Martin Penno

ZEKE-PEPICO investigations of dissociation energies in ionic reactions

Abstract A coincidence experiment between zero kinetic energy photoelectrons (ZEKE-PE) and the corresponding photoions (PI) has been set up. In this experiment electrons and ions are extracted from an ion source in a delayed pulsed field. The high electron energy resolution is based on time of flight discrimination of the electrons. This ZEKE-PEPICO technique has been applied to the threshold of the reactios: (i) C 2 H 2 + → C 2 H + + H and (ii) H 2 + → H + + H. The example of H loss from acetylene ions illustrates the power of the technique, while the dissociative ionization of H 2 + represent a critical test for the ZEKE-PEPICO spectrometer.

State selective predissociation spectroscopy of hydrogen chloride ions (HCl+) via the A2Σ+ ← X2Π3/2 transition

The photodissociation of hydrogen chloride ions (HCl+) has been investigated through the A2Σ+ (ν′ = 6, 7 and 8) ← X 2Π3/2 (ν″ = 0) transition. The spectra reveal that state selective photodissociation with complete resolution of the spin, orbital, and rotational angular momentum is possible in the A 2Σ+ (ν′ = 6) state. The analysis of these spectra yields the rotational and the spin-rotation coupling constant of the A 2Σ+ (ν′ = 6) state. The lifetime of HCl+ decreases significantly with increasing vibrational excitation in the 2Σ+ state. Within the experimental error limits no J dependence of the lifetime is observerd. The state selective photodissociation of the HCl+ ions is also shown to be a very sensitive probe for unexpected parity transitions in the 2 + 1 REMPI formation of the HCl+ ions in the X 2Π3/2 (ν″ = 0) state.

Synthesis of Cl2 induced by low energy (0–18 eV) electron impact to condensed 1,2-C2F4Cl2 molecules

The interaction of low energy electrons (0–18 eV) with C2F4Cl2 molecules condensed in multilayer amounts on a cryogenically cooled Au substrate generates Cl2, as can be seen from the energy and temperature dependence of the Cl− desorption signal. The cross section for Cl2 formation exhibits two pronounced resonant features with maxima near 0 and 10 eV, dropping to essentially zero in the energy range between the resonances (near 3 eV). The energy dependence of the reaction cross section qualitatively follows that of Cl− desorption, which itself can be correlated to dissociative electron attachment (DA) processes in the gas phase. While at 10 eV excited negative ion resonances and electronically excited states of neutral C2F4Cl2 may be involved in the process, in the low energy region (<2 eV) the reaction can only be initiated by dissociative electron attachment. Possible reaction pathways are discussed.

On the role of dissociative ionization in the formation of argon dimer ions

The formation of Ar2+ ions has been investigated by means of the threshold photoelectron photoion coincidence (TPEPICO) technique. Two pathways for the formation of Ar2+ ions are important. One is a direct path via excitation of Rydberg states of Ar2 with consecutive autoionization. The other path is dissociative ionization of larger argon clusters, in this case argon trimers. These two pathways lead to Ar2+ ions with different internal energy. The pathways are easily distinguished in the TPEPICO-TOF spectra by the kinetic energy released (KER) in the dissociative ionization. The KER for the reaction Ar3+ → Ar2+ + Ar was measured as a function of the photon energy and compared to the KER expected from statistical theory. The agreement is satisfying and confirms that Ar3+ ions do indeed dissociate at the thermochemical threshold. At higher photon energy the excited2Π(3/2)g state of Ar3+ is also detected from a second component in the KER. By applying a kinetic energy discrimination it is possible to measure cluster ion spectra in the presence of larger clusters but essentially without interference from the latter.

Rotational State Distribution of HBr+ Ions Formed by Resonance Enhanced Multiphoton Ionization

Abstract The rotational state distribution of HBr+ ions formed via a large number of pump lines in the f 3Δ2 ← 1Σ and the F 1Δ2 ← 1Σ REMPI spectrum has been derived from the analysis of state-selective predissociation spectra. For most pump lines in the two REMPI spectra molecular ions are formed in narrow state distributions with dominant contributions of two or three different rotational states. By choice of the appropriate pump line in the REMPI spectrum this state distribution can be shifted from N = 0 to about N = 10. The data provide evidence that REMPI spectroscopy is a viable tool for the formation of state-selected molecular ions.

Rotational effects in ionic reactions investigated by the ZEKE-PEPICO technique

Abstract The acetylene ion has been investigated by the zero kinetic energy photoelectron photoion coincidence technique (ZEKE-PEPICO). ZEKE-PES and ZEKE-PEPICO spectra recorded in the range of 500 meV above the IP reveal bending vibrations of the acetylene ion with low Franck-Condon factors and thus illustrate the high resolution and sensitivity of the experiment. The H loss reaction from acetylene ions has been investigated at different temperatures by measuring the breakdown curves in a ZEKE-PEPICO experiment. The shift between these breakdown curves has been analysed in terms of the difference in vibrational and rotational energy. The data show that the dissociation energy E 0 ( T rot ) is lowered by 2 2 R Δ T with increasing temperature. This result is in agreement with the simple bond fission of a molecule which is linear in the neutral state. The dissociation energy for the H loss reaction from acetylene ions at 0 K is E 0 ( T rot = 0 K) = 17.360±0.005 eV referenced to neutral acetylene.

Electron stimulated desorption of O− and O2− from condensed ozone: Possible implications for the heterogeneous photochemistry of stratospheric O3

We have studied the desorption of negative ions following low energy electron impact (0–15 eV) to ozone condensed in multilayer amounts on a cryogenically cooled gold surface. Intense desorption of O− is observed at low electron energies (below 4 eV) ia pronounced resonances with an estimated desorption cross section of the order of 10−18 cm2 (peak value). The resonant features in the ion yield indicate that O− desorption is driven by dissociative electron attachment to O3 molecules at or near the surface. While dissociative electron attachment (DA) from gas phase ozone generates the fragment ions O− and O2− at comparable intensities, desorption of O2− from condensed ozone is strongly suppressed, being only weakly observed within a resonance located at 7 eV. Possible implications for the heterogeneous chemistry of ozone on polar stratospheric cloud (PSC) particles, namely photo-induced dissociative electron transfer from the substrate, are discussed in the light of the effective O− desorption observed at low electron energies.

Time Resolved Photoelectron Spectroscopy After Laser Ionization

In this contribution we will report on recent experiments aimed at studying the spectroscopy and the dynamics of state selected molecular ions. One of the major sources for kinetic and energetic information on unimolecular reactions of ions has been the one photon ionization of molecules with consecutive reaction. In this large field the most direct results emerged from investigating energy and/or state selected ions. In those experiments which do not inherently produce state selected ions the photoelectron photoion coincidence (PEPICO) technique [1] has to be applied. Typically these experiments have been performed at a resolution of about 30 meV (240 cm− l) limited by the contribution from the photons and the electron energy. Applying this technique we have recently studied the J dependence of the dissociation energy Eo(J) [2,3]. It was at the same time the advantage and the limitation of this experiment that it did not require J selectivity but simply used the variation of the rotational temperature.