Coupled-cluster studies of the electronic excitation spectra of silanes

The electronic excitation spectra of unsubstituted linear silanes (n-Si(m)H(2m+2), m = 1-6), cyclopentasilane (c-Si5H10), and neopentasilane (neo-Si5H12) have been studied at the coupled-cluster approximate singles and doubles (CC2) level using Dunning's quadruple-zeta basis sets augmented with diffuse functions (aug-cc-pVQZ). Comparisons with measured ultraviolet spectra for Si2H6 and n-Si3H8 show that CC2 calculations using these basis sets yield excitation energies in good agreement with experiment. The calculated excitation thresholds for Si2H6 and n-Si3H8 of 7.61 and 6.68 eV are only 0.05 eV larger than the gas-phase values of 7.56 and 6.63 eV, respectively. For n-Si4H10, n-Si5H12, and neo-Si5H12, the calculated excitation thresholds of 6.51, 6.14, and 6.87 eV for the lowest dipole-allowed transitions are about 0.4 eV larger than the corresponding liquid-phase data of 6.05, 5.77, and 6.53 eV; the discrepancy can mainly be attributed to solvent effects. The obtained excitation thresholds for n-Si6H14 is 5.85 eV, whereas no experimental data are available for its optical gap. Calculations using the Karlsruhe triple-zeta valence basis sets augmented with single and double sets of polarization functions show that very large basis sets augmented with diffuse functions are needed for obtaining accurate excitation energies. The optical gaps for silanes obtained using the triple-zeta polarization basis sets were found to be 0.4 and 0.2 eV larger than those obtained using Dunning's quadruple-zeta basis sets. Excitation thresholds calculated at density functional theory levels using generalized gradient approximation are 0.7-1.0 eV smaller than the experimental values and by employing hybrid functionals they are 0.3-0.4 eV below the experimental thresholds. By adding the present basis-set correction and environmental effects to the previously calculated CC2 value for the excitation threshold of the Si29H36 silicon nanocluster, the extrapolated absorption threshold is 4.0 eV as compared to the recently reported experimental value of 3.7 eV.     

Photoelectron spectroscopy of phosphorus hydride anions

Negative-ion photoelectron spectroscopy is applied to the PH-, PH2-, P2H-, P2H2-, and P2H3-molecular anions. Franck-Condon simulations of the photoelectron spectra are used to analyze the spectra and to identify various P2H(n)- species. The simulations employ density-functional theory calculations of molecular geometries and vibrational frequencies and normal modes, and coupled-cluster theory calculations of electron affinities. The following electron affinities are obtained: EA0(PH) = 1.027 +/- 0.006 eV, EA0(PH2) = 1.263 +/- 0.006 eV, and EA0(P2H) = 1.514 +/- 0.010 eV. A band is identified as a mixture of trans-HPPH- and cis-HPPH-. Although the trans and cis bands cannot be definitively assigned from experimental information, using theory as a guide we obtain EA0(trans-HPPH)= 1.00 +/- 0.01 eV and EA0(cis-HPPH) = 1.03 +/- 0.01 eV. A weak feature tentatively assigned to P2H3- has a vertical detachment energy of 1.74 eV. The derived gas-phase acidity of phosphine is delta(acid)G298(PH3) < or = 1509.7 +/- 2.1 kJ mo1(-1).     

Photoelectron spectra of dihalomethyl anions: testing the limits of normal mode analysis

We report the 364-nm negative ion photoelectron spectra of CHX(2)(-) and CDX(2)(-), where X = Cl, Br, and I. The pyramidal dihalomethyl anions undergo a large geometry change upon electron photodetachment to become nearly planar, resulting in multiple extended vibrational progressions in the photoelectron spectra. The normal mode analysis that successfully models photoelectron spectra when geometry changes are modest is unable to reproduce qualitatively the experimental data using physically reasonable parameters. Specifically, the harmonic normal mode analysis using Cartesian displacement coordinates results in much more C-H stretch excitation than is observed, leading to a simulated photoelectron spectrum that is much broader than that which is seen experimentally. A (2 + 1)-dimensional anharmonic coupled-mode analysis much better reproduces the observed vibrational structure. We obtain an estimate of the adiabatic electron affinity of each dihalomethyl radical studied. The electron affinity of CHCl(2) and CDCl(2) is 1.3(2) eV, of CHBr(2) and CDBr(2) is 1.9(2) eV, and of CHI(2) and CDI(2) is 1.9(2) eV. Analysis of the experimental spectra illustrates the limits of the conventional normal mode approach and shows the type of analysis required for substantial geometry changes when multiple modes are active upon photodetachment.     

