1B2 State Research Articles

The Negative Ion Photoelectron Spectrum of meta-Benzoquinone Radical Anion (MBQ•–): A Joint Experimental and Computational Study

Negative ion photoelectron (NIPE) spectra of the radical anion of meta-benzoquinone (MBQ, m-OC6H4O) have been obtained at 20 K, using both 355 and 266 nm lasers for electron photodetachment. The spectra show well-resolved peaks and complex spectral patterns. The electron affinity of MBQ is determined from the first resolved peak to be 2.875 ± 0.010 eV. Single-point, CASPT2/aug-cc-pVTZ//CASPT2/aug-cc-pVDZ calculations predict accurately the positions of the 0-0 bands in the NIPE spectrum for formation of the four lowest electronic states of neutral MBQ from the (2)A2 state of MBQ(•-). In addition, the Franck-Condon factors that are computed from the CASPT2/aug-cc-pVDZ optimized geometries, vibrational frequencies, and normal mode vectors, successfully simulate the intensities and frequencies of the vibrational peaks in the NIPE spectrum that are associated with each of these electronic states. The successful simulation of the NIPE spectrum of MBQ(•-) allows the assignment of (3)B2 as the ground state of MBQ, followed by the (1)B2 and (1)A1 electronic states, respectively 9.0 ± 0.2 and 16.6 ± 0.2 kcal/mol higher in energy than the triplet. These experimental energy differences are in good agreement with the calculated values of 9.7 and 15.7 kcal/mol. The relative energies of these two singlet states in MBQ confirm the previous prediction that their relative energies would be reversed from those in meta-benzoquinodimethane (MBQDM, m-CH2C6H4CH2).

Chiral structure of vector and axial-vector tetraquark currents

We investigate the chiral structure of local vector and axial-vector tetraquark currents, and study their chiral transformation properties. We consider the charge-conjugation parity and classify all the isovector vector and axial-vector local tetraquark currents of quantum numbers I(G)J(PC)=1(-)1(-+), I(G)J(PC)=1(+)1(--), I(G)J(PC)=1(-)1(++) and I(G)J(PC)=1(+)1(+-). We find that there is a one to one correspondence among them. Using these currents, we perform QCD sum rule analyses. Our results suggest that there is a missing b1 state having I(G)J(PC)=1(+)1(+-) and a mass around 1.47-1.66 GeV.

Topology of conical/surface intersections among five low-lying electronic states of CO2: Multireference configuration interaction calculations

Multi-reference configuration interaction with single and double excitation method has been utilized to calculate the potential energy surfaces of the five low-lying electronic states (1)A1, (1)A2, (3)A2, (1)B2, and (3)B2 of carbon dioxide molecule. Topology of intersections among these five states has been fully analyzed and is associated with double-well potential energy structure for every electronic state. The analytical potential energy surfaces based on the reproducing kernel Hilbert space method have been utilized for illustrating topology of surface crossings. Double surface seam lines between (1)A1 and (3)B2 states have been found inside which the (3)B2 state is always lower in potential energy than the (1)A1 state, and thus it leads to an angle bias collision dynamics. Several conical∕surface intersections among these five low-lying states have been found to enrich dissociation pathways, and predissociation can even prefer bent-geometry channels. Especially, the dissociation of O((3)P) + CO can take place through the intersection between (3)B2 and (1)B2 states, and the intersection between (3)A2 and (1)B2 states.

Using Anisotropy Measurements from A-Band Photodissociation to Interrogate the Excited States of H2Se

The A-band photodissociation of H2Se has been studied by measuring H-atom velocity-aligned Doppler spectroscopy (VADS) spectra at five wavelengths from 210 to 266 nm. These spectra have been subsequently simulated by assigning cross-sections and anisotropy β values to the two SeH spin-orbit exit channels. While the SeH((2)Π3/2) exit channel has a β value close to -1 throughout the studied wavelength range, the spin-orbit excited SeH((2)Π1/2) exit channel's β value switches from near -1 to near +0.5 when the photolysis wavelength increases from 210 to 266 nm. These results have been examined in the light of available ab initio calculations. Throughout the studied wavelengths, the contribution from excitation to the 1(1)B1 state predominates and provides the source of the -1 β value. In order to account for the +0.5 β value, it is necessary to assume that the 2(1)A1 state as well as the 4A' and 5A' states (both originating from a (3)B1 state) also contribute at short wavelengths. More interestingly, at the longer wavelength end (266 nm), contribution of a +0.5 β value from the 3A' state (originating from a (3)A2 state) exceeds the contribution of the -1 β value for the SeH((2)Π1/2) channel.

