Jon A. Booze

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 photoionization and dissociation dynamics of energy‐selected acetylene dimers, trimers, and tetramers

The photoionization and dissociation characteristics of (C2H2)n (n=2, 3, and 4) have been studied by the threshold photoelectron–photoion coincidence technique applied to a molecular beam. For photon energies between 10.20 and 11.27 eV, only dissociative ionization pathways were observed for the clusters; no direct ionization of (C2H2)2, (C2H2)3, and (C2H2)4 to the stable ions C4H4+, C6H6+, and C8H8+ was observed. The shape of the time of flight peaks is consistent with statistical dissociation dynamics, and statistical analysis of the fragment peak shapes suggests that the dimer, trimer, and tetramer produce the fragments C4H2+/C4H3+, C4H4+, and C6H6+, respectively. Consistent with these observations, ab initio calculations are presented which indicate that the neutral T‐shaped dimer and triangular trimer geometries are unstable on the ionic potential energy surface, and indeed have vertical ionization energies well above the dissociation limits of many of their possible product channels. The statistical...

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...

On the determination of cluster properties by ionization techniques

Experimental differentiation between similar cluster ions of Acetylene produced by the chemical reactions is reported. (AIP)

Transition state structures and angular momentum effects in the dissociation dynamics of energy‐selected C4H+8 ions

The photoionization and dissociation dynamics of energy‐selected 1‐butene ions have been investigated by the technique of threshold photoelectron photoion coincidence (TPEPICO) time of flight mass spectrometry. The absolute dissociation rates for the reactions leading to the loss of H, CH3, and CH4 have been measured for two samples prepared with very different internal energy and angular momentum distributions. First rotationally cold ions were prepared by photoionizing 1‐butene molecules cooled in a seeded molecular beam. These rates were analyzed within the framework of RRKM theory with vibrator transition state structure for all three channels. Excellent agreement between theory and experiment was obtained when ab initio calculated transition state frequencies were used for the H loss and the CH3 loss transition states. A variational transition state theory (VTST) analysis shows that the CH3 loss transition state lies about 11 kJ/mol below the dissociation limit. Second, dissociation rates using an ef...

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 metastable formation of di-ethylchloronium ions from ethylchloride dimer ions in a seeded molecular beam

Cluster ions of ethylchloride and their dissociation products have been produced in a supersonic expansion of ethylchloride seeded in Ar and energy selected by the threshold photoelectron photoion coincidence (TPEPICO) method. The peak widths of the ion time of flight distribution indicate that all of the clusters are produced by dissociative photoionization of higher order clusters. Thus, trimer ions dissociate to form dimer ions and an ethylchloride monomer. This dimer ion was found to be metastable with respect to the formation of the di-ethylchloronium ion and a chlorine atom. The measured dissociation rate as a function of the dimer ion internal energy was compared to the calculated rates based on the statistical RRKM/QET theory. Good agreement was found when the dimer adiabatic IP was assumed to be 10.2 eV. The Cl loss from the ethylchloride dimer ion is associated with a reverse activation energy of about 0.32 eV.

The dissociative ionization of ethylene dimers, trimers, and tetramers studied by photoelectron photoion coincidence

The dissociation dynamics of energy selected ethylene dimer ions is investigated by photoelectron photoion coincidence. The derived dissociation rates as a function of the ion internal energy were found to be identical to those previously measured for rotationally cold C4H+8 ions. This demonstrates that the ethylene dimer ions rearrange to the 2‐butene isomers prior to dissociation. The ethylene dimer ions are produced by ionization of neutral ethylene dimer as well as by dissociative ionization of the neutral trimer. The statistical theory analysis of the kinetic energy released in the dissociative photoionization of the ethylene trimers indicates that these ions dissociate to the 2‐butene ion. At the energies investigated, the evaporation of a single monomer unit from the trimer ion removes sufficient energy to prevent further dissociation of the C4H+8 ions. All of the ethylene trimer ion signal was a result of dissociative ionization of higher order clusters. Finally, we show that the ethylene tetramer...

Nondissociative one-photon ionization of clusters--is it possible?

A new experimental technique, based on high resolution time of flight analysis of ions, has demonstrated that one photon ionization of dimers, trimers, and tetramers, is accompanied by dissociation of the cluster ion to form either clusters of lower order, new ion structures, or other products formed by intra-cluster ion molecule reactions. The method is based measuring the inevitable kinetic energy released in the dissociation process. By maintaining a low electric field across the ionization region, and optimizing the instrumental parameters, it is possible to measure kinetic energy releases as low as 2 meV. Examples of systems investigated include clusters of acetylene, ethylene, NO, CH3OH, CH3SH, CH3Cl, and C2H5Cl. The results showed that even at the lowest photon energies used, ionization without dissociation consisted of at most 5% of the total signal. In some ions, such as C2H2 clusters, 100% of the one photon ionization process is via dissociation.