High-resolution Translational Energy Research Articles

Photodissociation of pyrrole-ammonia clusters below 218 nm: Quenching of statistical decomposition pathways under clustering conditions

The excited state hydrogen transfer (ESHT) reaction in pyrrole-ammonia clusters (PyH·(NH(3))(n), n = 2-5) at excitation wavelengths below 218 nm down to 199 nm, has been studied using a combination of velocity map imaging and non-resonant detection of the NH(4)(NH(3))(n-1) products. Special care has been taken to avoid evaporation of solvent molecules from the excited clusters by controlling the intensity of both the excitation and probing lasers. The high resolution translational energy distributions obtained are analyzed on the base of an impulsive mechanism for the hydrogen transfer, which mimics the direct N-H bond dissociation of the bare pyrrole. In spite of the low dissociation wavelengths attained (~200 nm) no evidence of hydrogen-loss statistical dynamics has been observed. The effects of clustering of pyrrole with ammonia molecules on the possible statistical decomposition channels of the bare pyrrole are discussed.

Calculation of the effect of collisional broadening on high-resolution translational energy loss spectra

The collisional broadening of peaks in the spectra arising from translational energy spectroscopy (TES) studies is theoretically investigated. A numerical calculation, based on the TRIO matrix ion-beam transport computer programme, is used to simulate the collision event and its effects on the focussing properties of a number of ‘double-focussing’ instrument designs. The ion-optical models utilised include a commercial mass spectrometer and two novel high-resolution energy spectrometers (TESI and TESII), incorporating quadrupole and hexapole field lenses to focus the beam and reduce image aberrations. For a given design of spectrometer, peak broadening is evaluated in terms of the translational energy change suffered by the ion during collision and the angle through which it is scattered.

Single electron capture by B 2+ ions in Ne, Ar, Kr and Xe

High resolution translational energy spectra have been obtained for state-selective single-electron capture by 8 keV B 2+ in Ne, Ar, Kr and Xe. The Ne spectrum is used to determine accurately the zero energy change position for subsequent analysis of the three other systems. Six capture channels have been unambiguously identified for Ar, all producing Ar + in the ground [3p 5 2P 0] state. Kr exhibited eight capture channels and Xe seven, with both systems exhibiting spin-orbit splitting of the 2 P 3 2 , 1 2 state, Kr + being split by 0.67 eV and Xe + by 1.3 eV. The Kr + spectrum is neatly ordered into four well separated doublet peaks, whereas the relationship between doublets for Xe + is less obvious. Using a statistical model to compare the expected populations of the spin-orbit states of Kr + and Xe + to the observed populations, a novel method for calculating the gradient of the reaction window for systems which exhibit spin-orbit splitting has been developed. By comparing partial cross section data from all collision systems under study the results were found to be in good agreement with the Landau-Zener curve-crossing model.

The design and performance of a high resolution translational energy loss spectrometer

An instrumental development is described that achieved high energy resolving power when studying collisions between fast ion beams, at keV energies, interacting with thermal molecular and atomic gaseous targets. This technique is referred to as translational energy spectroscopy (TES) and can reveal spectroscopic information on the states of the participating species, the dynamics of the collision, lifetimes of species formed, state-to-state collision cross-sections and populations of states. The instruments described are based on an unique symmetrical arrangement of cylindrical electrostatic analysers. These are placed in a double focusing ion-optical geometry leading to very small first and second order image aberrations, sufficient to obtain a primary energy resolving power of 105. Two such instruments were developed and their relative performances are compared experimentally and through computer modelling; the second, and larger, instrument has an improved sensitivity by an order of magnitude and a three times increase in resolving power. This has enabled collision spectroscopy to achieve performance similar to electron energy-loss spectroscopy and photoelectron spectroscopy, with a resolution on the unscattered ion beam measured to be 0.01 eV, at 3 keV collision energy, on energy loss processes of ≥ 0.03 eV. These developments allow TES to observe, (i) the vibrational/electronic spectroscopy of keV ion/molecule collisions; (ii) spin-selective spectra; (iii) the highly excited states close to and beyond the ionization limit of the target species; and (iv) involatile targets using a high temperature cell. The validity of optical quantum selection rules, relying on the electric-dipole interaction governing photon-induced transitions, has been tested for TES, which relies on the more general ion/neutral interaction potential responsible for collisional excitation. Spin conservation was found to be strongly adhered to, whilst orbital angular momentum and total angular momentum were not. The specific selection rules for homonuclear diatomics g<–>g and u<–>u were found to be allowed in TES, in contradiction to the case of optical spectroscopy, whilst there was some preliminary evidence that the optical Σ+–Σ− symmetry-forbidden transition was also forbidden for the TES reactions studied. For H2+ projectiles, excitation to triplet states is strongly favoured over excitation to singlet states. © 1997 John Wiley & Sons, Ltd.

