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Current research interests lies in the following areas:
A. Molecular Halogen Elimination Using Cavity Ringdown Absorption Spectroscopy
Atmospheric halogen chemistry has drawn much attention, for the halogen atom (X) playing a catalytic role may cause severe stratospheric ozone depletion. atomic X elimination from X-containing hydrocarbons is recognized as the major primary dissociation process upon UV-light irradiation, whereas a direct elimination of X2 product has been seldom discussed or remained as a controversial issue. For the past years, we have detected X2 primary products using cavity ring-down absorption spectroscopy (CRDS) in the photolysis at 248 nm of a variety of X-containing compounds, focusing on bromomethanes (CH2Br2, CF2Br2, CHBr2Cl, and CHBr3), dibromoethanes (1,1-C2H4Br2 and 1,2-C2H4Br2) and dibromoethylenes (1,1-C2H2Br2 and 1,2-C2H2Br2), acyl bromides (BrCOCOBr and CH2BrCOBr), diiodomethane (CH2I2), thionyl chloride (SOCl2), and sulfuryl chloride (SO2Cl2). The optical spectra, quantum yields, and vibrational population distributions of the X2 fragments have been well characterized. With the aid of ab initio calculations of potential energies and rate constants, the detailed photodissociation mechanisms may be comprehended. Such studies are fundamentally important to gain insight into the dissociation dynamics and may also practically help to assess the halogen-related environmental variation. A review will soon appear in Phys. Chem. Chem. Phys. (Perspective, 2014)
As an emerging absorption technique, CRDS is based upon the measurement
of the decay rate of light trapped in an optical cavity with high
reflectance. When a pulsed laser radiation is guided into an optical
cavity, the small amount of light trapped inside the cavity reflects
between two highly reflective mirrors (R>99.9%) with a small fraction
transmitting through each mirror for each pass. The decay rate of the
light leaking out of the cavity is related to the absorption coefficient
of the sample in the cavity. Therefore, the CRDS method may ignore
fluctuation of incident radiation intensity, with better sensitivity
than conventional absorption methods due to a longer optical path.
 Schematic of experimental apparatus for cavity ring-down absorption spectroscopy.
A portion of Br2 spectra acquired in the photolysis of CH2BrC(O)Br at 248 nm.
(a) trace acquired experimentally for the bands of v=0 and 1,
(b) the simulated counterpart with the population ratio of Br2(v=1)/Br2(v=0) optimized at 0.5,
(c) simulated counterpart with the transition involving only v=0, and
(d) simulated counterpart with the transition involving only v=1.
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B. Roaming as an alternative pathway in molecular photodissociation
An abnormal CO rotational distribution appeared in photodissociation of
formaldehyde (H2CO) by Moore and co-workers. This phenomenon was later
thoroughly characterized as an alternative photodissociation mechanism
so-called “roaming” atom or radical by Suits and Bowman and their
co-workers about a decade ago. Since then, more and more molecular
photodissociation phenomena were explored to carry such a roaming
signature. We have presented the first example of methyl formate (HCOOCH3) in the ester family that involves roaming dynamics following ultraviolet photodissociation.
As the excitation energy increases, the energetic HCO moiety may further decompose into H and CO. Such a triple fragmentation can be an accompanying phenomenon to the roaming process. In the study of methyl formate, the CO-product formation is extensively inspected as a function of the photolysis wavelength ranging from 225 to 255 nm. Then, the efficiency of HCO decomposition is examined at 248 nm using time-resolved Fourier-transform infrared emission spectroscopy (FTIR). Further, from a theoretical point of view, we collaborate with Prof. Aquilanti’s group, Perugia Univ., Italy, who perform quasi-classical trajectory (QCT) calculations on a reduced-dimensional potential energy surface (PES) of ground state HCOOCH3 to obtain the CO kinetic energy distributions for triple fragmentation and molecular production. Then, the QCT results are compared with the experimental data in attempt to gain insight into the role of roaming pathway played in production dynamics of CO fragments. The results have been published in Phys. Chem. Chem. Phys. 13, 7154 (2011) and 16, 2854 (2014).
