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The properties of the lowest excited triplet states of quinoxaline derivatives 2,3-dimethylquinoxaline (DMQ) and 2,3,6,7-tetramethylquinoxaline (TMQ) in acetonitrile have been investigated by using time-resolved laser flash photolysis at 266 nm. The transient absorption spectra and the self-quenching rate constants (ksq) of the excited sensitizers have been obtained. The excited triplet states of DMQ and TMQ are efficiently quenched by some electron-poor alkenes, such as fumaronitrile (FN), dimethyl fumarate (DMF), diethyl fumarate (DEF), α-acetoxyacrylonitrile (AAN), acrylonitrile (AN) and crotononitrile (CrN). Obtained kq values decrease significantly from FN (2.50 × 109 M−1 s−1) to CrN (1.70 × 105 M−1 s−1) with DMQ and from FN (1.77 × 109 M−1 s−1) to CrN (1.20 × 105 M−1 s−1) with TMQ, which are in accordance with the reduction potential (Ered) and electron cloud density of these electron-poor alkenes. Plots of log kq versus free energy change ΔGct for different alkenes are linear and yield slopes −2.75 eV−1 for DMQ, −2.69 eV−1 for TMQ and −2.99 eV−1 for DMN in the endergonic region (ΔGct > 0). These shallower slopes indicate processes involving only partial charge transfer. The failure to detect the ion radicals also supports that triplet exciplex is formed during the processes. In addition, the electronic effects of ortho-substituents are estimated according to the good linear relation between log kq and Hammett σ constant.

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.

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.

CCl2 free radicals were produced by a pulsed dc discharge of CCl4 (in Ar). The ground electronic state CCl2 (X) radical was electronically excited to the A 1 B 1 (0 4 0) vibronic state with a Nd:YAG laser pumped dye laser at 541.52 nm. Experimental quenching data of excited CCl2( A 1 B 1 and a 3 B 1 ) by ketone molecules were obtained by observing time-resolved total fluorescence signal of the excited CCl2 radical, which showed a superposition of two exponential decay components under the presence of quencher. The quenching rate constants KA of CCl2 (A) state and Ka of CCl2 (a) state were derived by analyzing the experimental data according to a proposed three-level-model involving X 1 A 1 , A 1 B 1 , and a 3 B 1 states. On the whole, KA and Ka increase with increasing the number of C H bonds in ketone molecules.

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].

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 ).