(a) Chemical Dynamics and Molecular Spectroscopy

Under the outstanding leadership of Dr. Yuan-Tse Lee, the IAMS has become a world-renowned research institute. In particular, the chemical dynamics group is recognized as one of the leading groups in the world. One of the key factors for achievements in cutting-edge research involves the development of unique apparatuses for obtaining important experimental data. A new-generation universal crossed-molecular-beam apparatus has been designed and constructed by Xueming Yang and Jim Jr-Min Lin at the IAMS. This machine has the highest sensitivity and the lowest detector background among similar machines in the world. The machine’s most notable distinction is its ability to routinely detect and measure complicated reactions with multiple reaction pathways.  Chemical reactions of O(¹D) with saturated hydrocarbon molecules and other species have been investigated in detail using this machine.  Many triple product channels have been experimentally observed for the first time, which open enormous opportunities for dynamical studies on complicated chemical reactions.

In addition, a new crossed-molecular-beam apparatus based on photoionization using VUV synchrotron radiation or lasers has also been built by Xueming Yang and Jim Jr-Min Lin.  The main purpose of this project is to utilize the third-generation synchrotron radiation as a soft ionization source to study chemical reaction dynamics. Significant improvements have been achieved for this new end-station over a similar apparatus at the Advanced Light Source in the Lawrence Berkeley National Laboratory. Photodissociation dynamics of a few interesting molecular systems have been studied by using this end-station. There is also a great potential for crossed-molecular beam dynamical studies of complicated reaction dynamics using this apparatus.

Recently, a state-of-the-art Rydberg-tagging TOF apparatus has been developed by Xueming Yang for the study of reaction dynamics and elementary chemical reactions at the state-to-state level. Many interesting dynamical effects have been observed in the photodissociation of H₂O from the B surface at 121 nm. Quantum interference in a chemical reaction process through dissimilar conical intersections has been identified, which resembles the famous Young’s double slit experiment.  Other interesting dynamical phenomena observed include  single rotational product propensity in the HOD photodissociation and the production of highly rotational-excited OH products from HOD photodissociation.

Rotational-state resolved differential cross sections for the important O(¹D) + H₂ ® OH + H insertion reaction have been successfully measured for the first time. Studies of the reactions of O(¹D) with HD and D₂ have provided clear experimental evidence that the abstraction pathway at higher collisional energy in this reaction is due to a collinear abstraction mechanism. Recent work on the effect of single quantum rotational excitation on the state-to-state dynamics of the O(¹D) + H₂ reaction has provided a classic example on the rotational excitation effect in this important experiment. These experimental results provide an excellent test ground for quantum dynamical investigations on this benchmark insertion reaction.

The crossed-molecular-beam experiments performed by Kopin Liu with detailed quantum dynamical analyses have provided unambiguous observation and elucidation of dynamical resonance phenomena in chemical reactions.  In the F + HD reaction, a reactive resonance localized in the transition-state region is found to be responsible for the observed remarkable features in the integral as well as differential cross-section measurements. At collision energies below 1 kcal/mol, the reaction proceeds almost exclusively through resonant tunneling with very little contribution from the more conventional direct mechanism. In the Cl + HD reaction, a strong preference for the production of DCl is seen in crossed-molecular-beam experiments.  Exact quantum mechanical calculations of reactive scattering on a new ab initio potential energy surface with van der Waals forces support this experimental finding. The study of chemical reaction dynamics has now advanced to the stage where even comparatively weak van der Waals interactions can no longer be neglected the calculations of the potential energy surface of a chemical reaction. Recently, in collaboration with Jim Jr-Min Lin, a direct three- dimensional velocity-imaging technique has been developed and provides the first experimental demonstration of state-resolved, pair-correlated angular distribution for a polyatomic reaction.  This type of correlation information holds great promise to open up a new way to unravel the ‘extra-atom’ complexity of chemical events of a complex system.

To study photofragment translational spectroscopy, a new method, which is called multimass ion imaging technique, has been developed by Chi-Kung Ni. The major advantage of this technique is that it can record the translational energy spectra of various photo-fragments simultaneously. Photodissociation dynamics studies have been conducted for several aromatic molecules, including benzene, fluoro-substituted benzene, alkylbenzene, fluoro-substituted alkylbenzene, picoline and azulene.  The experimental results of toluene show that the carbon and hydrogen atoms belonging to the alkyl group are involved in an exchange with those atoms in the aromatic ring during isomerization.

