Institute of Atomic and Molecular Sciences

In July 1982, during the Fifteenth Meeting of members of Academia Sinica, fifteen members led by Dr. Y. T. Lee recommended that an institute of atomic and molecular sciences be founded. This recommendation was unanimously accepted and subsequently endorsed in the Sixth Meeting of the Eleventh Council of Academia Sinica.

After the case was approved, a preparatory office was inaugurated on September 23, 1982 to lay the foundations for the new institute. In the meantime, an advisory board, headed by Dr. Y. T. Lee, was organized to provide guidance on its future development. On December 11, 1982, Dr. C. T. Chang was appointed director of the preparatory office. From July 1993 on, Dr. S. H. Lin has been the director of the Preparatory Office. The Institute of Atomic and Molecular Sciences was formally established in April 1995. There are five research groups in the Institute, namely, Molecular Reaction Dynamics, Theoretical Atomic/Molecular Sciences, Condensed Matter Sciences, Photochemistry and Molecular Dynamics, and Surface Science.

RESEARCH

GAS PHASE REACTION DYNAMICS GROUP

Gas Phase

The gas phase dynamics group focuses on the study of reaction dynamics of simple transient species on molecular level. Research areas include electronic, vibrational and rotational energy transfer, state-to-state molecular dy-namics, reaction kinetics and mechanism, and cluster formation and photodissociation. Experimental techniques involve molecular beams, lasers and mass spectrometers. Several specific subjects have currently been explored:

• investigation of highly vibrationally excited products of chlorine atoms after photodissociation and subsequent recombination,
• reaction of alkaline earth atoms with hydrogen mole-cules,
• vibrational energy redistribution of internally excited cyanide radicals,
• reaction of oxygen atoms with fluorocarbons,
• photodissociation of molecular and ionic clusters, and
• fragmentation of simple neutral molecules.

THEORETICAL GROUP

Ultrafast Laser Laboratory

Research may be divided into three general areas: atomic physics, molecular physics and physical chemistry. Atomic physics focuses on the development of new many-body dynamic theories and on the investigation of atomic collision processes. Projects include atomic resonance phenomena, collisions of charged particles (e.g., electron, positron, photon, etc.) with atoms, photonionization of atoms, spin-polarization and angular correlations in electron-atom collisions, and compilation of spectral data. Molecular physics is concerned with vibration-rotational spectrometry of diatomic and small polyatomic molecules. The physical chemistry research covers ultrafast biological electron transfer, nonlinear optical processes, optical mode vibration coherence created with a fs laser pulse, solvent dynamics effects of nonadiabatic electron transfer reaction.

CONDENSED MATTER GROUP

• Nuclear Magnetic Resonance
  Current research can be divided into three major directions:
  1. NMR relaxation studies on the dynamics of molecular aggregates in solutions.
  2. NMR studies on the relaxation of multiple quantum and spin orders in solutions.
  3. Relaxation studies on the molecular dynamics of sorbate molecules on absorbent.
• Solid State Nuclear Magnetic Resonance
  Enhancing NMR sensitivity by optical pumping. On the physical and chemical properties of porous materials.
• Condensed Phases Laser Spectroscopy
  The study of the vibrational dynamics in mixed molecular crystals, specifically of how the guest molecules perturb the host vibrons.
  The investigation of the guest-host interaction at low temperature glasses.
  The study of the DNA-chromophore complexes.
• Near Field Optics and Light Emitting Polymers

ADVANCED LASER-OPTICS TECHNIQUES AND INSTRUMENTATION GROUP

The objectives include extension of laser frequency, enhancement of laser resolution in either the temporal or the spectral domain, and development of advanced optical instrumentation. In the extension of frequency, we employ laser pulses with high peak power and nonlinear tech-niques such as sum-frequency generation, difference-frequency generation, harmonics generation, parametric oscillation, Raman conversion, and supercontinuum generation to obtain coherent radiation from the mid-infrared to the vacuum ultraviolet. In the enhancement of resolution we use pulse compression and group-velocity-compensation techniques to obtain adjustable Fourier-transform limited ultrashort pulses, and injection seed cw single-longitudinal-mode laser into laser amplifiers to obtain Fourier-transform limited narrowband nanosecond pulses. In optical instrumentation we combine advanced light sources described above with modern optical techniques such as interferometry, holography, photon-counting correlation, laser-induced fluorescence, Fourier-transform spectroscopy, near-field optics, nonlinear mixing, saturation spectroscopy, and confocal feedback to develop new advanced experimental techniques.