 J selected CO fragment kinetic energy distributions from experiments (left-hand and middle panels) and from dynamics simulations (right-hand panels) are shown at the same nominal energies corresponding to 234 nm. The left column shows the imaging results of CO fragments. The speed distributions of the CO fragment are then converted into their corresponding translational energy distributions in the center-of-mass frame. The imaging results are treated by deconvolution (middle column) into separate contributions from triple fragmentation (orange curve), roaming (blue), and molecular mechanisms (red). (The same colour convention is adopted for the results of dynamics simulations shown in the right column). The global kinetic energy distribution curve (green) appears to be representative of a bimodal distribution, made up by a slow sharp component and a broad and faster one. Phase space theory calculations and dynamics simulations indicate that at energies above the opening of the triple fragmentation channel, the latter plays a significant role in contributing to the slow peak. Although there is not quantitative agreement between experiments and simulated distributions, due to the fact that a reduced dimension model has been used for the simulations, the two sets of data express the same qualitative picture.
Other than the ester family, aliphatic aldehyde is a second molecular series that we are looking into this newly found pathway. Aliphatic aldehydes are known to be present in the atmosphere through automobile exhaust or photooxidation of organic compounds. Investigating their photodissociation mechanisms has been a main theme in photochemistry. Among these aldehydes, formaldehyde (H2CO) and acetaldehyde (CH3CHO) were the only two that have been found to carry roaming signature in the photodissociation. Like the tight transition state (TS) mechanism, there exists a first-order saddle point for the roaming process, showing one small imaginary (harmonic) frequency along with some small frequency modes. Propionaldehyde (CH3CH2CHO), as a member of aliphatic aldehydes, shows similar chemical properties to its smaller counterparts, but its photochemistry is much less investigated. In the past years, we employed time-resolved Fourier-transform infrared (FTIR) emission spectroscopy to probe the HCO and CO fragments in propionaldehyde at 248 nm. Finally, the theoretical methods are performed in conjunction with the experimental findings to clarify dynamical complexity in photodissociation of propionaldehyde. We have found that the roaming pathway dominates the molecular products of CO + C2H6. This work implies that roaming mechanism plays an increasingly important role in aliphatic aldehydes, as the molecular size becomes larger. The results are published in J. Phys. Chem. Lett. 5, 190 (2014) and J. Chem. Phys. 140, 064313 (2014)
 (a) Threshold energies (kcal/mol) and reaction pathways of radical and molecular channels on C2H5CHO S0 surface. (b) Roaming saddle points and its intrinsic reaction coordinate (IRC). The formyl group proceeds flipping motion along the reaction path. Geometry optimization were performed via CASSCF(6,6)/6-311++G(d,p). Energy values with and without parentheses were calculated by means of CCSD(T)/6-311++G(d,p) and CASSCF (10,9)/CASPT2/6-311++G(d,p), respectively. The IRC calculations in (b) were performed at level of CASSCF(6,6)/6-31++G(d,p).
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C. Chemical and Biological Sensing on Silica Surface using Evanescent Wave-Cavity Ringdown Absorption Spectroscopy
Evanescent Wave-Cavity Ringdown Absorption Spectroscopy (EW-CRDS) combines the CRDS technique with attenuated total reflectance method. EW-CRDS is based on the measurement of the decay rate of a pulsed laser light that is reflected back and forth between two highly reflective mirrors, thereby causing a small fraction of light leaking out of the rear mirror to be detected. The intensity of the light decreases every round trip, creating an exponential decay curve. The time for the light to decay to 1/e of its initial intensity is defined as ring-down time (τ). A shorter ring-down time (τ) is produced in the presence of an absorbing species within the cavity, as compared to that for an empty cavity. A specially designed prism placed in the center of the cavity allows the laser beam to pass through generating a total internal reflection (TIR) within the prism; evanescent wave is thus developed at the surface of the TIR point. Evanescent wave is very sensitive to small changes in absorption, making EW-CRDS particularly useful for investigating interfacial processes.