Coupled-cluster studies of the lowest excited states of the 11-cis-retinal chromophore

The first few excited states of the 11-cis-retinal (PSB11) chromophore have been studied at the coupled-cluster approximative singles and doubles (CC2) level using triple-zeta quality basis sets augmented with double sets of polarisation functions. The two lowest vertical excitation energies of 2.14 and 3.21 eV are in good agreement with recently reported experimental values of 2.03 and 3.18 eV obtained in molecular beam measurements. Calculations at the time-dependent density functional theory (TDDFT) level using the B3LYP hybrid functional yield vertical excitation energies of 2.34 and 3.10 eV for the two lowest states. Zero-point vibrational energy (ZPVE) corrections of -0.09 and -0.17 eV were deduced from the harmonic vibrational frequencies for the ground and excited states calculated at the density functional theory (DFT) and TDDFT level, respectively, using the B3LYP hybrid functional.     

Photoelectron imaging and theoretical studies of silver monohalides AgX- (X = Cl, Br, I) and AuCl-

Photodetachment of AgX(-) (X = Cl, Br, I) and AuCl(-) is studied by a photoelectron velocity map imaging technique and theoretical calculations. Photoelectron spectra (PES) and photoelectron angular distributions (PADs) were obtained. The vibrationally resolved spectra provided approximately equal electron affinities (EAs) for AgX: 1.593(22) eV for AgCl, 1.623(21) eV for AgBr, and 1.603(22) eV for AgI, respectively. Franck-Condon simulations of these spectra gave the equilibrium bond lengths and vibrational frequencies of the title anions. Relativistic density functional theory (DFT) calculations using BLYP, PW91, PBE, and BP86 functionals have been performed to predict the EAs of the AgX (X = Cl, Br, I) molecules. The computed EAs at the BP86 level of theory are in good agreement with the experimental values. Energy partitioning analyses (EPA) at the BP86(ZORA)/QZ4P level of theory of both anions and their neutrals were reported.     

Ab initio calculation of the vibrational and electronic spectra of trans- and cis-azobenzene

We report accurate geometries and harmonic force fields for trans- and cis-azobenzene determined by second-order M?ller-Plesset perturbation theory. For the trans isomer, the planar structure with C(2h) symmetry, found in a recent gas electron diffraction experiment, is verified. The calculated vibrational spectra are compared with experimental data and density functional calculations. Important vibrational frequencies are localized and discussed. For both isomers, we report UV spectra calculated using the second-order approximate coupled-cluster singles-and-doubles model CC2 with accurate basis sets. Vertical excitation energies and oscillator strengths have been determined for the lowest singlet n(pi)* and (pi)(pi)* transitions. The results are compared with the available experimental data and second-order polarization propagator (SOPPA) and density functional (DFT) calculations. For both isomers, the CC2 results for the excitation energies into the S(1) and S(2) states agree within 0.1 eV with experimental gas-phase measurements.     

Photoelectron spectroscopy of 1-nitropropane and 1-nitrobutane anions

We present low-energy velocity map photoelectron imaging results for bare and Ar-solvated 1-nitropropane and 1-nitrobutane anions. We report the adiabatic electron affinity of 1-nitropropane as (223 ± 6) meV and that of 1-nitrobutane as (240 ± 6 meV). The vertical detachment energies of these two species are found to be (0.92 ± 0.05) and (0.88 ± 0.05) eV, respectively. The photoelectron spectra are discussed in the framework of Franck-Condon simulations based on density functional theory. We observe unusual resonances in the photoelectron spectra of both ions under study, whose kinetic energy is independent of the photon energy of the detaching radiation. We discuss possible origins of these resonances as rescattering phenomena, consistent with the experimental photoelectron angular distributions.     