Photodesorption of Na atoms from rough Na surfaces

We investigate the desorption of Na atoms from large Na clusters deposited on dielectric surfaces. High-resolution translational energy distributions of the desorbing atoms are determined by three independent methods, two-photon laser-induced fluorescence, as well as single-photon and resonance-enhanced two-photon ionization techniques. Upon variation of surface temperature and for different substrates (mica vs lithium fluoride) clear non-Maxwellian time-of-flight distributions are observed with a cos2 θ angular dependence and most probable kinetic energies below that expected of atoms desorbing from a surface at thermal equilibrium. The half-width of the time-of-flight distribution decreases with increasing surface temperature. A quantitative description of the energy and angular distributions is presented in terms of a model which assumes that following the initial surface plasmon excitation neutral atoms are scattered by surface vibrations. Recent experiments providing time constants for the decay of the optical excitations in the clusters support this model. The excellent agreement between experiment and theory indicates the importance of both absorption of the laser photons via direct excitation of surface plasmons as well as energy transfer with the substrate during the desorption process.

Excitation of CO+ and NO+ Electronic States Via Single-electron Capture by B2+ Ions

High-resolution translational energy loss spectrometry was performed to obtain the spectra for single-electron capture by 6 keV B2+ ions in collision with CO and NO molecules, to complement a previous investigation involving O2 and N2 targets. The spectrum for CO consists of eight well defined peaks, three of which correspond to capture into the ground state of CO+[X2Σ+] dominated by the 0–0 molecular transition. One intense peak and a subsidiary one are identified as capture into the first excited electronic state of CO+, responsible for subsequent emission of the comet-tail system CO+[A 2Πi→X 2Σ+]. The B 2Σ+ state of CO+, the upper electronic state of the Baldet-Johnson [B 2Σ+→A 2Σi] and the first negative [B 2Σ+→X 2Σ+] bands, is also accessed through two capture channels. The NO spectrum exhibits one intense narrow peak dominated by capture into the second triplet state of NO+ [b 3Π], together with subsidiary features which may involve capture into the ground X 1Σ+ and excited singlet (A 1Π,A′ 1Σ+, W 1Δ) and triplet (b 3Π, a 3Σ+, b′ 3Σ−) manifolds. The NO spectrum is complicated by energy overlap of relevant capture channels, which makes unambiguous identification difficult. © 1997 John Wiley & Sons, Ltd.

Quintet States in Atomic Systems

Quintet states in atomic and ionic systems are fascinating and are rare and elusive to optical studies. This brief communication reviews our observation of specific quintet states in the N+, O2+ and Ar4+ systems via electron-capture processes in high-resolution translational energy spectroscopy. In particular, the metastable [3s3p3 5S0] state of Ar4+, which is not listed in comprehensive spectroscopic tables, has been identified and located at an energy of 7.7 eV above the 3P ground state.

Collisional Excitation of O +2 and N +2 Electronic States via Single Electron Capture by B 2+ Ions

High-resolution translational energy spectra have been obtained for single-electron capture by 4-keV B 2+ ions in collision with Ne, O 2, and N 2. The Ne spectrum is used for accurate determination of the zero energy-change position for subsequent analysis of the complex molecular spectra. Six capture channels have been unambiguously identified for the O 2 spectrum; two of these involve capture-excitation, which leaves the O + 2 ion in its fourth excited state, responsible for subsequent emission of the first negative band system [ b 4Σ − g → a 4Π u ] of O + 2. The N 2 spectrum consists of seven narrow spectral features, four of which are identified with capture channels from which the first and second excited electronic states of the N + 2 ion are accessed. These constitute the upper states of the Meinel [ A 2Π u → X 2Σ − g ] and first negative [ B 2Σ + u → X 2Σ + g ] band systems of the N + 2 ion.