The EW-CRDS technique is composed of a linear optical cavity, a prism specially designed to allow for TIR occurrence, and a flow cell in which the flowing adsorbate may absorb the evanescent wave penetrating through the prism base. Trans-4-[4-(dibutylamino)-styryl] -1-(3-sulfopropyl) pyridinium (DP) and trans-4-[4-(dibutylamino)-styryl] -1-methylpyridinium iodide (DMP+I-) are adopted as neutral and charged species. Their adsorption behaviors at the silica/CH3CN interface are characterized. At the low concentration range, Langmuir isotherm model is applied to determine saturated surface density, equilibrium constant and free energy of adsorption of DP and DMP+ at the interface. Similar investigation has attracted wide attention, especially in the field of chromatography. Researches are actively focused on the interfacial interaction between stationary and mobile phases in order to improve the separation efficiency. The EW-CRDS technique is also employed to characterize the isolated silanol groups on a planar fused-silica surface, by using a CV+ molecular probe to determine the surface density distributions and its alignment at the CH3CN/silica interface. The Langmuir fit of the adsorption isotherms, as acquired in terms of both s- and p-polarized radiation, manifests two different types of isolated silanol distributions. The corresponding surface densities along with the orientation angles of CV+ adsorbed on each type surface are evaluated. Several papers have been published in Anal. Chem. 78, 3583 (2006), 79, 3654 (2007) and 82, 868 (2010)
Gold nanoparticles (Au NPs) exhibit unique size- and shape-dependent optical properties, biocompatibility, non-toxicity and high stability, making them ideal signaling elements for sensitive biosensors. Through covalent bonding or physical adsorption, Au NPs can be conjugated with various kinds of functional species. The biomolecule-conjugated Au NPs can offer desired properties for specific recognition and biocompatibility in biosensing. Among the functional species adopted, deoxyribonucleic acids (DNA) are commonly used for detecting small biological analytes, for their capabilities of specific binding affinities, catalytic activities, and chemical stability. Single-stranded DNAs (ssDNAs) can be easily modified with a mercapto or amino group in the 5'- or 3'-end. Thiol-modified DNA is readily assembled onto the surface of Au NPs through Au–S bonding. Similarly, amino group-modified DNA can be assembled onto the bare or carboxyl groups-modified silica surfaces. However, DNA has no optical properties in the visible region and thus labeling elements such as organic chromophores or metallic NPs are commonly applied. The color of metallic NPs originates from the surface plasmon effect. These NPs have much higher extinction coefficients than organic dyes. Therefore, Au NPs are commonly used for the preparation of DNA functionalized optical sensors, mainly because of their strong surface plasmon resonance (SPR) absorption with extremely high absorption coefficients (108–1010 M-1 cm-1) in the visible region, along with easy preparation, simple modification, high stability, and biocompatibility. DNA-conjugated Au NPs become powerful optical probes for various targets in the field of optical biosensors.
By taking advantage of EW-CRDS merits, the interaction and binding kinetics of DNA strands have been investigated by using Au NPs as sensitive reporters. Since complementary sequences of DNA (cDNA) have high affinities to form double-stranded DNA, these Au NPs are connected to target DNA that hybridizes with the cDNA fixed on the TIR surface, where evanescent waves occur. By the absorbance of Au NPs, we can examine the interaction between two DNA strands. This approach is also applied to the label-free detection of the DNA sequence of sickle-cell disease using the sandwich binding assay, where the DNA of interest partially hybridizes with the reporter DNA and the other half hybridizes to the capture DNA on the surface. Sickle-cell disease (also called sickle-cell anemia) is a hereditary blood disorder due to a mutation in the hemoglobin gene, characterized by red blood cells that adopt an abnormal, rigid, sickle shape. The advantage of the EW-CRDS method over conventional absorption spectroscopy measurement appears to have great potential for the investigation of the kinetics of a wide range of biological reactions. We foresee this method could be applied in real sample and versatile DNA disease studies. The results have been published in Anal. Chim. Acta, in press (2014) (selected as Featured Article and Cover Picture) Another subject on interaction between crystal violet and anionic surfactants at silica/water interface has been published in J. Coll. Interf. Sci. 379, 41 (2012).
Adsorption isotherm of CV+ at the CH3CN/silica interface. The adsorption isotherm is fitted by a two-site Langmuir equation, yielding two types of saturation surface densities and adsorption equilibrium constants of  = (7.4±0.5)x1012 cm-2 and KI = as well as  = (3.1±0.4)x1013 cm-2 and KII = M-1.
(A) Schematic diagram of the apparatus setup for the evanescent wave cavity ring-down absorption spectroscopy.
(B) Detailed diagram for the flow cell which is mounted on the top of an isosceles-triangle prism.
Experimental model for (a) DNA hybridization and (b) DNA sensing.