Two-color visible/vacuum ultraviolet photoelectron imaging dynamics of Br2

An experimental two-color photoionization dynamics study of laser-excited Br2 molecules is presented, combining pulsed visible laser excitation and tunable vacuum ultraviolet (VUV) synchrotron radiation with photoelectron imaging. The X 1Sigmag + -B 3Pi0+u transition in Br2 is excited at 527 nm corresponding predominantly to excitation of the v' = 28 vibrational level in the B 3Pi0+u state. Tunable VUV undulator radiation in the energy range of 8.40-10.15 eV is subsequently used to ionize the excited molecules to the X 2Pi32,12 state of the ion, and the ionic ground state is probed by photoelectron imaging. Similar experiments are performed using single-photon synchrotron ionization in the photon energy range of 10.75-12.50 eV without any laser excitation. Photoelectron kinetic energy distributions are extracted from the photoelectron images. In the case of two-color photoionization using resonant excitation of the intermediate B 3Pi0+u state, a broad distribution of photoelectron kinetic energies is observed, and in some cases even a bimodal distribution, which depends on the VUV photon energy. In contrast, for single-photon ionization, a single nearly Gaussian-shaped distribution is observed, which shifts to higher energy with photon energy. Simulated spectra based on Franck-Condon factors for the transitions Br2(X 1Sigmag+, v" = 0)-Br2 +(X 2Pi12,32, v+) and Br2(B 3Pi0+u, v' = 28)-Br2 +(X 2Pi12,32, v+) are generated. Comparison of these calculated spectra with the measured images suggests that the differences in the kinetic energy distributions for the two ionization processes reflect the different extensions of the vibrational wave functions in the v" = 0 electronic ground state (X 1Sigmag+) versus the electronically and vibrationally excited state (B 3Pi0+u, v' = 28).     

Photoionization cross-section weighted DFT simulations as promising tool for the investigation of the electronic structure of open shell metal-phthalocyanines

The valence band structure of different metal-phthalocyanines was investigated by comparing ultraviolet photoelectron spectra at different excitation energies with simulated spectra that take the different photoionization cross-sections at these energies into account. The Kohn-Sham eigenvalue spectra, derived from density functional theory calculations, using hybrid exchange-correlation functionals, were weighted with the photoionization coefficients in accordance with the used excitation energy. By applying these techniques, the differences in the photoelectron spectra using He I and He II radiation can be reproduced and investigated. It will be shown that the 3d-orbitals of the used metal central atom of these molecules have a major influence. The changes at different excitation energies were studied for Fe, Co, and Cu central atoms to describe the chemical tailoring effects.     

Electronic spectroscopy and ultrafast energy relaxation pathways in the lowest Rydberg States of trimethylamine

Resonance-enhanced multiphoton ionization photoelectron spectroscopy has been applied to study the electronic spectroscopy and relaxation pathways among the 3p and 3s Rydberg states of trimethylamine. The experiments used femtosecond and picosecond duration laser pulses at wavelengths of 416, 266, and 208 nm and employed two-photon and three-photon ionization schemes. The binding energy of the 3s Rydberg state was found to be 3.087 +/- 0.005 eV. The degenerate 3p x, y states have binding energies of 2.251 +/- 0.005 eV, and 3p z is at 2.204 +/- 0.005 eV. Using picosecond and femtosecond time-resolved experiments we spectrally and temporally resolved an intricate sequence of energy relaxation pathways leading from the 3p states to the 3s state. With excitation at 5.96 eV, trimethylamine is found to decay from the 3p z state to 3p x, y in 539 fs. The decay to 3s from all the 3p states takes place with a 2.9 ps time constant. On these time scales, trimethylamine does not fragment at the given internal energies, which range from 0.42 to 1.54 eV depending on the excitation wavelength and electronic state.     

