Translational Energy Spectroscopy Research Articles

Ethylcarbene versus Direct Propene Formation in the Near-UV Photodissociation of Ethylketene

The competing pathways in the photodissociation of gaseous ethylketene at excitation wavelengths of 320.0, 340.0, and 355.1 nm were studied using photofragment translational energy spectroscopy. The primary dissociation channel was C═C bond fission producing ethylcarbene (CH3CH2CH; also known as propylidene) and CO. Product translational energy distributions are consistent with theoretical predictions that ground state ethylcarbene lies ∼34 kJ/mol higher in energy than its isomer dimethylcarbene (CH3CCH3). A second dissociation channel involved direct formation of propene prior to or concurrent with CO elimination. The measured product branching ratios indicate that the effective potential energy barrier for the direct propene channel lies below the energetic threshold for ethylcarbene formation. A minor C-C bond fission channel was also observed, leading to CH3 + CH2CHCO products. Comparisons are made to the results of our recent studies of methylketene and dimethylketene photodissociation.

Dimethylcarbene versus Direct Propene Formation in Dimethylketene Photodissociation.

Highly reactive carbenes are usually produced by photolysis of ketenes, diazoalkanes, or diazirines. Sequential kinetic pathways for deactivation of nascent carbenes usually involve bimolecular reactions in competition with isomerization producing stable products such as alkenes. However, the direct photolytic production of stable products, effectively bypassing formation of free carbenes, has been postulated for over 50 years but remains very poorly understood. Often termed "rearrangement in the excited state" (RIES), examples include 1,2-hydrogen migration within photoexcited carbene precursors yielding alkenes and the Wolff rearrangement in photogenerated carbonyl-substituted carbenes producing ketenes. In this study, the two competing CO elimination channels from photoexcited gaseous dimethylketene, producing dimethylcarbene and propene, were studied as a function of electronic excitation energy, under collision-free conditions, by using photofragment translational energy spectroscopy with vacuum ultraviolet photoionization of the products. A significant fraction of the dimethylcarbene → propene isomerization exothermicity (∼300 kJ/mol) was released as propene + CO translational energy, indicating that propene is formed prior to or concurrent with CO elimination. An increase in the propene yield with increasing excitation energy suggests that the effective potential energy barrier for this channel lies ∼24 kJ/mol above the energetic threshold for dimethylcarbene formation via C═C bond fission. Possible mechanisms for direct propene elimination are discussed in light of the observed energy dependence for the competing pathways.

Subpicosecond HI elimination in the 266 nm photodissociation of branched iodoalkanes.

The 266 nm photodissociation dynamics of 1-iodopropane and 2-iodopropane were studied using photofragment translational energy spectroscopy using vacuum ultraviolet (VUV) photoionization and electron impact ionization detection of products. The photochemistry of 1-iodopropane was found to be similar to that of iodomethane and iodoethane, with dominant production of I*(2P1/2), and no evidence (<0.21%) for HI + alkene formation. Significantly different behavior was observed for 2-iodopropane, with dominant production of ground state I(2P3/2), and a HI yield >10.5%. The anisotropy (β) parameters for all channels approached the limiting value of 2.0, indicating that 1,2-HI elimination occurs on subpicosecond timescales, like direct C-I bond fission, following excitation to 3Q0. The HI translational energy and angular distributions were similar to those for I(2P3/2), suggesting that motion of the heavy I atom in HI is largely derived from the repulsive nature of the 1Q1 surface correlating to R + I with the light H atom picked up by ground state I late in the exit channel producing highly vibrationally excited HI.

