Translational Energy Spectroscopy Research Articles

A unified view of the spin conservation rule for disparate collisional processes

High resolution translational energy spectroscopy has been applied to a number of collisional processes, involving energy loss, charge transfer, charge stripping and charge inversion reactions. The role of the spin conservation rule has been assessed over this range of reactions and been found to hold, playing a vital part in the interpretation of these collision processes. Instances where the rule might be expected to break down, because of spin-orbit coupling, have been investigated and examples shown. In circumstances where free electrons are produced, and the spin of the ejected electron is indeterminate, it has been shown that application of the rule can be meaningful. The unexpected observation of intense simultaneous reactions (not arising from double collisions) is illustrated for a number of cases, which could only be clearly rationalised on the basis of the application of the spin rule.

Collisional excitation of the Comet-Tail system of CO + [ A2Π i- X2Σ +

High-resolution translational energy spectroscopy (0.1 eV full width at half maximum) has been performed on 3-keV CO + ions scattered off Ar, N 2, O 2, CO, and NO. Excitation of the Comet-Tail system [ A 2Π i ← X 2Σ +] of CO + is prominent in all the spectra. Characteristic singlet-triplet electronic transitions of the target molecules are also present with N 2, O 2, and CO as target gases.

Dissociation of doubly charged SH radicals. An experimental and theoretical study

Ion translational energy spectrometry has been applied to study unimolecular and collision-induced dissociation of dications of the SH radical. Experimentally determined values of the kinetic energy released upon dissociation are satisfactorily correlated with results of high-level ab initio calculations of potential energy curves of the ground and low-lying excited electronic states of SH 2+ ions. Quantum-chemical methods in which configuration interaction effects are accounted for via the coupled-cluster approach are employed.

Determination of metastable atomic ion beam fractions by TES

Translational energy spectroscopy (TES) has been used to measure the relative populations of several of the long-lived excited states of six atomic (C+, N+, O+, Cl+, F+ and B+) ion beams formed by electron impact ionization using 70 eV electrons. The method used is based on the excitation/de-excitation method originally devised by Moore and is discussed in detail with particular reference to the choice of O2 and NO as collision gases on spin-conservation grounds. The relative metastable populations of C+, N+, O+ and B+ reported in this paper are similar to other reports found in the literature. However, there are known reports of any measurements of the fractional abundances of the metastable states in Cl+ and F+ ion beams formed from BCl3 and CHF3 precursors respectively.

Collision spectroscopy of Ar8+ + He at low velocities (v

Single- (SC) and double-electron capture (DC) have been studied for collisions of Ar{sup 8+} with He using x-ray and vacuum ultraviolet (vuv) spectroscopy at a collision energy of 80 keV as well as translational-energy spectroscopy (TES) at lower collision energies (8 keV). It is shown that both processes involving the ground-state ion terminate in vuv photon emission, i.e., nonautoionizing levels are populated. The SC, by the long-lived metastable species Ar{sup 8+*} (2{ital p}{sup 5}3{ital s}){sup 3}{ital P}{sub 0,2}, populates mostly the Na-like core-excited levels with {ital n}=4: Ar{sup 7+*} (2{ital p}{sup 5}3{ital s} {sup 3}{ital P} 4{ital l}){sup 2,4}{ital L}{sub {ital J}}. With the use of calculated decay and autoionization rates, the measured vuv spectra are analyzed and compared with the TES results and the Auger spectra measured by other authors. It is shown that the stabilization of these core-excited states is both radiative and autoionizing. The DC by the metastable projectile is not yet fully understood.

Probing interatomic potentials by ion translational energy spectrometry: A new crossed molecular beams apparatus

This report presents details of the development of a new crossed molecular beams apparatus designed and fabricated to carry out high sensitivity ion translational energy spectrometric investigations of the potential energy surfaces of small molecular species. The translational energy spectrometer is used to carry out experimental studies of ion-neutral reactions resulting in charge stripping of CS+ radicals and dissociation of metastable CO2+ dications. These results are interpreted in the light of high-levelab initia molecular orbital calculations of the pertinent molecular potential energy functions. New results for the double ionisation energy of CS and the kinetic energy released upon dissociation of specific electronic states of CO2+ are presented.

