TPEPICO

Similar values have been obtained for AHffMesSi ) from two independent studies. The bond dissociation enthalpy DHfMeaSi-SiMea) = 332 +12 kJ moC was obtained from a kinetic study on the very low pressure pyrolysis of hexamethyldisilane and the enthalpy of formation of trimethylsilyl ion, AHf (MeaSi ) = 617.3 + 2.3kJmor, was determined using threshold photoelectron-photoion coincidence spectroscopy (TPEPICO). Both data are related to AHf°(Me3Si ). 

A number of techniques have been used previously for the study of state-selected ion-molecule reactions. In particular, the use of resonance-enhanced multiphoton ionization (REMPI) [21] and threshold photoelectron photoion coincidence (TPEPICO) [22] has allowed the detailed study of effects of vibrational state selection of ions on reaction cross sections. Neither of these methods, however, are intrinsically capable of complete selection of the rotational states of the molecular ions. The TPEPICO technique or related methods do not have sufficient electron energy resolution to achieve this, while REMPI methods are dependent on the selection rules for angular momentum transfer when a well-selected intermediate rotational state is ionized in the most favorable cases only a partial selection of a few ionic rotational states is achieved [23], There can also be problems in REMPI state-selective experiments with vibrational contamination, because the vibrational selectivity is dependent on a combination of energetic restrictions and Franck-Condon factors. 

Special techniques in addition to the UPS measurements, namely vacuum pyrolysis and TPEPICO, have been applied in the cases of (CF3)3Sb and Me3Bi. 

TPEPICO threshold photoelectron-photoion coincidence TPES threshold photoelectron spectroscopy... 

The study of ion-molecule reactions using state-selected reagents has become a very exciting area of molecular dynamics. We have developed an experimental apparatus in Orsay which utilizes the properties of our tunable synchrotron radiation source at LURE to prepare ions in selected vibronic levels and then to study their reactions. The ions are state-selected using the TPEPICO (threshold-photoelectron/photoion coincidence) method [1]. 

Examination of Fig. 7 shows that the reactive cross section rises monotonically for v = 0 to 5 but levels off for v = 6. The dependence of the cross section on v agrees well with the earlier work of Chupka [24], The agreement with the data of Turner et al. [25] is less satisfactory their cross sections level off at v = 3 and 4 at values considerably below our results. We attribute the difference to the manner in which they prepare their ions and estimate the distribution of vibrational levels in their beam. By comparison the TPEPICO technique prepares ions in a single vibrational level. The theoretical calculations of Whitton and Kuntz [26] and of... 

FIGURE 5. TPEPICO time-of-flight mass spectra of supersonically expanded acetylene at five photon energies. Lower right HC=CH (monomer) mass peak indicating narrow shape of translationally cold parent ions. Reproduced from Reference 28 by permission of the American Institute of Physics... 

Lago and Baer have studied the photo-ionization of bromine species that play an important role in the catalytic depletion of the Earth s protecting ozone layer by the threshold photoelectron photo-ion coincidence (TPEPICO) technique... 

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