Studies concerning competing dissociation of chlorofluoroethylenes have been conducted by Shen-Maw Lin.  When these molecules are radiated at 193 nm, the dissociation processes include the cleavage of the C=C and C-Cl bonds.  The branching ratios of these channels have been determined. In addition, when the C-Cl bond fission proceeds by two distinct reaction routes, the resulting radicals are highly unstable and dissociate into a pair of secondary products. In contrast, 1,1-dibromoethylene undergoes an asynchronous concerted reaction to produce two Br atoms at unequal velocities. The measured anisotropy indicates that both weak C-Br bond ruptures occur in a fraction of a rotational period.

To probe the properties of protonated cluster ions, an ion-trap tandem mass spectrometer in combination with a pulsed infrared laser system has been utilized by Huan-Cheng Chang. The isomeric structures of protonated water clusters H⁺(H₂O)_n in a molecular beam have been investigated by vibrational predissociation spectroscopy and ab initio calculations. The results prove that the gaseous H₅O₂⁺ ion can indeed survive in a symmetric environment. In addition, the important proton transfer intermediate H₅O₂⁺(H₂O)₄ has been identified for the first time from its characteristic bonded-OH stretching absorptions at 3178 cm^-1. This fundamental study has led to a better understanding of proton transfer reactions not only in gas-phase water clusters but also in aqueous acids and in relevant biological systems as well.

Concerning the detailed cationic properties of molecular species, Wen-Bih Tzeng has developed a two-color two-photon-resonant mass-analyzed threshold-ionization (MATI) spectroscopic method. This species-selected state-specified and mass-analyzed technique is very suitable foe studying spectroscopy and dynamics of molecules, radicals, clusters, and various isomers. Although MATI spectroscopy is an ideal approach for recording high-resolution ion spectra, there are still less than 15 research groups in the world who employ this technique due to the complexity and sophistication of the apparatus.  Since its introduction in 1991, less than 70 experimental research papers (till Nov. 2002) have been published using this sophisticated high-resolution spectroscopic method to probe the detailed properties of molecular cations. After developing this method in 1999, 12 papers in the major international journals have been published. In addition to providing new experimental data, these results allow us to investigate (1) the site-specific electronic transition, (2) the isotope effect, (3) the vicinal substitution effect, (4) the long alkyl chain effect, (5) the heavy atom effect, and many other properties of aromatic molecules. Because these experiments give very precise ionization energy and new data on cation vibration, all publications are cited in the world-wide internet ZEKE-web site (http://www.zeke.org) and the National Institute of Science and Technology (http://webbook.nist.gov).

Various laser spectroscopic techniques have been used to study neutral species, radicals and van der Waals complexes. In particular, the laser-induced fluorescence (LIF) technique has been utilized by Yen-Chu Hsu to probe the nascent product-state distribution in the 193.3-nm photolysis of acetylene. The results lead to information about the energy level structure of the bending vibration of the CCH radical in the ground electronic state. In addition, the geometry, ro-vibrational structure, and predissociation processes of its electronic excited state,, have also been determined for the first time. In the study of C₃-Rg (Rg, rare gas atom) van der Waals complexes, the C₃-bend levels up to 900 cm^-1 above the zero-point energy have been observed. A perturbation model has been developed to interpret the experimental findings. This model suggests that the Ar or Kr atom prefers to stay in the plane of bent C₃, but not the Ne atom. The latter molecule is classified as an intermediate case between a free C₃ and an in-plane complex. In order to obtain rotationally-resolved spectra of larger polyatomic molecules, two near Fourier transform-limited laser systems have been constructed by Chi-Kung Ni.  The first one is a CW Ti:Al₂O₃ master oscillator/dye preamplifier/ Ti:Al₂O₃ power amplifier system that generates laser pulses with a duration of <2 ns and a pulse energy of 100 mJ with a tunable range of 750 - 890 nm, a repetition rate of 30 Hz, and an output bandwidth of about 200 MHz. The second system is similar to the first one except that it has a duration of 4 ns, and an output pulse bandwidth of 275 MHz. This apparatus has also been used to successfully record the high-resolution LIF spectra of biacetyl in A¹A_u– X¹A_g, yielding information about the interaction of two torsional motions.

