Search Results - (Author, Cooperation:C. Y. Ng)

2018-07-19

American Physical Society (APS)

0556-2821

1089-4918

Physics

Astrophysics and astroparticle physics

2018-09-26

American Physical Society (APS)

0031-9007

1079-7114

Physics

Gravitation and Astrophysics

2018-09-26

American Physical Society (APS)

0556-2821

1089-4918

Physics

Astrophysics and astroparticle physics

2013-05-31

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

2014-10-04

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Absolute total cross sections for the reactions, O+(4S) + H2(X 1Σ+g)→O+H+2 [reaction (1)] and O+H+H+ [reaction (2)] have been measured in the center-of-mass collision energy (Ec.m.) range of 1.33–22.22 eV. The appearance energies for H+2 (1.70±0.10 eV) and H+ (4.50±0.10 eV) are in excellent agreement with the thermochemical thresholds for reactions (1) and (2), respectively. At Ec.m. higher than approximately 9 eV, the total cross sections for reactions (1) and (2) are greater than that for the exothermic channel forming OH++H.

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1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

A detailed three-dimensional quantum mechanical study of the (Ar+H2)+ system along the energy range 0.4 eV≤Etot≤1.65 eV is presented. The main difference between this new treatment and the previously published one [J. Chem. Phys. 87, 465 (1987)] is the employment of a new version of the reactive infinite-order sudden approximation (IOSA), which is based on the ordinary inelastic IOSA carried out for an optical potential. In the numerical treatment we include three surfaces (only two were included in the previous treatment), one which correlates with the Ar+H+2 system and two which correlate with the two spin states of Ar+(2Pj); j=3/2,1/2. The results are compared with both trajectory-surface-hopping calculations and with experiments. In most cases, very good agreement is obtained.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Absolute total cross sections for the reactions, Ar+(2P3/2,1/2)+N2→N++N+Ar [reaction (1)] and ArN++N [reaction (2)], have been measured in the center-of-mass collision energy (Ec.m.) range of 6.2–123.5 eV. The appearance energy for the formation of N+ (Ec.m.=8.65±0.21 eV) is in agreement with the thermochemical threshold for reaction (1). The comparison of the collision energy dependence of the N+ cross section with the photoionization efficiency spectrum of N+ from N2 suggests that the predissociative multielectron states of N+2, C˜ 2Σ+u, F˜ 2Σ+g, G˜ 2Σ+g, and 2Σ+g (2σg)−1, which are responsible for the dissociative photoionization of N2, also play a role in the formation of N+ via reaction (1). Product ArN+ ions of reaction (2) are only observed in the Ec.m. range of 8.2–41.2 eV. At Ec.m. slightly above the thermochemical thresholds of reactions (1) and (2), the majority of ArN+ and N+ ions are scattered backward and forward with respect to the center-of-mass velocity of reactant Ar+, respectively. This observation is rationalized by a charge transfer predissociation mechanism which involves the formation of ArN+ and N+ ions via nearly collinear Ar+-N-N collision configurations at Ec.m. near the thresholds of reactions (1) and (2). At Ec.m.≥11 eV, more than 92% of the charge transfer product N+2 ions are found to be slow ions formed mostly by the long-range electron jump mechanism.

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1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The reaction between oxygen ions and nitrogen molecules is studied by using tandem photoionization mass spectroscopy. This reaction is pertinent to the study of reaction occuring in the inosphere. (AIP)

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

We have measured the time-of-flight (TOF) spectra for SCH3, CH3, and SSCH3 formed in the photodissociation processes, CH3SSCH3+hν(193 nm)→2SCH3 and CH3+SSCH3. The dissociation energies for the CH3S–SCH3 and CH3SS–CH3 bonds determined at 0 K by the TOF measurements are 72.4±1.5 and 55.0±1.5 kcal/mol, in agreement with the literature values. The threshold value for the formation of S2 measured by the TOF spectrum for S2 is in accord with the thermochemical threshold for the process, SSCH3+hν(193 nm) →S2+CH3. The threshold energy determined from the TOF spectrum for S is found to be consistent with the thermochemical threshold for the photodissociation process, SCH3+hν(193 nm) →S(1D)+CH3, an observation supporting that S atoms are not produced in the ground S(3P) state in the 193 nm photodissociation of SCH3. This observation is rationalized by symmetry correlation arguments applied between the S+CH3 product and SCH3 states.