BeCH2: The Simplest Metal Carbene. High Levels of Theory

The simplest metal carbene, BeCH2, is experimentally unknown. Its isomer, HBeCH, lies higher in energy, but has been detected by the infrared matrix isolation [J. Am. Chem. Soc. 1998, 120, 6097]. In the present study the ground and low-lying excited states of the BeCH2 and HBeCH isomers were investigated using state-of-the-art ab initio methods, including coupled-cluster theory with up to full quadruple excitations (CCSDTQ), and complete active space self-consistent field (CASSCF) with multireference configuration interaction with single and double excitations (MRCISD). The relative energies were obtained using the focal point analysis combined with large correlation-consistent cc-pCVXZ basis sets (X = D, T, Q, 5) and were extrapolated to the complete basis set (CBS) limit. The (3)B1 state of BeCH2 (C(2v) symmetry) is the global minimum on the ground triplet potential energy surface (PES). The (3)Σ(-) state of the linear isomer HBeCH is located 4.9 kcal mol(-1) above the global minimum, at the CCSDTQ/CBS level of theory. The BeCH2 and HBeCH isomers are connected through the (3)A" transition state lying 46.1 kcal mol(-1) above the global minimum. The higher-lying energy HBeCH structure has much larger Be-C bond dissociation energy (126.6 kcal mol(-1), cf. BDE(BeCH2) = 62.1 kcal mol(-1)). The lowest excited state of BeCH2 is the open-shell (1)B1 state, with a relative energy of only 4.9 kcal mol(-1) above the global minimum, followed by (1)A1 state (16.8 kcal mol(-1)) at the MRCISD/cc-pCVQZ level of theory. For the HBeCH isomer the lowest-energy excited states are (1)Δ and (1)Σ(+), lying about 30 kcal mol(-1) above the global minimum. For the ground state of BeCH2 the fundamental vibrational frequencies computed using second-order vibrational perturbation theory (VPT2) at the CCSD(T)/cc-pCVQZ level are reported. We hope that our highly accurate theoretical results will assist in the experimental identification of BeCH2.

Ground and excited states of BF3+. Coupled cluster and density functional studies

Ab initio (CCSD, CCSD(T), EOM-CCSD, MRCI) and DFT calculations were performed on BF3+. Ab initio methods predict a double well potential with two C2v minima (2B2) and a D3h transition state (2A2′). The minimum with two short and one long B–F distances (2S1L) is lower in energy than 1S2L by 0.10eV (CCSD(T)). DFT methods with common hybrid functionals only find 1S2L. Long-range corrected functionals are needed for the other C2v state. Excited 2B2 states which vibronically interact with the ground state have minima close to D3h symmetry. The double well potential is caused by strong pseudo Jahn–Teller coupling, due in part to very low excitation energies. Vibrational frequencies, vertical excitation energies, hyperfine coupling constants and g-factors are presented for both C2v isomers.

Excitation of the six lowest electronic transitions in water by 9–20 eV electrons

We report differential and integral cross sections for excitation of the 3B1, 1B1, 3A2, 1A2, 3A1 and 1A1 states of H2O by 9–20 eV electrons. The measurements are taken by conventional differential electron energy loss spectroscopy techniques, while the calculations employ the Schwinger multichannel method within a ten-channel approximation. The new data are compared with previous experimental and theoretical results. The present measurements and calculations agree reasonably well both with each other and with prior theoretical efforts but show discrepancies with prior measurements. Reasons for those discrepancies are considered and discussed.

