Dependence of the Ultrafast S2−S1 Internal Conversion Rate of β-Carotene

The solvent dependence of the steady-state and time-resolved fluorescence emission from the S2 (11Bu) state of all-trans-β-carotene was investigated in a range of polar and nonpolar solvents and in hexane/carbon disulfide mixtures. The steady-state absorption and fluorescence emission maxima in both polar and nonpolar solvents showed parallel shifts with increasing solvent polarizability, indicating that the emitting state is the S2 (11Bu) state. The lifetime of the S2 state was determined using the fluorescence upconversion technique, and lifetimes varying from 120 fs in quinoline to 177 fs in hexane were found. An intramolecular relaxation process occurring on a similar time scale was observed as a broadening in the reconstructed time-dependent emission spectra in hexane and as emission wavelength-dependent lifetimes in all the solvents studied. Correlations were observed between the S2 lifetimes, the π* solvatochromic parameter, and the ratio of the absorption fine structure. A correlation between the ...

Optical charge carrier generation in a conjugated polymer

Stationary photoconduction was measured on 100 nm thick films of poly(methylimino-1,4-phenylene-1-phenyl-1,2-ethenylene-2,5-dimethoxy-1,4-phenylene-2-phenyl-1,2-ethenylene-1,4-phenylene) sandwiched between indium tin oxide and aluminium electrodes. Bulk photoionization was observed upon irradiating through the positively biased aluminium electrode. For photon energies 2.6 eV < hv < 3.2 eV either defect catalyzed or intrinsic dissociation of relaxed singlet excitations prevails. For hv > 3.2 eV the carrier yield increases with excess photon energy due to intrinsic photoionization. The functional dependencies on electric field and photon energy are in accord with the 3D version of Onsager's theory of geminate pair dissociation implying a geminate pair distance that increases with hv. As an ionizing event, coupling between the non-relaxed initial Franck-Condon state with an mAg state is involved. Further increase of the yield correlates with the onset of optical absorption to the third optically allowed Bu state.

Optical charge carrier generation in a conjugated polymer

Stationary photoconduction was measured on 100 nm thick films of poly(methylimino-1,4-phenylene-1-phenyl-1,2-ethenylene-2,5-dimethoxy-1,4-phenylene-2-phenyl-1,2-ethenylene-1,4-phenylene) sandwiched between indium tin oxide and aluminium electrodes. Bulk photoionization was observed upon irradiating through the positively biased aluminium electrode. For photon energies 2.6 eV < hv < 3.2 eV either defect catalyzed or intrinsic dissociation of relaxed singlet excitations prevails. For hv > 3.2 eV the carrier yield increases with excess photon energy due to intrinsic photoionization. The functional dependencies on electric field and photon energy are in accord with the 3D version of Onsager's theory of geminate pair dissociation implying a geminate pair distance that increases with hv. As an ionizing event, coupling between the non-relaxed initial Franck-Condon state with an mAg state is involved. Further increase of the yield correlates with the onset of optical absorption to the third optically allowed Bu state.

A coupled cluster study of the 1 1A1g and 1 1B2u states of benzene

A theoretical investigation of the equilibrium structures and harmonic frequencies of the 1 1A1g and 1 1B2u states of benzene is presented. The performance of coupled cluster singles (CCS), the recently proposed CC2 model, and coupled cluster singles and doubles (CCSD) is compared. The CC2 ground and excited states frequencies are a significant improvement of the CCS results and are relatively close to the CCSD results. A comparative analysis of the vibrations in the two electronic states of both C6H6 and C6D6 is presented. The reliability of predicted shifts in harmonic frequencies between the two states and isotopic shifts is estimated on the basis of the convergence in the CCS, CC2, and CCSD hierarchy of models, and through comparison with related theoretical work. The shifts are used in a critical comparison with experiment. Inconsistent experimental assignments have been scrutinized on the basis of the predicted shifts. The complete harmonic force fields are given for both states.