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1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The photoion–photoelectron coincidence spectra for C2H+ and C2H+2 have been measured in the wavelength range of 645–765 A(ring). The C2H+2(A˜ 2Ag,B˜ 2∑+u) ions prepared with internal energies above 17.39 eV are found to dissociate completely into C2H++H in the temporal range 〈12 μs. An upper bound of 17.33±0.05 eV is determined for the appearance energy of the process C2H2+hν→C2H++H+e− at 0 K.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The dynamics of S(3P2,1,0;1D2) production from the 193 nm photodissociation of CH3SCH3 has been studied using 2+1 resonance-enhanced multiphoton ionization techniques. The 193 nm photodissociation cross section for the formation of S from CH3S initially prepared in the photodissociation of CH3SCH3 is estimated to be 1×10−18 cm2. The branching ratio for S(3P)/S(1D) is found to be 0.15/0.85. The fine-structure distribution observed for product S(3P2,1,0) is nearly statistical. Possible potential energy surfaces involved in the 193 nm photodissociation of CH3S(X˜) have been examined theoretically along the CH3–S dissociation coordinate in C3v symmetry. These calculations suggest that predissociation of CH3S(C˜ 2A2) via the repulsive CH3S(E˜ 2E) surface is most likely responsible for the efficient production of S(1D). For vibrationally excited CH3S(X˜), a viable mechanism for the dominant production of S(1D) may involve direct dissociation via the CH3S(E˜ 2E) state formed in the 193 nm photoexcitation.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The ionization energy of SO3 is determined to be 12.828±0.010 eV from photoionization mass spectrometric measurements of SO3. Two autoionization Rydberg series converging to the SO3(2E') state (17.901 eV) are identified. Three window resonances observed in the region 610–690 A(ring) are assigned as members of an autoionizing Rydberg series converging to the SO+3(2A1) state (20.603 eV).