Photodissociation Dynamics of Gaseous CpCo(CO)2 and Ligand Exchange Reactions of CpCoH2 with C3H4, C3H6, and NH3

The photodissociation dynamics of CpCo(CO)(2) was studied in a molecular beam using photofragment translational energy spectroscopy with 157 nm photoionization detection of the metallic products. At 532 and 355 nm excitation, the dominant one-photon channel involved loss of a single CO ligand producing CpCoCO. The product angular distributions were isotropic, and a large fraction of excess energy appeared as product vibrational excitation. Production of CpCO + 2CO resulted from two-photon absorption processes. The two-photon dissociation of mixtures containing CpCo(CO)(2) and H(2) at the orifice of a pulsed nozzle was used to produce a novel 16-electron unsaturated species, CpCoH(2). Transition metal ligand exchange reactions, CpCoH(2) + L → CpCoL + H(2) (L = propyne, propene, or ammonia), were studied under single-collision conditions for the first time. In all cases, ligand exchange occurred via 18-electron association complexes with lifetimes comparable to their rotational periods. Although ligand exchange reactions were not detected from CpCoH(2) collisions with methane or propane (L = CH(4) or C(3)H(8)), a molecular beam containing CpCoCH(4) was produced by photolysis of mixtures containing CpCo(CO)(2) and CH(4).

Negative ions, energy loss, and electron emission during grazing scattering of fast H and He atoms from a clean and oxidized NiAl(1 1 0) surface

Negative ion fractions, projectile energy loss, and the emission of electrons is studied for grazing scattering of hydrogen and helium atoms/ions from a clean and oxidized NiAl(1 1 0) surface. Making use of translation energy spectroscopy and the coincident detection of the number of emitted electrons we have studied the electronic interaction mechanisms for the change from a clean metal target to an insulator surface via the preparation of a well defined ultrathin alumina film on top of the metal substrate. We find that already for a monolayer thick oxide film the characteristic different features of electronic processes for the surface of an insulator crystal are present.

Fragmentation and desorption in low-energy highly charged ion collisions with molecules and surfaces

In order to study secondary-ion emission in low-energy highly charged ion collisions with molecules and surfaces, we performed coincidence measurements of secondary ions and scattered ions, scattered neutral atoms or secondary electrons. Fragmentation and desorption processes induced by electron captures were successfully extracted by observing the scattered ions/atoms with small scattering angles. Momentum imaging of the secondary ions offers a new analysis method when combined with translational energy spectroscopy or energy-gain spectroscopy of scattered ions. This technique was successful in clarifying the reaction pathways of the electronic transitions of molecules and following the dissociation processes in collisions between Arq+ (q = 3–12) and CF4 and N2 molecules. We also successfully performed secondary ion mass spectroscopy of the topmost layers of the surfaces in glancing collisions between Ar8+ and both GaN (0001) and (000 1) surfaces.

Mechanisms of one-electron capture by slow He2+, C4+ and O6+ ions in collisions with CH4

Translational energy spectroscopy (TES) has been used to study one-electron capture by He2+, C4+, and O6+ ions in collisions with CH4 within the range 200 - 2000 eV amu-1. In each case the main collisions mechanisms and product channels have been identified. The measurements reveal significant differences in the way the dissociative and non-dissociative mechanisms contribute to electron capture. However, in all cases, the highly selective nature of the charge transfer process is confirmed in spite of the wide range of energy defects associated with possible product channels.

Charge Exchange Emission from Solar Wind Helium Ions

Charge exchange X-ray and far-ultraviolet (FUV) aurorae can provide detailed insight into the interaction between solar system plasmas. Using the two complementary experimental techniques of photon emission spectroscopy and translation energy spectroscopy, we have studied state-selective charge exchange in collisions between fully ionized helium and target gasses characteristic of cometary and planetary atmospheres (H2O, CO2, CO, and CH4). The experiments were performed at velocities typical for the solar wind (200-1500 km s-1). Data sets are produced that can be used for modeling the interaction of solar wind alpha particles with cometary and planetary atmospheres. These data sets are used to demonstrate the diagnostic potential of helium line emission. Existing Extreme Ultraviolet Explorer (EUVE) observations of comets Hyakutake and Hale-Bopp are analyzed in terms of solar wind and coma characteristics. The case of Hale-Bopp illustrates well the dependence of the helium line emission to the collision velocity. For Hale-Bopp, our model requires low velocities in the interaction zone. We interpret this as the effect of severe post-bow shock cooling in this extraordinary large comet.