Theoretical study of the π→π* excited states of linear polyenes: The energy gap between 11Bu and 21Ag states and their character

Theoretical study of the π→π* excited states of linear polyenes: The energy gap between 11Bu and 21Ag states and their character

Ab initio calculations on the diacetylene dimer: HCCCC(H)C(H)CCCH

Geometry optimizations were performed for singlet, triplet, and quintet states on the planar structures (in C2h and C2v symmetries) of the diacetylene dimer, using restricted open-shell Hartree–Fock (ROHF), unrestricted Hartree–Fock (UHF), and unrestricted hybrid density functional theory (UB3LYP) methods, with 6-31G(d) and 6-311G(d, p) basis sets. The 1Ag state of the planar van der Waals dimer is lower in energy than are any covalently bonded dimers. At our best B3LYP/6-311G(d, p) level, the most stable covalently bonded diacetylene dimer is the 3Bu state in C2h symmetry, 11 kcal mol−1 above the van der Waals dimer, followed by the 3B2 state in C2v symmetry with 13 kcal mol−1 above the van der Waals dimer. Both structures were confirmed to be local minima. The two diacetylene monomers of these structures are bridged through a single bond and they exhibit a small bend at the neighboring carbons to the bridge, trans to the hydrogens. The 1Bu and 5Ag states in C2h and the 1B2 and 5A1 states in C2v are between 39 and 43 kcal mol−1 above the van der Waals dimer. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 189–202, 1998

Theoretical Study of the Electronic Spectrum of trans-Stilbene

The electronic spectrum of trans-stilbene in the energy range up to 6 eV has been studied using multiconfigurational second-order perturbation theory (CASPT2). The study includes a geometry determination of the ground state. In all, 12 singlet and one triplet excited states were studied. The calculated spectrum makes it possible to assign the valence excited singlet states corresponding to the three bands observed in the low-energy region of the one-photon absorption spectrum. The most intense feature of the calculated spectrum corresponds to the 11Ag → 21Bu transition at 4.07 eV. The weakly allowed 11Bu state was found 0.3 eV below 21Bu. Transition to the 31Ag state, computed at 4.95 eV, is responsible for the main band observed in the two-photon absorption spectrum. This state has a large weight (around 43%) of doubly excited configurations. The first singlet−singlet Rydberg transition (3s) appears at 5.33 eV and is weak. Implications of the present findings on the trans → cis photoisomerization process...

The molecular and electronic states of 1,2,4,5-tetrazine studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio multi-reference configuration interaction studies

The VUV electronic absorption spectrum of 1,2,4,5-tetrazine has been re-investigated, and together with electron energy-loss spectra has led to identification of a number of new excited states. The valence and Rydberg excited states have been studied by multi-reference multi-root configuration interaction studies using MRDCI techniques. Initial studies with the RPA and TDA methods gave almost identical results for the excitation energies, but there is a substantial energy-lowering in the MRDCI calculations, which improves agreement with experiment substantially; these differences are a result of the double, triple and quadruple excited reference configurations included in the reference set of the latter method. The 1ππ ∗ excitations are calculated rather higher than experiment, except for the lowest-lying (weak) 1B 2u state at 5.0 eV. The calculated order for the next three ππ ∗ states is 1B 1u (weak) followed by 1B 2u (strong) and 1B 1u (strong), the last two bands being responsible for the dominant absorption near 7.5 and 8.5 eV. All of this group of four bands involve excitations from the pair of MOs 1b 2g and 1b 1g into the 1a u ∗ and 4b 3u ∗ VMOs. The sequence of nπ ∗ stakes are in a similar order to the ππ ∗ excitations, with respect to the upper state, and the two lowest singlet states, 1B 3u and 1A u are reasonably well determined. The triplet states follow a similar order to the singlets, and again the dominance of the effect of the two lowest VMOs is demonstrated, but considerable differences between the weighting of leading configurations occurs between singlet and triplet manifolds. The non-diagonal TDA method has been used to reconsider the UV-photoelectron spectrum. The ionisation potentials for tetrazine are reinterpreted with the first three bands being regrouped into 1, 2, 2 ionisations respectively. The ground state properties of tetrazine suggest that the NQR spectrum will show a principal axis 14N quadrupole coupling constant larger than the other known azines. The decline in diamagnetic susceptibility anisotropy in tetrazine, relative to benzene and pyridine, is indicative of lower resonance energy.