PHOTOCHEMISTRY AND SPECTROSCOPY GROUP

Major research interests reside in employing modern laser spectroscopic techniques to study the chemical kinetics of free radicals of atmospherical and combustion chemical interests, the isomerizations of reactive intermediates, and the photodissociation processes of small molecules. One of the long-term goals is to advance the detailed understandings of atmospherical and combustion chemistry. The investigations of photodissociation processes of small molecules by laser spectroscopic techniques provide more detailed information about product state and angular momentum distribution. The other ultimate goal is to achieve maximum interactions between theories and experiments in molecular spectroscopy.

SURFACE SCIENCE GROUP

Surface Science Laboratory

Research has been focused on studying the physical and chemical properties, and the interactions on solid surfaces to obtain a better understanding of reaction mechanisms and control of reaction steps at material interfaces, and also to search for possible applications in advanced materials and technologies. The systems of interest include metals, insulators and semiconductors with particular emphasis on materials directly relevant to micro- and opto-electronics. The experimental approaches are used on chemical treatment/processing under controlled condi-tions, and structural as well as chemical analyses with advanced instruments in ultrahigh vacuum chambers. The group members have initiated joint research projects with other institutes to develop non-linear optical spectroscopy in conjunction with scanning tunneling microscopy to investigate electrochemistry as well as gas liquid-solid systems. In addition, we are carrying out collaborative research with scientists at Synchrotron Radiation Research Center and other universities to build advanced soft X-ray spectromicroscope with synchrotron light source. This will be used to fabricate, modify and probe nanometer structures in microelectronics and biological systems. The combined expertise and experimental approaches are expected to yield unprecedented clarity and insight into heterogeneous interactions.

PERSONNEL AND FACILITIES

PERSONNEL

The Institute has 2 distinguished research fellows, 12 research fellows, 14 associate research fellows, 8 techni-cians and 7 administrative personnel.

FACILITIES

Major facilities include crossed beam apparatus, high vacuum reaction chambers, ion cyclotron resonance mass spectrometer, excimer lasers, Nd:YAG lasers, argon ion lasers, colliding-pulse modelocked ultrafast laser, picosecond laser and spectrometer, dye lasers, CO\AV;2\AV; lasers, time-of-flight mass spectrometer, quadruple mass spectrometer, pulsed-field zero-kinetic photoelectron spectrometer, low temperature circulating refrigerator (4K and 10K), focused ion beam, X-ray photoemission spectrometer, Auger electron spectrometer, low energy electron diffraction, electron energy loss spectrometer, Kelvin probe, second-harmonic and sum-frequency generators, microwave plasma reactor, micro-Raman spectrometer, IBM RS/6000 workstations, 500 MHz NMR nuclear magnetic resonance, 300 MHz NMR nuclear magnetic resonance, 90 MHz NMR nuclear magnetic resonance, and scanning probe microscope. IAMS has a joint library with the Chemistry Department of NTU. The library has more than 16,000 books and 207 periodicals. The computer system in IAMS is accessible to the Internet. The electronic shop, machine shop, and glass shop in IAMS support the scientific research with instrumentation designs.