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

We have measured the translational energy releases of the laser photodissociation processes CH3SCH3+hν (193 nm)→CH3+CH3S [process (1)] and CH3SCH2+H [process (2)]; and CH3S+hν (193 nm)→S+CH3 [process (3)]. The onsets of the translational energy distributions for photofragments of processes (1) and (2) allow the direct determination of 74.9±1.5 and 91±2.5 kcal/mol for the dissociation energies of the CH3–SCH3 and H–CH2SCH3 bonds at 0 K, respectively. The threshold observed for S formed by process (3) is consistent with the conclusion that the production of S(3P) is small compared to S(1D). The photoelectron–photoion coincidence (PEPICO) spectra for CH3SCH+3, CH3SCH+2, CH3S+ (or CH2SH+ ), and CH2S+ resulting from photoionization of CH3SCH3 have been measured in the wavelength region of 900–1475 A(ring). The PEPICO study allows the construction of a detailed breakdown diagram for the formation of CH3SCH+2, CH3S+ (or CH2SH+ ), and CH2S+ from energy-selected CH3SCH+3 ions.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The kinetic energy releases of the photodissociation processes, CH3SH+hν (193 nm)→CH3+SH, CH3S+H, and CH2S+H2, have been measured using the time-of-flight mass spectrometric method. These measurements allow the direct determination of the dissociation energies for the CH3–SH and CH3S–H bonds at 0 K as 72.4±1.5 and 90±2 kcal/mol, respectively. The further dissociation of SH according to the process SH+hν (193 nm)→S+H has also been observed. The appearance energy (AE) of S produced in the latter process is consistent with the formation of S(3P)+H. The photoelectron–photoion coincidence (PEPICO) spectra for CH3SH+, CH3S+ (or CH2SH+), and CH2S+ from CH3SH have been measured in the wavelength range of 925–1460 A(ring). The PEPICO measurements make possible the construction of the breakdown diagram for the unimolecular decomposition of internal-energy-selected CH3SH+ in the range of 0–83 kcal/mol. The AE measured for CH2S+ is consistent with the conclusion that the activation energy is negligible for 1,2-H2 elimination from CH3SH+.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Photoelectron–photoion coincidence (PEPICO) data for OH+(OD+), H+(D+), and H2O+ (D2O+) from H2O (D2O) have been obtained in the region of 625–700 A(ring). The PEPICO measurements allow the construction of breakdown diagrams for the unimolecular dissociation of energy-selected H2O+ and D2O+ in the B˜ 2B2 state. The breakdown diagrams for H2O+(B˜ 2B2) and D2O+(B˜ 2B2) in the internal energy range of 129–166 kcal/mol are essentially identical. The branching ratios observed for H+ (D+) are higher than those reported previously. About 3%–5% of stable H2O+ (D2O+) is observed in the time scale of ≈10 μs. These stable H2O+ (D2O+) ions are attributed to ultrafast B˜ 2B2→A˜2A1 nonradiative relaxation followed by the radiative stabilization from H2O+(A˜2A1) [D2O+(A˜2A1)] to H2O+ (X˜2B1) [D2O+(X˜2B1)]. This observation also supports that the formation of H+ (D+) via the H2O+ (A˜ 2A1)[D2O+(A˜2A1)] state is a viable process. The relative state-selected cross sections for the reaction H2O+ (X˜2B1,A˜2A1; ν1, ν2)+H2O→H3O+ +H at center-of-mass collision energies ≤1.9 eV have been examined using an effusive beam arrangement. Experimental evidence supports that vibrational and electronic excitations suppress the cross section of the latter reaction. We have also performed PEPICO measurements of the processes, (H2O)2+hν→(H2O)+2+e− and H3O+ +OH +e−, in the region of 1020–1110 A(ring). The sums of the PEPICO intensities for H3O+ and (H2O)+2 at corresponding photoionization wavelengths yield the photoelectron spectrum for (H2O)2.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Total state-selected and state-to-state absolute cross sections for the reactions Ar+(2P3/2,1/2)+H2(X,v=0)→Ar (1S0)+H+2(X˜,v') [reaction (1)], ArH++H [reaction (2)], and H++H+Ar [reaction (3)] have been measured in the center-of-mass collision energy Ec.m. range of 0.24–19.1 eV. Absolute spin–orbit state transition total cross sections (σ3/2→1/2,σ1/2→3/2) for the collisions of Ar+(2P3/2,1/2) with H2 at Ec.m.=1.2–19.1 eV have been obtained.The measured state-selected cross sections for reaction (1) [σ3/2,1/2(H+2)] reveal that at Ec.m.≤5 eV, σ1/2(H+2) is greater than σ3/2(H+2), while the reverse is observed at Ec.m.≥7 eV. The total state-to-state absolute cross sections for reaction (1) (σ3/2,1/2→v') show unambiguously that in the Ec.m. range of 0.16–3.9 eV the dominant product channel formed in the reaction of Ar+(2P1/2)+H2(X,v=0) is H+2(X˜,v'=2)+Ar. These observations support the conclusion that at low Ec.m. the outcome of charge transfer collisions is governed mostly by the close energy resonance effect. However, at sufficiently high Ec.m.(〉6 eV) the charge transfer of Ar+(2P3/2)+H2 is favored compared to that of Ar+(2P1/2)+H2.The relative values measured for X1/2→v'[≡σ1/2→v'/σ1/2 (H+2)] are in good accord with those predicted from calculations using the state-to-state cross sections for the H+2(X˜,v'=0–4)+Ar charge transfer reaction and the relation based on microscopic reversibility. The experimental values for X3/2→v'[≡σ3/2→v'/σ3/2 (H+2)] and those predicted using the microscopic reversibility argument are also in fair agreement. The spin–orbit effect for the cross section of reaction (2) [σ3/2,1/2(ArH+)] is significantly less than that for reaction (1). Both σ3/2(ArH+) and σ1/2(ArH+) decrease rapidly as Ec.m. is increased, and become essentially identical at Ec.m. ≈3.8 eV. The cross sections for reaction (3) observed in the Ec.m. range of 2.5–12 eV are ≤3% of σ3/2,1/2(H+2).The onset for the formation of H+ by reaction (3) is consistent with the thermochemical threshold. The values for σ3/2→1/2 and σ1/2→3/2 observed here are nearly a factor of 2 greater than those measured by the energy loss spectroscopic method. However, the kinetic energy dependencies for σ3/2→1/2 and σ1/2→3/2 are in accord with the previous measurements. Theoretical cross sections for the charge transfer and spin–orbit state transition reactions are calculated at Ec.m.=19.3 eV using the nonreactive infinite-order sudden approximation for comparison with experimental values.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

