G. D. Flesch

A 193 nm laser photofragmentation time‐of‐flight mass spectrometric study of CS2 and CS2 clusters

A crossed laser and molecular beam photofragmentation apparatus is described. The apparatus is equipped with a rotatable molecular beam source and a translationally movable ultrahigh vacuum mass spectrometer for time‐of‐flight (TOF) measurements. Using this apparatus we have measured the TOF spectra of S and CS resulting from the photofragmentation processes, CS2+hν(193 nm)→CS(X,v)+S(1D or 3P). The translational energy distributions of photofragments derived from the S and CS TOF spectra are in good agreement. This observation, together with the finding that the TOF spectra of S and CS are independent of laser power in the 25–150 mJ range, shows that the further absorption of a laser photon by CS to form C(3P)+S(3P) within the laser pulse is insignificant. The TOF spectra of S obtained at electron ionization energies of 20 and 50 eV are indiscernible, indicating that the contribution to the TOF spectrum of S from dissociative ionization of CS is negligible at electron impact energies ≤50 eV. The thermoche...

A state‐to‐state study of the electron transfer reactions Ar+(2P3/2,1/2)+N2(X̃,v=0)→Ar(1S0) +N+2(X̃,v’)

The vibrational state distributions of N+2(X,v’) ions resulting from the reactions, Ar+(2P3/2)+N2(X,v=0)→Ar(1S0) +N+2(X,v’) [reaction (1)] and Ar+(2P1/2)+N2(X,v=0)→Ar(1S0) +N+2(X,v’) [reaction (2)], over the center‐of‐mass collisional energy (Ec.m.) range of 0.25–41.2 eV in a crossed ion–neutral beam experiment have been probed by the charge exchange method. The experimental results obtained for reaction (1) are in accord with the predictions of the semiclassical multistate calculation of Spalburg and Gislason that N+2 ions are formed predominantly (≳85%) in the v’=1 state and that the production of N+2(X,v’=0) becomes more important as Ec.m. is increased. The experiment also supports the theoretical results for reaction (2) at Ec.m.=1.2 and 4.1 eV showing that ≳80% of N+2 product ions are in the v’=2 state. However, the calculation is found to either over‐estimate the populations for N+2(v’ 2) resulting from reaction (2) at Ec.m.=10.3and 41.2 eV. Absol...

A state-selected study of the ion–molecule reactions O+(4S,2D,2P)+N2

Absolute state-selected cross sections for the reactions O+(4S,2D,2P)+N2→N2++O, NO++N, and N++NO (and/or N++N+O) have been measured in the center-of-mass collision energy (Ec.m.) range of 0.06–40 eV employing the differential retarding potential method and the O+(2D) and O+(2P) ion state-selection schemes we developed recently. Charge transfer is the overwhelming product channel for the O+(2D)+N2 and O+(2P)+N2 reactions. Contrary to the results of previous experiments, the charge transfer cross sections for O+(2P)+N2 are found to be 30%–100% greater than those for O+(2D)+N2. This observation suggests that N2 is an excellent quenching gas for O+(2D,2P). While the Ec.m. dependencies for the cross sections of NO+ from O+(4S)+N2 and O+(2D)+N2 are similar, exhibiting a broad maximum in the Ec.m. range of 1.5–8 eV, the cross section for NO+ from O+(2P)+N2 is found to decrease as Ec.m. is decreased. The N+ signal observed in the O+(4S)+N2 reaction is attributed to the formation of N++N+O. The pathway of O++N2→N+...

Absolute state‐to‐state total cross sections for the reactions N+2(X̃,v’=0–2) +Ar(1S0)→N2(X,v)+Ar+(2P3/2,1/2)

Absolute state‐selected total cross sections σv’, v’=0 and 1, for the reaction N+2(X,v’=0,1) +Ar(1S0)→N2(X,v)+Ar+(2P3/2,1/2) [reaction (1)] over the center‐of‐mass collisional energy (Ec.m.) range of 1.2–140 eV have been measured using the photoionization mass spectrometric and radio frequency ion guide methods. These measurements, together with the relative values for σv’, v’=0–2, and spin‐orbit‐state distributions of product Ar+ ions determined using the crossed ion‐neutral beam photoionization apparatus, allow the determination of the absolute values for σ2 and partial state‐to‐state cross sections σv’→J, v’=0–2, for reaction (1). Absolute values for σv’, v’=0–2, at Ec.m.=8 and 20 eV are in good agreement with those determined previously by the threshold photoelectron secondary ion coincidence method. Absolute values for σv’→J, v’=0–2, at Ec.m.=8 and 20 eV are also found to be in satisfactory accord with the predictions of the semiclassical multistate calculation which uses the ab initio potential ene...