Time-dependent density functional theory based upon the fragment molecular orbital method

Time-dependent density functional theory (TDDFT) was combined with the two-body fragment molecular orbital method (FMO2). In this FMO2-TDDFT scheme, the system is divided into fragments, and the electron density for fragments is determined self-consistently. Consequently, only one main fragment of interest and several fragment pairs including it are calculated by TDDFT. To demonstrate the accuracy of FMO2-TDDFT, we computed several low-lying singlet and triplet excited states of solvated phenol and polyalanine using our method and the standard TDDFT for the full system. The BLYP functional with the long-range correction (LC-BLYP) was employed with the 6-31G(*) basis set (some tests were also performed with 6-311G(*), as well as with B3LYP and time-dependent Hartree-Fock). Typically, FMO2-TDDFT reproduced the full TDDFT excitation energies within 0.1 eV, and for one excited state the error was about 0.2 eV. Beside the accurate reproduction of the TDDFT excitation energies, we also automatically get an excitation energy decomposition analysis, which provides the contributions of individual fragments. Finally, the efficiency of our approach was exemplified on the LC-BLYP6-31G(*) calculation of the lowest singlet excitation of the photoactive yellow protein which consists of 1931 atoms, and the obtained value of 3.1 eV is in agreement with the experimental value of 2.8 eV.     

Probing the electronic structure and property of neutral and charged arsenic clusters (As(n)(+1,0,-1), n≤8) using Gaussian-3 theory

The structures and energies of As(n) (n = 2-8) neutrals, anions, and cations have been systematically investigated by means of the G3 schemes. The electron affinities, ionization potentials, binding energies, and several dissociation energies have been calculated and compared with limited experimental values. The results revealed that the potential surfaces of neutral As(n) clusters are very shallow, and two types of structural patterns compete with each other for the ground-state structure of As(n) with n ≥ 6. One type is derived from the benzvalene form of As(6), and another is derived from the trigonal prism of As(6). The previous photoelectron spectrum (taken from J. Chem. Phys. 1998 , 109 , 10727 ) for As(3) has been reassigned in light of the G3 results. The experimental electron affinities of As(3) were measured to be 1.81 eV, not 1.45 eV. We inferred from the conclusion of G3 and density functional theory that the experimental electron affinities of 1.7 and 3.51 eV for As(5) are unreliable. The reliable electron affinities were predicted to be 0.83 eV for As(2), 1.80 eV for As(3), 0.54 eV for As(4), 3.01 eV for As(5), 2.08 eV for As(6), 2.93 eV for As(7), and 2.02 eV for As(8). The G3 ionization potentials were calculated to be 9.87 eV for As(2), 7.33 eV for As(3), 8.65 eV for As(4), 6.68 eV for As(5), 7.97 eV for As(6), 6.58 eV for As(7), and 7.65 eV for As(8). The binding energies per atom were evaluated to be 1.99 eV for As(2), 2.01 eV for As(3), 2.61 eV for As(4), 2.39 eV for As(5), 2.51 eV for As(6), 2.55 eV for As(7), and 2.67 eV for As(8). These theoretical values of As(2), As(3), and As(4) are in excellent agreement with those of experimental results. Several dissociation energies were carried out to examine relative stabilities. This characterized the even-numbered clusters as more stable than the odd-numbered species.     

The Kohn-Sham density of states and band gap of water: from small clusters to liquid water

Electronic properties of water clusters (H2O)(n), with n=2, 4, 8, 10, 15, 20, and 30 molecules were investigated by sequential Monte Carlo/density-functional theory (DFT) calculations. DFT calculations were carried out over uncorrelated configurations generated by Monte Carlo simulations of liquid water with a reparametrized exchange-correlation functional that reproduces the experimental information on the electronic properties (first ionization energy and highest occupied molecular orbital-lowest unoccupied molecular orbital gap) of the water dimer. The dependence of electronic properties on the cluster size (n) shows that the density of states (DOS) of small water clusters (n>10) exhibits the same basic features that are typical of larger aggregates, such as the mixing of the 3a1 and 1b1 valence bands. When long-ranged polarization effects are taken into account by the introduction of embedding charges, the DOS associated with 3a1 orbitals is significantly enhanced. In agreement with valence-band photoelectron spectra of liquid water, the 1b1, 3a1, and 1b2 electron binding energies in water aggregates are redshifted by approximately 1 eV relative to the isolated molecule. By extrapolating the results for larger clusters the threshold energy for photoelectron emission is 9.6+/-0.15 eV (free clusters) and 10.58+/-0.10 eV (embedded clusters). Our results for the electron affinity (V0=-0.17+/-0.05 eV) and adiabatic band gap (E(G,Ad)=6.83+/-0.05 eV) of liquid water are in excellent agreement with recent information from theoretical and experimental works.     

