Browsing by Author "Wang, NS"

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Experimental and theoretical studies of the rate coefficients of the reaction O(P-3)+HCl at high temperatures
(2002-10-31)
Hsiao, CC
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Lee, YP
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Wang, NS
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Wang, JH
;
Lin, MC
Rate coefficients of the reaction O(P-3) + HCl in the temperature range 1093-3197 K were determined using a diaphragmless shock tube. O atoms in the ground electronic P-3 states were generated either by photolysis Of SO2 with a KrF excimer laser at 248 nm or by pyrolysis of N2O with the shock wave. Their concentrations were monitored via atomic resonance absorption excited by emission from a microwave-discharged mixture of O-2 and He. Rate coefficients determined in this work show non-Arrhenius behavior, with values consistent with previously reported. measurements for T < 1486 K; they fit well with the equation k(1)(T) = (9.27 +/- 0.03) X 10(-24) T3.67+/-0.18 exp[-(1030 +/- 160)/T] cm(3) molecule(-1) s(-1); listed errors represent one standard deviation in fitting. Theoretical calculations at the CCSD(T)/6-311 +G(d, p) level locate a bent (3)A" (TS1) and a linear (3)A' (TS3) transition state characterized previously. On the basis of the results computed by CCSD(T)/ 6-311 +G(3df,2p)//CCSD(T)/6-311 +G(d,p), the rate coefficients predicted with conventional transition-state theory, including Eckart-tunneling corrections and with variational transition-state theory including zero- and small-curvature tunneling corrections, all agree satisfactorily with experimental observations.
Kinetics of the CH3S+O-3 reaction.
(1997-09-07)
Wang, NS
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Chew, EP
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Lin, JT
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Lee, YY
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Lee, YP
Kinetics of the CH3S+O-3 reaction.
(1997-09-07)
Wang, NS
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Chew, EP
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Lin, JT
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Lee, YY
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Lee, YP
Luminescence and energy transfer of Eu- and Mn-coactivated CaAl2Si2O8 as a potential phosphor for white-light UVLED
(2005-07-26)
Yang, WJ
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Luo, LY
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Chen, TM
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Wang, NS
;
應用化學系
A series of Eu2+- and Mn2+-coactivated CaAl2Si2O8 phosphors have been synthesized at 1400 degrees C under a reduced atmosphere and their luminescence properties have been investigated as a function of activator and coactivator concentrations. We have discovered that energy transfers from Eu2+ to Mn2+ by directly observing significant overlap of the excitation spectrum of Mn2+ and the emission spectrum of Eu2+ as well as the systematic relative decline and growth of emission bands of Eu2+ and Mn2+, respectively. The critical distance and average separation of Eu2+ and Mn2+ have also been calculated. By utilizing the principle of energy transfer, we have also demonstrated that with appropriate tuning of activator content CaAl2Si2O8:Eu2+ Mn2+ phosphors exhibit great potential to act as a phosphor for white-light ultraviolet light-emitting diodes (UVLEDs).
Photochemistry and photodissociation of benzosultine and naphthosultine: electronic relaxation of sultines and kinetics and theoretical studies of fragment o-quinodimethanes
(2005-02-28)
Tang, KC
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Lee, SJ
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Chi, SH
;
Lu, KL
;
Chen, WC
Fluorescence decays of benzosultine and naphthosultine excited at 263 nm are detected with time-correlated single-photon counting (TCSPC). Both molecules display biexponential decay with a rapid component (time constants 90 and 350 ps for benzosultine and naphthosultine, respectively) assigned to an electronically excited state S-n at high energy and the slow one (constants 7.5 and 9.0 ns for benzosultine and naphthosultine, respectively) to the S-1 state. Dissociation of both molecules was investigated with nanosecond laser flash photolysis combined with transient absorption to detect the intermediates and products. With excitation at 266 nm, benzosultine yields a transient with absorption maximum at lambda(max) = 370 nm; this transient has a short-lived component with lifetime around 1 mus and a long-lived component. Both components are insensitive to molecular oxygen. The short-lived component is tentatively assigned to the dissociation intermediate and the long-lived to be singlet o-benzoquinodimethane (o-BQDM). Photolysis of naphthosultine yields two transient species with absorption at; lambda(max) = 420 and 520 nm; we assign the former band to triplet-triplet absorption of naphthosultine and the latter to absorption by the product singlet o-naphthoquinodimethane. Optimal geometries, energetics, and vertical transitions of benzosultine, naphthosultine, o-benzoquinodimethane, and o-naphthoquinodimethane are calculated using methods based on density-functional theory (B3LYP/6-311++G**) and time-dependent density-functional theory (TD-DFT). The results of these calculations imply that the ground electronic state of these two o-quinodimethanes (o-QDM) is singlet with a structure of diene form. Their triplet states display a biradical structure. The energy separation between singlet and triplet states of o-benzoquinodimethane is calculated to be ca. 93.6 kJ/mol, but for o-naphthoquinodimethane, it is only ca. 27.8 kJ/mol. (C) 2004 Elsevier B.V. All rights reserved.
