Thomas R. Govers

Non‐Franck–Condon transitions in resonant autoionization of N2O

Autoionization of N2O between 12.89 and 16.4 eV was investigated by photoionization using the pulsed synchrotron radiation from ACO, Orsay’s storage ring. Measurements were performed of threshold photoelectron spectra, photoionization spectra, and of photoelectron energy spectra. The latter were obtained from photoelectron time of flight distributions at selected wavelengths. The results suggest that autoionization in the Franck–Condon gap between the ? 2Π and the ? 2Σ+ states of N2O+ proceeds via two distinct mechanisms. The major autoionization process (?90%) produces the ? 2Π state in its low vibrational levels, while a resonant autoionization path (?10%) produces vibrationally excited ? 2Π ions. The latter process is associated with the production of low energy electrons with a distribution peaking sharply at zero energy. This resonant autoionization process appears to be a general phenomena for polyatomic molecules.

State-selected ion-molecule reactions: N2+(X, v″), N2+(A, v′) + Ar → N2 + Ar+

Abstract The total, absolute cross sections for charge transfer between N 2 + (X, A, v ) and Ar have been measured at 8, 14 and 20 eV center of mass translational energy. The internal energy of the N 2 + ions was selected by threshold photoelectron-photoion coincidence using pulsed synchrotron radiation from the ACO storage ring. The vibrational levels investigated were v ″ = 0−4 for the X state, and v ′ = 0−6 for the A state. The data for the A state were corrected for the fraction of ions which fluoresced to the X state prior to reaction with Ar. The N 2 + (X, v ″ = 0) state was found to be much less reactive (by a factor ≥ 10) than the other X-state levels, at all three translational energies. The levels N 2 + (A, v ′ ⩾ 3) were found to react with cross-sections which depend strongly on the relative translational energy. The data are interpreted in terms of the interaction between vibronic curves as discussed by Bauer, Fisher and Gilmore. This model accounts well for the low reactivity of the N 2 + (X, 0) level. It is proposed that the variation in the cross section of the N 2 + (A, v ′ ⩾ 3) + Ar reactions is a result of competition with a radiationless transition which converts N 2 + (A, v ′) ions into N 2 + (X, v ″) ions.

A threshold photoelectron–photoion coincidence study of the N2O+ dissociation between 15 and 20.5 eV

Branching ratios and the kinetic energy released in the various fragmentation channels of energy selected N2O+ (15–20.5 eV) were investigated by the technique of threshold photoelectron–photoion coincidence. Pulsed synchrotron radiation from ACO, Orsay’s storage ring, dispersed by a monochromator, was used as a photon source. Threshold electrons were energy selected on the basis of angular and temporal discrimination against energetic electrons. The energy region below 16.388 eV and the ? state were investigated in detail. Below the ? state the most abundant fragment ion is O+, while above the ? state NO+ dominates. Results for the ? and ? states are also reported.

Observation of dissociative states of O2+ by threshold photoelectron-photoion coincidence

Using monochromatised synchrotron radiation as an excitation source, threshold photoelectron-photoion coincidence experiments have been performed on energy-selected states of O2+. Predissociation is observed for the b4 Sigma g- and c4 Sigma u- states. Two dissociation continua are observed between 23 and 24.5 eV.

O+2 ions dissociation studied by threshold photoelectron–photoion coincidence method

The threshold photoelectron–photoion coincidence method was used to study O+2 dissociation. Previous results were confirmed and a careful analysis of the time of flight peak shapes using a Monte Carlo simulation gave us new results. The lifetime of the B 2Σ−g state was measured to be 70∓25 ns independently of the vibrational quantum number. The III 2∏u state which dissociates towards the O+(2D0)+O(3P) and O+(2P0)+O(3P) limits shows anisotropic distribution of fragment ions. The c 4Σ−u(v=0) state which was previously observed to dissociate into O+(4S0)+O(1D), its adiabatic limit, is seen to predissociate also about 40% towards the ground state limit.