Vibronically induced decay paths from the $\tilde C$C̃1B1-state of water and its isotopomers

The photochemistry of the water molecule has revealed a wealth of quantum phenomena, which arise from the involvement of several coupled electronic states with very different potential energy surfaces. Most recently, dissociation from single rotational levels of its C̃(1)B1 state near 124 nm has been attributed to a vibronically coupled decay via the lower Ã-state surface, despite a large vertical energy gap of 2.8 eV. Similar conclusions have been reached for subsequent experimental data for D2O. The present paper presents further experimental data for HOD and for both the H+OD(X) and D+OH(X) products. Unlike the cases for H2O and D2O, the vibrational populations for hydroxyl products do not follow a smooth distribution with v(OH∕OD). In particular, for OH there is a clear alternation in population for all the strong peaks, with odd v favoured over even v. These experimental data are analysed using new MRCI+Q calculations, which have been used to generate potential surfaces and associated non-adiabatic matrix elements for transition from the adiabatic C̃-state to lower unbound potential surfaces; and hence, to guide dynamical calculations using time-dependent wavepackets. It is concluded that although there is a minor contribution from the C̃→ à decay route, the major route follows C̃→ (1)A2 → Ã. This is mediated through two regions of near degeneracy of the elusive (1)A2 surface with C̃ for short bonds ca. 0.8 Å; and between (1)A2 and à with long bonds ≥2 Å, thereby bridging the vertical energy gap. The striking population alternation for the D+OH(X) products is attributed to dynamic symmetry breaking on the (1)A2 surface.

Neutral-Fragmentation Paths of Methane Induced by Intense Ultrashort IR Laser Pulses: Ab Initio Molecular Orbital Approach

Instantaneous (laser-field-dependent) potential energy curves leading to neutral fragmentations of methane were calculated at several laser intensities from 1.4 × 10(13) to 1.2 × 10(14) W/cm(2) (from 1.0 × 10(10) to 3.0 × 10(10) V/m) using ab initio molecular orbital (MO) methods to validate the observation of neutral fragmentations induced by intense femtosecond IR pulses (Kong et al. J. Chem. Phys. 2006, 125, 133320). Two fragmentation paths, CH(2) + 2H and CH(2) + H(2), in (1)T(2) superexcited states that are located in the energy range of 12-16 eV were considered as the reaction paths because these states are responsible for Jahn-Teller distortion opening up reaction paths during ultrashort pulses. As field intensity increased, the low-lying excited (1)A(1) states originated from the Jahn-Teller (1)T(2) states were substantially stabilized along the neutral-fragment path CH(4) → CH(2) + 2H and were located below the ionization threshold. On the other hand, the low-lying excited (1)B(1) states, which also originate from the Jahn-Teller (1)T(2) states, were embedded on the ionized state along the dissociation path to CH(2) + H(2). This indicates that ionic fragments, rather than neutral ones, are produced along the CH(2) + H(2) path. The computational results support neutral fragmentations through superexcited states proposed by Kong et al.

Theoretical study on low-lying states of Ga2X (X = P, As) with coupled-cluster approaches

Low-lying states of Ga2P and Ga2As are investigated with the equation-of-motion coupled-cluster approach for ionized states at the singles and doubles level (EOMIP-CCSD) as well as at the CCSDT-3 level together with CCSD, CCSD(T), and DFT. Except for the asymmetric stretching b2 mode of the (2)B2 and (2)A1 states, all these approaches provide structures, frequencies and adiabatic electron affinities that are in reasonable agreement with each other. According to our results, the lowest-energy state of these two molecules is the (2)A' state of C(s) symmetry and the (2)B2 state is the ground electronic state with C(2v) symmetry. As for the b2 mode, CCSD and CCSD(T) afford real frequencies for the (2)B2 state, while EOM approaches and DFT with most exchange-correlation functionals give rise to imaginary frequencies. The (2)B2 and (2)A1 states couple with each other due to distortion along b2 mode through the pseudo-Jahn-Teller effect. Analysis on results shows that EOM approaches afford reasonable b2 frequencies for the two states and DFT approaches, except for BP86 and PBE, provide qualitatively correct b2 frequencies for the (2)B2 state. In addition, a potential matrix is introduced to describe the vibronic coupling between the (2)B2 and (2)A1 states and parameters in the matrix are fitted to the adiabatic potential curves from EOMIP-CCSD results.