Collision mechanisms in one-electron capture byHe2+ions in collisions with hydrocarbons

Translational energy spectroscopy has been used to study one-electron capture by 200-2000 eV amu{sup -1} He{sup 2+} ions in CH{sub 4}, C{sub 2}H{sub 4}, and C{sub 2}H{sub 6}. The main He{sup +}(n,l) excited product channels have been identified and their relative importance assessed. Although significantly different patterns of behavior are observed, all three cases exhibit the highly selective nature of the electron capture process in spite of the large number of possible nondissociative and dissociative product channels associated with a wide spread in energy defects {delta}E.

Charge exchange and dissociative processes in collisions of slowHe2+ions withH2Omolecules

Experimental and theoretical studies of one-electron capture in collisions of He2+ ions with H2O molecules have been carried out in the range 0.025-12 keV amu(-1) corresponding to typical solar wind velocities of 70-1523 km s(-1). Translational energy spectroscopy (TES), photon emission spectroscopy (PES), and fragment ion spectroscopy were employed to identify and quantify the collision mechanisms involved. Cross sections for selective single electron capture into n=1, 2, and 3 states of the He+ ion were obtained using TES while PES provided cross sections for capture into the He+(2p) and He+(3p) states. Our model calculations show that He+(n=2) and He+(n=3) formation proceeds via a single-electron process governed by the nucleus-electron interaction. In contrast, the He+(1s) formation mechanism involves an exothermic two-electron process driven by the electron-electron interaction, where the potential energy released by the electron capture is used to remove a second electron thereby resulting in fragmentation of the H2O molecule. This process is found to become increasingly important as the collision energy decreases. The experimental cross sections are found to be in reasonable agreement with cross sections calculated using the Demkov and Landau-Zener models.

State Selective Electron Capture in the Collision of S3+ Ions in Atomic Hydrogen and Helium

A full theoretical treatment including an ab-initio molecular calculation of the potential energy curves and couplings followed by a semi-classical collision dynamics has been performed for the one-electron capture by S3+ ions in collision with atomic hydrogen. The present paper completes a previous letter [9] and displays the full results concerning this process in order to provide a detailed understanding of the mechanism. These calculations show evidence of the predominance of the S2+(3s23p3d)3F° capture level, already pointed out by translational energy spectroscopy experiments and confirms experimental measurements. A compared study of the behaviour of the S3+ projectile colliding both hydrogen and helium targets is also presented.

Ab initio molecular treatment for charge transfer by S 3+ ion on hydrogen

Potential energy curves and coupling matrix elements of the one-electron capture by S 3+ ions in collision with atomic hydrogen have been determined at the multi-reference configuration interaction level of theory. These calculations show evidence of an untabulated S 2+(3s 23p3d) 3F 0 capture level, already pointed out by translational energy spectroscopy experiments, which is necessary to explain the charge transfer mechanism. Calculated cross-sections are in good agreement with the experimental measurements.

Identification of single charge transfer channels in –N2 collisions by translational energy spectroscopy¶

Single charge transfer reactions in the collision of ions at a laboratory energy of 2 keV with N2 were observed using translational energy spectroscopy (TES). Within the limitation of the energy analyser resolution (∼ 8/q eV, q being the ionic charge number), the translational energy spectrum shows a single broad charge transfer peak with a rich substructure. The latter looks like a conglomeration of closely spaced channels clumped together, corresponding mainly to charge transfer into excited states, rather than into the ground state. The reaction channel assignments were made using data available in the literature. Our findings are in accord with a recent experimental result, but there are no theoretical calculations to compare our experimental measurements with.

State-selective and total single-electron capture for impact of slow He2+ (E ≤ 1 keV) on H2, O2 and CO

Projectile state-selective single-electron capture for impact of slow He2+ (impact energy≤1keV) on H2, O2 and CO has been studied experimentally by means of translational energy spectroscopy. Comparative measurements were made with a Ne gas target for absolute calibration of the electron capture cross-sections for the molecular targets. We compare the importance of “direct” single-electron capture (transition of one electron only without excitation or dissociation of the ionised target molecule) versus single-electron capture accompanied by target excitation (transfer excitation, possibly leading to molecular dissociation). Our measurements are extended to lower impact energies than earlier ones and are qualitatively interpreted on the basis of Landau–Zener transitions in the collisional quasimolecules. Similar as for direct single-electron capture, transfer excitation proceeds preferentially within a “reaction window” for the involved reaction energy defect.

