T.
Darrah Thomas
Distinguished
Professor of Chemistry
PROFESSOR EMERITUS
B.S.,
Ph.D.,
T.Darrah.Thomas@oregonstate.edu
Dr. Thomas studies the energy spectra of electrons ejected from the inner shells of molecules, using facilities for high-resolution electron spectroscopy at the Advanced Light Source in Berkeley, the MAX-II synchrotron in Sweden, and the SPring-8 synchrotron in Japan.. A major theme of this work has been a search for better understanding of chemical processes that involve the addition of charge to a molecule [1-7]. Other recent research has been concerned with the effects of molecular conformation on inner-shell photoelectron spectra [8] and on inner-shell ionization energies [9,10], and with photoelectron-recoil-induced excitation of rotational and vibrational motion [11,12].
Inner‑shell ionization energies in substituted benzenes
Inner‑shell ionization energies reflect the energy required to change the charge at a particular site in a molecule, as do such more common chemical properties as acidity, basicity, and rates of electrophilic reactions. Our work has taken advantage of the high‑resolution capability of synchrotron radiation to measure the carbon 1s ionization energies for hydrocarbons, where high-resolution makes it possible to resolve details that were previously inaccessible. A typical example can be seen in the carbon 1s photoelectron spectrum of methyl-substituted benzenes, shown to the right [6]. Each chemically inequivalent carbon atom in these molecules shows a unique pattern of vibrational excitation, which can be calculated theoretically. Fitting these calculated profiles to the overall spectra makes it possible to assign carbon 1s ionization energies to all of the carbon atoms in the molecule, and these can be correlated with other chemical properties, such as enthalpies of protonation and activation energies for a variety of chemical reactions. Similar result have been obtained for fluorobenzenes[5] and for fluoromethylbenzenes.[7]
The carbon 1s ionization energies are found to correlate linearly with enthalpies of protonation and with activation energies for electrophilic reactions. These correlations can be used to illustrate the role of fluorine as a p electron donor and of the added proton in the protonated species as a p electron acceptor.
Conformational effects on inner-shell photoelectron spectra
In general, it had not been expected that inner-shell ionization energies and inner-shell photoelectron spectra would have a significant dependence on molecular conformation. Recent experiments have shown that this is not the case and, that for certain classes of molecules these effects are quite apparent. One example is ethanol [8], which exists in both an anti and a gauche form. Upon carbon 1s ionization of the methyl carbon in the gauche form there is strong repulsion between the core-ionized carbon and the proton of the hydroxyl group, leading to considerable excitation of torsional motion in the ion and, consequently to broadening of the carbon 1s photoelectron spectrum. A second example is butyronitrile, which also exists in both anti and gauche forms. In the gauche form, the distance between the CN group and the methyl carbon is less than it is in the anti form. The Coulombic interaction between the negatively charged CN group and the carbon 1s electron leads to a noticeably smaller ionization energy for the methyl carbon in the gauche form than in the anti form [9]. The carbon 1s photoelectron spectrum of butyronitrile is shown in the figure to the right, where it can be seen that the peak with the lowest ionization energy, arising from the terminal methyl group, is split into two peaks, with the peak for the gauche configuration shifted to lower ionization energy by the interaction with the CN group. Similar results are seen for 1-fluoroethane [9], propanal [9], and 1-pentyne[10].
Recoil-induced internal excitation accompanying photoionization
The atom from which a photoelectron is emitted has a momentum that is equal and opposite to that of the photoelectron. For an atom in a molecule, this momentum is transferred to the molecule as a whole. However, since the molecule has a larger mass than any of its constituent atoms, the recoil kinetic energy of the molecule is less than the recoil energy of the atom from which the electron was ejected. The energy difference is taken up by the molecule as internal excitation, either vibrational or rotational. Although this effect was predicted theoretically more than 30 years ago it is only recently that advances in experimental techniques have made it possible to observe it. Our measurements of the carbon 1s spectrum of CF4 over a photon energy range from 330 eV (just above threshold) to 1500 eV show that this recoil effect leads to an energy-dependent excitation of the asymmetric stretching mode in the residual ion, as can be seen in the figure to the right. At a photon energy of 330 eV only the symmetric CF stretching mode is excited, whereas at 1500 eV there is significant excitation of the asymmetric CF stretching mode, which is the dominant excitation at high energy. The degree of excitation is found to increases linearly with the kinetic energy of the photoelectron at a rate that is in excellent agreement with the predictions of theory [11].
For ionization of a core electron it is reasonable that the momentum is taken up initially by the single atom from which the electron is emitted as in the case discussed above. For a valence electron, which is delocalized over the molecule, it is less apparent that this should be the case. To investigate this question we have measured the valence photoelectron spectra of N2 over a range of photon energies from 60 to 800 eV. For the B state of N2+ we find that the recoil-induced rotational excitation increases linearly with the photoelectron kinetic energy at a rate that is consistent with the electron having been emitted from a single atom [12]. Thus, in this case, the electron acts as if it is localized.