Joachim Mähnert

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.

Statistical and Non-statistical Reactions in Energy Selected Fluoromethane Ions.

The unimolecular reactions of the fluoromethane ion have been investigated by the threshold photoelectron photoion coincidence technique (TPEPICO). The breakdown curves have been measured in the energy range between the adiabatic ionization potential of 12.53 eV and 21 eV. In this energy range the formation of the CH 2F + , CHF + , CH 2 + , CH 3 + , and CF + fragment ions is observed. The appearance energies (AE) for these ions at 300 K are 13.2, 13.91, 13.93, 14.51 and < 17.7 eV respectively. The 0 K threshold energy for the formation of CH 2F + ions is 13.37 eV. Two different pathways for the unimolecular reaction of the fluoromethane ion can be distinguished. The reaction proceeding through the electronic ground state of the ion is entirely statistical. In contrast to this the reaction occurring through the first electronic excited state shows nonstatistical behaviour. The two pathways are distinguished via the kinetic energy released in the respective dissociation channels. While all thermochemically allowed reaction channels are observed on the ion ground state potential, the first excited state seems to decay predominantly by F loss reaction.

Investigation of the ArN+2 ion by dissociative ionization of argon/nitrogen clusters

The ArN+2 ion has been investigated by means of photoionization of an argon/nitrogen cluster beam in a threshold photoelectron photoion coincidence experiment. Two pathways for the formation of ArN+2 ions have been observed: (i) the nondissociative ionization of ArN2 neutrals and (ii) the dissociative ionization of Ar2N2. The two pathways are distinguished by the kinetic energy released (KER) in the dissociative ionization. The KER for the reaction Ar2N+2→ArN+2+Ar has been measured as a function of the excitation energy. The comparison of the measured KER with the statistically expected KER allows us to extrapolate to the thermochemical threshold of the reaction under investigation. A consistent picture is obtained under two assumptions: (i) the ArN+2 ion is linear and (ii) the ionization potential of ArN2 is 14.486±0.05 eV. The former assumption is confirmed by high level ab initio calculations (QCISD/6‐311G*).

The investigation of the (CO)+2 ion by dissociative ionization of argon/carbon monoxide clusters

The formation of (CO)+2 ion has been investigated by the dissociative ionization of Ar(CO)2 clusters in a threshold photoelectron photoion coincidence experiment. The kinetic energy released (KER) in the reaction Ar(CO)+2→(CO)+2+Ar has been measured as a function of the internal energy of the Ar(CO)+2 ion. The comparison between the experimental KER and the statistically expected KER allows one to extrapolate back to zero KER which corresponds to the thermochemical threshold for the formation of (CO)+2. The ionization potential (IP) of (CO)2 is determined to be 12.24±0.15 eV. This leads to a binding energy of the (CO)+2 of 1.80 eV one of the strongest known for any ionized van der Waals dimer. Our data are in line with a trans planar structure of the CO dimer ion as suggested by our ab initio calculations at the CI level.

THE DIRECT MEASUREMENT OF THE RATE CONSTANT K(E) FOR THE REACTION C2H+6 C2H+4 + H2

The reaction C2H,,* —> C,H4+ + H2 has been investigated in an experiment with energy selected ions at a resolution of 10 meV The experiment unambiguously proofs that this H2 loss reaction is metastable in a certain energy range. The rate constant k(E) for the reaction has been determined for the first time by comparison of calculated and experimental time of flight spectra. k(E) is found to vary by two orders of magnitude within about 25 meV of ion internal energy. This very steep increase is in line with recent calculations [Int. J. Mass Spectrom. Ion Proc. 136 (1994) 1] which predicted a large kinetic shift and a very steep k(E) curve. This finding explains why the metastability of the reaction has not been evidenced in previous studies with energy selected ions and solves a long standing discrepancy with MIKES experiments which had reported the metastability of the reaction. The data are in agreement with a model according to which the H, loss reaction from ethane ions proceeds by tunneling of H atoms through a transfer barrier with a height of 0.86 ± 0.05 eV.

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.

The formation of ArCO+ ions by dissociative ionization of argon/carbonmonoxide clusters

The formation of ArCO+ ions has been investigated in a photoelectron photoion coincidence experiment by dissociative ionization of Ar2CO clusters. The kinetic energy released (KER) in this dissociation has been measured as a function of the internal energy of the parent cluster ion and compared to the KER expected on the basis of statistical theory of unimolecular reaction, e.g., phase space theory. The latter calculations are based on high level ab initio calculations of the ArCO+ ion and the Ar2CO+ ion. The ab initio calculations show a nonlinear equilibrium geometry of the ArCO+ ion. The comparison of experimental and theoretical KER leads to an adiabatic ionization potential of the ArCO of 13.03 eV and a dissociation energy of the ArCO+ ion of 1.00 eV.

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.