Electronic excited states and electronic spectra of biphenyl: a study using many-body wavefunction methods and density functional theories

The low-lying electronic excited states of biphenyl were studied using the symmetry-adapted cluster-configuration interaction (SAC-CI), complete active space self-consistent field (CASSCF), complete active space perturbation theory of the second-order (CASPT2), and the time-dependent density functional theory (TDDFT). The molecular geometries in the ground and excited states were optimized using the SAC-CI and TDDFT for singlet and triplet states. The energies of vertical excitations, emissions, and adiabatic transitions were calculated. The TDDFT calculations significantly underestimated the excitation energy of the 1(1)B1 state, while the SAC-CI and CASPT2 provided essentially similar results. The present SAC-CI and CASPT2 calculations concluded that the lowest singlet state of isolated biphenyl is the 1(1)B3 state that takes a planar geometry and the second lowest state is the 1(1)B2 state with a twisted geometry. The present results were consistent with the previous experimental findings. The 1(1)B1 state that has a charge-separated biracial character in the vertical excitation relaxed into a planar quinoid structure in which bond alternations were emphasized. The other states took a benzenoid structure. The ultraviolet (UV) absorption and circular dichroism (CD) spectra below 7 eV were calculated with the SAC-CI method. The valence-Rydberg mixings were found to be significant in the second and higher series of excited states.

Probing ultrafast dynamics in photoexcited pyrrole: timescales for 1πσ* mediated H-atom elimination

The heteroaromatic ultraviolet chromophore pyrrole is found as a subunit in a number of important biomolecules: it is present in heme, the non-protein component of hemoglobin, and in the amino acid tryptophan. To date there have been several experimental studies, in both the time- and frequency-domains, which have interrogated the excited state dynamics of pyrrole. In this work, we specifically aim to unravel any differences in the H-atom elimination dynamics from pyrrole across an excitation wavelength range of 250-200 nm, which encompasses: (i) direct excitation to the (formally electric dipole forbidden) 1(1)pisigma* (1A2) state; and (ii) initial photoexcitation to the higher energy 1 pipi* (1B2) state. This is achieved by using a combination of ultrafast time-resolved ion yield and time-resolved velocity map ion imaging techniques in the gas phase. Following direct excitation to 1(1)pisigma* (1A2) at 250 nm, we observe a single time-constant of 126 +/- 28 fs for N-H bond fission. We assign this to tunnelling out of the quasi-bound 3s Rydberg component of the 1(1)pisigma* (1A2) surface in the vertical Franck-Condon region, followed by non-adiabatic coupling through a 1(1)pisigma*/S(0) conical intersection to yield pyrrolyl radicals in their electronic ground state (C4H4N(X)) together with H-atoms. At 238 nm, direct excitation to, and N-H dissociation along, the 1(1)pisigma* (1A2) surface is observed to occur with a time-constant of 46 +/- 22 fs. Upon initial population of the 1pipi* (1B2) state at 200 nm, a rapid 1pipi* (1B2) --> 1(1)pisigma* (1A2) --> N-H fission process takes place within 52 +/- 12 fs. In addition to ultrafast N-H bond cleavage at 200 nm, we also observe the onset of statistical unimolecular H-atom elimination from vibrationally hot S(0) ground state species, formed after the relaxation of excited electronic states, with a time-constant of 1.0 +/- 0.4 ns. Analogous measurements on pyrrole-d1 reveal that these statistical H-atoms are released only through C-H bond cleavage.

Cross sections for below threshold electron excitation of furan

Experimental and theoretical cross sections are present for electron impact excitation of the 3B2 and 3A1 electronic states of furan by sub 20eV electrons.

Cross sections for electronic excitation of water by low-energy electrons

New experimental and theoretical data are presented for excitation of the 3B1 and 1B1 states of water by low-energy (9-15 eV) electron impact.

The magnetic circular dichroism (MCD) and absorption studies of 1,8-naphthalimide. The theoretical analysis in terms of density functional (DF) and coupled cluster (CC) theories

The vibronic coupling theory based on the Franck–Condon (FC) approach was applied to study the absorption and magnetic circular dichroism (MCD) spectra of 1,8-naphthalimide in the excitation region corresponding to the low-energy 1A1→2A1 and 1A1→1B1 transitions. The computations carried out in terms of PBE0 and CC2 methods have shown that the vertical excitation energies of 2A1 and 1B1 states and Franck–Condon parameters from CC2/aug-cc-pVDZ computation lead to a very good agreement with the experiment. MCD analysis suggests that the vibronic coupling effects between 2A1 and 1B1 states may be responsible for observed minor discrepancies between the theoretical and empirical MCD spectra.