Probing the electronic and structural properties of chromium oxide clusters (CrO3)n(-) and (CrO3)n (n = 1-5): photoelectron spectroscopy and density functional calculations

Photoelectron spectroscopy has been conducted for a series of (CrO3)n(-) (n = 1-5) clusters and compared with density functional calculations. Well-resolved photoelectron spectra were obtained for (CrO3)n(-) (n = 1-5) at 193 nm (6.424 eV) and 157 nm (7.866 eV) photon energies, allowing for accurate measurements of the electron binding energies, low-lying electronic excitations for n = 1 and 2, and the energy gaps. Density functional and molecular orbital theory (CCSD(T)) calculations were performed to locate the ground and low-lying excited states for the neutral clusters and to calculate the electron binding energies of the anionic species. The experimental and computational studies firmly establish the unique low-spin, nonplanar, cyclic ring structures for (CrO3)n and (CrO3)n(-) for n > or = 3. The structural parameters of (CrO3)n are shown to converge rapidly to those of the bulk CrO3 crystal. The extra electron in (CrO3)n(-) (n > or = 2) is shown to be largely delocalized over all Cr centers, in accord with the relatively sharp ground-state photoelectron bands. The measured energy gaps of (CrO3)n exhibit a sharp increase from n = 1 to n = 3 and approach to the bulk value of 2.25 eV at n = 4 and 5, consistent with the convergence of the structural parameters.     

Electron Affinities of the Early Lanthanide Monoxide Molecules

The photoelectron imagings of LaO^-, CeO^-, PrO^-, and NdO^- at 1064 nm are reported. The well resolved photoelectron spectra allow the electron a±nities to be determined as 0.99(1) eV for LaO, 1.00(1) eV for CeO, 1.00(1) eV for PrO, and 1.01(1) eV for NdO, respectively. Density functional calculations and natural atomic orbital analyses show that the 4f electrons tend to be localized and suffer little from the charge states of the molecules. The photodetached electron mainly originates from the 6s orbital of the metals. The ligand field theory with the δ=2 assumption is still an effective method to analyze the ground states of the neutral and anionic lanthanide monoxides.     

Wavelength dependence of electronic relaxation in isolated adenine using UV femtosecond time-resolved photoelectron spectroscopy

Electronic relaxation pathways in photoexcited nucleobases have received much theoretical and experimental attention due to their underlying importance to the UV photostability of these biomolecules. Multiple mechanisms with different energetic onsets have been proposed by ab initio calculations yet the majority of experiments to date have only probed the photophysics at a few selected excitation energies. We present femtosecond time-resolved photoelectron spectra (TRPES) of the DNA base adenine in a molecular beam at multiple excitation energies between 4.7-6.2 eV. The two-dimensional TRPES data is fit globally to extract lifetimes and decay associated spectra for unambiguous identification of states participating in the relaxation. Furthermore, the corresponding amplitude ratios are indicative of the relative importance of competing pathways. We adopt the following mechanism for the electronic relaxation of isolated adenine; initially the S(2)(ππ*) state is populated by all excitation wavelengths and decays quickly within 100 fs. For excitation energies below ～5.2 eV, the S(2)(ππ*)→S(1)(nπ*)→S(0) pathway dominates the deactivation process. The S(1)(nπ*)→S(0) lifetime (1032-700 fs) displays a trend toward shorter time constants with increasing excitation energy. On the basis of relative amplitude ratios, an additional relaxation channel is identified at excitation energies above 5.2 eV.     

