Coauthored Publications with: Fan

Journal Article

Chen, Y-J, Tzeng H-Y, Fan H-F, Chen M-S, Huang J-S, Lin K-C.  2010.  Photoinduced electron transfer of oxazine 1/TiO2 nanoparticles at single molecule level by using confocal fluorescence microscopy. Langmuir. 26:9050-9060., Number 11 AbstractWebsite

Kinetics of photoinduced electron transfer (ET) from oxazine 1 dye to TiO2 nanoparticles (NPs) surface is studied at a single molecule level by using confocal fluorescence microscopy. Upon irradiation with a pulsed laser at 630 nm, the fluorescence lifetimes sampled among 100 different dye molecules are determined to yield an average lifetime of 2.9 ± 0.3 ns, which is close to the value of 3.0 ± 0.6 ns measured on the bare coverslip. The lifetime proximity suggests that most interfacial electron transfer (IFET) processes for the current system are inefficient, probably caused by physisorption between dye and the TiO2 film. However, there might exist some molecules which are quenched before fluorescing and fail to be detected. With the aid of autocorrelation analysis under a three-level energy system, the IFET kinetics of single dye molecules in the conduction band of TiO2 NPs is evaluated to be (1.0 ± 0.1)×104 s-1 averaged over 100 single molecules and the back ET rate constant is 4.7 ± 0.9 s-1. When a thicker TiO2 film is substituted, the resultant kinetic data do not make a significant difference. The trend of IFET efficacy agrees with the method of fluorescence lifetime measurements. The obtained forward ET rate constants are about ten times smaller than the photovoltage response measured in an assembled dye-sensitized solar cell. The discrepancy is discussed. The inhomogeneous and fluctuation characters for the IFET process are attributed to microenvironment variation for each single molecule. The obtained ET rates are much slower than the fluorescence relaxation. Such a small ET quantum yield is yet feasibly detectable at a single molecule level. © 2010 American Chemical Society.
Wu, C-C, Lin H-C, Chang Y-B, Tsai P-Y, Yeh Y-Y, Fan H, Lin K-C, Francisco JS.  2011.  Br 2 molecular elimination in photolysis of (COBr) 2 at 248 nm by using cavity ring-down absorption spectroscopy: A photodissociation channel being ignored. Journal of Chemical Physics. 135, Number 23 AbstractWebsite

A primary dissociation channel of Br 2 elimination is detected following a single-photon absorption of (COBr) 2 at 248 nm by using cavity ring-down absorption spectroscopy. The technique contains two laser beams propagating in a perpendicular configuration. The tunable laser beam along the axis of the ring-down cell probes the Br 2 fragment in the B 3Π + ou-X 1Σ g + transition. The measurements of laser energy- and pressure-dependence and addition of a Br scavenger are further carried out to rule out the probability of Br 2 contribution from a secondary reaction. By means of spectral simulation, the ratio of nascent vibrational population for v = 0, 1, and 2 levels is evaluated to be 1:(0.65 ± 0.09):(0.34 ± 0.07), corresponding to a Boltzmann vibrational temperature of 893 ± 31 K. The quantum yield of the ground state Br 2 elimination reaction is determined to be 0.11 ± 0.06. With the aid of ab initio potential energy calculations, the pathway of molecular elimination is proposed on the energetic ground state (COBr) 2 via internal conversion. A four-center dissociation mechanism is followed synchronously or sequentially yielding three fragments of Br 2 + 2CO. The resulting Br 2 is anticipated to be vibrationally hot. The measurement of a positive temperature effect supports the proposed mechanism. © 2011 American Institute of Physics.
Fan, H, Tsai P-Y, Lin K-C, Lin C-W, Yan C-Y, Yang S-W, Chang AHH.  2012.  Molecular elimination of Br2 in photodissociation of CH 2BrC(O)Br at 248 nm using cavity ring-down absorption spectroscopy. Journal of Chemical Physics. 137, Number 21 AbstractWebsite