Theoretical study of the ethylene electronic spectrum and extraction of an r-dependent Hubbard Hamiltonian

The iterative difference-dedicated configuration interaction method has been applied to study the vertical spectrum of ethylene below 9.5 eV. The deviation from experimental estimations is lower than 0.1 eV, including the strongly mixed Rydberg valence 1B 1u states. The potential energy curves of the three lowest valence states as a function of the CC bond length are calculated and provide an r-dependent Hubbard Hamiltonian.

ChemInform Abstract: A Kekule‐Crossing Model for the “Anomalous” Behavior of the b2u Modes of Aromatic Hydrocarbons in the Lowest Excited 1B2u State

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The Electronic Spectra of Ethylene

Abstract The SAC (Symmetry Adapted Cluster) and SAC-CI methods have been applied to the calculation of the valence and Rydberg electronic states of ethylene using large basis sets. Three singlet valence states are computed to lie below 10 eV: the V(1B1u) state and the σπ* 21B1g and 21B1g states. The excitation energies obtained for the low-lying Rydberg states are in excellent agreement with experiment, for the most part within 0.1 eV. The assignment of the 3R band to two Rydberg transitions is confirmed.

The difference bandsv11-v4andv10-v18of benzene at high resolution

The very weak difference bands v 11-v 4 and v 10-v 18 of the benzene molecule have been recorded by a high resolution Fourier transform spectrometer equipped with a long path absorption cell. Analysis of the spectra yielded much improved data for the Raman active states v 11 (species E1g ) and v 18(E2g ), and for the inactive B2u state v 10.

Evolutions of Singlet Excited-State Absorption and Fluorescence of all-trans-1,6-Diphenyl-1,3,5-hexatriene in the Picosecond Time Domain

Temporal behaviors of singlet excited-state absorption and fluorescence of all-trans-1,6-diphenyl-1,3,5-hexatriene in solution are reported in the picosecond time domain. Contrary to an earlier study, the absorbance ratio at the two λmax (∼460 and ∼650 nm) is time (∼10 ps resolution) and, except for small shifts, temperature (238−298 K) independent. Similarly, in contrast to an earlier report, identical time evolution of fluorescence intensity is found at short (405 ± 10 nm) and long (≥500 nm) λ. We conclude that the initially formed 11Bu state completely relaxes to a 11Bu/21Ag equilibrium mixture within our time resolution (∼10 ps). Since the latter is favored overwhelmingly, all transient absorption is assigned to n1Bu ← 21Ag transitions, consistent also with small red shifts in λmax with increased medium polarizability. We discern no spectral manifestations of phenyl−vinyl torsional motions, or any associated stabilization of the 11Bu state, at times longer than 10 ps.

Comprehensive Evaluation of the Absorption, Photophysical, Energy Transfer, Structural, and Theoretical Properties of α-Oligothiophenes with One to Seven Rings

A large basis set of α-oligothiophenes with two to seven rings (α2−α7), also including thiophene, α1, have been investigated in five solvents regarding absorption, fluorescence and phosphorescence, quantum yields of fluorescence (φF) and triplet formation (φT), lifetimes of fluorescence and the triplet state, quantum yields of singlet oxygen production (φΔ), all rate constants kF, kIC, kISC, and several of the foregoing as a function of temperature. Ten different theoretical calculations across several levels including three levels of ab initio have been carried out regarding which conformer is lowest in energy and the ΔH's among all conformers of α2, α3 and α5, as well as calculations of transitions energies of the α-oligothiophenes. We have shown that the (l) 1Bu state is the lowest singlet state for all α2−α7 in any solvent, in contradiction to previous predictions for the higher members. Based on absorption and fluorescence data and calculations of atomic charges in S0 and S1, the ground state is twis...