Yuan-Pern Lee (jointly appointed with NTHU) leads a group which is the first to establish time-resolved Fourier-transform spectroscopy (TR-FTS) in the absorption mode for gas phase studies. They have successfully demonstrated the advantage of this technique with the observation of reaction intermediates such as ClCO, CH₄ (n₂ or n₄=1), ClSO, and cis-HOCO. They also pioneered the application of resonant four-wave mixing to spectroscopic studies of the highly predissociative B, C, and D electronic states of CH and the n₁, n₅, and 2n₆ vibrations of the A state of CH₃S. With the selective laser photolysis and matrix isolation techniques, they produced and characterized several novel species, such as cyclic-CS₂, cyclic-S₂O, SOO, OSOO, and ClSO₂ which are important reaction intermediates in atmospheric and planetary chemistry.  In addition, they have used a synchrotron radiation source to determine the cross sections of H₂O, HCl, C₂H₄, CH₃OH, and their isotopomers to provide information for studying isotopic disproportionation in planetary chemistry and for characterizing their Rydberg states. Recently, a second-generation diaphragm- less shock tube coupled with laser photolysis was installed to study chemical kinetics at high temperatures.

Tzu-Min Su (jointly appointed with NTU) has conducted absolute asymmetric synthesis of chiral compounds through macroscopic translation-rotational motions.  The optical rotamers of the alkanes are generated in the gas phase and detected by an ultra-sensitive polarization detection system. Although some optically active phenomena have been understood qualitatively, more quantitative theoretical and computational efforts are underway. A new experimental system is being set up for single molecule detection experiments. The new project is aiming at the preparation of the desired bio-samples. King-Chuen Lin (jointly appointed with NTU) leads a group to investigate the reaction dynamics of excited alkali metal atoms (Li, K, Mg, Ca) with H₂ and CH₄. The information obtained from these experiments includes product energy disposal, state reactivity, and reaction mechanism.  Recent, they have designed a laser-induced breakdown technique incorporating an electrospray needle to detect trace metal contents in the sprayed droplets.  Yit-Tsong Chen (jointly appointed with NTU) has investigated the Rydberg states and ionization of propyne, allene, vinyl chloride, allyl-h₅ radical, allyl-d₅ radical, and acetyl radical in the gas phase. Another project concerning the electronic properties, surface structures, defects, exciton and phonon dynamics of silica-based nanoparticles and nanopores has been undertaken. Silicon nanowires and carbon nanosprings have been fabricated and characterized by photo- luminescence, micro-photoluminescence, and micro-Raman spectroscopic methods.

(B) Surface Science

The Surface Sciences Group was started in 1991 with selected research topics including surface nanostructures (Yuh-Lin Wang), surface knintics and dynamics (Jiing-Chyuan Lin), and adsorbate induced self organization (Ker-Jar Song). Surface chemistry and spectroscopy (Tung-Jung Chuang), and advanced- and nano-materials (Kuei-Hsien Chen) were added in 1993. These plus the subsequent joint appointment of Woei Wu Pai (SPM for surface morphological evolution) and Chia-Chun Chen (nanomaterials synthesis and surface modification) have made the surface science group one of the most competitive teams in the field of surface sciences and nano-structural materials.

Yuh-Lin Wang is the first to discover surface magic clusters (SMC) and has carried out pioneering work on focused ion-beam (FIB) related research.  The discovery of magic clusters on a Si surface has opened up the field of SMC, an analogy of fullerenes in three-dimensional space, and self-organized two-dimensional lattices of SMC thus produced.

Dr. Wang has been conducting both experimental and theoretical studies to determine the precise atomic structure of the lattice of SMC. Meanwhile, he is also exploiting FIB for the fabrication of various nanostructures including the pre-patterning of aluminum surfaces to form arrays of long-range ordered anodic alumina (AAO) nano-channels (in collaboration with K. J. Chao, NTHU), and Ga⁺ ion-implanted GaN nanowires (in collaboration with K. H. Chen, IAMS). These approaches combine the power of top-down nanolithography and bottom-up self-assembly, and therefore have high potential for the creation of nanomaterials with a unique functionality.