Dynamics and mode specificity in OClO photodissociation

The photodissociation of OClO in a molecular beam was studied using photofragment translational energy spectroscopy at wavelengths between 350 and 475 nm. Although the dominant products are ClO(x{sup 2}II) + O({sup 3}P), the authors observe formation of Cl({sup 2}P) + O{sub 2}({sup 3}{Sigma}{sub g}{sup {minus}}) and find strong evidence for a substantial yield of Cl({sup 2}P) + O{sub 2}({sup 1}{Delta}{sub g}). The total yield reaches a maximum of 3.9 {plus_minus} 0.8% near 404 nm and decreases to <0.2% under the absorption peaks in the wavelength range 350-370 nm. The branching ratios display mode specificity: the yield of Cl + O{sub 2} from OClO with symmetric stretching excitation or symmetric stretching + bending is {approximately} 10 times greater than from states at nearly the same energy with asymmetric stretching excitation. The Cl + O{sub 2} results from concerted unimolecular decomposition with release of up to {approximately} 68 kcal/mol in relative translational energy. 25 refs., 6 figs.

Electron capture in He2+ collisions with aligned Na*(3p) atoms.

State-selective single-electron capture in slow (0.5\char21{}3 keV/amu) collisions of ${\mathrm{He}}^{2+}$ with laser-excited ${\mathrm{Na}}^{\mathrm{*}}$(3p) atoms has been studied experimentally by means of translational energy spectroscopy, and theoretically by 38-state atomic orbital (AO) close-coupling calculations. The initial orbital alignment of the ${\mathrm{Na}}^{\mathrm{*}}$(3p) state has been controlled via polarization of the exciting laser light. Electron capture into the dominant ${\mathrm{He}}^{+}$(n=4) state is strongly favored for linear light polarization perpendicular to the ${\mathrm{He}}^{2+}$ beam. The impact energy dependence of the observed alignment behavior can be quantitatively explained by the AO calculations.

Spin—orbit and other electronic transitions in collisions of Ar + and B + ions with O 2 molecules

High-resolution translational energy spectroscopy (0.05 eV fwhm) has been performed on 3 keV Ar + and B + ions scattered off O 2 molecules. The Ar + spectra display the fine structure transitions between the doublet states 2P 1 2 ↔ 2P 3/2 together with simultaneous transitions between these states coupled with electronic excitation to the molecular states of a 1Δ g and b 1Σ + g of O 2. The B + spectra display a multitude of transitions between four atomic states of B + together with simultaneous transitions involving the a 1Δ g and b 1Σ + g states of O 2. Pure molecular transitions between the triplet ground state and these singlet excited states are suppressed on account of the singlet nature of the B + ground state.

State-selective electron capture by slow C4+ ions in collisions with H2 and O2

Translational energy spectroscopy has been used to study state-selective one-electron capture by C4+ ions in collisions with H2 and O2 at impact energies of 4, 8 and 16 keV. The main excited product channels have been identified and their relative importance assessed. In the case of H2, capture into the n=3 states of C3+ is predominant and separate cross sections for capture into the C3+(1s2)3s, 3p and 3d states have been determined by reference to previously measured total cross sections. The authors' measured cross sections are in good agreement with previous measurements based on photon emission spectroscopy and with the predictions of quantal calculations. In the case of O2, where no previous measurements exist, capture into the n=3 states of C3+ is again predominant with a small n=4 contribution at 4 and 8 keV. The authors' measurements in both H2 and O2 show H+2 and O+2 product ion formation via Franck-Condon transitions.

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

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 2Π r) → NO(A 2Σ +, 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 +( 3P) + NO(X 2Π r) → N + ( 5S) + NO(X 2Π r) and N + 3P) + NO(X 2Π r) → N +( 1D) + NO +( 1Σ + + e − stood out. The use of the Wigner spin conservation rule greatly facilitated spectral interpretation by reducing the number of candidate reaction channels available.

Collisions of doubly charged nitrogen molecules with rare gas atoms

The authors report on single electron capture in slow collisions of doubly charged nitrogen molecules with the rare gases He, Ne and Ar. The vertical double ionization energy of N2 as well as vertical excitation energies of N22+ have been calculated by means of the multiconfiguration coupled electron pair approximation (MC-CEPA) method. Furthermore, experiments have been carried out applying the technique of translational energy spectroscopy. The energy gain spectra of the resulting N2+ molecules have been analysed, showing the single electron capture to be dominated by N22+ (X 1 Sigma +g) projectile ions, as well as by N22+ ions in the low-lying metastable c 3 Sigma +u state. The position of the experimentally observed reaction window is discussed in terms of Franck-Condon transitions and is compared with predictions of classical descriptions.

