Near-edge Soft X-ray Research Articles

Near-Edge Soft X-ray Absorption Mass Spectrometry of Protonated Melittin

We have investigated the photoionization and photofragmentation yields of gas-phase multiply protonated melittin cations for photon energies at the K-shell absorption edges of carbon, nitrogen, and oxygen. Two similar experimental approaches were employed. In both experiments, mass selected [melittin+qH]q+ (q=2–4) ions were accumulated in radiofrequency ion traps. The trap content was exposed to intense beams of monochromatic soft X-ray photons from synchrotron beamlines and photoproducts were analyzed by means of time-of-flight mass spectrometry. Mass spectra were recorded for fixed photon energies, and partial ion yield spectra were recorded as a function of photon energy. The combination of mass spectrometry and soft X-ray spectroscopy allows for a direct correlation of protein electronic structure with various photoionization channels. Non-dissociative single and double ionization are used as a reference. The contribution of both channels to various backbone scission channels is quantified and related to activation energies and protonation sites. Soft X-ray absorption mass spectrometry combines fast energy deposition with single and double ionization and could complement established activation techniques.Graphical ᅟ

Probing ultrafast \u03c0\u03c0*/n\u03c0* internal conversion in organic chromophores via K-edge resonant absorption

Many photoinduced processes including photosynthesis and human vision happen in organic molecules and involve coupled femtosecond dynamics of nuclei and electrons. Organic molecules with heteroatoms often possess an important excited-state relaxation channel from an optically allowed ππ* to a dark nπ* state. The ππ*/nπ* internal conversion is difficult to investigate, as most spectroscopic methods are not exclusively sensitive to changes in the excited-state electronic structure. Here, we report achieving the required sensitivity by exploiting the element and site specificity of near-edge soft X-ray absorption spectroscopy. As a hole forms in the n orbital during ππ*/nπ* internal conversion, the absorption spectrum at the heteroatom K-edge exhibits an additional resonance. We demonstrate the concept using the nucleobase thymine at the oxygen K-edge, and unambiguously show that ππ*/nπ* internal conversion takes place within (60 ± 30) fs. High-level-coupled cluster calculations confirm the method’s impressive electronic structure sensitivity for excited-state investigations.

New theoretical investigation resolving discrepancies of atomic form factors and attenuation coefficients in the near-edge soft X-ray regime

Reliable knowledge of the complex X-ray form factor and the photoelectric attenuation coefficient is required for crystallography, medical diagnosis, radiation safety and XAFS studies. Discrepancies between currently used theoretical approaches of 200% exist for numerous elements for X-ray energies from 1 to 3 keV. This work addresses key discrepancies and derives theoretical results in near-edge soft X-ray regions. DHF wave functions are employed and computational and convergence issues are of direct concern. Comparisons with simpler wave functions, including additional relativistic corrections to the form factors, are insightful. The current result improves upon the theoretical uncertainty in these regions to an estimated standard deviation of 20–30%.

Detailed Tabulation of Atomic Form Factors, Photoelectric Absorption and Scattering Cross Section, and Mass Attenuation Coefficients in the Vicinity of Absorption Edges in the Soft X-Ray (Z=30–36, Z=60–89, E=0.1 keV–10 keV), Addressing Convergence Issues of Earlier Work

Reliable knowledge of the complex x-ray form factor [Re(f ) and f″] and the photoelectric attenuation coefficient (σPE) is required for crystallography, medical diagnosis, radiation safety, and XAFS studies. Discrepancies between currently used theoretical approaches of 200% exist for numerous elements from 1 to 3 keV x-ray energies. The key discrepancies are due to the smoothing of edge structure, the use of nonrelativistic wave functions, and the lack of appropriate convergence of wave functions. This paper addresses these key discrepancies and derives new theoretical results of substantially higher accuracy in near-edge soft x-ray regions. The high-energy limitations of the current approach are also illustrated. The energy range covered is 0.1 to 10 keV. The associated figures and tabulation demonstrate the current comparison with alternate theory and with available experimental data. In general, experimental data are not sufficiently accurate to establish the errors and inadequacies of theory at this level. However, the best experimental data and the observed experimental structure as a function of energy are strong indicators of the validity of the current approach. New developments in experimental measurement hold great promise in making critical comparisons with theory in the near future.

Resolution of discrepancies of atomic form factors and attenuation coefficients in the near-edge soft X-ray regime

Abstract X-rays interactions with atoms should be well understood quantitatively. Reliable knowledge of the complex X-ray form factor (Re( f ) and f ″) and the photoelectric attenuation coefficient σ PE is required for crystallography, medical diagnosis, radiation safety and XAFS studies. Discrepancies between currently used theoretical approaches of 200% exist for numerous elements from 1 to 3 keV X-ray energies. This paper addresses key discrepancies and derives new theoretical results in near-edge soft X-ray regions of recent interest.

Quarter-Wave Plate in Near-Edge Soft X-Ray Region Based on the Total External Reflection Principle

A new way for the development of an x-ray quarter-wave plate is suggested. The idea is based on an effect of total external reflection from an anisotropic crystal in an anomalous dispersion region. It is found that in the case of reflection of plane polarized synchrotron radiation from a uniaxial crystal near the total external reflection angle the reflected radiation can be circularly polarized (right or left handed). The reflected beam polarization is calculated for a hexagonal BN crystal near the boron $K$ absorption edge and for a graphite crystal near carbon $K$ absorption edge. It is shown that the method developed gives a simple way to construct an effective x-ray quarter-wave plate.

Hybridization of the 3d states of transition metals with the states of the ZnS matrix

The near-edge soft x-ray absorption spectra of ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{M}}_{\mathrm{x}}$S compounds (where M is Mn, Fe, Co, and Ni, and x ranges to the solubility limit) have been obtained by measuring the total electron yield (photoelectron, Auger and secondary) from the sample in an ultrahigh vacuum (UHV) sample chamber using x-rays from beam line 6.3.2, a bending magnet beamline at the Advanced Light Source (ALS). The high spectral resolution of the beamline in the 50--1000 eV region allowed detailed measurements of S-${\mathrm{L}}_{2,3}$ and transition metal ${\mathrm{L}}_{3}$ absorption spectra. The study of d and s antibonding states in ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{M}}_{\mathrm{x}}$S compounds provided evidence that 3d states of transition metals participate in the formation of ionic-covalence bonding between M-S. Furthermore, it was found that the ability of 3d electrons to participate in bonding decreases with increasing 3d orbit filling. As the 3d orbit becomes more closed, the 3d electrons assume a more corelike character. The evidence of these behaviors can be seen in the metal and sulfur L absorption spectra as well as in the charge transfer values calculated from the energy shifts in transition metal K edges. These findings provided an explanation for the drastic decrease in the solubility of these particular metals in the ZnS matrix.