Charge exchange between xenon ions and xenon atoms is the source of a detrimental low energy plasma in the vicinity of electrostatic spacecraft thrusters. Proper modeling of charge-exchange induced spacecraft interactions requires knowledge of the respective charge-exchange cross sections. Guided-ion beam measurements and semiclassical calculations are presented for xenon atom charge-exchange collisions with Xe+ and Xe2+ at energies per ion charge ranging from 1 to 300 eV. The present measurements for the symmetric Xe++Xe exchange system are in good agreement with several earlier experimental studies and semiclassical calculations based on the most recently computed Xe2+ interaction potentials. The cross sections are ∼30% higher than predictions by the Rapp and Francis model [D. Rapp and W. E. Francis, J. Chem. Phys. 37, 2631 (1962)]. The present Xe2++Xe symmetric charge exchange measurements are the first to cover the ion energy range from 40 to 600 eV. The cross sections are in good agreement with low-energy drift tube measurements and are significantly lower than previous higher energy measurements. A simple model for symmetric two-electron transfer is proposed that is in good agreement with the present measurements. The onset for the asymmetric charge-exchange process, Xe2++Xe→2Xe+, is observed to be at 10 eV. For this process, a cross section of 2.8±0.9 Å2 is measured for a Xe2+ energy of 600 eV.

The endothermic proton transfer reaction, H2+(v+,N+=1)+Ne→NeH++H(ΔH=0.54 eV), is investigated over a broad range of reactant vibrational energies using the pulsed-field ionization–photoelectron–secondary ion coincidence (PFI–PESICO) scheme. For the lowest vibrational levels, v+=0 and 1, a detailed translational energy dependence is also presented using a continuous approach for preparing reactant ions with monochromatic VUV. Sharp threshold onsets are observed, suggesting the importance of long-lived intermediates or resonances. At a translational energy, ET=0.7 eV, absolute state-selected reaction cross sections are measured for all reactant vibrational levels v+=0–17. For levels v+=0–6, the cross sections grow rapidly with vibrational quantum, above which the cross sections saturate at a value of ∼13±4 Å2. At levels v+>13, the cross sections decline, probably due to competition with the dissociation channel. At a translational energy, ET=1.7 eV, absolute state-selected reaction cross sections are measured for reactant vibrational levels spanning the range between v+=0 and 14. Cross section growth is observed from v+=0–7, above which the cross sections no longer exhibit a steady trend. At ET=4.5 eV, cross sections are reported for vibrational levels covering the range between v+=0 and 12. The cross sections are substantially lower at this high translational energy, however, they still exhibit a substantial vibrational enhancement below v+=8. The present measurements are compared with quasiclassical trajectory (QCT) calculations. The comparison can be categorized by three distinct total energy (Etot=ET+Evib) regimes. For Etot<1 eV, the experimental cross sections exceed the QCT results, consistent with important quantum effects at low energies. For 1<Etot<3 eV, excellent agreement is observed between the PFI–PESICO cross sections and the QCT calculations. At total energies exceeding 3 eV, the experimental results are generally higher, probably because QCT overpredicts competition from the dissociation channel.

We present results on the energy dependence of the vibrational branching ratio for the bending mode in CO 2 3 u Ϫ1 photoionization. Specifically, we determine the v ϩ ϭ(0,1,0)/v ϩ ϭ(0,0,0) intensity ratio by detecting dispersed fluorescence from the electronically excited photoions. The results exhibit large deviations over a very wide energy range, 18Ͻh exc Ͻ190 eV. Production of the v ϩ ϭ(0,1,0) level of the ion from the v 0 ϭ(0,0,0) ground state is forbidden by symmetry, and while observations of such features are well established in photoelectron spectroscopy, their appearance is normally ascribed to vibronic coupling in the ionic hole state. In this case, we find that such explanations fail to account for the energy dependence of the branching ratio. These deviations indicate that the continuum photoelectron participates in transferring oscillator strength to the nominally forbidden vibrational transition. A theoretical framework is developed for interpreting the experimental data, and Schwinger variational calculations are performed. These calculations demonstrate that the continuum electron is responsible for the observation of the excited bending mode as well as its energy dependence. This is an intrachannel effect that is best described as photoelectron-induced vibronic symmetry breaking. This appears to be a general phenomenon, and it may be useful in illuminating connections between bond angle and photoionization spectroscopies. The magnitude of these deviations display the utility of vibrationally resolved studies, and the extent over which these changes occur underscores the necessity of broad range studies to elucidate slowly varying characteristics in photoionization continua.

