15 research outputs found
Molecular double core-hole electron spectroscopy for chemical analysis
We explore the potential of double core hole electron spectroscopy for
chemical analysis in terms of x-ray two-photon photoelectron spectroscopy
(XTPPS). The creation of deep single and double core vacancies induces
significant reorganization of valence electrons. The corresponding relaxation
energies and the interatomic relaxation energies are evaluated by CASSCF
calculations. We propose a method how to experimentally extract these
quantities by the measurement of single and double core-hole ionization
potentials (IPs and DIPs). The influence of the chemical environment on these
DIPs is also discussed for states with two holes at the same atomic site and
states with two holes at two different atomic sites. Electron density
difference between the ground and double core-hole states clearly shows the
relaxations accompanying the double core-hole ionization. The effect is also
compared with the sensitivity of single core hole ionization potentials (IPs)
arising in single core hole electron spectroscopy. We have demonstrated the
method for a representative set of small molecules LiF, BeO, BF, CO, N2, C2H2,
C2H4, C2H6, CO2 and N2O. The scalar relativistic effect on IPs and on DIPs are
briefly addressed.Comment: 35 pages, 6 figures. To appear in J. Chem. Phys
Observation of electron transfer mediated decay in aqueous solution
Photoionization is at the heart of X ray photoelectron spectroscopy XPS , which gives access to important information on a sample s local chemical environment. Local and non local electronic decay after photoionization in which the refilling of core holes results in electron emission from either the initially ionized species or a neighbour, respectively have been well studied. However, electron transfer mediated decay ETMD , which involves the refilling of a core hole by an electron from a neighbouring species, has not yet been observed in condensed phase. Here we report the experimental observation of ETMD in an aqueous LiCl solution by detecting characteristic secondary low energy electrons using liquid microjet soft XPS. Experimental results are interpreted using molecular dynamics and high level ab initio calculations. We show that both solvent molecules and counterions participate in the ETMD processes, and different ion associations have distinctive spectral fingerprints. Furthermore, ETMD spectra are sensitive to coordination numbers, ion solvent distances and solvent arrangemen
Exploring Protonation and Deprotonation Effects with Auger Electron Spectroscopy
Auger electron spectroscopy is demonstrated to be a very
efficient
tool to probe alterations in local chemical environment due to changes
in protonation states. We show that electronic and geometric structure
changes induced by protonation or deprotonation are well reflected
in Auger spectra through characteristic chemical shifts and spectral
shape variations. We also present evidence that Auger spectra are
sensitive to relative concentrations of compounds in different protonation
states. Special attention is paid to the high kinetic energy spectral
regions that exhibit remarkable features resulting from core ICD-like
transitions in normal species and Auger transitions in deprotonated
fragments. The latter contribution was so far ignored when explaining
Auger spectra of species embedded in the environment. This contribution
should be reconsidered, taking into account the recently discovered
possibility of ultrafast dissociation of core-ionized hydrogen-bonded
systems in media
The molecules for which the IR energy of the tsDCH states has been studied
<p><strong>Figure 1.</strong> The molecules for which the IR energy of the tsDCH states has been studied.</p> <p><strong>Abstract</strong></p> <p>The interatomic relaxation (IR) effects of two-site double core hole (tsDCH) states in selected molecules with a polarizable unit have been systematically investigated using <em>ab initio</em> calculations. The IR effects are analysed by varying the size of this polarizable unit and its position relative to the DCHs. The systems with the DCHs located on the opposite sides of the polarizable unit show large negative IR energies, while those on the same side of the polarizable unit have smaller negative IR effects. Here, the IR energies can even be positive if the polarizable unit is large enough. The generalized Wagner plots of tsDCH states are used to visualize the trend of the IR effects in the molecules studied.</p
Core-level tsDIP, SIP, IR energy, modified second IP (IP*) (in eV) and polarizability (α) calculated by the <em>ab initio</em> SCF method with the cc-pCVTZ basis set
