Jinghua Dai

à 2Πu state-intermediated two-photon dissociation of CS2+ via the first channel

The [1+1] A 2Πu-state resonance enhanced two-photon dissociation process of CS2+ molecular ions has been investigated by measuring the photofragment S+ excitation (PHOFEX) spectrum in the wavelength range of 424–482 nm, where the CS2+ molecular ions were prepared purely by [3+1] multiphoton ionization of the neutral CS2 molecules at 483.2 nm. The PHOFEX spectrum was assigned essentially to the CS2+(A 2Πu)←CS2+(X 2Πg) transition, and the dissociation mechanism of CS2+ was preliminarily attributed to (i) CS2+(X 2Πg)→CS2+(A 2Πu) through one-photon excitation, (ii) CS2+(A 2Πu)→CS2+(X†) via internal conversion process due to the vibronic coupling between the A and X states, (iii) CS2+(X†)→CS2+(B 2Σu+) through the second photon excitation, and (iv) CS2+(B 2Σu+)→S++CS owing to the potential curve crossing with the repulsive 4Σ− state correlated with the first dissociation limit.

Investigation of the Mechanism of the Reaction between Atomic Oxygen Radical Anion and Benzene

Abstract The reaction mechanism between atomic oxygen radical anion and benzene has been investigated using the density functional theory (DFT). Geometries of the reactants, products, complexes, and transition states involved have been optimized at the B3LYP/6-31+G(d, p) level, and their vibrational frequencies and zero-point energies (ZPEs) have been calculated subsequently at the same level. The multichannel pathways, e.g., the H atom abstraction, oxide ion formation, H2+ transfer, and proton transfer, are confirmed by the calculated potential energy surface of this reaction. Based on the G2MP2 energies, a reasonable description has been proposed qualitatively to explain the inconsistency of previous experimental conclusions.

Study on resonance-enhanced multiphoton ionization (REMPI) of SO2 in the range 365–405 nm

Abstract The resonance-enhanced multiphoton ionization (REMPI) of SO 2 in the range 365–405 nm has been studied using mass-distinguished excitation spectra. Most of the resonance peaks for both parent ion SO 2 + and fragment ions SO + , S + and O + in the excitation spectra have been assigned to the transition SO 2 ( a 3 B 1 ) ← SO 2 ( X 1 A 1 ) . It has been reasonably assumed that SO 2 (C 1 B 2 ), produced by the subsequent excitation of SO 2 (a 3 B 1 ), plays a key role in generating the fragment ions. It seems that tragment ions come mainly from the photoionization of neutral fragments dissociated from SO 2 (C 1 B 2 ) while the parent ion is formed directly by further photoionization of SO 2 (C 1 B 2 ). That the resonance peaks of the parent ion appear generally narrower than those of fragment ions can also be explained by the known spectral properties of SO 2 (C 1 B 2 ).

State resolved rotational relaxation of CS2 in argon free jet

Rotational relaxation of CS2 in argon free jet was studied using laser‐induced fluorescence (LIF). The rotational populations were obtained from LIF detection at 19 different points along the jet axis from 3 to 15 nozzle diameters. On the basis of the energy gap power law, the master equation was solved to fit the experiment results, and the two temperature‐independent parameters in the law were determined. The results show that the state‐to‐state relaxation cross sections vary as the inverse 1.7 power of the energy gap between the initial and the final states. The expressions for the state‐to‐state relaxation cross sections and the rate constants were written. The relaxation cross sections decrease with increasing temperature over the range of 2–60 K for the CS2+Ar collision.

A laser flash photolysis study of amino acids and dipeptides using 4-nitroquinoline 1-oxide as a photosensitizer: The pH dependence

The pH effects on the photochemical reaction of amino acids and related dipeptides with 4-nitroquinoline 1-oxide (4NQO) as a photosensitizer have been investigated by laser flash photolysis. The obtained kinetic parameters show that the electron transfer from Tryptophan (Trp), Tyrosine (Tyr) as well as dipeptides containing Trp and/or Tyr residue to triplet 4NQO (T4NQO) are efficient, but inefficient from methionine (Met) and dipeptides containing neither Trp nor Tyr. The result was supported by the calculated values of the free energy change from measured oxidation potentials for the electron transfer. It was demonstrated that Trp and Tyr residues are initial reaction sites with T4NQO, while Tyr/O⋅ radical may be final species for Trp-Tyr dipeptide. In acidic aqueous solutions, the self-quenching rate constants of T4NQO and the rate constants of electron transfer from amino acids to T4NQO decrease with decreasing pH. In alkaline solutions, amino acids are easily oxidized by 4NQO under irradiation of laser pulse, and no transient absorption signal was observed.