Probing potential energy curves ofC2−by translational energy spectrometry

We present studies on collision induced dissociation of $\mathrm{C}_{2}{}^{\ensuremath{-}}$ with Ar at an impact energy of $15\phantom{\rule{0.3em}{0ex}}\text{keV}$. The ${\mathrm{C}}^{\ensuremath{-}}$ fragment ion kinetic-energy release (KER) distribution is measured and is used to compute the KER in the center of mass (c.m.) frame $({\text{KER}}_{\mathit{c.m.}})$. We employ the reflection method to deduce an effective repulsive potential-energy curve for the molecular anion that is otherwise difficult to evaluate from quantum computational methods. The nuclear wave packet of the molecular ion in the initial ground state is computed by the semiclassical WKB method using the potential-energy curve of the $^{2}\ensuremath{\sum}_{g}^{+}$ ground electronic state calculated by an ab initio quantum computation method. The ground-state nuclear wave packet is reflected on a parametrized repulsive potential-energy curve where the parameters are determined by fitting the measured ${\text{KER}}_{\mathit{c.m.}}$ with the calculated KER distribution.

Studies of collision mechanisms in electroncapture by slow multiply charged ions

We review measurements based on translational energy spectroscopy which are beingused to identify and assess the relative importance of the various collision mechanisms involvedin one-electron capture by slow multiply charged ions in collisions with simple atoms andmolecules.

The initiation of EBIS/T-based HCIresearch in Japan

This article reviews early EBIS/T-based HCIresearch in Japan designed to understand atomic processes of highly charged impurity ions in plasmas. In particular the work of the NICE (Naked Ion Collision Experiments) project is discussed. The NICE experiment showed for the first time that translational energy spectroscopy would give useful information about the intermediate multiple-excitedstates in the transfer ionization processes, and therefore it would be a powerful methodfor spectroscopy of so-called hollow atoms.

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.

Collision mechanisms in one-electron capture by slow H and He-like ions of C, N and O in H andH2

Translational energy spectroscopy (TES) has been used to study state-selectiveone-electron capture by H and He-like ions of C, N and O in both H and H2within the range 250–900 eV amu−1.The main collision mechanisms leading to state-selective electron capture havebeen identified, their relative importance assessed and compared, where possible,with theoretical predictions and with any previous measurements basedon photon emission spectroscopy. For one-electron capture in H2,the relative importance of contributions from non-dissociative and dissociativecapture as well as from two-electron capture into autoionizing states isfound to be strikingly different for the cases considered. Our TESmeasurements in atomic hydrogen provide an important extension of previousmeasurements to energies below 1000 eV amu−1 andshow that, as the impact energy decreases, electron capture becomes more selective untilonly a single nproduct channel is significant. These product main channels are well described byreaction windows calculated using a Landau–Zener approach. However, the sameapproach applied to the more complex energy-change spectra observed in H2is found to be less successful.

Studies of state selective electron capture processes by helium-like ions in H and H2

The technique of translational energy spectrometry has been used to study state selective one electron capture 214–900 eVamu−1 helium-like C4+, N5+ and O6+ in atomic and molecular hydrogen. In the case C4+ and O6+ in an atomic hydrogen target electron capture occurs predominantly into C3+n=3 levels and O5+n=4 levels. Both results are in agreement with theoretical semiclassical calculations. For N5+ in the energy range capture is predominantly into n=4 levels confirming theoretical predictions. In the case of a molecular hydrogen target capture by C4+ and O6+ ions is dominated by non-dissociative one electron capture channels. In the case of N5+/H2 collisions autoionising double electron capture becomes dominant as the collision energy is reduced below 1 keVamu−1. A simple attenuation technique was used in this case to obtain cross section values for individual capture channels which are compared to recent experimental and theoretical values.