Sheng Hsien Lin

Advances in multi-photon processes and spectroscopy

In view of the rapid growth in both experimental and theoretical studies of multiphoton processes and multiphoton spectroscopy of atoms, ions, and molecules in chemistry, physics, biology and material sciences, this series contains review papers exploring the growth. They are readable not only by active researchers in these areas but also by those who are not experts in the field but intend to enter the field.

Density matrix method and femtosecond processes

This book originates from lectures given at Shandong University and Nanjing University and the special topics course offered at Arizona State University. It is prepared at a level intended for chemistry and physics graduate students.

C/B codoping effect on band gap narrowing and optical performance of TiO2 photocatalyst: a spin-polarized DFT study

The electronic and optical properties of several possible C/B-codoped models of anatase and rutile TiO2 have been investigated systematically using spin-polarized density functional theory calculations. Our calculated results show that the synergistic effect of C/B codoping can induce some hybridized energy states appearing in the forbidden gap and the band gap has a narrowing in anatase and rutile TiO2, which can lead to a decrease of the photon excitation energy and an obvious red-shift of the optical absorption edge. These results lead to an excellent photocatalytic activity in C/B-codoped TiO2. Moreover, with the increase of C and B impurities’ concentration in anatase and rutile TiO2, we find that the intensity of impurity states has a strengthening in the band gap, the position of impurity states changes, and the visible-light absorption performance improves gradually.

Theoretical design of polythienylenevinylene derivatives for improvements of light-emitting and photovoltaic performances

Poly(thienylene vinylene) (PTV) is a low-bandgap polymer but shows poor performance in both light-emitting and photovoltaic applications. Recently a derivative of PTV with carboxylate substitution, poly(3-carboxylated thienylenevinylene) (P3CTV), has been synthesized and was shown to be fluorescent. The photovoltaic power conversion efficiency based on P3CTV has been found to be much larger than PTV, indicating an intrinsically concomitant relationship between light-emitting and photovoltaic properties. Employing quantum chemistry calculations coupled with our correlation function formalism for excited state decay and optical spectra, we have investigated a series of side-chain substituted PTVs targeting optimal optoelectronic performance. We predict that the carbonyl substituted PTV is a strongly fluorescent polymer with low bandgap, long exciton lifetime, and large spectral overlap between emission and absorption. It is expected that carbonyl PTV is a promising light-emitting and photovoltaic polymer. Methodology wise, we find that (i) our correlation function approach to calculate the optical spectrum has much lower computational scaling with respect to the system size than the conventional method; (ii) the harmonic oscillator approximation for the nonadiabatic decay works better for a large system than for a small system.

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.

Coherent Ring Currents in Chiral Aromatic Molecules Induced by Linearly Polarized UV Laser Pulses

In recent years, laser control of electrons in molecular system and condensed matter has attracted considerable attention with rapid progress in laser science and technology [. In particular, control of π-electron rotation in photo-induced chiral aromatic molecules has potential utility to the next-generation ultrafast switching devices. In this paper, we present a fundamental principle of generation of ultrafast coherent ring currents and the control in photo-induced aromatic molecules. This is based on quantum dynamics simulations of π-electron rotations and preparation of unidirectional angular momentum by ultrashort UV laser pulses properly designed. For this purpose, we adopt 2,5-dichloro [(3,6) pyrazinophane (DCPH) fixed on a surface, which is a real chiral aromatic molecule with plane chirality. Here π electrons can be rotated along the aromatic ring clockwise or counterclockwise by irradiation of a linearly polarized laser pulse with the properly designed photon polarization direction and the coherent ring current with the definite direction along the aromatic ring is prepared. This is contrast to ordinary ring current in an achiral aromatic ring molecule with degenerate electronic excited state, which is prepared by a circularly polarized laser [2]. In this case, π electrons rotate along the Z-axis of the laboratory coordinates, while for the present case electrons rotate along the z-axis in molecular Cartesian coordinates. It should be noted that signals originated from the coherent ring currents prepared by linearly polarized ultrashort UV lasers are specific to the chiral molecule of interest.

Density matrix method and ultrafast processes

When femto-second (fs) time-resolved experiments are used to study ultrafast processes, quantum beat phenomena are often observed. In this paper, to analyze the fs time-resolved spectra, we will present the density matrix method, a powerful theoretical technique, which describes the dynamics of population and coherence of the system. How to employ it to study the pump-probe experiments and fs ultrafast processes is described. The ππ*→nπ* transition of pyrazine is used as an example to demonstrate the application of the density matrix method. Recently, Suzuki’s group have employed the 22 fs time resolution laser to study the dynamics of the ππ* state of pyrazine. In this case, conical intersection is commonly believed to play an important role in this non-adiabatic process. How to treat the effect of conical intersection on non-adiabatic processes and fs time-resolved spectra is presented. Another important ultrafast process, vibrational relaxation, which takes place in sub-ps and ps range and has never been carefully studied, is treated in this paper. The vibrational relaxation in water dimer is chosen to demonstrate the calculation. It should be noted that the vibrational relaxation of (H2O)2 has not been experimentally studied but it can be accomplished by the pump-probe experiments.

Wolf Prize in Chemistry: An Epitome of Chemistry in 20th Century and Beyond

Bioorganic Chemistry (C Djerassi) Synthetic Transition Metal Chemistry (J Chatt) Electron Transfer Reactions (R A Marcus) Quantum Chemical Methods (J A Pople) Femtochemistry (A H Zewail) Oligosaccharides (R U Lemieux) Transition Metal Chemistry (F A Cotton) Long-Range Electron Transfers (H B Gray) Laser in Analytical Chemistry (R N Zare) and other papers.