T. Darrah Thomas

Professor of Chemistry

PROFESSOR EMERITUS(P)

B.S., Haverford College, 1954

Ph.D., University of California, Berkeley, 1957

 thomast@chem.orst.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, 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

 

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. Recent 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 propyne,[10] shown above. The contributions from the three inequivalent carbons as well as the unique vibrational structure associated with each carbon is clearly visible. Comparing the observed vibrational structure with that predicted theoretically makes it possible to assign the three peaks to the chemically inequivalent carbon atoms in propyne: the peak to the left arises from ionization of the HC≡ carbon and that to the right from ionization of the CH3 carbon. In this figure, the green curve shows the spectrum of propyne that was previously available – the improvement in quality with the new facilities is striking.


 

Vibrational structure in inner-shell photoelectron spectroscopy

Inner-shell ionization of molecules is generally accompanied by vibrational excitation, as can be seen in the spectrum for propyne. A more complex example, the carbon 1s photoelectron spectrum of 1,3-cyclohexadiene, which, like propyne, has three different types of carbon atom is shown above. In addition to the experimental data, shown as circles, the figure also shows the vibrational excitation spectra for each carbon (colored sticks) calculated ab initio using electronic structure theory, the same calculated spectra dispersed with the known line shape and experimental resolution (colored curves), and a least squares fit of these spectra to the data with only the energy position and the height of each group as adjustable parameters (solid line through the data). It is clear that this procedure provides an excellent description of the experimentally observed spectrum and that with this we can obtain accurate ionization energies for the inequivalent carbon atoms, even in a rather complex spectrum.

 


[1]. Carbon 1s photoelectron spectroscopy of CF4 and CO: Search for chemical effects on the carbon 1s hole-state lifetime,  T. X. Carroll, K. J. Børve, L. J. Sæthre, J. D. Bozek, E. Kukk, J. A. Hahne, and T. D. Thomas,  J. Chem. Phys. 116, 10221 (2002).

[2]. Anomalous natural linewidth in the 2p photoelectron spectrum of SiF4,  T. D. Thomas, C. Miron, K. Wiesner, P. Morin, T. X. Carroll, and L. J. Sæthre,  Phys. Rev. Lett., 89, 223001 (2002).

[3]. Line shape and lifetime in argon 2p electron spectroscopy, T. X. Carroll, J. D. Bozek, E. Kukk, V. Myrseth, L. J. Sæthre, and T. D. Thomas, J. Electr. Spectrosc. Relat. Phenom., 120, 67 (2001).

[4]. Vibronic structure in the carbon 1s photoelectron spectra of HCCH and DCCD,  K. J. Børve, L. J. Sæthre, T. D. Thomas, T. X. Carroll, N. Berrah, J. D. Bozek, and E. Kukk, Phys. Rev. A 63, 012506 (2001).

[5]. Vibrational structure and vibronic coupling in the carbon 1s photoelectron spectra of ethane and deuteroethane,  T. Karlsen, L. J. Sæthre, K. J. Børve, N. Berrah, E. Kukk, J. D. Bozek, T. X. Carroll, and T. D. Thomas,  J. Phys. Chem. 105, 7700 (2001).

[6]. Molecular-field splitting and vibrational structure in the phosphorus 2p photoelectron spectrum of PF3,  K. J. Børve, L. J. Sæthre, J. D. Bozek, J. True, T. D. Thomas,  J. Chem. Phys., 111, 4472 (1999).

[7]. Molecular-field splitting of the 2p3/2 peak in x-ray photo­electron spectroscopy of second-row atoms: A theoretical study of phosphine and phosphorus trifluoride,  K. J. Børve and T. D. Thomas,  J. Chem. Phys. 111, 4478 (1999).

[8]. Adiabatic and vertical carbon 1s ionization energies in representative small molecules, V. Myrseth, J. D. Bozek, E. Kukk, L. J. Sæthre, and T. D. Thomas, J. Electr. Spectrosc. Relat. Phenom., 122, 57 (2002).

[9]. Xenon N4,5OO Auger spectrum – a useful calibration source, T. X. Carroll, J. D. Bozek, V. Myrseth, L. J. Sæthre, T. D. Thomas, and K. Wiesner, J. Electr. Spectrosc. Relat. Phenom., 125, 127 (2002).

[10]. Chemical insights from high-resolution x-ray photoelectron spectroscopy and ab initio theory: Propyne, trifluoropropyne, and ethynylsulfur pentafluoride,  L. J. Sæthre, N. Berrah, J. D. Bozek, K. J. Børve, T. X. Carroll, E. Kukk, G. L. Gard, R. Winter, and T. D. Thomas,   J. Am. Chem. Soc. 123, 10729 (2001).