Absolute state-selected and state-to-state total cross sections for the reaction Ar+(2P3/2,1/2)+O2

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 ...

Theoretical and experimental total state-selected and state-to-state cross sections. III, The (Ar+H2)+ system

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.

Experimental and theoretical total state‐selected and state‐to‐state absolute cross sections. II. The Ar+(2P3/2,1/2)+H2 reaction

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....

Experimental and theoretical total state-selected and state-to-state absolute cross sections. II. The Ar sup + ( sup 2 P sub 3/2,1/2 )+H sub 2 reaction

Total state-selected and state-to-state absolute cross sections for the reactions Ar{sup +}({sup 2}{ital P}{sub 3/2,1/2})+H{sub 2}({ital X},{ital v}=0){r arrow}Ar ({sup 1}{ital S}{sub 0})+H{sup +}{sub 2}({ital {tilde X}},{ital v}{prime}) (reaction (1)), ArH{sup +}+H (reaction (2)), and H{sup +}+H+Ar (reaction (3)) have been measured in the center-of-mass collision energy {ital E}{sub c.m.} range of 0.24--19.1 eV. Absolute spin--orbit state transition total cross sections ({sigma}{sub 3/2{r arrow}1/2},{sigma}{sub 1/2{r arrow}3/2}) for the collisions of Ar{sup +}({sup 2}{ital P}{sub 3/2,1/2}) with H{sub 2} at {ital E}{sub c.m.}=1.2--19.1 eV have been obtained.

Absolute state-selected total cross sections for the ion–molecule reactions O+(4S,2D,2P)+H2(D2)

Absolute total cross sections for the state-selected reactions of O+(4S,2D,2P)+H2 (D2) have been measured in the center-of-mass collision energy (Ec.m.) range of 0.02–12 eV. The cross sections for OH+ (OD+) from O+(2D)+H2 (D2) are slightly higher than those from O+(4S)+H2 (D2), whereas the OH+ (OD+) cross sections from O+ (2P)+H2 (D2) are ≈40% lower than those from O+(4S)+H2 (D2) and O+ (2D)+H2 (D2). At Ec.m.<0.5 eV, the total cross sections for OH+ (OD+) from O+ (4S)+H2 (D2) and O+(2D)+H2 (D2) are in accord with those predicted by the Langevin–Gioumousis–Stevenson model. Significantly higher cross sections are observed for H+ (D+) and H2+ (D2+) from O+(2D)+H2 (D2) and O+(2P)+H2 (D2), as compared to those from O+(4S)+H2 (D2). The exothermic nature of the O+(2D,2P)+H2 (D2) charge transfer collisions accounts for the high cross sections observed for H2+ (D2+). While the H+ (D+) ions observed in the O+(4S)+H2 (D2) reaction are identified with the H+ (D+)+O+H channel, the H+ (D+) ions from the reactions invol...

Absolute state‐selected and state‐to‐state total cross sections for the Ar+(2P3/2,1/2)+CO2 reactions

Absolute spin–orbit state‐selected total cross sections for the reactions, Ar+(2P3/2,1/2)+CO→CO++Ar [reaction (1)], C++O+Ar [reaction (2)], O++C+Ar [reaction (3)], and ArC++O [reaction (4)], have been measured in the center‐of‐mass collision energy (Ec.m.) range of 0.04–123.5 eV. Absolute spin–orbit state transition total cross sections for the Ar+(2P3/2,1/2)+CO reactions at Ec.m. have also been obtained. The appearance energies (AE) for C+(Ec.m.=6.6±0.4 eV) and O+(Ec.m.=8.6±0.4 eV) are in agreement with the thermochemical thresholds for reactions (2) and (3), respectively. The observed AE for reaction (4) yields a lower bound of 0.5 eV for the ArC+ bond dissociation energy. The kinetic energy dependence of the absolute cross sections and the retarding potential analysis of the product ions support that ArC+, C+, and O+ are formed via a charge transfer predissociation mechanism, similar to that proposed to be responsible for the formation of O+ (N+) and ArO+ (ArN+) in the collisions of Ar+(2P3/2,1/2)+O2(N2).