Bibliothek

Notizen: State-selected absolute cross sections for H2O+ and OH+ formed by the O+(2D,2P)+H2O reactions have been measured in the center-of-mass collision energy (Ec.m.) range of (approximate)0.10–30 eV. The charge transfer cross sections for O+(2D)+H2O are significantly higher than those for O+(4S)+H2O. This observation is attributed to the increased number of accessible exothermic product channels for O+(2D)+H2O. While the H2O+ cross sections for O+(2P)+H2O are comparable to those from O+(4S)+H2O at Ec.m.≥1 eV, the H2O+ cross sections for O+(2P)+H2O at Ec.m.〈1 eV are substantially lower than those for O+(4S)+H2O. The lower H2O+ cross sections observed for O+(2P)+H2O are rationalized as due to further dissociation of excited charge transfer H2O+ ions and/or the efficient competition of the OH++OH product channel. The cross sections for OH+ from O+(2D,2P)+H2O are significantly greater than those from O+(4S)+H2O. The majority of OH+ ions from O+(2D,2P)+H2O are associated with exothermic channels corresponding to the formation OH+(X 3Σ−,1Δ,A 3Π)+OH. The comparison of the sum (σT) of the cross sections for H2O+ and OH+ from O+(4S)+H2O to those from O+(2D)+H2O and O+(2P)+H2O shows that σT's for O+(4S)+H2O and O+(2P)+H2O are comparable, whereas the σT values for O+(2D)+H2O are greater than those for O+(4S)+H2O and O+(2P)+H2O. The σT values are found to conform with the 1/Ec.m. dependence at low Ec.m.'s, indicating that the ion–dipole interaction plays an important role in the formation of the long-lived collision complexes. The high cross sections for H2O+ and OH+ from O+(2D,2P)+H2O observed here suggest that these reactions should be included in the simulation of the H2O+ and H3O+ ion density data obtained in space-borne mass spectrometric experiments. © 1997 American Institute of Physics.

Notizen: 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 (approximate)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 involving O+(2D) and O+(2P) are associated mostly with the H+ (D+)+OH (OD) channel, the formation of which obeys the spin-conservation rule. The comparison of the sum (σT) of cross sections for OH+ (OD+), H2+ (D2+), and H+ (D+) from O+(4S)+H2 (D2) to those from O+(2D)+H2 (D2) and O+(2P)+H2 (D2) shows that the σTs for O+(4S)+H2 (D2), O+(2D)+H2 (D2), and O+(2P)+H2 (D2) at Ec.m.〈0.5 eV are comparable. At Ec.m.〉0.5 eV, the σTs for O+(2P)+H2 (D2) are greater than those for O+(2D)+H2 (D2), which in turn are greater than those for O+(4S)+H2 (D2). This observation is attributed to the increase in the number of accessible product channels for reactions involving the excited O+(2D) and O+(2P) reactant ions. © 1997 American Institute of Physics.

Notizen: 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++NO to generate N+ is strongly suggested as the major channel in the reactions of O+(2D,2P)+N2, as evidenced by the observation of N+ well below the thermochemical thresholds of O+(2D,2P)+N2→N++N+O. © 1997 American Institute of Physics.

Notizen: We present a differential retarding potential (DRP) method for improving the kinetic energy resolution of a reactant ion beam for scattering experiments. This method allows ion-molecule reaction absolute total cross-section measurements to be performed down to thermal energies using the simple electrostatic aperture ion lenses of a tandem quadrupole mass spectrometric ion-molecule reaction apparatus, even though the reactant ions are formed originally with a broad kinetic energy distribution. To illustrate the principle of the DRP method, examples are given for its application to reactant ion beams prepared in an electron impact ion source and in an ion-molecule reaction ion source. © 1994 American Institute of Physics.

