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Notes: 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.

Notes: 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).

Notes: 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.

Notes: 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.

Notes: Total state-selected and state-to-state absolute cross sections for the reactions, H+2(X˜,v'=0–4)+Ar→H2(X,v) +Ar+(2P3/2,1/2) [reaction (I)], ArH++H [reaction (II)], and H++H+Ar [reaction (III)], have been measured in the center-of-mass collision energy (Ec.m.) range of 0.48–100 eV. Experimental state-selected cross sections for reactions (I) and (II) measured at Ec.m.=0.48–0.95 eV are in agreement with those reported previously by Tanaka, Kato, and Koyano [J. Chem. Phys. 75, 4941 (1981)]. The experiment shows that prominent features of the cross sections for reactions (I) and (II) are governed by the close resonance of the H+2(X˜,v'=2)+Ar and H2(X,v=0)+Ar+(2P1/2) vibronic states. At Ec.m.≤3 eV, the vibrational state-selected cross section for the charge transfer reaction (I) is peaked at v'=2.The enhancement of the charge transfer cross section for v'=2 as compared to other v' states of reactant H+2 increases as Ec.m. is decreased. The state-to-state cross sections for reaction (I),measured at Ec.m.≤3 eV, show that the enhancement for the charge transfer cross section for v'=2 is due to the preferential population of Ar+(2P1/2). At Ec.m.=0.48–0.95 eV and v'=2, nearly 80% of the charge transfer product Ar+ ions are formed in the 2P1/2 state. However, at Ec.m.〉5 eV, the intensity for charge transfer product Ar+(2P3/2) is greater than that for Ar+(2P1/2). Contrary to the strong vibrational dependence of the cross section for reaction (I), the cross section for reaction (II) is only weakly dependent on the vibrational state of H+2. At Ec.m.≤3 eV, the cross section for the formation of ArH+ is the lowest for v'=2 compared to other v' states, an observation attributed to the competition of the nearly resonant Ar+(2P1/2)+H2(X,v=0) charge transfer channel. The cross section for reaction (II) decreases with increasing Ec.m..At Ec.m.≥20 eV, the cross sections for the formation of ArH+ become negligible compared to those for Ar+. The appearance energies for the collision-induced dissociation H+2(X˜,v'=0–4) are consistent with the thermochemical threshold for reaction (III). The cross sections the formation of H+ are ≤20% of those for H+2. Theoretical state-to-state cross sections for reaction (I) at Ec.m.=19.3 and 47.6 eV calculated using the nonreactive infinite-order sudden approximation are found to be in fair agreement with experimental results.

Notes: 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)