Congxiang Chen

Reactions of N(4S) atoms with NO and H2

Reactions of N(4S) atoms with NO and H2 have been investigated using direct detection of N atoms by the atomic resonance absorption technique in a shock tube apparatus, where N(4S) is generated by photodecomposition of NO by 193 nm laser radiation behind reflected shock waves. The rate constant of the reaction, N+NO→N2+O (1) has been determined using pseudo first‐order kinetic analysis to be k1=(1.3±0.3)×1013 (cm3 mol−1 s−1) over 1600–2300 K temperature range, which agrees very well with the estimation by Baulch et al. [Evaluated Kinetic Data for High Temperature Reactions (Butterworths, London, 1973), Vol. 2]. No (or very small) activation energy of this process was confirmed. Also, the rate constant of the reaction, N+H2→NH+H (2) has been decided by adding H2 to NO–Ar mixtures; it is k2=(2.8±0.2)×1014 exp(−Ea/RT) (cm3 mol−1 s−1), where Ea =33±7 kcal/mol. A quantum mechanical calculation performed in order to determine the mechanism of this reaction suggests that the reaction N(4S)+H2→NH+H proceeds via a...

Quenching of CH(A 2Δ and B 2Σ−) by NO, CHBr3 and amine molecules

Abstract The rate constants of quenching of CH(A 2 Δ) by CHBr 3 and CH(A 2 Δ and B 2 Σ − ) by NO, (CH 3 ) 2 NH, (C 2 H 5 ) 2 and (CH 3 ) 3 N were measured by using a direct time-resolved technique at 290 K. CH(A 2 Δ and B 2 Σ − ) radicals were produced by laser photolysis of CHBr 3 at 266 nm. A mechanism for this photolysis process is proposed. The time-resolved emissions (CH(A→X) and CH(B→X) were monitored as a function of time using a fast digital storage oscilloscope coupled with a microcomputer. For quencher CHBr 3 , the preliminary result shows that the quenching rate constant decreases with increasing rotational quantum number of CH(A 2 Δ, ν′=0). A comparison of the quenching rate constant of CH(A) with that of CH(B) is given.

Collisional quenching of CH(A 2Δ) at 300 K by O2, alkane, chloromethane and CS2 molecules

Abstract Rate constants for the quenching of CH(A 2 Δ, ν′=0) by O 2 , cyclo-C 6 H 12 , n -C 5 H 12 , n -C 6 H 14 , n -C 7 H 16 , CH 2 Cl 2 , CHCl 3 , CCl 4 and CS 2 molecules were measured by using the direct time-resolved technique. CH(A 2 Δ) was produced by 266 nm ultraviolet laser photolysis and the time-resolved emission of CH(A 2 Δ→X 2 Π) was then monitored as a function of time using a fast digital storage oscilloscope coupled with a microcomputer. For the partner molecule O 2 , the quenching rate constant is consistent with previous measurements. For alkane molecules, the quenching rate constants increase approximately in proportion to the number of CH bonds in the alkane. The results for chloromethane molecules are interpreted using a collision complex model. For the collision partner CS 2 , CH(A 2 Δ) is more reactive than CH(X 2 Π).

Investigation of the collisional quenching of CH(A 2Δ and B 2Σ−) by Ar, O2, CS2, alcohol, and halomethane molecules

The quenching rate constants of CH(A 2Δ and B 2Σ−) by Ar, O2, CS2, alcohol, and halomethane molecules have been measured at 290 K by using the direct time‐resolved technique. The electronically excited CH radicals were produced by laser photolysis of CHBr3 at 266 nm and the time‐resolved emissions of CH(A 2Δ and B 2Σ−–X 2Π) were then monitored as a function of time using a fast digital storage oscilloscope coupled with a microcomputer. For a given partner, the collisional removal rate of CH(B 2Σ−) is faster than that of CH(A 2Δ) except for O2.

Laser-induced fluorescence excitation spectrum of CCl2 cooled in a supersonic free jet

Abstract Laser-induced fluorescence (LIF) excitation spectrum for gas-phase dichlorocarbene (CCl 2 ) cooled in a supersonic free jet has been observed in the wavelength range of 497–517 nm. The K-structure was clearly resolved. About 80 subbands were assigned to ( v 1 , v 2 0)←(0, 0, 0) vibronic transitions. The vibrational frequencies of ν′ 1 =643.5 cm −1 , ν′ 2 =308,0 cm −1 and their anharmonic constants were obtained. Furthermore, the rotational data A ′− B ′=3.67 cm −1 , A ″− B ″=1.80 cm −1 were deduced from the spectral analysis.