The primary elimination channel of bromine molecule in one-photon dissociation of CH2BrC(O)Br at 248 nm is investigated using cavity ring-down absorption spectroscopy. By means of spectral simulation, the ratio of nascent vibrational population in v 0, 1, and 2 levels is evaluated to be 1:(0.5 ± 0.1):(0.2 ± 0.1), corresponding to a Boltzmann vibrational temperature of 581 ± 45 K. The quantum yield of the ground state Br2 elimination reaction is determined to be 0.24 ± 0.08. With the aid of ab initio potential energy calculations, the obtained Br2 fragments are anticipated to dissociate on the electronic ground state, yielding vibrationally hot Br2 products. The temperature-dependence measurements support the proposed pathway via internal conversion. For comparison, the Br2 yields are obtained analogously from CH3CHBrC(O)Br and (CH3)2CBrC(O)Br to be 0.03 and 0.06, respectively. The trend of Br2 yields among the three compounds is consistent with the branching ratio evaluation by Rice-Ramsperger-Kassel-Marcus method. However, the latter result for each molecule is smaller by an order of magnitude than the yield findings. A non-statistical pathway so-called roaming process might be an alternative to the Br2 production, and its contribution might account for the underestimate of the branching ratio calculations. © 2012 American Institute of Physics.
Hu, E-L, Tsai P-Y, Fan H, Lin K-C.  2013.  Photodissociation of gaseous CH3COSH at 248 nm by time-resolved Fourier-transform infrared emission spectroscopy: Observation of three dissociation channels. Journal of Chemical Physics. 138, Number 1 AbstractWebsite

Upon one-photon excitation at 248 nm, gaseous CH3C(O)SH is dissociated following three pathways with the products of (1) OCS + CH 4, (2) CH3SH + CO, and (3) CH2CO + H 2S that are detected using time-resolved Fourier-transform infrared emission spectroscopy. The excited state 1(nO, π *CO) has a radiative lifetime of 249 ± 11 ns long enough to allow for Ar collisions that induce internal conversion and enhance the fragment yields. The rate constant of collision-induced internal conversion is estimated to be 1.1 × 10-10 cm3 molecule -1 s-1. Among the primary dissociation products, a fraction of the CH2CO moiety may undergo further decomposition to CH2 + CO, of which CH2 is confirmed by reaction with O2 producing CO2, CO, OH, and H2CO. Such a secondary decomposition was not observed previously in the Ar matrix-isolated experiments. The high-resolution spectra of CO are analyzed to determine the ro-vibrational energy deposition of 8.7 ± 0.7 kcal/mol, while the remaining primary products with smaller rotational constants are recognized but cannot be spectrally resolved. The CO fragment detected is mainly ascribed to the primary production. A prior distribution method is applied to predict the vibrational distribution of CO that is consistent with the experimental findings. © 2013 American Institute of Physics.
Chen, M-S, Fan H-F, Lin K-C.  2010.  Kinetic and thermodynamic investigation of rhodamine B adsorption at solid/solvent interfaces by use of evanescent-wave cavity ring-down spectroscopy. Analytical Chemistry. 82:868-877., Number 3 AbstractWebsite

Evanescent-wave cavity ring-down spectroscopy is applied to investigate the adsorption behavior of rhodamine B at three different interfaces. The adsorption equilibrium constant (Kads) and adsorption free energy of rhodamine B at the silica/methanol interface are determined to be (1.5 ± 0.2) × 104 M-1 and -23.8 ± 0.4 kJ/mol by use of a Langmuir isotherm model. A Langmuir-based kinetic model is also developed to determine the corresponding adsorption and desorption rate constants of (1.02 ± 0.03) × 102 M-1 s-1 and (7.1 ± 0.2) × 10-3 s-1, from which Kads is obtained to be (1.45 ± 0.09) × 104 M-1, in agreement with the value determined under equilibrium conditions. Similarly, when rhodamine B is at the chlorotrimethylsilane-immobilized silica/methanol interface, the adsorption and desorption rate constants are determined to be (1.7 ± 0.2) × 102 M-1 s-1 and (5.0 ± 1.0) × 10-3 s-1· The subsequent Kads is (3.6 ± 0.4) × 104 M-1, which is larger than that at the silica/methanol interface. The former adsorption is dominated by hydrophobic interaction, while the latter is subject to electrostatic attraction. When rhodamine B is at the silica/water interface, there exist three chemical forms, including zwitterion (R+B -), cation (RBH+), and lactone (RBL). A combination of double-layer and Langmuir competitive models is used to fit the adsorption isotherm as a function of solution pH, yielding Kads of (2.5 ± 0.2) × 104 M-1 and (1.1 ± 0.2) × 105 M-1 for R+B- and RBH +, respectively. RBL is considered to have the same Kads value as R+B-. © 2010 American Chemical Society.