Kueih-Tzu Lu

Understanding barriers to internal rotation in substituted toluenes and their cations

Author Institution: Lawrence Berkeley Laboratory, M/S 2-300, 1 Cyclotron Road, Berkeley, CA 94720; University of Wisconsin-Madison, Madison, WI 53706-1396.

Band-gap narrowing of TiO2 doped with Ce probed with x-ray absorption spectroscopy

The electronic structure of TiO2 doped with Ce was investigated with x-ray absorption and photoluminescence (PL) spectra. Ce doping narrows the band gap of TiO2, as evidenced by a redshift in the absorption edge in the O K-edge spectrum and PL. Narrowing is ascribed to the formation of an impurity band, composed of Ce 4f (Ce3+/Ce4+) and oxygen defect states, that induces tailing of the conduction band. Band gap narrowing extends the photoactivity of TiO2 to visible light. The impurity band located below the minimum of the conduction band traps excited electrons and suppresses recombination, enhancing the photocatalytic activity.

INTENSITIES OF FORBIDDEN PURE TORSIONAL BANDS IN S1-S0 SPECTRA OF TOLUENES

In toluenelike molecules with sixfold internal rotation symmetry, three weak, Franck–Condon forbidden, pure torsional transitions m21, m30, and m41 invariably appear in S1–S0 absorption spectra. The intensities are typically 1%–5% of the allowed bands, m00 and m11. Determination of the preferred conformer in S1 as staggered or eclipsed relies on proper assignment of the upper state of the m30 band as 3a‘1 or 3a‘2 under the molecular symmetry group G12. In addition, inferences of the preferred conformer in S0 and D0 (ground state cation) from band intensities of fluorescence and threshold photoionization spectra have also relied on the same assignment. For a set of six molecules having sixfold rotor potentials, including both –CH3 and –CD3 rotors, we present experimental relative intensities for the three forbidden S1–S0 absorption bands. Within an adiabatic electronic representation, we show how a Fourier expansion of the three components of the S1–S0 electric dipole transition moment predicts relative fo...

Silyl group internal rotation in S1 phenylsilane and phenylsilane cation: Experiments and ab initio calculations

Resonant two‐photon ionization (R2PI) and pulsed field ionization (PFI) were used to measure S1–S0 and cation–S1 spectra of internally cold phenylsilane. We measure the adiabatic ionization potentials IP(phenylsilane)=73 680±5 cm−1, IP(phenylsilane ⋅Ar)=73 517±5 cm−1 and IP(phenylsilane ⋅Ar2)=73 359±5 cm−1. We assign many low lying torsion–vibration levels of the S1 (A 1A1) state and of X 2B1 of phenylsilane+. In both states, the pure torsional transitions are well fit by a simple sixfold hindered rotor Hamiltonian. The results for the rotor inertial constant B and internal rotation potential barrier V6 are, in S1, B=2.7±0.2 cm−1 and V6=−44±4 cm−1; in the cation, B=2.7±0.2 cm−1 and V6=+19±3 cm−1. The sign of V6 and the conformation of minimum energy are inferred from spectral intensities of bands terminating on the 3a‘1 and 3a‘2 torsional levels. In S1 the staggered conformation is most stable, while in the cation ground state the eclipsed conformation is most stable. For all sixfold potentials whose ab...

THRESHOLD PHOTOIONIZATION SPECTRA OF BENZYL RADICAL : CATION VIBRATIONAL STATES AND AB INITIO CALCULATIONS

We have measured threshold photoionization spectra of benzyl+‐h7, benzyl+‐αd2, and benzyl+‐d7 in the ground electronic state (X+ 1A1) using resonant two‐photon excitation and detection of electrons by pulsed field ionization. The adiabatic ionization potentials of benzyl‐h7, benzyl‐αd2, and benzyl‐d7 are 58 468±5 cm−1, 58 418±5 cm−1, and 58 386±5 cm−1. Excitation through a variety of vibronically mixed A 2A2–B 2B2 neutral excited states allows observation of cation vibrations of both a1 and b1 symmetries. We directly measure in‐plane fundamentals and infer the frequencies of certain out‐of‐plane fundamentals from their involvement in combinations or overtones. By comparison with harmonic frequencies from ab initio calculations, we assign 35 of 48 observed levels in the ‐h7 isotopomer, 15 of 22 levels in ‐αd2, and 25 of 30 levels in ‐d7. Ab initio calculations permit a detailed comparison of the geometry, chemical bonding, and vibrational frequencies in the benzyl anion, neutral, and cation. The anion a...