Kinetics of C2(a3Πu) radical reactions with alkanes by LIF

The reactions of C2(a3Piu) radicals with a series of alkanes have been studied at room temperature and 6.5 torr total pressure using the pulsed laser photolysis/laser-induced fluorescence technique. C2(a3Piu) radicals were generated by photolysis of C2Cl4 with the focused output from the fourth harmonic of a Nd: YAG laser at 266 nm. The relative concentration of C2(a3Piu) radicals was monitored on the (0,0) band of the C2(d3Pig <-- a3Piu) transition at 516.5 nm by laser-induced fluorescence. From the analysis of the relative concentration-time behavior of C2(a3Piu) under pseudofirst-order conditions, the rate constants for the reactions of C2(a3Piu) with alkanes (C1-C8) were determined. The rate constant increases linearly with the increasing of the number of CH2 groups in the alkanes. The experimental results indicate that the reaction of C2(a3Piu) with small alkanes (C1-C8) follows the typical hydrogen abstraction process. Based on the correlation of the experimental results with the bond dissociation energy of the alkanes, the reactions of C2(a3Piu) with small alkanes likely proceed via the mechanism of hydrogen abstraction.

Time‐resolved kinetic studies on quenching of CH(A 2Δ and B 2Σ−) by (CX3)2CO, CF3COOX, and CXCl3 (X=H or D) and alkane molecules

The quenching rate constants kq of CH(A 2Δ and B 2Σ−) radicals by (CX3)2CO, CF3COOX, and CXCl3 (X=H or D), and some alkane molecules have been measured using laser photolysis of CHBr3 at 266 nm to produce CH radicals and time‐resolved fluorescence quenching measurements. An isotope effect is found on quenching of both A and B states by deuterated and hydride molecules and the quenching rate constants of both A and B states increase steadily with increase of the number of C–H bonds contained in the alkane molecules. The formation cross sections of complexes of electronically excited CH radicals and alkane molecules were calculated by means of a collision complex model. It is shown that the dependence of the formation cross section of complex on the number of C–H bonds contained in alkane molecules is in agreement with that of the measured quenching cross section.

Experimental and theoretical study of the photoionization and dissociative photoionizations of carbon tetrachloride

Abstract The photoionization and dissociative photoionizations of carbon tetrachloride have been studied both experimentally and theoretically. In experiments, we have combined the techniques of synchrotron radiation, molecular beam and mass spectrometry to obtain the photoionization efficiency spectra for CCl3+, CCl2+, CCl+, C+ and Cl+. Thus we have measured the appearance energies of these ion fragments, which in turn lead to the various dissociation energies (D0) for CCln+ and CCln. Computationally, we have also obtained the D0 values by the ab initio Gaussian-2 method. The two sets of D0 values are in good agreement with each other.

Time-resolved kinetic studies on quenching of NCO (A 2Σ+) by alkanes and substituted methane molecules

NCO radicals were produced by laser photolysis of CHBr3 at 266 nm followed by the reaction of CH with N2O. The radicals were then electronically excited from the ground electronic state to the A 2Σ+ state with a Nd:YAG laser pumped dye laser at 438.6 nm in the Q subband of A 2Σ+(0000)←X 2Πi(0010). The rate constants kq and thermally averaged cross sections σq for collision quenching of NCO (A 2Σ+) by n-CnH2n+2 (n=1,5–8), c-C6H12, CH4−nCln (n=1–4), CH3OH, CH3NO2, and CCl2F2 were measured at room temperature (298 K) by observing the time-resolved fluorescence signals of the excited NCO in a cell at total pressure of about 20 Torr. Formation cross sections of complexes of the electronically excited NCO radicals and quenchers were calculated by means of a collision complex model. It was shown that the quenching rates of NCO (A 2Σ+) by alkane molecules increase with the number of C–H bonds of the molecules, and that the attractive forces play a main role in the collisional quenching processes of NCO (A 2Σ+) by...

Investigation of collisional quenching of CCl2 (Ã 1B1 and ã 3B1) by alkanes

CCl2 free radicals were produced by a pulsed dc discharge of CCl4 (in Ar). Ground electronic state CCl2(X) radical was electronically excited to A 1B1(0,4,0) vibronic state by Nd:YAG laser pumped dye laser at 541.52 nm. The laser induced fluorescence signal is observed to be a monoexponential decay curve corresponding to the decay of the A state under supersonic jet conditions and the lifetime of CCl2(A→X) transition was measured being 0.83±0.03 μs. Experimental quenching data of excited CCl2(A 1B1 and a 3B1) by alkanes were obtained by observing the time-resolved total fluorescence signal of the excited CCl2 radical in a cell, which showed a superposition of two exponential components under the presence of a quencher. The quenching rate constants of A(1B1) and a(3B1) states of CCl2, KA, and Ka, were derived by analyzing the experimental data according to a proposed three-level model to deal with the CCl2(X 1A1,A 1B1,a 3B1) system. KA and Ka increase steadily by increasing the number of C–H bonds conta...