5638ciated with 1 ', k o h s d exceeds the extrapolated k o h s d for benzanilide by a factor of 6 X IO5. The rate constant for the bimolecular reaction between phosphate and 3 is very small. No reaction of 3 in 1 M potassium phosphate buffer (pH 6) was observed when the solution was maintained at 90" for 1 month. Therefore, comparison of the rate of the intramolecular reaction of the internal phosphonate residue with the amide with that of external phosphate and 3 can only be estimated to favor the internal reaction by an apparent concentration of more than I O 3 M , assuming we could detect a concentration change of 3%. The solvent isotope effect in both plateau regions (below pH 2 and above pH 5), k H 9 0 / k D I 0 , is 1.0, indicative of nucleophilic ~a t a l y s i s . ~ We propose the mechanism in Scheme I as a likelyTherefore k in eq 1 = route for the hydrolysis reaction of 1 which is consistent with the experimental data and kinetic forms (an analogous route can be written for the monoanion). The proposed cyclic acyl phosphonate intermediate (B) is a compound known to hydrolyze rapidly under the conditions of the amide hydrolysis reaction.*lg In Scheme I, hydrolysis of the acyl phosphonate B occurs after the aniline molecule has left the addition intermediate and thus after the chromophore being observed has been eliminated. Nucleophilic catalysis by neighboring un-ionized carboxyl functions (or the zwitterionic equivalent) is a favored mechanism in the hydrolysis of amides lo-l 6 (7) (a) A, R. Butler and V. Gold, Proc. Chem. Soc., London, 15 (1960); (b) C. A. Bunton and V. although a mechanism involving electrostatic catalysis has also been c o n~i d e r e d .~~~'~~~~ The observed rate constant for hydrolysis of these compounds shows a plateau region similar to that which we have observed for 1, where the plateau ends as dissociation of the carboxylic acid occurs. Solvent isotope effects on these rate constants for carboxyl participation are also near unity. This argues for a similarity of mechanism between the phosphoric and carboxylic acid cases. Since dissociation of the proton from the phosphonate monoacid occurs at higher pH than does the corresponding carboxylic acid, the phosphonate group is an apparently more effective internal catalyst in neutral solution. Our finding further emphasizes that the amide functionality is reactive toward noncarboxylic functional groups as well as carboxylic acids. The very large rate ratio of the inter-and intramolecular reactions of phosphate with the amide gives further indication of a mechanism involving nucleophilic rather than general acid or base catalysis. l7 We are examining the requirements for interaction of phosphates and amides in further detail and are extending our kinetic studies on these and related systems.Acknowledgments. We note our appreciation for support provided by the National Institute of Arthritis, Metabolism and Digestive Diseases (AM-I 5013-03) and the Research Corporation.(13) Sir:We wish to report here (for the first time to our kno...

Public reoortiiKi buitien for this collection of information is estimated to average 1 hour per response, including the lime for reviewing instructois, searching easting data sounces galhenng and rnaintaimg the Sl^^,^S^^nd <^c*Snq and reviewing this collection of informatfon. Send comments regarding this burden estimate or any other aspect of this collecbon of information, ineludKig suggestions for reducing ^.^^to^^^^^^^/^alNngt^Headquarteis Services, Directorate for Infomiation . 1215 Jeffeison Davis H^hv^y, Surte 12M, Arlington, VA 22202-^R^^S^d bea«lre that notvSthstandind any other provision of law, no person shall be subject to any penalty for foiling to comply with a coHecbon of infomnaton if it does not display a cunently SM OMR^Mitroi nirober. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. REPORT DATE (DD-MM-YYYY) AFRL-VS-HA-TR-2004-1100 SPONSOR/MONITOR'S ACRONYM(S)AFRL/VSBXT SPONSOR/MONITOR'S REPORT NUMBER(S) DISTRIBUTION / AVAILABILITY STATEMENT ABSTRACTWe have developed an octopole-quadrapole photoionization apparatus at the Advanced Light Source for absolute integral cross-section measurements of rovibrational-state-selected ion-molecule reactions. This apparatus consists of a high-resolution photoionization ion source, a wired ion gate lens, a dual radio-frequency (rO octopole ion guide reaction gas cell, and a quadmpole mass spectrometer for reactant and product ion detection. The unique feature of this apparatus is the implementation of the high-resolution pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence (PFI-PEPICO) technique, which has allowed the rotational-state selection of diatomic ions for ion-molecule reaction studies, The novel application of the wired ion gate lens for the rejection of false coincidence background ions is described. This application, along with the difFerenti?d-ion-gate scheme, has made possible the measurements of rovibrationai-state-selected absolute integral reaction cross sections for ion-molecule collisions using the PFI-PE-secondaiy ion coincidence (PFI-PESICO) method. ' We have developed an octopole-quadrupole photoionization apparatus at the Advanced Light Source for absolute integral cross-section measurements of rovibrational-state-selected ion-molecule reactions. This apparatus consists of a high-resolution photoionization ion source, a wired ion gate lens, a dual radio-frequency (rf) octopole ion guide reaction gas cell, and a quadrupole mass spectrometer for reactant and product ion detection. The unique feature of this apparatus is the implementation of the high-resolution pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence (PFI-PEPICO) technique, which has allowed the rotational-state selection of diatomic ions for ion-molecule reaction studies. The novel application of the wired ion gate lens for the rejection of false coincidence background ions is described. This application, along with the diflferential-ion-gate scheme, has made possible the measurements of rovibrational-state-selected absolute integral reacti...