Effect of spin—orbit states of Kr + and Xe + on the single-electron capture spectra of C 2+ ions

High resolution translational energy spectra have been obtained for state-selective single-electron capture by 8 keV C 2+ in Ar and 6 keV C 2+ in Kr and Xe. The Ar spectrum displays 12 well-resolved peaks which fall into two sequences corresponding to capture from the ground 1S state and the metastable 3P 0 state of the C 2+ ion. The only two capture channels which violate spin conservation in this range of energy gain are notably absent from this spectrum. The effect of spin—orbit splitting (0.18 eV) of the 2P 3/2,1/2 ground state of the Ar + ion is not apparent in this spectrum. By contrast the spin—orbit splitting of the 2P 3/2,1/2 states of the Kr + (at 0.67 eV) and Xe + (at 1.31 eV) ions is strongly reflected in the respective spectra. The Kr + spectrum reveals six pairs of neatly arranged doublet peaks, while the Xe + spectrum displays a less obvious relationship between the doublets.

Translational energy spectroscopy of single-electron capture by C 2+ ions in He, Ne and Ar

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.

Spin conservation in single-electron capture by N2+ in He, Ne, and Ar.

High-resolution translational energy spectra (0.2 eV at full width at half maximum) have been obtained for single-electron capture by 4-keV N[sup 2+] ions in collision with He, Ne, and Ar. Seven capture channels have been unambiguously identified for each of the He and Ne spectra, many of which have not been previously reported. This allows clarification of earlier studies. Twelve reaction channels have been identified for capture by N[sup 2+] in Ar, three of which involve excitation of the target product Ar[sup +] ion. All the observed channels are spin conservative. Competing channels which violate spin conservation are seen to be rigorously suppressed.

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

High-resolution translational energy spectroscopy has been performed of N + 2 ions scattered off Ar, N 2, O 2 and NO. Intense excitations and de-excitations of the Meinel system of N + 2 (A 2Π u ← X 2∑ + 2) were observed in all collision gases. Transitions in the first negative system N + 2 (B 3∑ + u ← X 2 g) were also detected, and well-resolved in the case with Ar as collision gas. Simultaneous transitions involving excitation of the Meinel system and the a 1Δ g state of O 2 were observed with O 2 as collision gas.

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.

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.

Cross Sections for Excitation and De-Excitation Processes among3P2,3P1,3P0,1D2and1S0of Ne2+and Ar2+in Collisions with Rare Gases

Cross sections for excitation and de-excitation in Ne 2+ and Ar 2+ have been measured with a high resolution translational energy spectrometer. For Ar 2+ , the cross sections for the transitions 3 P 2 ↔ 3 P 1 , 3 P 2 ↔ 1 D 2 and 1 D 2 ↔ 1 S 0 have been measured in collisions with He and Ne in the energy range from 0.8 to 5 keV. For Ne 2+ , no peak due to the transitions has been observed.

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.

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.

Electronic excitation of atmospheric molecules by proton impact

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 1Σ + g → a 1Π g) in N 2 and ( X 1Σ + → A 1Π) 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 3Σ − u and A 3Σ + 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′ 2Δ i (7.48 eV) to the S 2Σ + (8.29 eV) state. Three well-defined peaks observed at 5.5 eV to 6.1 eV have been assigned to the transitions X 2Π r ( v = 0) → A 2Σ + ( v = 0, 1, 2) of NO.

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

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 X 1Σ + → A 1Π and X 1Σ + → a 3Π r 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: Δ S = 0, ± 1 with singlet-singlet transition forbidden. In every instance where CO molecule undergoes a X 1Σ + → a 3Π r transition, this selection rule appears to be observed rigorously by the atomic ion.