Jiing-Chyuan Lin’s interest is in surface structure and surface chemistry of silicon, germanium, and diamond.  His work on hydrogen bonding and its effects on the structure of diamond surfaces have contributed to much understanding in diamond surface chemistry. Detailed FTIR spectra of the C-H bonding were resolved at different temperatures and led to the understanding of surface vibrational dephasing dynamics on the surfaces (in collaboration with J. K. Wang, NTU). Subsequent studies of the IR spectra of nanocrystalline diamond particles (in collaboration with H. C. Chang, IAMS) have led to better understanding of interstellar diamond dust in space.  Meanwhile, Dr. Lin has extended his research to surface photochemistry via the newly implemented low-temperature scanning tunneling microscope (LT-STM).  The interfacial structures of trans-stilbene and cis-stilbene on Ag/Ge(111) surface were molecularly resolved and led to the observation of UV stimulated paired cis-trans isomerization and to the understanding of its mechanism. This work opened up the study of surface photo-isomerizations utilizing molecular- resolved STM techniques and led to much detailed understanding of their mechanisms.

Kuei-Hsien Chen started as a thin film developer in the IAMS. His current work is on the synthesis, characterization, and application of light covalent bonded materials such as diamond, SiCN, BCN, and III-Nitride compound semiconductors.  He also extended his field of research to the synthesis and functionalization of nanomaterials, e.g., carbon nanotubes, Si, GaN and InN nanowires. His pioneer work in diamond synthesis has led to the observation of the quantum confinement effect of nanocrystalline diamonds. He is also the first to discover SiCN, a wide bandgap, super-hard compound semiconductor (in collaboration with L. C. Chen, NTU). This work has led to potential applications in high temperature hetrojunction diodes and UV photoconducting diodes (Y. K. Gang, NCKU).  Meanwhile, Dr. Chen is currently conducting a team effort to investigate the growth, property and applications of one-dimensional nanomaterials.  Selective area growth of well-aligned carbon nanotubes (CNTs) for designated applications, including field emission, gas sensing, Li⁺ ion storage, and molecular electronics, can be routinely performed from the investigations. 

The integration of CNTs with thin-film transistors (TFT), in particular, has demonstrated highly stable electron beam source for many applications in vacuum electronics (in collaboration with H. C. Cheng, NCTU).  Moreover, the studies of III-Nitride nanowires showed interesting optoelectronic phenomena different from their thin film counterparts. Many important issues, e.g., superconductivity of CNTs (in collaboration with C. T. Chen, Academia Sinica), nanometer scale fabrication of GaN nanowires utilizing FIB (in collaboration with Y. L. Wang, IAMS), size-control and selective-area growth of nanowires (in collaboration with C. Y. Mou, NTU) have been realized.  

The major area of interest for Ker-Jar Song is the control of the morphology and of the growth mode of ultrathin-film-covered systems. His work on the reversible phase transitions between faceted and planar forms of adsorbate covered W(111) surfaces nicely demonstrates the rich possibility in adsorbate-induced self-organization of the surface. In particular, the Pd-induced faceting of W(111) surface has later been utilized by others to reproducibly prepare well-defined atomically sharp tips, which is crucial for scanning tunneling microscopy. More recently he has been studying the "interfactant" assisted epitaxy. With his own temperature- programmed Auger instrumentation, he has found that the strong tendency of potassium to agglomerate on W(111) can be suppressed by introducing a submonolayer of Ag at the interface. He proposes that Ag acts like an anti-reflection coating for the electrons in the potassium film, and it is the "de-confinement" of the electronic states that makes the planar form the most stable one. The "interfactant assisted epitaxy" could be applied to and should be of practical value to the making of photocathodes. Most recently, he has started a new effort to create superstructures with large unit cell and growth of magnetic nanostructures based on such templates (in collaboration with M. T. Lin, NTU).

