C.S. Enos

Collisional excitation of CO: a study of the wigner spin rule

Abstract Vibrational excitation of CO by 3 keV H+, He+, N+, O+, Cl+ and Ar+ ions has been investigated by high resolution translational energy spectroscopy. The vibrational structures for the observed X1Σ+ → A1Π and X1Σ+ → a3Πr transitions of CO are clearly resolved, and the various reaction channels are unambiguously identified. The projectile ions are deliberately chosen for their distinctly different valence electronic configurations in a systematic study of the Winger spin conservation rule. In all ion-atom/molecule collisions involving simultaneous transitions in both partners, spin conservation dictates that a single-triplet (or doublet-quartet) transition in one partner necessitates observance of the following selection rule by the other collision partner: ΔS = 0, ± 1 with singlet-singlet transition forbidden. In every instance where CO molecule undergoes a X1Σ+ → a3Πr transition, this selection rule appears to be observed rigorously by the atomic ion.

Electronic excitation of atmospheric molecules by proton impact

Abstract Electronic and vibrational excitation of N 2 , O 2 , CO and NO by 3 keV protons has been investigated by means of high resolution translational energy spectroscopy. The dominant transitions ( X 1 Σ + g → a 1 Π g ) in N 2 and ( X 1 Σ + → A 1 Π) in CO are well resolved and the relative intensities for the vibrational excitations have been determined. The dominant structures in O 2 result from excitation to the B 3 Σ − u and A 3 Σ + u states which leads to dissociation of the O 2 molecule. The most prominent feature in NO is difficult to identify because of the existence of eleven doublet electronic states from the B ′ 2 Δ i (7.48 eV) to the S 2 Σ + (8.29 eV) state. Three well-defined peaks observed at 5.5 eV to 6.1 eV have been assigned to the transitions X 2 Π r ( v = 0) → A 2 Σ + ( v = 0, 1, 2) of NO.

Translational energy spectroscopy of single-electron capture by C2+ ions in He, Ne and Ar

Abstract High resolution (0.3 eV at FWHM) translational energy spectroscopy has been employed to study single-electron capture by 4 keV C 2+ ions in collision with He, Ne and Ar atoms. Our results are generally in good agreement with previous measurements. Our spectra reveal an additional weak channel for C 2+ in He and two additional weak channels for C 2+ in Ne, not previously reported. One of these capture channels in Ne occurs at a relatively large endothermic energy change. Eight transitions are identified for the single-electron capture by C 2+ in Ar.

Collisional excitation of the Meinel and first negative systems of N+2

Abstract High-resolution translational energy spectroscopy has been performed of N+2 ions scattered off Ar, N2, O2 and NO. Intense excitations and de-excitations of the Meinel system of N+2 (A2Πu ← X2∑+2) were observed in all collision gases. Transitions in the first negative system N+2 (B3∑+u ← X2g) were also detected, and well-resolved in the case with Ar as collision gas. Simultaneous transitions involving excitation of the Meinel system and the a1Δg state of O2 were observed with O2 as collision gas.

Spin conservation in collision of C+ ions with atomic and molecular targets

Abstract High resolution translation energy spectroscopy (TES) has been performed on C+ ions in collision with Ar, CO, O2 and NO. The target species included atoms and molecules in singlet, triplet and doublet ground states. The observed reaction channels were discussed in the light of a simple selection rule based on the Wigner spin conservation requirements. All the identified channels were found to be in accord with these requirements. The fractional population of the 4P state in the C+ ion beam has been estimated from the relative intensities for the C+ (2P ↔ 4P) transitions obtained from the TES spectrum of C+ on O2. The fraction of C+ (4P) was found to be about 10% of ions produced from CH4 precursor gas at a nominal electron ionizing energy of 100 eV.

Single electron capture by C3+ ions in He, Ne and Ar

Abstract Translational energy spectra have been obtained for 6 keV C 2+ ions resulting from single-electron capture by 6 keV C 3+ ions in collision with He, Ne and Ar. Our data for He and Ne are in good agreement with previous measurements, while data for the Ar target have not appeared in the literature. The spectrum for C 3+ in Ar is complex and appears to contain many strong spectral features which involve capture with excitation of the target product ion in Ar + .

Collisional excitation of NO by H+, N+ and O+ ions

Abstract Despite the low atmospheric abundance of NO, it is one of the more important components, playing a vital role in many atmospheric processes. The excitation of NO by the atmospheric ions H + , N + and O + has been investigated using high resolution translational energy spectroscopy conducted at 3 keV. The first three vibrational levels of the γ system, NO(X 2 Π r ) → NO(A 2 Σ + , ν′ = 0–2) were clearly resolved. The main features of the spectra could not be unambiguously assigned as they comprised of several (11) overlapping processes, but two features N + ( 3 P) + NO(X 2 Π r ) → N + ( 5 S) + NO(X 2 Π r ) and N + 3 P) + NO(X 2 Π r ) → N + ( 1 D) + NO + ( 1 Σ + + e − stood out. The use of the Wigner spin conservation rule greatly facilitated spectral interpretation by reducing the number of candidate reaction channels available.

Breakdown of spin conservation for collisions of chlorine ions with atomic and molecular gases

High resolution translational energy spectroscopy has been performed on 3 keV Cl+ ions in collisions with Ar, He, N2, O2 and CO. The target species included atoms and molecules in singlet and triplet ground states. Spectroscopically, N+ and Cl+ ions are very similar, both have 3P ground states and have similar low-lying metastable states 1S and 1D. Correspondingly, their energy loss spectra are very similar. In collisions of Cl+ with N2, CO and Ar a weak structure is observed which is identified as the spin non-conserved transition 3P → 1D in the Cl+ ion.

Electron spin correlation in single charge stripping of C+ ions

Abstract Translational energy spectra have been measured for C 2+ ions resulting from single electron stripping of C + ions in collision with O 2 , NO, Ne and N 2 targets. A pronounced feature corresponding to the projected transition C + ( 2 P) → C 2+ ( 3 P 0 ) appears in the spectra with O 2 and NO as targets, but is absent in the spectra with Ne and N 2 as targets. A pseudo two-step mechanism involving a virtual intermediate state C + ( 4 P) is proposed to explain these observations which are then consistent with the requirements of spin conservation. This implies that the spin of the stripped electron is strongly correlated with the spins of the unremoved electrons in the parent C + ( 4 P) ion which may be considered to be in a generic state for the production of C 2+ ( 3 P 0 ).

STATE-SELECTIVE SINGLE-CHARGE STRIPPING OF METASTABLE AR+ IONS

Abstract Translational energy spectroscopy has been performed to investigate the single-charge stripping of metastable Ar+ ions in collision with O2, NO, N2 and HCl targets. The spectra exhibit fine-structure peaks which have not been previously observed. The positions of the five peaks are independent of the choice of target gases, but two of these are only present when the target molecules are in non-singlet ground states. This apparent anomaly can be explained on the basis of a pseudo-two-step mechanism consistent with spin-conservation requirements. The two anomalous peaks are identified with the processes Ar + [3s 2 3p 4 ( 3 P)4p, 4 S 0 ] → Ar + [3s 2 3p 4 ( 1 D)4p, 2 p 0 ] → Ar 2+ [3s 2 3p 4 , 1 D] and Ar + [3s 2 3p 4 ( 3 P)3d, 4 D] → Ar + [3s 2 3p 4 ( 1 D)3d, 4 P] → Ar 2+ [3s 2 3p 4 , 4 P] , in which the initial step in each case requires an electron spin flip in the core configuration, and is prohibited for singlet targets which remain in singlet states after collisions.