On the formation of rotationally hot H2+. by dissociation of CH42+ dications

Dissociation of CH into CH ion pairs results in formation of H ions possessing a degree of rotational excitation which is high enough to enable formation of several rotational resonance states. These resonances give rise to unimolecular dissociation into H+ + H· fragments. Ion translational energy spectrometry has been applied to measure such dissociation spectra, and a number of rotational resonances are observed. Rotational resonances are not observed when H ions are generated from H2 precursors. Two models based upon ideas of impulsive energy release are investigated which attempt to correlate the centre-of-mass kinetic energy released upon dissociation of CH with the observation of the rotational resonances. There is good accord between the experimental results and the prediction of our models.

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, N 2, O 2 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 N 2, CO and Ar a weak structure is observed which is identified as the spin non-conserved transition 3P → 1D in the Cl + ion.

An experimental and theoretical study of SF q+ ( q = 1−3) ions

Ion translational energy spectrometry has been applied to study collision induced excitation/de-excitation and dissociation of positively charged ions of the radical SF q+ for q = 1, 2 and 3. Doubly and triply charged molecular ions are found to be metastable, with the lower limit of their lifetimes against dissociation being ≈ 1 μs. Ab initio molecular orbital calculations of the potential energy curves of the ground and low-lying excited electronic states of neutral SF and SF q+ ions have been calculated using quantumchemical methods in which configuration interaction effects are accounted for by perturbative techniques and via the coupled cluster approach.

Detection of long-lived highly excited ions by means of charge-stripping processes

Translational energy spectroscopy is being used for detection of long-lived highly excited Ar 2+ ions, produced in an electron impact ion source, by means of state-selective single-electron stripping processes in collisions of 2–8 keV Ar 2+ ions with rare-gas atoms. Translational energy-loss spectra indicate that stripping of a single electron from Ar 2+ occurs predominantly by a reaction channel due to ionization of long-lived highly excited states with excitation energies very close to the ionization potential of Ar 2+. This reaction channel is found to be overwhelmingly dominant at electron impact energies, E e ≥ 87 eV, in the electron impact ion source. Weaker processes are also observed due to ionization of highly excited metastable, low-lying metastable, and the ground state of Ar 2+ ions.

Energy studies of atomic atmospheric ions in collisions with molecular nitrogen and oxygen

AbstractStudies of internal energy changes between the partners in ion‐molecule collisions can be facilitated by high‐resolution translational energy spectroscopy. Collisions of keV H+, N+ and O+ on molecular nitrogen and oxygen have shown that for 30 reaction channels all are consistent with spin conservation, which has not been generally acknowledged in previous studies. Estimations of the fractional metastable composition in each ion beam have also been made.

Single electron capture for impact of (0.5-9 keV) C2+ on He, Ar and H2

Absolute total cross section measurements and state-selective translational energy spectroscopy have been performed for single electron capture by 0.5-9 keV C2+ from H2, He and Ar, for the C2+ ground state as well as the C2+ metastable state. The experimental results are compared with available data obtained with undefined metastable primary ion beam fractions. For the particular case of C2+-H2 collisions, experimental results could be explained by close-coupling calculations involving empirically derived coupling matrix elements and Franck-Condon factors for H2 ionization via electron capture.

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

High resolution translation energy spectroscopy (TES) has been performed on C + ions in collision with Ar, CO, O 2 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 4 P state in the C + ion beam has been estimated from the relative intensities for the C + ( 2 P ↔ 4 P) transitions obtained from the TES spectrum of C + on O 2. The fraction of C + ( 4 P) was found to be about 10% of ions produced from CH 4 precursor gas at a nominal electron ionizing energy of 100 eV.

Translational energy spectroscopy of N + and O + in collision with O 2

High-resolution translation energy spectroscopy has been performed on N + and O + ions scattered off O 2 molecules. Twenty-one reaction channels have been unambiguously identified, all of which are consistent with the Wigner spin conservation rule. A simple estimate of the ion-beam constitution reveals the presence of significant fractions of the metastable 1D state in the N + beam and the 2D and 2P states in the O + beam obtained under our source conditions.