Luminescence in near-thermal charge exchange, I: He++N2

Abstract A pulsed Ion Cyclotron Resonance cell fitted with synchronous photon-counting equipment is used to study the emission produced by near-thermal (≲0.1

Contamination by nickel, copper and zinc during the handling of euro coins.

The introduction of the euro has revived interest in the risk of nickel allergy due to the handling of coins. In the present work, the transfer of metallic contamination during the manipulation of coins is examined by means of leaching experiments and manipulation tests. It is shown that pre‐existing metallic species present on the surface of the coins are the major source of contamination during manipulation, and that friction inherent to everyday usage contributes predominantly to their transfer to the hands. The comparison of coins as to their relative risks of metal contamination should therefore rely on tests that simulate the friction inherent in everyday human handling. Carrying out such tests with the newly issued 1€ and 2€ pieces, we find, contrary to long‐term leaching measurements, that the euros release less nickel than previously circulated pure‐nickel coins, but that this decrease is less pronounced than might have been hoped for on the basis of their surface composition. When the coins are rubbed to a shiny polish before manipulation, contamination of the fingers is reduced by more than a factor of 10. A comparison of coins used in France indicates that the introduction of the common currency has led to a fourfold reduction in contamination by nickel, while causing a 45% increase in contamination by copper.

Excitation and decay of the C2Σ+u state of N+u in the case of electron impact on n2

Abstract This paper deals with the mechanism for electron-impact excitation of the C2Σ+u and with the competition between the two observed C state decay modes: “2nd negative” radiation (C2Σ+u→X2Σ+g) and predissociation. The relevant data were obtained from a quantitative analysis of the 2nd negative Δu = −6 and Δu = −7 emission sequences; these were produced by the bombardment of a low-pressure N2 target with an electron beam whose energy could be varied up to 800 eV. The signal from interfering Lyman-Birge-Hopfield radiation was suppressed by means of a chopped-beam technique. The data are consistent with the recent proposal by Lorquet and Desouter that the C state can be excited through intensity borrowing by configuration interaction. For the C state vibrational levels υ> 3 in 14NS14NS+ the ratio of the probability for predissociation to that for 2nd negative emission is found to increase from 9.6-1.5+2.6 for υ = 3 to more than 60 for υ = 7. For 14N15N+ predissociation becomes about 6 times less probable, and for 15N15N+ about 10 times. These observations can possibly be explained by the accidental predissociation mechanism suggested by Lorquet and Desouter.

Collision-induced dissociation of 10 keV N2+ ions: Evidence for predissociation of the C2Σu+ state

Abstract An analysis is made of the laboratory velocity distribution of forward and backward scattered N+ fragments produced by the impact of a 10 keV N2+ beam on He. The discrete structure observed is ascribed to a collision-induced population of the C2Σu+ state of N2+, followed by a predissociation of its v′ ⩾ 3 levels. The lifetime of the levels v′ ⩾ 4 is estimated to be 5 × 10−9 sec; that for the v′ = 3 is considerably longer.

State-selected ion-molecule reactions: H+2(ν) + He → HeH+ + H and He + H+ + H

Abstract Total cross sections for production of HeH+ and H+ in the reaction of state-selected H+2 (v = 0 to 6) with He at 3.1 eV c.m. collision energy are measured by means of the threshold-photoelectron/photoion coincidence method, using pulsed synchrotron radiation. Both reaction cross sections are observed to rise with vibrational energy. The H+/HeH+ branching ratio, which is determined directly, remains approximately constant at about 0.3 for v ⩽ 3 and rises gradually for higher levels to reach the value 1.3 for v = 6. For v ⩽ 3 both reactions involve hard-type collisions and result in large-angle scattering. In contrast, at higher v levels, the HeH+ becomes essentially forward scattered with respect to the incident He direction, but with a velocity greater than that expected from the spectator stripping model. The H+ products are backward scattered with respect to the incident H+2 for v ⩽ 1 and receed faster from the He atom than the H products. This observation directly leads to the conclusion that collision-induced dissociation from v = 0 and 1 involves transitions to the first excited potential-energy surface.