Structural and magnetic properties of epitaxial Co2FeAl films grown on MgO substrates for different growth temperatures

We report the correlation between the crystalline structure, electronic structure and magnetic properties of Co2FeAl films as a function of growing temperature both experimentally and theoretically. The Co2FeAl film grown at room temperature is initially in the partially disordered B2 state, but then it gains a much higher ordered structure with increasing growing temperature due to its transition from short-range to long-range crystallographic order by surface diffusion. Electron energy loss spectroscopy measurements reveals that the increase in the I(L3)/I(L2) ratio of Co can be attributed to the enhanced ferromagnetic exchange interaction between neighboring Co atoms and the fact that the Co contribution is more dominant than the Fe contribution. As the growing temperature increases, many more unoccupied 3d states in Co are observed, hence the Gilbert damping constant increases due to a strong spin–orbit interaction. We also present the results of highly accurate quasiparticle self-consistent GW calculations and confirm that Co2FeAl in an ideal L21 structure is indeed a half-metal with a well-defined band gap in the minority spin channel.

Excitation of the and states of H2O by low-energy electron impact

We report measured and calculated differential cross-sections for inelastic scattering of low-energy electrons by water leading to excitation of the dissociative (1b1 → 4a1) 1, 3B1 states. The measurements were taken using conventional energy-loss spectroscopy at incident energies of 9, 10, 12, 15, and 20 eV for scattering angles from 10° to 130°. The calculations were carried out using the Schwinger multichannel method, with a Born-dipole correction applied in the singlet excitation channel. Integral excitation cross sections for the and states are also derived from the differential cross section results.

Relativistic Jahn–Teller and pseudo-Jahn–Teller couplings in [formula omitted] systems

The Hamiltonian for linear and quadratic Jahn–Teller (JT) and pseudo-JT (PJT) coupling as well as zeroth-order and linear spin–orbit (SO) coupling in the 2E+2B2 electronic multiplet in D2d systems is derived. The SO coupling is described by the microscopic Breit–Pauli operator. It is shown that the 2E state exhibits a 2E×e JT effect which is of relativistic origin, that is, it arises from the SO operator. The relativistic PJT coupling of the 2E and 2B2 states involves the normal modes of b1,b2 and e symmetry. The 2E×(b1+b2+e) JT Hamiltonian is analytically transformed to a SO-adapted electronic basis in which the zeroth-order SO operator is diagonal. In the special case of 2E-2B degeneracy, the (2E+2B2)×(b1+b2+e) JT+PJT Hamiltonian also is transformed to a SO-adapted basis.

Mass-analyzed threshold ionization of an excited state of lanthanum dioxide

LaO(2) was produced in a pulsed laser-vaporization molecular beam source and studied by mass-analyzed threshold ionization (MATI) spectroscopy and ab initio electronic structure calculations. The calculations included density functional theory, second-order perturbation theory, coupled cluster theory, and complete active space self-consistent field methods. The adiabatic ionization energy of the molecule and vibrational frequencies of the molecule and its cation were measured accurately for the first time from the MATI spectrum. Numerous ionization processes of lanthanum dioxide, peroxide, and superoxide were considered; the (3)B(2) ← (4)B(2) electronic transition of the dioxide was assigned upon comparison with the observed spectrum. The ionization energy and O-La-O bending frequency of the (4)B(2) neutral state are 4.9760 (6) eV and 92 cm(-1), respectively. The La-O stretching and O-La-O bending frequencies of the (3)B(2) cationic state are 656 and 122 cm(-1), respectively. The (4)B(2) state is formed by two electron transfer from lanthanum to oxygen atoms, and the (3)B(2) state is produced by the further removal of a lanthanum 6s-based electron.

Enhancement of interfacial specific resistance by the insertion of Fe(001) layers between alternate monatomic [Fe/Co]n superlattices and Ag spacer

The bulk and interface spin scattering asymmetric coefficients, βF and γF/N, respectively, as well as the interfacial specific resistance, AR*F/N, contribute to the total resistance change-area product, ΔRA, in current-perpendicular-to-plane (CPP) geometry. In this paper, we report the effect of the insertion of a 1 nm Fe layer into ferromagnetic (F)/non-ferromagnetic (N) interfaces on the spin dependent transport properties. Using the Valet-Fert theory based on the two-current model in which ΔRA is obtained as a function of thicknesses of the ferromagnetic layer (3, 4, and 5 nm), we experimentally deduced the values of βF, γF/N, and AR*F/N of exchange-biased spin valves (EBSVs) for an artificially ordered B2 state Fe50Co50 alloy as a magnetic layer and an Ag spacer layer.