The most senior member of the group, Tung-Jung Chuang (jointly appointed with NTU), focuses primarily on surface chemistry and spectroscopy. Basic surface chemical processes and potential applications, with particular emphasis on nanoscopic phenomena, have been explored utilizing full-fledge surface science tools such as XPS, AES, TDS, LEED, HREELS and excimer lasers, which contributed to basic understanding of primary steps and mechanisms involved in the hydrocarbon surface chemistry relevant to heterogeneous catalysis and deposition of carbon-based materials.  Meanwhile, in a major efford, Dr. Chuang has led a team to build a scanning photoemission spectromicroscopy (SPEM) end-station at SRRC in Hsinchu, one of the most advanced facilities in the world.  The facility has started to yield excellent results on surface microchemistry (down to 100-nm resolution) such as the oxidation of Si₃N₄ (in collaboration with S. C. Gwo, NTHU) and the electronic structure of CNTs (in collaboration with L. C. Chen, NTU and W. F. Pong, Damkang University).  Moreover, efforts including STM/AFM analyses and chemical manipulation of atoms and molecules on Si, diamond and metal surfaces, as well as SPEM characterization/fabrication of ultrafine structures are underway to elucidate the heterogeneous reaction mechanisms relevant to catalysis and materials processing and to promote nanotechnology-related skills.

The joint appointment of Woei-Wu Pai (jointly appointed with the Center for Condensed Matter Sciences, NTU) and Chia-Chun Chen (jointly appointed with the Chemistry Dept., National Taiwan Normal University) significantly extended the scope of the Surface Science Group.  W. W. Pai has established a state-of-the-art temperature-controlled UHV STM system enabling time-sequenced measurements to study the dynamical behavior of 2D clusters.  The system is equipped with a mini molecular beam epitaxy (MBE) system that allows controlled growth of a series of heterostructures and explore specific issues including (1) diffusion, coarsening, and reshaping of nanoclusters, (2) adsorption structure of radicals on metal surfaces and associated electronic screening, and (3) morphological instability of strained superlattice and its application in optoelectronics.  C. C. Chen focuses on solution synthesis and CVD of semiconductor nanomaterials such as Au and CdSe quantum dots, GaN and GaP nanowires.  He has extended his research to potential application of these nanomaterials in bio-labeling, diagnostics, and assays, allowing studies of the binding/interaction between peptides/carbohydrates functionalized nanoparticles and specific receptors.

(C) Theoretical Atomic and Molecular Sciences

Sheng Hsien Lin in collaboration with Alexander M. Mebel have undertaken the theoretical work concerning the Duschinsky effect on molecular photophysical processes. Non-adiabatic photophysical processes like absorption, emission, radiationless transitions, electronic energy transfer, electron transfer etc. involve the potential surfaces of at least two different electronic states. The Duschinky effect appears in the case where the molecular symmetries of these different electronic states are different causing mode-mixing in the observed spectra. Theoretical treatment has been accomplished to successfully interpret the extraordinary bandwidths resulting from the p®p* transitions of ethylene and acetone, which had puzzled the spectroscopic community for a number of years. A technique has been developed to take into account the normal mode mixing (rotation) in the excited electronic states to address the role of the Duschinsky effect.  The calculated wavefunctions are also used to determine matrix elements for vibronic coupling, non-adiabatic coupling, and spin-orbit coupling between different electronic states. The coupling elements are utilized in the calculations of intensities of forbidden transitions in vibronic spectra and for computations of the rate of internal conversion and intersystem crossing, relevant to the photodissociation mechanisms. Branching ratios of electronic states of photodissociation products can be also addressed and evaluated based on ab initio calculations of excited state PES and vibronic spectra.

Ab initio/RRKM calculations have been performed by Alexander M. Mebel  to investigate gas phase chemical reactions. The results provide accurate data on the geometric and electronic structure, vibrational frequencies, rotational constants, and relative energies for all possible reaction intermediates and transition states. These data are then used for the RRKM and TST calculations to predict the rate constants of various reaction steps as well as the product branching ratios. The method has been applied to study numerous reactions of interest in combustion, atmospheric and interstellar chemistry, including reactions of vinyl, phenyl and other radicals. This ab initio/RRKM theoretical approach combining ab initio and kinetic calculations from the first principles has become a widely used tool in the literature.