<p><b>Table 1.</b> Core-level tsDIP, SIP, IR energy, modified second IP (IP*) (in eV) and polarizability (α) calculated by the <em>ab initio</em> SCF method with the cc-pCVTZ basis set.</p> <p><strong>Abstract</strong></p> <p>The interatomic relaxation (IR) effects of two-site double core hole (tsDCH) states in selected molecules with a polarizable unit have been systematically investigated using <em>ab initio</em> calculations. The IR effects are analysed by varying the size of this polarizable unit and its position relative to the DCHs. The systems with the DCHs located on the opposite sides of the polarizable unit show large negative IR energies, while those on the same side of the polarizable unit have smaller negative IR effects. Here, the IR energies can even be positive if the polarizable unit is large enough. The generalized Wagner plots of tsDCH states are used to visualize the trend of the IR effects in the molecules studied.</p
Proton-Transfer Mediated Enhancement of Nonlocal Electronic Relaxation Processes in X‑ray Irradiated Liquid Water
We have simulated the oxygen 1s Auger-electron
spectra of normal
and heavy liquid water using <i>ab initio</i> and quantum
dynamical methods. The computed spectra are analyzed and compared
to recently reported experimental data. The electronic relaxation
in liquid water exposed to ionizing X-ray radiation is shown to be
far more diverse and complex than anticipated and extremely different
than for an isolated water molecule. A core-level ionized water molecule
in the liquid phase, in addition to a local Auger process, relaxes
through nonlocal energy and charge transfer, such as intermolecular
Coulombic decay and electron-transfer mediated decay (ETMD). We evaluate
the relative efficiencies for these three classes of relaxation processes.
The quantitative estimates for the relative efficiencies of different
electronic decay modes help determine yields of various reactive species
produced by ionizing X-rays. The ETMD processes which are considered
here for the first time in the core-level regime are found to have
a surprisingly high efficiency. Importantly, we find that all nonlocal
electronic relaxation processes are significantly enhanced by ultrafast
proton transfer between the core-ionized water and neighboring molecules
Generalized Wagner plots for the molecules shown in figure 1
<p><strong>Figure 3.</strong> Generalized Wagner plots for the molecules shown in figure <a href="http://iopscience.iop.org/0953-4075/46/16/164012/article#jpb463173f1" target="_blank">1</a>. The F1s SIPs are plotted against the modified second IPs defined by equation (<a href="http://iopscience.iop.org/0953-4075/46/16/164012/article#jpb463173eqn04" target="_blank">4</a>). The diagonal dashed lines with the slope +1 correspond to the levels of constant IR energies.</p> <p><strong>Abstract</strong></p> <p>The interatomic relaxation (IR) effects of two-site double core hole (tsDCH) states in selected molecules with a polarizable unit have been systematically investigated using <em>ab initio</em> calculations. The IR effects are analysed by varying the size of this polarizable unit and its position relative to the DCHs. The systems with the DCHs located on the opposite sides of the polarizable unit show large negative IR energies, while those on the same side of the polarizable unit have smaller negative IR effects. Here, the IR energies can even be positive if the polarizable unit is large enough. The generalized Wagner plots of tsDCH states are used to visualize the trend of the IR effects in the molecules studied.</p
Ionic-Charge Dependence of tie Intermolecular Coulombic Decay Time Scale for Aqueous Ions Probed by the Core-Hole Clock
Auger electron spectroscopy combined with theoretical calculations has been applied to investigate the decay of the Ca 2p core hole of aqueous Ca2+. Beyond the localized two-hole final states on the calcium ion, originating from a normal Auger process, we have further identified the final states delocalized between the calcium ion and its water surroundings and produced by core level intermolecular Coulombic decay (ICD) processes. By applying the core-hole clock method, the time scale of the core level ICD was determined to be 33 +/- 1 fs for the 2p core hole of the aqueous Ca2+. The comparison of this time constant to those associated with the aqueous K+, Na+, Mg2+, and Al3+ ions reveals differences of 1 and up to 2 orders of magnitude. Such large variations in the characteristic time scales of the core level ICD processes is qualitatively explained by different internal decay mechanisms in different ions as well as by different ion solvent distances and interactions