Photoionization spectra and ionization thresholds of CH3SO, CH3SOH, and CH3SS(O)CH3
(1996-11-01)
Hung, WC
;
Shen, MY
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Lee, YP
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Wang, NS
;
Cheng, BM
Photoionization efficiency (PIE) spectra of CH3SO, CH3SOH, and CH3SS(O)CH3 were measured in the spectral range 110-150 nm by means of a discharge flow system coupled to a photoionization mass spectrometer that employs a synchrotron radiation source, These species were generated bq reacting O atoms with CH3SSCH3 in the flow tube; a microwave discharge was employed to produce O atoms from O-2, Alternatively, C2H5SCH3 replaced CH3SSCH3 to form CH3SO, CH3SOH, and other products. Ionization thresholds of 8.99+/-0.02, 8.67+/-0.03, and 8.82+/-0.05 eV were determined for the first time for CH3SO, CH3SOH, and CH3SS(O)CH3, respectively, The former two values agree well with theoretical calculations with the G2 method, The latter value fits satisfactorily with those predicted for CH3SS(O)CH3 with the G2MP2 method; the G2 method was not employed in this case due to the numerous atoms in this molecule. Ionization thresholds of CH3S and CH3S2 were also determined to be 9.27+/-0.03 and 8.62+/-0.05 eV, respectively. experimental reports. (C) 1996 American Institute of Physics.
Photolysis of oxalyl chloride (ClCO)(2) at 193 nm: Emission of CO(v <= 6, J <= 60) detected with time-resolved Fourier-transform spectroscopy
(2004-04-15)
Wu, CY
;
Lee, YP
;
Wang, NS
;
應用化學系
;
Department of Applied Chemistry
Upon photolysis of oxalyl chloride at 193 nm, time-resolved and rotationally resolved emission of CO(vless than or equal to6, Jless than or equal to60) in the spectral region 1850-2350 cm(-1) was detected with a step-scan Fourier-transform spectrometer under nearly collisionless conditions. Boltzmann-type rotational distributions of CO correspond to temperatures 3520+/-110 (v=1) to 2300+/-610 K (v=6), with an average rotational energy of 23+/-2 kJ mol(-1). The average vibrational energy of CO is estimated to be 26+/-4 kJ mol(-1) according to observed vibrational populations of v=1-6 and that of v=0 predicted with a surprisal analysis. Combining the average internal energy of CO determined in this work and average translational energies of photofragments Cl and CO determined previously by Hemmi and Suits, we propose a four-body dissociation mechanism producing one pair of translationally rapid and internally excited CO and one pair of translationally rapid Cl, each with similar energies, to account for the energy balance. Formation of translationally slow ClCO, Cl, and CO reported previously by Hemmi and Suits might be rationalized with a second channel involving emission of electronically excited intermediates. We observed no emission of ClCO near 1880 cm(-1), indicating that surviving ClCO has little vibrational excitation in the C-O stretching mode. (C) 2004 American Institute of Physics.
Photolysis of oxalyl chloride (ClCO)(2) at 248 nm: Emission of CO(v '<= 3, J '<= 51) detected with time-resolved Fourier transform spectroscopy
(2003-04-10)
Wu, CY
;
Lee, YP
;
Ogilvie, JF
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Wang, NS
;
應用化學系
Upon photolysis of oxalyl chloride at 248 nm, rotationally resolved emission of CO (v less than or equal to 3) in the spectral region 1900-2300 cm(-1) was detected with a step-scan time-resolved Fourier transform spectrometer under nearly collisionless conditions. Boltzmann-like rotational distributions of CO (v = 1 and 2) correspond to temperatures similar to2290 +/- 100 and 1910 +/- 130 K, respectively, with average rotational energy of 16 +/- 2 kJ mol 1; several lines of CO (v = 3) were also observed, but their weak intensity precludes quantitative analysis. The average vibrational energy of CO is estimated to be 10 +/- 3 kJ mol(-1) according to observed vibrational populations of v = 1 and 2 and that of v = 0 and 3 predicted with surprisal analysis. Combining the average internal energy of CO determined in this work and average translational energies of photofragments Cl, CO, and ClCO determined previously by Suits and co-workers, we derived a revised energy balance. We observed no emission of ClCO near 1880 cm(-1), indicating that ClCO (v(1) = 1) decomposes rapidly and that surviving ClCO has little vibrational excitation in the CO stretching mode. Most CO produced from decomposition of ClCO is in its vibrational ground state with small rotational excitation, undetectable with our technique.
Temperature dependence of rate coefficients of reactions of NO2 with CH3S and C2H5S
(2000-06-15)
Chang, PF
;
Wang, TT
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Wang, NS
;
Hwang, YL
;
Lee, YP
Rate coefficients of reactions (1) CH3S + NO2 and (2) C2H5S + NO2 were determined using laser photolysis and laser-induced fluorescence, CH3S and C2H5S radicals were generated on photolysis of CH3SSCH3 and C2H5SSC2H5, respectively, with a KrF excimer laser at 248 nm. Their concentrations were monitored via fluorescence excited by emission from a dye laser at 371.4 nm (for CH3S) or 410.3 nm (for C2H5S) pumped with a pulsed XeCl excimer laser at 308 nm. Our measurements show that k(1) (297 K) = (1.01 +/- 0.15) x 10(-10) cm(3) molccule(-1) s(-1) is similar to a value reported by Balla ct al. and approximately 50% greater than other previously reported values; listed errors represent 95% confidence limits. Reaction 1 has a negative activation energy: k(1) = (4.3 +/- 1.3) x 10(-11) exp[(240 +/- 100)/T] cm(3) molecule(-1) s(-1) for T = 222-420 K. Reaction 2 has a rate coefficient similar to reaction 1 at room temperature: k(2)(296 K) = (1.05 +/- 0.16) x 10(-10) cm(3) molecule(-1) s(-1) but has a small positive activation energy with k(2) = (2.4 +/- 0.7) x 10(-10) exp[(-210 +/- 80)/T] cm(3) molecule(-1) s(-1) for T = 223-402 K. The temperature dependence of k(2) is determined for the first time. Reactions of NO2 with HS, CH3S, and C2H5S are compared.