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D. Single Molecule Spectroscopy
Single molecule spectroscopy (SMS) has emerged as an important method for the studies of the excited state electronic and molecular dynamic processes of the molecules, such as intersystem crossing, energy transfer, and spectral diffusions. Very recently, SMS has been extended to probe photo-induced electron transfer in complicated systems including intramolecular and interfacial electron transfers (IMET and IFET). Although similar experiments have been studied in solutions, these ET processes appear to be too slow relative to radiative decay to be followed using bulk techniques. Single molecule spectroscopy turns out to be a suitable method for investigating low quantum yield excited state deactivation processes. By taking advantage of the SMS merits, the IFET phenomena of organic dye on the TiO2 NPs film has been investigated using confocal fluorescence microscopy. We apply time-correlated single-photon counting to measure changes in the single molecule fluorescence lifetime. We also acquire single molecule fluorescence trajectories, characteristic of “on” and “off” blinking with time, in which the intensity fluctuation is attributed to the IFET behavior. The discrete fluorescence intensity jumps between “on” and “off” intensity level are analyzed by using autocorrelation function. The “on” and “off” lifetimes and the subsequent rate constants of forward and backward electron transfers are then determined.
Quantum dots (QDs) have potential to be an alternative as electron donors, for their unique properties such as size-dependent tunable energy gap, a broad absorption band with large absorption cross sections, and multiple exciton generation. When QDs absorb a photon to form an electron-hole pair, the electrons may have chance to transfer to an accepting species such as TiO2, if the conduction band edge of QDs is tuned higher than the conduction band of TiO2. Such a kinetic behavior of electron transfer (ET) between QDs and TiO2 is one of the key roles to achieve a high energy-conversion efficiency. The bulk measurements yield ensemble-averaged information and sometimes could mask or overlook specific phenomena occurring at the sensitizer-semiconductor interfaces. As a result, SMS has emerged as a powerful tool for investigating the dynamic processes of excited molecules in heterogeneous surrounding. We have attempted to understand ET dynamics of QDs adsorbed on the TiO2 thin film at a single molecule level. We have observed the fluorescence intermittency of CdSe/ZnS (core/shell) QDs with three different sizes adsorbed individually on bare coverslip or TiO2 NPs thin film using SMS method and investigated the subsequent size-dependent IFET kinetics. We have applied time-correlated single-photon counting (TCSPC) to measure fluorescence lifetimes among a quantity of single QDs for each size. The ET processes from QDs to TiO2 will lead to further fluorescence fluctuation of trajectory and shorten the on-state lifetimes of fluorescence decay. The fluorescent QD lies in an off-blinking state when it becomes ionized with an electron ejection to the TiO2 conduction band. After the electron recombines with the charged state, the neutral QD blinks “on” again following photoexcitation. The fluorescence lifetime becomes shorter and the resultant “off” time is prolonged with decreased size of QDs. The smaller size of QDs results in a more rapid ET rate. Finally, the off-time and on-time probability densities are estimated and then fitted appropriately. With the aid of Marcus model, the theoretical ET rate constants are calculated for comparison and the ET process may thus be gained insight.
For studying the above ET phenomena, electron donor may be deposited directly on electron acceptor. Despite easy preparation, such a system is not specifically defined and the factors affecting the ET process are sometimes difficult to assess. For instance, an average ET rate constant from a carboxylic acid-capped CdSe/ZnS QDs adsorbed on TiO2 nanoparticles (NPs) was found to be about five times larger than that from octadecylamine-capped CdSe/ZnS on TiO2NPs. The ET kinetic discrepancy may not be easily explained for the same composite materials if without defining the capped functional group. To better control the factors played in the ET dynamics, the method by using a bifunctional linker to bridge both electron donor and acceptor species forming a well-defined system has received attention. We have probed the linker length effect on the ET kinetics of single QD bound to TiO2 NPs via varying mercaptocarboxylic acids (MAA, HS-(CH)n-COOH, n=2, 5, and 10). PL blinking in the three QD-MAA-TiO2 complexes is monitored by single QD fluorescence spectroscopy. The fabricated QD-TiO2 complexes provide an appropriate model system for the single-molecule exploration of photo-induced ET between QDs and TiO2 NPs with well control of the distance through the MAA linkers. The fluorescence intermittency of CdSe/ZnS (core/shell) QDs tethered to TiO2 NPs via three different linkers has been probed and the subsequent linker-length dependent interfacial ET kinetics is investigated. The details of ET events in each QD-MAA-TiO2 complex can be gained insight by analyzing the fluctuation of fluorescence intensity and lifetime. The outcome has resulted in several papers published in Langmuir, 26, 9050 (2010),ChemPhysChem 13, 2711 (2012), and Electroanalysis 25, 1064 (2013).