MAJOR RESULTS OF RESEARCH

MOLECULAR REACTION DYNAMICS

Molecular Beam Laboratory

• Studied detailed dynamics of simple elementary reac-tions and photochemical processes that play important roles in macroscopic systems. Molecular beams of reactants were used to study individual reactive encounters, between molecules, or to monitor photodissociative events in a collision-free environment. Most of the information was derived from measurement of the energy, angular and state distributions of the products and fragments using a pulsed cross molecular beam machine.
• Additional investigation is being made on vibrational spectroscopy and intramolecular dynamics of ions and ionic clusters using an ion trap vibrational predi-ssociation spectrometer. Research will center on the mechanisms of elementary chemical reactions involving free radicals, oxygen and hydrogen atoms with various hydrocarbons, on primary photochemistry of unsaturated organic molecules and their isomers, and on the spectroscopy, structure and function of protonated organic and biologically important molecules.
• Characterized the detailed dynamics of elementary radical reactions to provide a better understanding of chemical reactivity in general. Our experimental approach is to use photolysis to generate a cold radical beam, and then to use either LIF or REMPI technique to characterize the product state, angle distributions and their correlations under the crossed-beam conditions. We studied the detailed intramolecular dynamics of the "intermediate case" polyatomic molecules or radicals. The unimolecular processes to be studied include isomerization, tunneling, intramolecular energy redistribution, internal conversion and intersystem crossings, etc., under external field or field-free condition.
• Studied molecular dynamics and kinetics in the gas phase such as chemical reaction, nonreactive energy transfer, mechanistic processes and orbital alignment effect in the collisions of metal atoms with small molecules and photodissociation behavior of small molecules. We applied laser spectrometers to analytical chemistry such as trace analysis, atomization efficiency determination and reduction of electrical interference with signals.
• We used a molecular beam multiphoton ionization time-of-flight mass spectrometer to study intracluster reaction, cluster ion stability, and stable structure of ammonia, amines, and mixed benzene-ammonia clusters. Research subjects also include spectroscopy of aniline and other aromatic molecules and their clusters. From measured REMPI and fluorescence spectra and theoretical calculations, we can gain insights into intramolecular vibration, structure of aromatics, nature of intermolecular interaction, and structure of size-specific aromatic molecular clusters at both ground and excited electronic states.
• A rotatable beam source machine was employed to study the photodissociation process of simple molecules by translational spectroscopy. In these experiments, molecules in a beam were crossed by polarized light, and the mass, translational energy, and direction of the resulting fragments were determined by the time-of-flight technique. From these measurements of the translational energy distributions and angular dependence of the recoiling products, information on the excited state involved in the transition and energy partitioning in the dissociation process will be derived.
• We applied laser desorption ionization (LDI) and elec-trospray ionization techniques for the vaporization of ionic transition metal complexes for studying their properties in the gas phase with a magnetic ion trap also known as Fourier transform mass spectrometry (FTMS) and time-of-flight mass spectrometry.
• We have recently studied the 266 nm laser desorption dynamics of frozen aqueous solutions containing 0.01M CeCl where CeO(H₂O)n with n=0, 1, 2, ... up to more than 22 dominated the mass spectra. These experimental results could be satisfactorily analyzed by a simple microcanonical desorption model. This model is currently examined in more detail by studying the wavelength-dependence of the desorption dynamics.

THEORETICAL GROUP

Theoretical Chemistry

• More than a hundred research papers have been pub-lished in research journals in recent years.
• Recently in collaborating with experimental groups we have reported the three-dimensional experimental and calculated fs transient spectra electronictransfer in the bacterial reaction center of Rb. sphaeroides.

  We have shown that the wavelength-dependent kinetics observed experimentally can be modeled in terms of the multi-step ET mechanism described by the excitionic-vibronic coupling model developed by us.

  In collaborating with Prof. Schlag's group, we reported the matrix-assisted desorption of gramicidin D dissolved in a platinum matrix. To interpret the experimental data, we have developed a microcanonical model of desorption. That is, we have employed the RRKM theory to treat laserinduced desorption. We have calculated the effect of variation of the gramicidin D to the platinum ratio (i.e., sample to matrix ratio) at laser-desorption wavelengths of 266 and 532 nm on the laser-desorption yields. We have also proposed two "giant" optical mode vibrations created by a fs laser pulse and the other project is the pre-desorption induced by a fs UV-visible laser. We have studied the calculation of single-vibronic level rate constants of electronic processes for displaced-distorted potential sur-faces. The type of rate constants is important not only in isolated molecules but also in fs time-resolved experiments.

• We have predicted the existence of doubly excited shape resonance in e-H scattering lying above the hydrogen N=3 and N=4 thresholds. Our results have recently been confirmed by other calculations.

  We have carried out a calculation for 1P¢X autoionization states in He. Our resonance energies and autoionization widths compare well with experimental observations using synchrotron light sources around the globe. The ground state energy of positronium hydride (PsH) was calculated using elaborate Hylleraas-type wave functions. Our results for the dissociation energy into a positronium atom and a hydrogen atom, as well as the poitron-electron annihilation rate are improvements of available data in the literature. Doubly excited 1, 3 De and 1, 3D¢X states of positronium ions have recently been calculated. Results play an important role in the investigations of symmetry of atomic Hamiltonians, and electron correlation effects of three-body atomic systems.