Theories have been developed by Sheng Hsien Lin to provide selection rules, band-shapes, temperature effect and other possible interpretation for the findings from sum-frequency generation (SFG) experiments, which may be divided into four classes, i.e, resonant/off-resonant, off-resonant/resonant, off-resonant/ off-resonant, and resonant/resonant processes. The resonant-off/resonant IR-UV SFG spectra of acetone have been analyzed and the associated molecular dynamics has been derived. For the resonant-resonant IR-UV (visible) SFG method, which is a new experimental technique and shown to be closely related to infrared and resonance Raman spectroscopic techniques, the theory has been successfully used to analyze the doubly resonant IR-Vis SFG spectra of Rhodamin 6G on fused silica.

Theoretical work has been conducted to account for the ultrafast dynamics phenomenon and spectroscopy of bacterial photosynthetic reaction center (RC). The photosynthetic RC can initiate a series of electron transfer (ET) reactions subsequent to energy transfer events. The initial ET process occurs from the special pair (P) to bacteriopheophytin (15 Å apart) in 1-4 ps, and exhibits an inverse temperature dependence. The absorption spectra of RC and the temperature effect have been analyzed in detail. In addition, the femtosecond time-resolved spectra have been calculated to determine the mechanism of initial ET and the role played by various vibrational modes to interpret the observed quantum beat.

Theoretical work related to biophysical and biochemical problems have been performed by Dah-Yen Yang. To study the electron transfer mechanism, a bi-functional model has been proposed to address the charge conductivity of polypeptide. The theoretical part contains the stochastic motion of a virtual Brownian particle moving inside the Ramachandran plot between an amino acid pair, and illustrates its escape process.

This model is further proven by molecular dynamic simulation that shows exact agreement with the experimental distance decaying factor. In addition, the one-gate escape theory has been developed to account for the uni-direction flow nature for investigating the ligand escape process in the myoglobin-ligand system. Since this new theory shows good agreement with Frauenfelder’s experimental results, it is further extended to the two-gate and channel model. As a result, selectivity becomes a major factor in dominating the escape pathway due to the gate size and gate modulation asymmetry. For an ion channel with fluctuating gates, a sequential connected blob model is proposed. The theory predicts that the entire uni-directional flow process does not rely on any external applied fluctuation forces and is entropy driven. 

Keh-Ning Huang has carried out theoretical investigations on the spin polarization and angular distribution correlations of photoelectrons in photoionization processes. Asymmetric and spin polarization parameters of photoelectrons from some neutral atoms were obtained by using the relativistic random-phase approximation (RRPA). These parameters in the energy range near the first ionization threshold are very sensitive to valence-core correlations. The results are in good agreement with recent experimental measurements. A complete kinematics analysis of radiations of the residual ion after photoionization has been carried out for polarized incident photons on polarized targets. Photoionization of polarized atoms has been analyzed kinematically. Future experiments on polarized targets were suggested. Also, accurate autoinozation resonance positions and widths for singly- and doubly-excited states of various atoms were deduced from theoretical photoionization spectra. 

Collision processes of charged particles with atoms has been under investigation by K.-N. Huang. Double differential, single differential, and total cross sections for electron-impact and positron-impact ionization of hydrogen-like, helium-like, and lithium-like ions have been calculated.  Cross-section universal curves have been deduced for these ions. These detailed systematic studies have broadened our understanding of the Z-dependence, the relativistic effects, and the difference between isoelectronic sequences. The near-threshold cross sections of the electron-impact ionization of the hydrogen atom has been parametrized by the power g and proportionality constant a₀ for models with various asymptotic charges.  Polarization correlations of radiations from electron-impact excited atoms have been analyzed, and physical interpretations of the five independent dynamic parameters in the electric dipole approximation were also given.

In recent years, Yew Kam Ho has carried out theoretical studies of atomic resonance phenomena using the method of complex-coordinate rotation.  The results on the doubly-excited ²S^e resonance in e^--He scattering can be treated as standard references for other works.  Studies of interactions of positrons and positronium with atoms were also carried out. By employing highly-correlated Hylleraas wave functions, Y. K. Ho (in collaboration with Z.-C. Yan, University of New Brunswick, Canada ) has calculated different partial wave resonances in Ps-H scattering.  Doubly-excited states (L8) of positronium ions were also calculated. The results play an important role in studies of symmetries in three-body atomic systems, and reveal supermultiplet structures for the doubly-excited intra-shell states of Ps^-, exhibiting ro-vibrational characters very much similar to those of a linear X-Y-X tri-atomic molecule.  Also, for the first time in the literature, high-angular- momentum resonances in positron-hydrogen and e⁺-He⁺ scattering have been identified. Using the stabilization method, Y. K. Ho and coworkers have calculated the energy positions and widths for resonances in e⁺-Li scattering, and the doubly-excited resonance states in four-electron systems such as Be and Be-like ions.  The complex absorbing potential (CAP) technique has been applied to investigate the electric-field effects on Li.