Laser-induced fluorescence studies of jet-cooled CF2: determination of Ã-state stretching frequencies

Abstract Laser-induced fluorescence (LIF) excitation spectra of the CF2 A 1B1←X 1A1 system around 250 nm has been measured under supersonic free-jet conditions. CF2 radical was produced by DC-discharge of CF4 seeded in argon. Previously ambiguous assignments of the A-state stretching frequencies have been experimentally determined to be ν1′=1012.1 cm−1 and ν3′=1180.2 cm−1, by analyzing the subband spectra attributed to some of the observed progressions (20n, 10120n−2 and 20n311 (n≤6)) in which 21 vibronic bands were unambiguously labeled. The measured A-state stretching frequencies do not support King et al.s assignments [D.S. King, P.K. Schenck, J.C. Stephenson, J. Mol. Spectrom. 78 (1979) 1], but are in good agreement with Cameron et al.s theoretical assertions [M.R. Cameron, S.H. Kable, G.B. Bacskay, J. Chem. Phys., 103 (1995) 4475].

Theoretical studies on mechanism for the reaction of the excited nitrogen atom and chloromethane

Abstract The probable reaction mechanism for the reaction of excited nitrogen atom and chloromethane has been studied using the G2MP2 method. Based on the calculated reaction processes, the excited nitrogen atom will insert into the CCl bond firstly to form the intermediate trans -CH 3 NCl, which can subsequently decompose and isomerize to further products. The present result suggests that CH 2 NCl is the most feasible among all possible products. The reaction mechanisms for the N ( 2 D )+ CH 3 X (X=H, F, Cl) system are contrasted in detail.

High-resolution Absorption Spectra of Acetylene in 142.8–152.3 nm

The absorption spectra of acetylene molecules was measured under jet-cooled conditions in the wavelength range of 142.8152.3 nm, with a tunable and highly resolved vacuum ultraviolet (VUV) laser generated by two-photon resonant four wave difference frequency mixing processes. Due to the sufficient vibrational and rotational cooling effect of the molecular beam and the higher resolution VUV laser, the observed absorption spectra exhibit more distinct spectral features than the previous works measured at room temperature. The major three vibrational bands are assigned as a C-C symmetry stretching vibrational progress (u2 = 02) of the C1 IIu state of acetylene. The observed shoulder peak at 148.2 nm is assigned to the first overtone band of the trans-bending mode u4 of the C1 IIustate of acetylene. Additionally, the two components, 4o2(1IIu) and 4o2(1 IIuare suggested to exhibit in the present absorption spectra, due to their RennerTeller effect and transition selection rule. All band origins and bandwidths ...

Collisional quenching of NCO (A2Σ+) by some inorganic molecules

Abstract Collisional removal of the (0,00,0) level of the A 2 Σ + state of the NCO radical has been studied at room temperature (298 K). NCO radicals were produced by laser photolysis of CHBr3 at 266 nm followed by the reaction of CH with N2O, then electronically excited with Nd:YAG laser pumped dye laser. The quenching rate coefficients kq and thermally averaged cross-sections σq were determined by the pressure dependence of the time-resolved fluorescence. Colliders investigated are Ar, N2, O2, CO, NO, N2O, CO2, SO2, H2O and photochemical precursor CHBr3. The quenching rates increase on the whole with the increasing of polarities of the colliders. By comparison with the formation cross-sections of complexes calculated by means of a collision complex model, it is believed that the attractive forces play main roles in the collision quenching processes of NCO ( A 2 Σ + ) by the quenchers studied in this work.

Photochemistry of naphthosultine and naphthosulfolene in acetonitrile solutions

The photochemistry of naphthosulfolene ( 1 ) and naphthosultine ( 2 ) was investigated using time-resolved laser flash photolysis, conventional photochemical steady-state techniques and product analysis. Excited by a laser pulse at 266 nm using laser-induced transient absorption spectroscopy, both compounds 1 and 2 exhibit the typical excited triplet state peaks with maxima at 410 nm and 430 nm in acetonitrile solution under deoxygenated conditions. Interestingly, upon direct photolysis in acetonitrile solution, compound 2 has three additional peaks located at 380 nm, 500 nm and 525 nm, generated by two distinct mechanisms: (1) the cleavage of the C—O bond in the sultine molecule produces a singlet biradical with absorption bands located at 500 nm and 525 nm, which decays at a much lower rate than does the excited triplet state which absorbs at 410 nm and 430 nm; (2) a simultaneous cleavage of the C—O bond and C—S bond in the sultine molecule generates a triplet biradical with absorption band located at 380 nm. Steady-state product studies indicate that reaction (1) is much more favorable than reaction (2), and the decay of the singlet biradical generates naphthosulfolene 1 in high yields in the absence or presence of quenching agents (electron-poor alkenes). The reactivity of the excited triplet states, the singlet biradical and the triplet biradical are discussed.Time-resolved transient absorption spectra have been obtained first for mesitylene by 355 nm laser flash photolysis at ambient temperature. Mono- and biradicals were observed in the experiment, in which the transient absorption peak at about 370 nm was assigned to the biradical and the peak at about 580 nm to the monoradical; the biradical has a longer lifetime than the monoradical. According to the representative value of the C—H bond dissociation energy for aromatic compounds monoradical formation might be a single-photon process. For comparison with the absorption spectra the time-resolved fluorescence spectra were obtained and also the biradical was assigned to about 393 nm.