• The most notable recent developments are the deriva-tions of novel methods to evaluate the electric and magnetic properties of molecules in particular, the electric dipole moment, the rotationalg factor and the contribution to the paramagnetic susceptibility and magnetisability independent of temperature that arise from a correct treatment of the nonadiabatic rotational and vibrational effects discernible in the vibration-rotational spectra of diatomic and triatomic molecules measured in the absence of externally applied electric and magnetic fields.

CONDENSED MATTER GROUP

Thin Film Laboratory

The research goal of condensed matter group can be divided into two main areas: magnetic resonance and laser spectroscopy. In the former, studies on liquid and solid matter are covered; and in the latter, new techniques in the sub-micron and molecular cross interaction in condensed phases are developed.

• Development and Application on Magnetic Resonance Spectroscopy

  Enhancing NMR sensitivity by optical pumping:
  A new experimental scheme for dynamic nuclear pola-rization (DNP) through optical pumping has been developed. The technique invokes the photo-excited triplet states of organic solids (e.g., pentacene) created by pulsed laser; detailed electron-nuclear cross-polarization dynamics at the level anticrossing (LAC) region has been examined. Our objective is to enhance the NMR sensitivity by cross-polarization technique in a simple optical pumping process. This should prove to be a viable technique for significantly enhancing NMR sensitivity of solid samples, especially for those in rare spin system and/or for organic doped on surface and polymer thin-film.

  On the physical and chemical properties of porous materials:
  High resolution and solid state NMR experiments in conjunction with other experimental techniques such as ESR, IR, XRD, ICP-AES, TEM, and thermal analyses are used to investigate fundamental properties of porous materials, such as zeolites, (silico-, alumino-) phosphates, and pillared clays. Major research interests include: synthesis and characterization of porous materials, molecular absorption, catalytic reaction and related coking effect, and ion-exchange properties.

• The Relaxation Processes Studied by NMR Spectroscopy

  NMR null point spectral analysis:
  NMR null point spectral analysis, developed in our laboratory, has been applied in studying the binding of chloried ions to human serum albumin. The density matrix formalism was used to simulate the null point spectra. This study shows that analysis of the fine spectral structure at the null point provides a more detailed source of motional information than the normally used measurements of longitudinal and transverse relaxation. This method also allows more accurate determination of correlation times and enables the interpretation of quadrupolar ion binding data when the relaxation is too nonexponential to allow the use of usual methods.

  Resolution enhancement in Mg NMR spectroscopy:
  Spectral estimation of many kinds of signals in time or distance domain is usually based on the Fourier transform. This method is computationally very efficient but suffers from some well-known drawbacks such as sinc wiggling artifacts by zero filling, phase distortion due to windowing or apodization processes, its inability to distinguish between signal and noise, and, particularly, the limited resolution constrained by strongly damped or truncated data. To overcome the above problems inherent in 25Mg NMR as well as many other FT spectroscopies, we present a novel approach to achieve a resolution in chemical shifts far exceeding the performance of the conventional FT method by directly analyzing the frequency correlation in free induction decay. Our approach is based on the modification of linear prediction with singular value decomposition (LPSVD) method. However, the LPSVD method is less satisfactory at low signal to noise ratio (S/N) mainly because of significant contamination of noise in the signal subspace. To alleviate this problem presented here for NMR experiments using nature abundant 25Mg in dilute solutions, a signal-enhancement algorithm based on the extension of the novel mathematical methods of composite matrix property mappings can be invoked prior to the LPSVD. The main idea lies in finding a "noiseless" data matrix which possesses the inherent matrix properties such as Hankel structure and a prespecified rank and which lies closest to the noise-corrupted data matrix. A dramatic enhancement in sensitivity and resolution can then be achieved through the iterative procedures of slightly modifying the matrix constructed from the measured noisy signal.

