Photoelectron spectroscopy ionization energy determination

The ionization energies determined by photoelectron-photoion coincidence spectroscopy are plotted as a function of the cube root of the reciprocal number of atoms x in the cluster. The latter is determined by in situ mass spectroscopy of the ionized clusters. 

Unlike the stable molecule N2O, the sulfur analogue N2S decomposes above 160 K. In the vapour phase N2S has been detected by high-resolution mass spectrometry. The IR spectrum is dominated by a very strong band at 2040 cm [v(NN)]. The first ionization potential has been determined by photoelectron spectroscopy to be 10.6 eV. " These data indicate that N2S resembles diazomethane, CH2N2, rather than N2O. It decomposes to give N2 and diatomic sulfur, S2, and, hence, elemental sulfur, rather than monoatomic sulfur. Ab initio molecular orbital calculations of bond lengths and bond energies for linear N2S indicate that the resonance structure N =N -S is dominant. 

The ionization energy of gaseous disulfane has been determined by photoionization efficiency spectroscopy as 9.40 0.02 eV [25] and by photoelectron spectroscopy as 9.41 eV [61]. Recently, XANES spectra of H2S and H2S2 have been reported which show distinct differences [62]. 

Even the photoelectron spectroscopy of closed-shell molecules is valuable for the physical chemistry of radicals because a difference between the nth and the first adiabatic ionization potentials determines the excitation energy in a radical cation for a transition from the ground doublet state to the (n — 1) excited doublet state. 

A second role for mass spectrometry in the investigation of reactive intermediates involves the nse of spectroscopy. Althongh an important nse of ion spectroscopy is the determination of thermochemical properties, including ionization energies (addition or removal of an electron), as in photoelectron or photodetachment spectroscopy, and bond dissociation energies in ions, as in photodissociation methods, additional spectroscopic data can also often be obtained, inclnding structural parameters such as frequencies and geometries. 

Photoelectron spectroscopy has routinely been used to determine the ionization energies of stable molecules. It has also been adapted for the investigation of reactive species, including radicals, biradicals, and carbenes, usually generated chemically or by using pyrolysis. ... 

The probability of a particular vertical transition from the neutral to a certain vibrational level of the ion is expressed by its Franck-Condon factor. The distribution of Franck-Condon factors, /pc, describes the distribution of vibrational states for an excited ion. [33] The larger ri compared to ro, the more probable will be the generation of ions excited even well above dissociation energy. Photoelectron spectroscopy allows for both the determination of adiabatic ionization energies and of Franck-Condon factors (Chap. 2.10.1). 

A very useful thermodynamic cycle links three important physical properties homolytic bond dissociation energies (BDE), electron affinities (EA), and acidities. It has been used in the gas phase and solution to determine, sometimes with high accuracy, carbon acidities (Scheme 3.6). " For example, the BDE of methane has been established as 104.9 0.1 kcahmol " " and the EA of the methyl radical, 1.8 0.7 kcal/mol, has been determined with high accuracy by photoelectron spectroscopy (PES) on the methyl anion (i.e., electron binding energy measurements). Of course, the ionization potential of the hydrogen atom is well established, 313.6 kcal/ mol, and as a result, a gas-phase acidity (A//acid) of 416.7 0.7 kcal/mol has been... 

The highest occupied molecular orbital (HOMO) in formaldehyde and heteroaldehydes, H2C=E, is the lone pair at E (nE), and the second highest MO (SOMO) is the C=E 77-bonding orbital. The LUMO is the 77 CE orbital composed of the antibonding combination of pz(C) and pz(E). The ionization energy of the HOMO in formaldehyde is 10.88 eV and of the SOMO 14.5 eV, as determined by photoelectron spectroscopy.33 The ionization energy of the HOMO and the SOMO both decrease considerably when the oxygen atom in formaldehyde is replaced by sulfur or selenium (see Fig. 1, data are compiled from Refs. 33-37). 

D. -S. Yang, M. Z. Zgierski, D. M. Rayner, P. A. Hackett, A. Martinez, D. R. Salahub, P.-N. Roy, and T. Carrington Jr., J. Chem. Phys., 103, 5335 (1995). The Structure of NbjO and Nb30 Determined by Pulsed Field Ionization-Zero Electron Kinetic Energy Photoelectron Spectroscopy and Density Functional Theory. 

Figure 51 Vertical ionization energies (/v) for bands (J) determined by photoelectron spectroscopy for thiopyranylidene derivatives 242-245.

Important information on the similarities and differences of germanium, tin and lead compounds was obtained using two mutually complementary types of spectroscopy. Photoelectron spectroscopy is widely used to determine the first (Ip) and subsequent ionization potentials of molecules. According to Koopmans theorem, the Ip is equated with the HOMO energy (equation 18)128. 

Photoelectron spectroscopy has been extensively used for Ge, Sn and Pb compounds to understand the bonding of these systems and to determine ionization energies19-21 105 119. Mass spectrometry has also been used in this respect although in general with a lower energy resolution. The results are reviewed in Reference 5 and in many cases the main uncertainty in the reported ionization energies is related to the ability in determining the true adiabatic IE. 

Xiu et al. [23] reported in Bi-doped ZnO films by molecular-beam epitaxy that Bi-induced acceptor ionization energy was estimated to be 0.185-0.245 eV by photoluminescence measurements based on the donor-acceptor pair peak position in the Bi-doped ZnO films Bi in ZnO films had positive charge state determined by X-ray photoelectron spectroscopy measurements, indicating that BiZn at Zn sites, rather than Bio at O sites, was formed in the films. BiZn itself, however, is a donor. The origin of the shallow acceptor states was, therefore, identified as a donor-acceptor pair such as BiZn-VZn-0, or BiZn-2VZn complexes. 

---