A. K. Edwards

Dissociation of N2+2 ions into N+ fragments

Pairs of N+ ions emitted in the dissociation of N2+2 ions were observed in coincidence. Three distinct dissociation channels are reported with total kinetic energy releases of 7.8, 10.2, and 14.8 eV. The dissociative states of N2+2 were formed by bombardment of N2 by 1 MeV He+ ions. The kinetic energy spectra of the N+ ions were measured using the time‐energy spectroscopy technique, wherein the ions released in the dissociations can be separated according to their mass‐to‐charge ratios and their kinetic energy spectra can be recorded. Essentially all of the N+ ions observed resulted from the dissociation of N2+2. The results reported here are in generally good agreement with published reports of electron induced dissociation of N2+2.

Dissociative ionization of N/sub 2/ produced by 1-MeV H/sup +/ and He/sup +/: Time-energy spectroscopy

By measuring the energy and time of flight of fragment ions from molecular breakup, the fragments can be separated according to their mass to charge ratios, and the kinetic-energy spectra for each, recorded. This technique is applied to the dissociative ionization of N/sub 2/ caused by 1-MeV H/sup +/ and He/sup +/ projectiles. The energy spectra for N/sup +/ is not unlike that produced by electron bombardment, but the N/sup + +/ spectra show some high-energy structure that has not been previously observed. An N/sup 3 +/ spectrum is obtained with a maximum near 25 eV, and the reflection method is used to obtain a potential curve for a presumed N/sub 2/ /sup 3 +/ state. (AIP)

Dissociation of CH4 by 1 MeV H+, He+, and O+ projectiles

Methane is bombarded by 1 MeV H+, He+, and O+ projectiles. Kinetic‐energy spectra are presented for fragment ions with integer mass‐to‐charge ratios of 1,2,3,4,6, and 12. The H+ and H+2 kinetic‐energy spectra show well‐defined structure which is indicative of two‐body breakup of the CH4. In addition, the H+ spectra exhibit high‐energy tails which may be attributed to multibody breakup. This feature is greatly enhanced in the H+ energy distribution produced with O+ projectiles. Bombardment with O+ projectiles also produces carbon fragments Cq+(q=1,2,3), C4, and/or H+3.

Dissociation of CO2+2 ions into CO+ and O+ fragments

Dissociative ionization of CO2 is further studied by the bombardment of 1‐MeV He+ ions. The coincident detection of CO+ and O+ fragment pairs confirms the dissociative channel CO22+→CO++O++6.0 eV which is the principal contributor to the ion yield. (AIP)

Dissociation of CH3Cl by 1 MeV H+, He+, and O+

The dissociation of methyl chloride produced by bombardment of 1 MeV H+, He+, and O+ projectiles was investigated. H+, H+2, H+3, C+, CH+2, CH+3, and Cl+ fragments were observed and their respective kinetic energy spectra recorded. Results suggest that one particular channel involved in the dissociation is CH3Cl2+→CH+3+Cl+ with a dissociation energy of 6.6 eV. Results also indicate a possible mechanism in CH+n production involving the dissociation of a CHnClm+ fragment.

Angular correlation of the N+ ions produced in the dissociative double ionization of nitrogen

The double ionization of N2 by He+ projectiles was studied by measuring the angular correlation between the two N+ ions emitted in the dissociation of N2+2 molecular ions. If there were no recoil velocity imposed on the N2+2 ion in the initial ionizing collision, and thermal effects were neglected, the pair of N+ ions would have equal and opposite velocity vectors in the laboratory frame of reference. Measuring the coincidence yield of pairs of N+ ions as a function of the angle between their velocity vectors permits an estimate to be made of the component of momentum transferred to the N2+2 parent ions in the beam direction. The results presented in this report show the recoil velocity to be considerably less than the mean thermal velocity of N2 molecules at room temperature. We also report mesurements of the relative cross section for N+ production from N2+2 as a function of the orientation of the N2 target molecules relative to the projectile beam axis.

Dissociation of CO and NO by fast H+, He+, and O+ projectiles

In this work we report on studies of the dissociative ionization of CO and NO induced by 1‐MeV H+, He+, and O+ projectiles. The time‐energy spectroscopy (TES) technique is used to identify the dissociation fragments according to their mass‐to‐charge (m/q) ratios, and to record kinetic energy spectra for each fragment species. The spectra fragments with 1+, 2+, and 3+ charges show overlapping peaks for which energy assignments are made. In most case, the He+ and O+ projectiles appear to form dissociative states of CO2+ and NO2+ which decay into singly charged fragments. The H+ projectile appears to form CO+ and NO+ which subsequently dissociate into charged and neutral fragments. The dissociative states excited in this experiment are different from those reported in K‐shell ionization studies.

Excitation of the Q11Σg+ doubly excited state of H2 by electron impact

400 eV electrons were used to excite the autoionizing state Q11Σg+ of H2. The relative cross section was measured as a function of: (1) the angle between the internuclear axis and the momentum transfer vector of the collision; and (2) energy loss for a fixed orientation of the molecular axis in the laboratory frame. In the first set of measurements electrons scattered at 18° with an energy loss of 29 eV were detected in coincidence with a 2 eV H+ ion resulting from dissociation following autoionization. The angular distribution of the 2 eV ions was recorded and fitted to calculated values. A maximum was found when the internuclear axis was aligned with the momentum transfer vector. Secondly, the relative yield of 2 eV H+ ions was measured as a function of energy loss in the electron-H2 collisions. Energy loss ranged from 23 to 35 eV. The lowest lying Q11Σg+, 1Σu+ and 1Πg were included in the calculated fit to the data.

Limitations of the axial recoil approximation in measurements of molecular dissociation

The axial recoil approximation holds that when a diatomic molecular ion is formed in a dissociative state, the atoms produced in the dissociation process will move outward along the straight line defined by the internuclear axis of the molecule. Analysis of experiments measuring the angular distribution of Auger electrons emitted by N2 following K-shell ionization of N2 molecules shows that the axial recoil approximation is not strictly true. Significant corrections must be made for the rotation of the molecule during the time of dissociation. Smaller corrections must be made for the thermal distribution of the translational velocities of the target molecules, and for instrumental effects. In the analysis of the N2 data, the corrections have the effect of smoothing the predicted angular distribution functions. The amount of the smoothing depends primarily on the temperature of the target gas and the shape of the potential-energy curve for the N22+ final state involved in the Auger transition.

The dissociative ionization of CO2 by fast heavy ions

The time–energy spectroscopy (TES) technique has been used to study the dissociative ionization of CO2 induced by bombardment with 1 MeV H+, He+, and O+ ions. Fragments observed in the dissociation process include C+, O+, CO+, C2+, and O2+. The technique used permits separation and identification of the different fragment species, and measurement of a kinetic energy spectrum for each species. The kinetic energy spectra and relative intensities of the different fragments are reported. The O+ ions constitute more than half of the total charged fragment yield. The C+ and CO+ yields are nearly equal and the yield of doubly charged ions is small. The energy spectra and yields are compared to electron impact and neutral helium impact dissociation studies. The O+ and CO+ data suggest that the channel (CO2)2+→CO++O++6.0 eV is a major contributor to the observed spectra.