PHOTOCHEMISTRY AND SPECTROSCOPY GROUP

Photo-chemistry Laboratory

• High selectivity of grometric isomers of nitrate com-pounds: Utilizing the advantages of the cage effect of the matrix and laser photolysis technique, many new transient species can be generated and characterized by IR absorption. For example, photolysis of nitric acid produces cis-cis and trans-perp HOONO which have been observed by IR absorption for the first time. Irradiated by different wavelength light, a specific conformer of nitric acid can be optically selected. Photolysis of potassium nitrate can also generate cis- and trans-conformers, between which photoconversion can take place.
• Spectroscopy of high-lying vibrational levels of free radicals NH2: The highly excited vibrational states (0, 10, 0) and (0, 12, 0) of A2A1NH2 radical have been observed by laser-induced fluorescence excitation spectroscopy. The molecular constants, such as band origins, rotational constants, spin-orbit constant, and asymmetry parameters, of these two vibrational states have been determined. The obtained molecular constants provide a test for the accuracy of novel variational calculations on spinrovibronic levels of NH2 under Renner-Teller effect.

  Due to large vibronic interactions in the X and A states of CCH radical, it is difficult to analyze the high-resolution IR spectra alone. By simultaneous analysis of our UV spectra and some previously reported IR spectra, we have obtained the bending vibrational levels especially K=2 and 3 up to 5000 cm-1 for the first time. The obtained molecular parameters are useful for analyzing the vibronic interactions of the ethynyl radical and the photodissociation process of acetylene.

SURFACE SCIENCE GROUP

The scanning probe micorscopy laboratory

• Diffusion and aggregation of surface particles
  The oxidation process of liquid Ga was investigated using scanning ion microscopy. The surface diffusion and aggregation patterns of Ga oxide particles were determined for better understanding of the oxidation behavior of metals and various kinetic models of particle aggregation.
• Ultra thin metal films
  The effects on the structural stability of a Mo single crystal by ultra thin Au, Pd and oxygen overlayers have been studied with LEED, XPS and TPD. The phenomenon of faceting was observed, and the "faceted to planar" phase transition depended on the substrate temperature. The results provide important insight into the epitxial growth mechanism and the stability of nanostructures.
• Second-harmonic and sum-frequency generation spectroscopy
  A picosecond laser and optics system has been constructed for SHG and SFG non-linear spectroscopy to investigate interfacial properties and surface molecular vibrations. The in-situ techunique will be applied to study the reaction steps and mechanisms involved in diamond film growth and other processes.

  Surface Science Laboratory

• Deposition of diamond films
  Selective and oriented growth of diamond thin film was accomplished on a microscopic grating created on a Si surface by a focused ion beam. The results provide a better control of the nucleation step. Raman micro-probe was successfully used to determine the stress distribution in the deposited film.
• Laser spectroscopy in condensed phases

  Experimental set-up in laboratory:
  A pair of home-made sync-pumped dye lasers pumped by a cw mode-locked and Q-switched Nd: YAG laser were constructed for the picosecond coherent Stokes and anti- Stokes Raman spectroscopies. The pulse width is 30 ps and the band width is 2cm-1 at FWHM for the picosecond coherent Stokes and anti-Stokes Raman spectroscopies. The pulse width is 30 ps and the band width is 2cm-1 at FWHM for the dye lasers. The average power is 40 nW at 500 Hz repetition rate. Using this laser system, we can perform the CARS, photon echo, pump-probe, and transient grating. In the frequency domain experiments, we have used a ring dye laser pumped by a cw Ar+ laser to perform spectral hole burning for better understanding the dye-polymer interaction. Recently, we have constructed a hybridized sync-pumped dye laser pumped by a cw mode-locked Nd: YAG laser. The average power is 30 Mw and the pulse width is 200 femtosecond. This system will be employed for probing the ultrafast phenomena.

  Fundamental studies of vibrational dynamics in molecular crystals:
  Picosecond CARS has been widely utilized to study the vibrational dynamics in molecular crystals. Recently, we have demonstrated a new experimental method, multiplex picosecond CSRS spectroscopy, to measure vibrational decay times from different species simultaneously. In addition, we have investigated how the impurity (pentacene) excitation affects the decay processes of host vibration by multi-resonant time-resolved CSRS.

  Spectral hole burning study of dye-polymer interactions:
  We have performed temperature-annealing-cycling hole burning experiments of various chromophores imbedded in dpolyvinyl alcohol. We have measured the distribution of barrier heights and asymmetric energies in the two-level systems. We have also built an optical parametric oscillator (OPO) pumped by cw mode-locked Ti Sapphire laser.

LASER CENTER

• Tunable passive modelocked dye laser
• Synchronized multiwavelength femtosecond laser
• Mid-infrared tunable picosecond parametric oscillator
• Optical tomagraphy in random scattering media
• Confocal laser feedback microscopy

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