State-specific photochemistry of SiCl4 condensed on Si(100) with synchrotron radiation

State-specific photochemistry for SiCl4 adsorbed on Si(100) at ∼90 K following the Cl 2p and Si 2p core-level excitations has been investigated by photon-stimulated ion desorption, resonant photoemission, ion energy distribution, and soft x-ray absorption spectral measurements. The Cl+ photon-stimulated ion desorption spectrum and Cl L-edge x-ray absorption spectrum for condensed SiCl4 exhibit a clear dissimilarity unlike the other fragments, viz. SiCl3+, SiCl2+, SiCl+, and Si+. The Cl+ desorption yield shows a significant enhancement following the Cl 2p→8a1* excitation, while the Si 2p→8a1* excitation leads to the scarce enhancement of the Cl+ yield. The Cl+ ion energy distributions via the Cl 2p→8a1* excitation are shifted to higher energy (0.3–0.4 eV) compared to those via the Cl 2p→9t2* and Si 2p→8a1* excitations. The enhancement of the Cl+ desorption yield at the Cl(2p)−18a1* resonance is attributed to the higher ion escape energy. These results provide an insight into the comprehensive understanding...

Orbital structure of FeTiO3 ilmenite investigated with polarization-dependent X-ray absorption spectroscopy and band structure calculations

We explored the orbital structure of FeTiO3 with polarization-dependent x-ray absorption spectra complemented with electronic structure calculations. The electronic structure, near the bottom of conduction band, is composed of O 2p and Ti 3d orbitals. Ti 3d/4p hybridization dominantly lies on the ab plane. The highly delocalized Ti 4p orbital might hybridize with O 2p orbital and even extend to the next-neighbor Fe atom whereby establishing a linear orbital combination of Ti-O-Fe. A clear picture of the orbital construction in FeTiO3 will help to elucidate the paths of pressure-induced charge transfer and other physical or magnetic characteristics.

A combined first principle calculations and experimental study on the spin-polarized band structure of Co-doped PbPdO2

With x-ray spectroscopy and first-principles calculations, we expose the electronic structure, near the Fermi level, of Co-doped PbPdO2 composed of O 2p-Pd 4d hybridized states with an additional contribution of a spin-polarized Co 3d state at either a greater or smaller energy. The spin-polarized Co 3d states interacting with O 2p-Pd 4d hybridized states cause spin splitting at the band edge. Fascinating physical properties such as high-temperature ferromagnetism thus arise in Co-doped PbPdO2. Results will help in the design of materials with desired electronic structures and the control of spin polarization with chemical doping.

ELECTRONIC RELAXATION AND ION DESORPTION PROCESSES OF SOLID SI(CH3)2CL2 FOLLOWING SI 2P CORE-LEVEL EXCITATION

High-resolution Si L23-edge x-ray absorption near-edge structure (XANES) total electron yield (TEY) spectrum and photon stimulated ion desorption (PSID) spectra of condensed Si(CH3)2Cl2 have been measured in the energy range of 102–115 eV using synchrotron radiation. Excitation from Si 2p to a Si–C antibonding orbital enhances the CH3+ formation, while excitation to the Si–Cl antibonding orbital gives rise to a pronounced Cl+ production. This indicates that the character of the bound terminating orbital has a significant influence on the fragmentation processes. The selective enhancement of H+ yield at a peak of ∼106.0 eV is interpreted in terms of the excited electron in the bound orbital with strong C–H antibonding character. Applying resonant photoemission spectroscopy, the spectator Auger process was found to be the dominant decay channel for the resonantly excited Si(2p) core hole of condensed Si(CH3)2Cl2. The close resemblance of the TEY and PSID spectra of solid Si(CH3)2Cl2 was attributable to spec...

State-specific desorption in condensed Si(CH3)2Cl2 following resonant excitation at the Cl 2p edge

Photon-stimulated ion desorption (PSID) from solid Si(CH3)2Cl2 following Cl 2p core-level excitation has been investigated along with the resonant photoemission spectra using synchrotron radiation. The Cl L23-edge total-electron yield spectrum of solid Si(CH3)2Cl2 mainly exhibits the Cl 2p→15a1*, Cl 2p→10b1* and shape resonance peaks. A significant dissimilarity between the Cl L23-edge total-electron yield spectrum and Cl+ PSID spectrum of solid Si(CH3)2Cl2 is observed. The desorption of atomic Cl+ ions is detected predominantly at the Cl 2p→15a1* excitation, while scarcely any Cl+ ions are observed following the Cl 2p→10b1* excitation and Cl 2p→shape resonance excitation. This indicates that the character of bound terminating orbital has a significant influence on the desorption processes. The enhanced Cl+ desorption yield at the Cl 2p→15a1* resonance is attributed to the the fast desorption process originating from the localization of the spectator electron in the strong antibonding orbital.