Polarization of molecular x-ray fluorescence

Abstract

Polarization of CI Kfi x-ray fluorescence following selective excitation of gaseous CH3CI with synchrotron radiation is reported. The degree of polarization of the fluorescence depends sensitively on the chosen incident excitation energy in the CI A^-edge region. Theoretical considerations indicate that the fluorescence-polarization measurements can provide directly absorption and emission anisotropics, molecular-orbital symmetries, and relative fluorescence transition strengths.PACS numbers: 33.20. Rm, 33.50.Dq, 33.90.+h X-ray emission spectroscopy under conditions of unpolarized excitation and detection has a long history of elucidating core-level phenomena in a wide variety of contexts. ! Synchrotron-radiation sources in the x-ray range now provide both tunable and polarized exciting radiation throughout the spectral regions near core-level thresholds in atoms, molecules, and solids. 2 Combining the advantages offered by synchrotron radiation with polarization analysis of the x-ray fluorescence, we report here results of valence-electron x-ray fluorescencepolarization measurements following energy-selected and polarized excitation near the CI K edge of methyl chloride (CH3CI). Theoretical interpretation of the measured polarized-fluorescence spectra indicates that the anisotropy of the photoexcitation process and the orientations and relative magnitudes of fluorescent dipole transition moments may be obtained directly from experiment. The x-ray polarized-fluorescence technique reported here thus provides a new approach to molecular photoabsorption and emission, as well as molecularstructure studies.Previous work has used ultraviolet and visible fluorescence polarization to infer alignment of molecular ions and fragments following polarized excitation of valence electrons. 3 The present study reports initial observations of fluorescence polarization following core-level excitation. 4 For molecular systems, core-level studies offer two distinct advantages. First, because core levels undergo rapid Auger decay (< 10 ~1 4 s), fluorescence is observed on a time scale that is short compared with normal molecular tumbling periods (10 -11 -10 -12 s). Valenceshell hole states, in contrast, are sufficiently long lived (>10~1 0 s) that rotational motion can occur prior to radiative decay, complicating the interpretation of fluorescence-polarization measurements. 3 Second, corelevel spectroscopy is highly selective to a specific atomic absorption edge and, with sufficient resolution, to the same atomic species in different environments.Core-level fluorescence-polarization measurements require (i) a source of polarized x rays, (ii) highthroughput and high-resolution primary and secondary x-ray monochromators, and (iii) polarization sensitivity of the secondary spectrometer. The first requirement is satisfied by synchrotron radiation, which in this work was provided by beam line X-24A at the National Synchrotron Light Source (NSLS), where a large-aperture crystal-diffraction primary monochromator selects the desired incident...

Probing symmetry breaking upon core excitation with resonant x-ray fluorescence

“…Although it is an accepted fact that without additional interactions, such as vibronic coupling, the symmetry must be restored for the exact electronic core hole state wave function, direct experimental evidence has been lacking to prove just that. The introduction of high-brightness synchrotron radiation sources has opened new possibilities to measure resonant core-electron spectra [5] and thereby reach conclusions on this issue by purely experimental means. Resonant x-ray emission provides such a possibility because the dipole character of the absorption and emission leads to simple selection rules also for the full scattering process [4].…”

Resonant inelastic x-ray scattering at the limit of subfemtosecond natural lifetime

“…2,3 Polarization and anisotropy of x-ray emission can be used to study orbital components, bond directions, and molecular orbital symmetries. [4][5][6] Recently, molecular-field effects a) Author to whom correspondence should be addressed. Electronic mail: tatiana.marchenko@upmc.fr.…”

Resonant inelastic x-ray scattering of methyl chloride at the chlorine K edge

“…33 The second maximum at 2826.7 eV is attributed to transitions to Rydberg states of (1a 1 → np) symmetry converging to the threshold at 2828.4 eV, indicated by the vertical bar in Fig. 1.…”

“…It has been shown that polarized x-ray-emission spectroscopy allows direct determination of the symmetries of occupied/unoccupied valence molecular orbitals and the anisotropy in absorption and emission processes. 33,[42][43][44] Using the polarimeter capabilities of the ALS spectrometer, K α emission lines were recorded after resonant photoexcitation at the Cl 1s → 8a 1 resonance for ten different θ angles between incident ( e 1 ) and emitted ( e 2 ) polarization vectors. The experimental spin-orbit ratio were measured from a least-squares fit procedure of the K α emission lines by two Voigt profiles assuming identical emission profiles for the SO doublet 2p 3/2 and 2p 1/2 .…”