The CAP method is a new and effective method to study resonance phenomena. This method has also been used to study resonances in nuclear physics problems with short-range potentials, as well as electric-field effects on quantum-confined hydrogenic impurities in a spherical quantum dot.

 The combined electric- and magnetic-field effects on Li have also been investigated.  Y. K. Ho and coworkers used the method of complex-coordinate rotation to investigate field effects (electric, magnetic, and the combined fields) on the doubly-excited states of H^- and Ps^-. Studies for the electric-field effects on the Feshbach and shape resonances of H^- were also performed. The theoretical results were used to interpret the experimental observations carried out in the Los Alamos Meson Physics Factory (LAMPF) in the USA.  Calculations for the electric-field effects on the photoionization cross sections on He in the energy region with double electron excitation have been carried out (in collaboration with T.-K. Fang, Fu Jen Catholic Univ.). This work has motivated researchers in the Photon Factory, KEK in Japan, to perform experiments to investigate such electric-field effects on doubly-excited He.

(D) Condensed Matter Science

Two PIs, Shang-Bin Liu and Lian-Pin Hwang, focused on nuclear magnetic resonance (NMR) based research and related topics. S. B. Liu carried out systematic studies of porous materials such as microporous zeolites, silico-/alumino- phosphates and mesoporous molecular sieves utilizing the NMR facility. Issues such as coking, deactivation, regeneration, and product selectivity of porous catalysts have been investigated. In addition, a laser-polarized setup has been facilitated to produce hyperpolarized ¹²⁹Xe (in collaboration with C. Y. Mou, NTU), which enhanced nearly four orders of magnitude the ¹²⁹Xe signal and extended NMR study to a new era. Many interesting phenomena in nanomaterials and mesoporous materials can thus be studied.  Recently, S. B. Liu developed a new methodology utilizing solid-state ³¹P MAS NMR of adsorbed phosphine oxide probe molecules to quantify the acidic properties of porous catalysts.

Meanwhile, L. P. Hwang has developed new models to study the surface dynamics of solids using NMR relaxation mechanisms. The topics explored include NMR relaxation and inversion-recovery of biological systems, polymeric composite material, porous material, colossal magnetoresistance material, carbon nanotubes and carbon nano-onions.

To integrate laser spectroscopy with biotechnology, Ta-Chau Chang has developed non-resonant hole burning spectroscopy and combined it with other spectroscopies to serve as spectroscopic probes to determine the binding modes of dyes to various DNA structures, to identify different conformational structures of dye-DNA complexes, and to diagnose the protein-DNA recognition.

The results indicated that a labeling dye could influence the DNA structure, and perturb the DNA-protein recognition, providing a new direction for drug development and new channels for drug delivery. Based on the dye-DNA interaction results, he has attempted to develop new compounds for stabilizing the quadruplex structure of d(T₂AG₃)₄, the sequence of human telomere, for tumor inhibition study. One of the new compounds (20 uM) incubated with human epithelial cancer cells and human peritoneal fibroblasts normal cells for 24 hrs can inhibit their cell growths by 90 % and 45 %, respectively.  In addition, T. C. Chang has constructed apparatuses to perform single molecule spectroscopy and fluorescence microscopy which are aimed to measure fluorescence lifetime and triplet lifetime of individual molecules and to examine molecular diffusion and drug delivery.

Wunshain Fann has developed a sub-micron spectroscopic instrument of spatial resolution better than 100 nm and applied it for various problems in condensed matter physics. By combining laser spectroscopy with near-field optics, the instrument will find broad applications in nanometer sciences and technology. Meanwhile, W. Fann studied conjugated polymers that have both the plastic's flexibility and a semiconductor's electrical and optical properties. He is especially interested in the electronic properties of light emitting polymers and their relations to small organic molecules, and the interplay between condensed matter physics and organic chemistry. Moreover, W. Fann has also worked on single-molecule spectroscopy and biophysics to address issues in nanotechnology and protein folding, respectively.