The photoionization cross sections for the hydrogen atom were measured in the wavelength range from 700−920 Angstroms. It was found that the experimental results agree with the theoretical predictions.(AIP)

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Absolute spin–orbit state-selected total cross sections for the reactions, Ar+(2P3/2,1/2)+O2→O+2+Ar [reaction (1)], O++O+Ar [reaction (2)], and ArO++O [reaction (3)], have been measured in the center-of-mass collision energy (Ec.m.) range of 0.044–133.3 eV. Absolute spin–orbit state transition total cross sections for the Ar+(2P3/2,1/2)+O2 reaction at Ec.m.=2.2–177.6 eV have also been examined. The appearance energies for the formation of O+ (Ec.m.=2.9±0.2 eV) and ArO+ (2.2±0.2 eV) are in agreement with the thermochemical thresholds for reactions (2) and (3), respectively. The cross sections for O+2, O+, and ArO+ depend strongly on Ec.m. and the spin–orbit states of Ar+, suggesting that reactions (1)–(3) are governed predominantly by couplings between electronic potential energy surfaces arising from the interactions of Ar+(2P3/2)+O2, Ar+(2P1/2)+O2, and O+2+Ar.In the Ec.m. range of 6.7–22.2 eV, corresponding to the peak region of the O+ cross section curve, the cross sections for O+ are ≥50% of those for O+2. The production of O+ by reaction (2) is interpreted to be the result of predissociation of O+2 in excited states formed initially by reaction (1). The formation of charge transfer O+2(a˜ 4Πu) has been probed by the charge transfer reaction O+2(a˜ 4Πu)+Ar. The results indicate that in the Ec.m. range of 0.4–3.0 eV charge transfer product O+2 ions are formed mainly in the O+2(a˜ 4Πu) state. Experimental evidence is found supporting the conclusion that the vibrational distributions of O+2(a˜ 4Πu) formed in reaction (1) and by photoionization of O2 in the energy range between the O+2(a˜ 4Πu, v=0) and O+2(A˜ 2Πu, v=0) thresholds are similar.The population of O+(4S) formed by reaction (2) has also been measured by the reaction O+(4S)+N2→NO++N. In the Ec.m. range of 3–44 eV, product O+ ions of reaction (2) are shown to be dominantly in the O+(4S) ground state. At Ec.m.≥14 eV, the retarding potential energy analysis for O+2 shows that more than 98% of the charge transfer O+2 ions are slow ions formed mostly by the long-range electron jump mechanism. Product ArO+ ions are observed only in the Ec.m. range of 2.2–26.6 eV. At Ec.m. slightly above the thermochemical thresholds of reactions (2) and (3), the overwhelming majority of ArO+ and O+ ions are scattered backward and forward with respect to the c.m. velocity of reactant Ar+, respectively. This observation is rationalized by a charge transfer predissociation mechanism which involves the formation of ArO+ and O+ via nearly collinear Ar+–O–O collision configurations at Ec.m. near the thresholds of reactions (2) and (3).

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Photoelectron–photoion coincidence spectra for SO+2, SO+, and S+ resulting from the photoionization of SO2 over the photon energy range of 15.5–17.2 eV (720–800 A(ring)) have been obtained at an electron energy resolution of ≈40 meV (full width at half maximum). The breakdown diagram is presented for SO+2, SO+, and S+. For excited SO+2(C˜,D˜,E˜) ions formed initially with internal energies in the range of 4.18–4.87 eV, the breakdown curves for the three ions are relatively smooth. The relative yields for the formation of S+, SO+, and SO+2 via radiative stabilization are approximately 1:80:20. The lack of structure observed in the breakdown curves in this internal energy region is consistent with the conclusion that rapid internal conversion occurs prior to ionic decay. For the SO+2 internal energy range of 3.88–4.18 eV, the S+ breakdown curve is structured, suggesting that the formation of S+ follows a state-specific dissociation pathway. The results of the present experiment do not support the previous findings that the SO+2 and S+ product channels are only formed for photon energies ≤16.7 eV.

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