The single molecule experiments were performed with a confocal fluorescence microscope. A single-mode pulsed laser was used as the excitation source. An oil immersion objective was used both to focus the laser beam onto the sample, which was prepared on the surface of a silica coverslip, and to collect the fluorescence from the sample. After transmitting through a dichroic mirror, the fluorescence was refocused by a tube lens onto an optical fiber which was coupled to an avalanche photodiode (APD) detector. The fluorescence signal may also be reflected simultaneously to a charge-coupled device (CCD) by a beamsplitter. A notch filter and a long-pass filter were positioned in front of the detector to remove excitation background. Single molecule that fluoresced was readily moved to a particular position with respect to the laser focus using a x-y positioning stage. The fluorescence lifetime of single molecule was measured by time-correlated single-photon counting.
 Image of (a) 0.2 um fluorescent beads and, (b) 10-12 M R6G single molecules.
(c) (d)
 Fluorescence intensity trajectories recorded for single organic molecules (c) on bare coverslip, (d) on TiO2 NPs-coated coverslip.
 The energy diagram of TiO2 and QDs with different size.
(A) (B)
 
The on-state probability density of 10 single QDs each with a diameter of 4.6±0.7 nm on
(A) glass and (B) TiO2 film. The spots denote experimental data and lines denote simulation by truncated power law distribution.
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E. Photodissociation Dynamics Using Ion-Imaging Technique
Photochemistry of alkyl halides has received extensive attention for decades, mainly because of its potential effect on ozone depletion. These alkyl halides may undergo C-X (X=Cl, Br, I) bond fission to release halogen atoms following UV absorption to the A-band which comprises three overlapping electronic excited states, denoted as 3Q1, 3Q0, and 1Q1 in the ascending order by Mulliken, stemming from promotion of a lone-pair electron of the X atom into the antibonding orbital localized on the C-X bond. Photodissociation of ethyl iodide with an electron-donating CH3 group branched to the a-carbon atom is anticipated to yield different behavior from methyl iodide, and is worth to be treated as a model for substitution effect. By taking advantage of resonance-enhanced multiphoton ionization (REMPI) technique combined with velocity imaging detection, we look into the photodissociation dynamics of C2H5I over the A-peak region from 245 to 283 nm. The speed and angular distributions of the I, I*, and C2H5 moieties are analyzed to determine the subsequent spatial anisotropies and the relative quantum yields of the I*- and I-product channels at different photolyzing wavelengths. Given these data, the curve crossing probabilities between the 3Q0 and 1Q1 states are then evaluated. Finally, these results are compared to the case of methyl iodide to gain insight into the ethyl-substituted effect.
The apparatus consisted of two vacuum chambers for the molecular beam source and the detection regions. A <5% C2H5I sample (99% purity) was carried by helium gas through a pulsed valve with 0.6-mm diameter orifice, operating at 20 Hz, and expanded into the source chamber. After passing through a skimmer and a collimator, the molecular beam was intersected perpendicularly by a linearly polarized laser beam in a two-stage ion lens region. The resulting ions were extracted and accelerated into a 36-cm long field-free drift tube along the molecular beam direction, followed by velocity-mapping onto a two-stage microchannel plate (MCP) and a phosphor screen. The ion imaging on the phosphor screen was recorded by a charge coupled device (CCD) camera.
(a)
  
(b)
  Raw images, inverse-Abel-transformed images, and corresponding speed distributions of (a) I and (b) I* ions in the photolysis of C2H5I at 277 nm.
 Experimental setup.
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Lastest topics
A. Molecular Halogen Elimination Using Cavity Ringdown Absorption Spectroscopy
B. Roaming as an alternative pathway in molecular photodissociation
C. Chemical and Biological Sensing on Silica Surface using Evanescent Wave-Cavity Ringdown Absorption Spectroscopy
D. Single Molecule Spectroscopy
E. Photodissociation Dynamics Using Ion-Imaging Technique
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