Xingxiao Ma

Photodissociation dynamics of CH2I2 molecules in the ultraviolet range studied by ion imaging

The photodissociation dynamics of diiodomethane molecules has been investigated in the wavelength range of 277–305 nm by an ion imaging spectrometer operated under optimal conditions for velocity mapping, where the ions were generated from (2+1) multiphoton ionization of I(2P3/2) and I*(2P1/2) fragments with the same laser as that to dissociate the parent molecules. The speed and angular distributions of I* and I fragments were determined from the images. The translational energy distribution of I*(2P1/2) fragment consists of a single Gaussian component (named G*), while that of I(2P3/2) consists of two Gaussian components (named G1 and G2). It was found that the component G* and G2 show similar angular distributions and similar fragmentation energy partitioning ratios, indicating that these two components originate from dissociation at the same electronically excited state, while the component G1 is from another state. Three fragmentation pathways were employed to account for the experimental observation...

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).

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 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.

NH(A3Π → X3Σ−) emission from 193 nm two-photon photolysis of NH3

Abstract The NH(A 3 Π → X 3 σ − ) emission spectrum was observed via 193 nm NH 3 photolysis (NH 3 + h ν → NH 2 + H; NH 2 + h ν → NH(A 3 Π) + H). The high rotational temperature (7700 K) suggests a dramatic change of bending angle from NH 2 (A 2 A 1 ) to NH 2 (B 2 B 1 ), which dissociates. Quenching rate coefficients (10 −11 cm 3 molecule −1 s −1 ) for O 2 , NO, H 2 , CH 4 , and Ar are 2.9 ± 0.2, 7.3 ± 1.0, 4.8 ± 0.5, 6.6 ± 1.5, and ≈10 −2 respectively.

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.

Channel switching effect in photodissociating N2O+ ion at 312.5 nm

A experimental observation is presented on the N2O+ photodissociation process, which exhibits a complete channel switching effect in a narrow energy range. The N2O+ ions, prepared at the X2Pi (000) state by (3+1) multiphoton ionization of neutral N2O molecules at 360.6 nm, were excited to different vibrational levels in the A2Sigma+ state in a wavelength range of 275-328 nm. Based on the estimates of total released kinetic energies from the time-of-flight mass spectrum, it was found that the dissociation pathway of N2O+ (A2Sigma+), NO+ (X1Sigma+) + N(4S) with lower dissociation limit, changes abruptly and completely to NO+ (X1Sigma+) + N(2D) with higher dissociation limit, in a excitation energy range of merely 250 cm(-1) at lambda approximately 312.5 nm. This phenomenon was explained by competition between the two dissociation pathways across the special excitation energy region.

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.