(E) Optical Science and Technology

The Optical Science and Ultrafast Technology Laboratory under the leadership of Jyhpyng Wang and Szu-yuan Chen has constructed a 10-TW laser system that produces 55-fs, 550-mJ pulses at 10-Hz repetition rate to facilitate frontier research in high-field physics.  In collaboration with Chau-Hwang Lee, IASE, Academia Sinica, and Jiunn-Yuan Lin, NCCU, the laser is being used for research in tabletop X-ray lasers and laser wake-field accelerators.

Owing to their extremely large acceleration gradient, laser wake-field accelerators hold the promise of breaking the current limits in accelerator technology.  As a first notable step, relativistic self-channeling in plasma has been achieved, which guides the propagation of an optical pulse to break the limit of interaction length set by the Rayleigh range.  A collimated electron beam emitting from the plasma channel, with an energy currently limited by the phase-matching range of the plasma wave, has also been observed recently.  In searching for the X-ray lasing conditions, it is found that strong emission between 11-16 nm with a conversion efficiency approaching 10% can be obtained from a laser-driven nanoplasma formed by ionized Ar clusters.  The soft X-ray thus produced will be applied to the development of X-ray microscopy and lithography.  With the capability of coupling optical fields through laser-driven plasma wave, the group is also working on the first demonstration of plasma-wave high power optical amplifier that will not be limited by material breakdown.

This group has also developed differential confocal microscopy, a far-field technique with nanometer depth resolution. Combined with laser based optical pressure as a driving force, this advanced technique is used to measure the viscoelastic properties of living cells and biomembranes. It can also be used for rapid online inspection of semiconductor surfaces and optical disks.  Furthermore, in a separate endeavor, J. Wang has developed a theory of quantum integration by non-unitary canonical transformation and its application in solving the Schroedinger equation systematically with a unified algebraic method.

In the high-resolution laser laboratory, Andy Kung and Chi-Kung Ni have developed two pulse-amplified tunable lasers with high power transform-limited nanosecond pulses of over 20MW peak power and wavelength coverage from the near infrared to the deep ultraviolet.  The laboratory is also equipped with an excimer laser and a high vacuum experimental set-up that allow the execution of photodissociation and spectroscopic measurements using pulsed molecular beams probed by LIF, TOF-MS, and imaging techniques.  The facilities in this laboratory enabled the first demonstration of complete suppression of spontaneous amplification in high power pulsed laser systems, full rotational analysis and quantum beats of excited states close to the dissociation threshold in triatomic and medium size molecules, first observation of extremely stable states with ultra-narrow resonances in doubly-excited Mg atoms several eV above the first ionization threshold, and extension of high-resolution laser spectroscopy to the study of molecules which contain two torsional motions.  Using quantum beat finger-print identification, level assignment in the complex bands of biacetyl were made.  The band origin, rotational constants, and the splitting due to the tunneling in these three bands were accurately determined.

The development of a novel multimass ion imaging technique also took place in this laboratory.  Recently, transform-limited pulses of tunable radiation from 185 nm to 200 nm were generated in a solid-state set-up.  The approach has the potential to reach 165 nm, allowing molecular dissociation dynamics to be studied in the deep UV region.

A method for efficient fourth and fifth harmonic generation of medium power diode-pumped solid-state lasers was developed by Andy Kung.  The utility of the device has been demonstrated in an UV photochemical etching application.  In another development, a compact narrowband IR optical parametric oscillator using the quasi-phase-matched material PPLN was built, and applied to trace gas detection in a collaboration with Prof. Peter Hess of Heidelberg University, achieving detection sensitivity of 1 ppbV for methane and 10 ppbV for ethylene in a simple portable system which can be suitable for environmental monitoring and pollution emission control.  The OPO was also used to demonstrate for the first time its application with synchrotron radiation sources in two-photon photoionization studies of atoms and molecules (collaborator Cheuk Ng of UC Davis).  This opens up the use of synchrotron VUV sources in probing prohibited transitions.