Karl-Michael Weitzel

High-resolution pulsed field ionization photoelectron-photoion coincidence spectroscopy using synchrotron radiation

We have developed a sensitive and generally applicable scheme for performing pulsed field ionization (PFI) photoelectron (PFI-PE)-photoion coincidence (PFI-PEPICO) spectroscopy using two-bunch and multibunch synchrotron radiation at the Advanced Light Source. We show that this technique provides an ion internal state (or energy) selection limited only by the PFI-PE measurement. Employing a shaped pulse for PFI and ion extraction, a resolution of 0.6 meV [full width at half maximum (FWHM)] is observed in the PFI-PEPICO bands for Ar+(2P3/2,1/2). As demonstrated in the PFI-PEPICO study of the process, O2+hν→O2+(b 4Σg−, v+=4, N+)+e−→O+(4S)+O(3P)+e−, the dissociation of O2+(b 4Σg−, v+=4) in specific rotational N+ levels can be examined. The simulation of the experimental breakdown diagram for this reaction supports the conclusion that the threshold for the formation of O+(4S)+O(3P) from O2+(b 4Σg−, v+=4) lies at N+=9. We have also recorded the PFI-PEPICO time-of-flight (TOF) spectra of O+ formed in the dissoci...

Shifts in photoionization fragmentation onsets. A direct measure of cooling in a supersonic molecular beam

The dissociation of isobutane and ethyl chloride cooled in a supersonic expansion was investigated by photoelectron-photoion coincidence (PEPICO). The breakdown diagrams for various expansion conditions were compared to the breakdown diagram for a thermal source. The shift in the cross-over energy provides a direct measurement of the internal energy of molecules in the beam. For isobutane the lowest internal energy was observed for a seededbeam with 〈Eint〉 = 5 meV (corresponding to T< 100 K) compared to 〈Eint〉 = 123 meV at room temperature. Theexperiments revealed that rotational energy is available for dissociation in isobutane ions. For ethyl chloride the lowest internalenergy observed was 5 meV compared to 73 meV at room temperature. The results indicated that part of the rotational energy is available forovercoming the dissociation barrier. This is in spite of the tight transition state associated with the isomerization barrier.

The rates of HCl loss from energy‐selected ethylchloride ions: A case of tunneling through an H‐atom transfer barrier

The dissociation rates of energy‐selected ethylchloride and deuterated ethylchloride ions were measured as a function of the parent‐ion internal energy by the method of photoelectron photoion coincidence. Previously performed ab initio calculations indicated that the rate‐determining step for this reaction is an H‐atom transfer from the β carbon to the Cl atom via a substantial energy barrier of 92 kJ/mol (referenced to the zero‐point energy). The ion internal energy range in which the experimental rates varied between 105 and 107 s−1 was found to lie well below the calculated barrier for H‐atom transfer. The rates were modeled with the RRKM statistical theory which includes a tunneling step through an Eckart potential. The vibrational frequencies of both the normal and deuterated ethylchloride ions were determined by ab initio molecular‐orbital methods. The theory accounted very well for the absolute rates including the strong deuterium isotope effect. The measured kinetic‐energy release distribution app...

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.

The distinction of direct and pulsed-field ionized zero kinetic energy photoelectrons in electron/ion coincidence experiments

Abstract The direct formation of zero kinetic energy photoelectrons (ZEKE-PE) as well as the formation of PEs by delayed pulsed-field ionization (PFI-PE) has been investigated in photoelectron photoion coincidence (PEPICO) experiments employing synchrotron radiation. Direct ZEKE-PEs and delayed PFI-PEs can easily be distinguished by their time of flight if a small static background field is applied. Coincidence experiments have been performed between either one direct ZEKE-PE or one PFI-PE and the corresponding photoion in the photoionization of argon atoms. The experiments prove that previous ZEKE-PEPICO experiments of our group [Chem. Phys. Letters 224 (1994) 371; Chem. Phys. 187 (1994) 117] were free from the influence of delayed pulsed-field ionization. To our knowledge this is the first report of a pure PFI photoelectron spectrum (PFI-PES) recorded with synchrotron radiation.

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.

Tunneling RRKM calculations for the H2 loss reaction from ethane ions on an ab initio potential energy surface

Abstract The potential energy surface for the loss of H 2 from ethane cations has been investigated by means of ab initio calculations. Several energy minima and transition states and the reaction path connecting them have been identified. The highest energy part of the reaction path involves the transfer of H atoms. The energy of the transition states governing the H 2 loss reaction is significantly higher than the experimental dissociation threshold. This discrepancy can be resolved by assuming that the reaction proceeds by tunneling of H atoms through a transfer barrier. Based on the ab initio potential energy surface and specific rate constant for the reaction has been calculated including tunneling of H atoms through this transfer barrier with the threshold energy for the reaction given by the thermochemical energy of the products. Calculation of k ( E ) predicts a large kinetic shift compatible with TPEPICO experiments. The extremely steep slope of the k ( E ) curve explains why the metastability of the reaction is difficult to observe in a TPEPICO experiment.

Threshold photoelectron photoion coincidence study of the ethane loss from energy selected pentane ions cooled in a supersonic expansion

The loss of C2H6 from energy selected pentane ions has been investigated by the threshold protoelectron photoion coincidence method. Breakdown diagrams are reported at room temperature and for two different conditions in a molecular beam. From the observed shift in cross-over energies the 0 K onset for the C2H6 loss is determined to be 690 ± 20 meV. This result together with information about kinetic energy release and other thermochemical data supports the earlier conclusion that the corresponding ion in the C2H6 loss is the propene ion rather than the cyclopropane ion. It is shown that rotational and vibrational energies are both active in this dissociation. The rate constants of dissociation have been measured as a function of the ion internal energy and are shown to agree with the results of Rice-Rampsperger-Kassel-Marcus calculations. Evidence is given that isomerisation of pentane to methylbutane prior to dissociation does not play an important role.

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.

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.