Notizen: An experimental device is reported that utilizes time-correlated nanosecond light pulses in combination with photoionization mass spectrometry. A primary light pulse is generated by a tunable dye laser in the ultraviolet regime, which photolyzes neutral gas targets under collision free conditions. Subsequently, a time-correlated extreme ultraviolet-light pulse comes from a laser-produced plasma that is monochromatized in the 10–25 eV regime. The photolysis products are ionized by one-photon absorption, so that the cations are finally detected by time-of-flight mass spectrometry. The performance of this experimental approach is characterized by investigating the primary photolysis products of chlorine dioxide. Finally, possible applications of this approach are briefly discussed. © 2000 American Institute of Physics.

Notizen: By controlling the collision energies for dissociative charge transfer collisions of He+(Ne+,Ar+) +O2 in a rf octopole ion guide gas cell, and by applying appropriate effective ion trapping potentials to the rf octopole ion guide, we show that state-selected O+(4So), O+(2Do), and O+(2Po) reactant ion beams with high purities and usable intensities can be prepared for scattering experiments. This experimental scheme, which makes possible the enrichment of an ionic species with a lower kinetic energy distribution in a rf multipole ion guide, should be useful for state selection of other excited atomic ions by using appropriate dissociative charge transfer or dissociative photoionization processes. © 1995 American Institute of Physics.

Notizen: Sulfur 1s excitation of S2 and S8 is reported. The experiments made use of monochromatic synchrotron radiation from the BW-1 beam line at the HASYLAB (Hamburg, Germany), where total cation yields and photoionization mass spectra were investigated as a function of the source temperature that was used for evaporation of α-sulfur. The near-edge structure undergoes characteristic changes as a function of the source temperature, reflecting significant size- and geometry-dependent changes in electronic structure of the sulfur clusters. The detailed analysis of the near-edge features is obtained from ab initio (MR-SDCI) calculations that focus on S8 and S2. The present results are discussed in comparison to previous work on inner-shell excited sulfur clusters as well as condensed sulfur. The near-edge structure of S2 is compared to that of O2 and Se2, yielding characteristic differences in core–valence and valence–valence–exchange interaction. © 2002 American Institute of Physics.

Notizen: 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 ((approximately-greater-than)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 (approximately-greater-than)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) or underestimate the populations for N+2(v'〉2) resulting from reaction (2) at Ec.m.=10.3and 41.2 eV. Absolute spin-orbit-state-selected total cross sections for reactions (1) and (2), σ3/2 and σ1/2, respectively, at the Ec.m. range of 0.25–115.3 eV have also been measured using a tandem photoionization mass spectrometer which is equipped with a radio frequency (RF) octopole ion guide reaction gas cell. The measured values for σ3/2 at Ec.m.=4.1, 10.3, and 41.2 eV and σ1/2 at 41.2 eV are in reasonable agreement with the theoretical cross sections. However, the experimental values for σ3/2 at 1.2 eV and σ1/2 at 1.2, 4.1, and 10.3 eV are approximately a factor of 2 higher than the theoretical predictions. A model analysis, which takes into account possible collision-induced spin-orbit mixings of the reactant Ar+ states in the RF octopole gas cell, shows that the values for σ1/2/σ3/2 and σ1/2 determined using the ion beam–RF octopole gas cell arrangement can be strongly susceptible to gas cell pressure effects whereas the experimental values for σ3/2 are reliable. The values for σ1/2 deduced by multiplying the values for σ3/2 and the ratios σ1/2/σ3/2 determined in the crossed ion–neutral beam experiment are in agreement with the theoretical cross sections. Both σ3/2 and σ1/2 are found to increase as Ec.m. is increased from 41.2 eV. This observation is interpreted as due to the formation of N+2 in the A˜ 2Πu state at high Ec.m. . Combining the measured vibrational state distributions of product N+2(X˜,v') ions and the absolute state-selected total cross sections, absolute state-to-state total cross sections for reactions (1) and (2) at selected Ec.m. are determined.

Notizen: 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 energy surfaces of the [N2+Ar]+ system. Experimental state-to-state cross sections obtained in this study are consistent with those for the reaction Ar+(2P3/2)+N2(X,v=0)→Ar(1S0)+N+2 (X˜,v') from the consideration of microscopic reversibility.