Study of the photoelectron and electron momentum spectra of cyclopentene using benchmark Dyson orbital theories

A complete study of the valence electronic structure and related electronic excitation properties of cyclopentene in its C(s) ground state geometry is presented. Ionization spectra obtained from this compound by means of photoelectron spectroscopy (He I and He II) and electron momentum spectroscopy have been analyzed in details up to electron binding energies of 30 eV using one-particle Green's function (1p-GF) theory along with the outer-valence (OVGF) and the third-order algebraic diagrammatic construction [ADC(3)] schemes. The employed geometries derive from DFT/B3LYP calculations in conjunction with the aug-cc-pVTZ basis set, and closely approach the structures inferred from experiments employing microwave spectroscopy or electron diffraction in the gas phase. The 1p-GF/ADC(3) calculations indicate that the orbital picture of ionization breaks down at electron binding energies larger than approximately 17 eV in the inner-valence region, and that the outer-valence 7a' orbital is also subject to a significant dispersion of the ionization intensity over shake-up states. This study confirms further the rule that OVGF pole strengths smaller than 0.85 foretell a breakdown of the orbital picture of ionization at the ADC(3) level. Spherically averaged (e, 2e) electron momentum distributions at an electron impact energy of 1200 eV that were experimentally inferred from an angular analysis of EMS intensities have been interpreted by comparison with accurate simulations employing ADC(3) Dyson orbitals. Very significant discrepancies were observed with momentum distributions obtained from several outer-valence ionization bands using standard Kohn-Sham orbitals.     

Photoelectron spectroscopy of the molecular anions, ZrO-, HfO-, HfHO-, and HfO2H-

Negative ion photoelectron spectra of ZrO(-), HfO(-), HfHO(-), and HfO(2)H(-) are reported. Even though zirconium- and hafnium-containing molecules typically exhibit similar chemistries, the negative ion photoelectron spectral profiles of ZrO(-) and HfO(-) are dramatically different from one another. By comparing these data with relevant theoretical and experimental studies, as well as by using insights drawn from atomic spectra, spin-orbit interactions, and relativistic effects, the photodetachment transitions in the spectra of ZrO(-) and HfO(-) were assigned. As a result, the electron affinities of ZrO and HfO were determined to be 1.26 ± 0.05 eV and 0.60 ± 0.05 eV, respectively. The anion photoelectron spectra of HfHO(-) and HfO(2)H(-) are similar to one another and their structural connectivities are likely to be H-Hf-O(-) and O-Hf-OH(-), respectively. The electron affinities of HfHO and HfO(2)H are 1.70 ± 0.05 eV and 1.73 ± 0.05 eV, respectively.     

The merits of the frozen-density embedding scheme to model solvatochromic shifts

We investigate the usefulness of a frozen-density embedding scheme within density-functional theory [J. Phys. Chem. 97, 8050 (1993)] for the calculation of solvatochromic shifts. The frozen-density calculations, particularly of excitation energies have two clear advantages over the standard supermolecule calculations: (i) calculations for much larger systems are feasible, since the time-consuming time-dependent density functional theory (TDDFT) part is carried out in a limited molecular orbital space, while the effect of the surroundings is still included at a quantum mechanical level. This allows a large number of solvent molecules to be included and thus affords both specific and nonspecific solvent effects to be modeled. (ii) Only excitations of the system of interest, i.e., the selected embedded system, are calculated. This allows an easy analysis and interpretation of the results. In TDDFT calculations, it avoids unphysical results introduced by spurious mixings with the artificially too low charge-transfer excitations which are an artifact of the adiabatic local-density approximation or generalized gradient approximation exchange-correlation kernels currently used. The performance of the frozen-density embedding method is tested for the well-studied solvatochromic properties of the n-->pi(*) excitation of acetone. Further enhancement of the efficiency is studied by constructing approximate solvent densities, e.g., from a superposition of densities of individual solvent molecules. This is demonstrated for systems with up to 802 atoms. To obtain a realistic modeling of the absorption spectra of solvated molecules, including the effect of the solvent motions, we combine the embedding scheme with classical molecular dynamics (MD) and Car-Parrinello MD simulations to obtain snapshots of the solute and its solvent environment, for which then excitation energies are calculated. The frozen-density embedding yields estimated solvent shifts in the range of 0.20-0.26 eV, in good agreement with experimental values of between 0.19 and 0.21 eV.