High resolution x‐ray photoelectron spectroscopy study of Cr(CO)6in the gas phase

Investigation of the valence ionization spectrum of chromium carbonyl using an ab initio quantum dynamical approach

The nuclear dynamics of the chromium carbonyl cation following an ionization process corresponding to the ²T_2g ← ¹A_1g transition is studied theoretically, for the first time, using a fully quantal approach as well as high levels of the ab initio electronic structure and semiempirical density functional theory (DFT) methods. The photoelectron spectrum is calculated by the construction of a Hamiltonian model, in which the two totally symmetric modes ν₁₉ (the Cr-C stretching mode) and ν₃₉ (the C-O stretching mode) together with the spin-orbit (SO) coupling up to the zeroth-order SO splitting are treated. The potential energy curves along these two vibrational modes are computed by using the DFT. The simulated photoelectron spectrum is found to be in good agreement with the corresponding experimental one, leading to the conclusion that the potential energy surfaces and the diabatic population analysis are accurately determined. Our calculation confirms that the vibrational modes ν₁₉ and ν₃₉ are the vibrational progression of the valence ionization spectrum of the chromium carbonyl cation.

Energy-Dependent Relative Cross Sections in Carbon 1s Photoionization: Separation of Direct Shake and Inelastic Scattering Effects in Single Molecules

We demonstrate that the possibility of monitoring relative photoionization cross sections over a large photon energy range allows us to study and disentangle shake processes and intramolecular inelastic scattering effects. In this gas-phase study, relative intensities of the carbon 1s photoelectron lines from chemically inequivalent carbon atoms in the same molecule have been measured as a function of the incident photon energy in the range of 300-6000 eV. We present relative cross sections for the chemically shifted carbon 1s lines in the photoelectron spectra of ethyl trifluoroacetate (the "ESCA" molecule). The results are compared with those of methyl trifluoroacetate and S-ethyl trifluorothioacetate as well as a series of chloro-substituted ethanes and 2-butyne. In the soft X-ray energy range, the cross sections show an extended X-ray absorption fine structure type of wiggles, as was previously observed for a series of chloroethanes. The oscillations are damped in the hard X-ray energy range, but deviations of cross-section ratios from stoichiometry persist, even at high energies. The current findings are supported by theoretical calculations based on a multiple scattering model. The use of soft and tender X-rays provides a more complete picture of the dominant processes accompanying photoionization. Such processes reduce the main photoelectron line intensities by 20-60%. Using both energy ranges enabled us to discern the process of intramolecular inelastic scattering of the outgoing electron, whose significance is otherwise difficult to assess for isolated molecules. This effect relates to the notion of the inelastic mean free path commonly used in photoemission studies of clusters and condensed matter.

Dissociation kinetics of excited ions: PEPICO measurements of Os3(CO)12 - The 7-35 eV single ionization binding energy region

In this article, we study the photoinduced dissociation pathways of a metallocarbonyl, Os₃(CO)₁₂, in particular the consecutive loss of CO groups. To do so, we performed photoelectron-photoion coincidence (PEPICO) measurements in the single ionization binding energy region from 7 to 35 eV using 45-eV photons. Zero-energy ion appearance energies for the dissociation steps were extracted by modeling the PEPICO data using the statistical adiabatic channel model. Upon ionization to the excited ionic states above 13 eV binding energy, non-statistical behavior was observed and assigned to prompt CO loss. Double ionization was found to be dominated by the knockout process with an onset of 20.9 ± 0.4 eV. The oscillator strength is significantly larger for energies above 26.6 ± 0.4 eV, corresponding to one electron being ejected from the Os₃ center and one from the CO ligands. The cross section for double ionization was found to increase linearly up to 35 eV ionization energy, at which 40% of the generated ions are doubly charged.

Charge transfer in the Cl−CO cluster induced by core ionization

Ab initio calculations of core-ionization spectra of the anion-molecule Cl-CO cluster are performed. Particular attention is paid to the investigation of charge-transfer screening processes accompanying core ionization of the CO molecule in the cluster. The charge-transfer processes are very efficient and favored by the presence of a low-lying unoccupied pi* orbital in CO capable of accepting an electron from Cl-. The O1s(-1) and C1s(-1) core-ionization spectra are calculated and compared. Both reveal a breakdown of the quasiparticle picture of core ionization caused by the charge-transfer processes. Remarkable differences between these two spectra are found which manifest themselves in distinct intensity distributions in the prominent low-energy spectral bands. The underlying reason for these differences is elucidated and linked with the preference of the pi* orbital to localize mainly on carbon. Core-ionization spectra of anion-molecule clusters are very sensitive to the type of the molecule involved as the comparative analysis of the O1s(-1) core-ionization spectra of the Cl-CO and Cl-H(2)O clusters show.

Vibrational progressions in the valence ionizations of transition metal hydrides: evaluation of metal-hydride bonding and vibrations in (eta(5)-C(5)R(5))Re(NO)(CO)H [R = H, CH(3)]

The first examples of vibrational structure in metal-ligand sigma-bond ionizations are observed in the gas-phase photoelectron spectra of CpRe(NO)(CO)H and CpRe(NO)(CO)H [Cp = eta(5)-C(5)H(5), Cp = eta(5)-C(5)(CH(3))(5)]. The vibrational progressions are due to the Re-H stretch in the ion states formed by removal of an electron from the predominantly Re-H sigma-bonding orbitals. A vibrational progression is also observed in the corresponding ionization of the deuterium analogue, CpRe(NO)(CO)D, but with lower vibrational energy spacing as expected from the reduced mass effect. The vibrational progressions in these valence ionizations are directly informative about the nature of the metal-hydride bonding and electronic structure in these molecules. Franck-Condon analysis shows that for these molecules the Re-H or Re-D bond lengthens by 0.25(1) A when an electron is removed from the Re-H or Re-D sigma-bond orbital. This bond lengthening is comparable to that of H(2) upon ionization. Removal of an electron from the Re-H or Re-D bonds leads to a quantum-mechanical inner sphere reorganization energy (lambda(QM)) of 0.34(1) eV. These observations suggest that even in these low symmetry molecules the orbital corresponding to the Re-H sigma bond and the Re-H vibrational mode is very localized. Theoretical calculations of the electronic structure and normal vibrational modes of CpRe(NO)(CO)H support a localized two-electron valence bond description of the Re-H interaction.