X-Ray Raman Scattering
X-ray physics constitutes one of a few branches which has remained vital through the whole 20th century. The modern history of X-ray spectroscopy has though spanned a comparatively short time period, as it has tightly followed the development of synchrotron radiation sources. The brightness of these sources is of the order 1000000 larger than standard X-ray sources, like X-ray tubes. The high intensity of synchrotron radiation implies that the monochromatization can be driven far. This new source of polarized and tunable monochromatic X-ray radiation constitutes the background of current studies of resonant X-ray Raman scattering (RXS) with super high spectral resolution. In spite of the short time span of investigations, many new physical phenomena in the RXS field have been unraveled and studied for gas and condensed phases. A set of fundamental results in RXS has been obtained in our group. While one has witnessed many exciting breakthroughs in the field of RXS in the recent years, revealing several underlying physical principles. The goal of our studies in this field is to gain more of the necessary understanding of the physical and chemical processes involved, and to develop efficient computational approaches to simulate the spectra of resonant X-ray Raman scattering.
The polarization of light plays very important role in RXS measurements due to space and symmetry selectivity of core excitation. The problem of the electronic selection rules in RXS is untrivial and is strongly related with interchannel interference. The dynamics of resonant X-ray Raman scattering is a main direction of our research. The first impression is that it is impossible to study dynamics due to long pulses of synchrotron radiation. We recognized, however, that scattering is limited in time due to the duration of RXSwhich can be changed by tuning the photon frequency off the resonance. The possibility to manipulate processes at different microscopic time scales by detuning the frequency and thereby shortening the duration time is a novel aspect of the spectroscopy in this respect, leading to effects like ''symmetry restoration'', ''vibrational collapse'', and ''control of dissociation''. Rather often the electronic selection rules breaks down due to electon-vibronic coupling. We have found that the electronic selection rules are restored when the duration of RXS is shorter than the period of vibrations. Nowadays, this effect constitutes a powerful tool to interprete RXS spectra.
When the wave length of radiation becomes comparable with the molecular size, the electronic selection rules breaks down due to the diffractional scattering of photons. An intristic feature of RXS is its coherence due to which different scattering channels interfere strongly. The destructive interference can completely quench some vibrational lines for certain detuning. This opens new ways for geometry determination. The important interference effect is the lifetime vibrational interference (LVI). One of the manifestation of LVI is the collapse of vibrational structure. The vibrational broadening of resonances makes the spectrum complicated. However, it was found that the electron-vibrational band can collapse in single narrow peak when the RXS duration is short. The vibrational structure changes drastically the Raman dispersion law. Due to LVI the center of gravity of the RXS band depends nonlinearly on the excitation energy. The spectral shape of RXS is very sensitive to the spectral function of incident radiation due to the Stokes doubling effect. In general, the electronic transition matrix elements depend on internuclear distances, especially at avoided crossings. We have found many intriguing aspects of RXS involving dissociative states and the appearance of molecular and fragment contributions to the RXS cross section, the super narrow interference hole. being one example.
The large energy release under dissociation results in strong enhacement of the electronic Doppler shift. The electronic Doppler effect is now widely used to study the dissociative states. The resonant scattering of photons through equivalent atoms of symmetrical molecules has intrictic features. Indeed, these scattering channels interfere due to the indistinguishability of atoms. Under certain conditions such an interference results in super narrow hole or peak on the top of a Doppler broadened atomic peak.
Our interest in resonant X-ray scattering is motivated also by different applications of RXS to determination of geometrical structures of molecules. One of the powerful techniques to study the geometry of disordered systems is the EXAFS technique (Extended X-ray Absorption Fine Structure). However, this technique allows to measure only internuclear distances. We found that EXAFS studied in X-ray Raman mode gives unique possibility to measure also bond angles.
The study of the many-electron effects in RXS has been a recent goal in our research. Here we pay special attention on the dynamical aspects of relaxation of electrons in the field of a core hole.
With lasers one can get a glimpse on molecular excitations at freezing times making use pump-probe technique. X-ray spectra are modified by a strong laser field. Laser assisted X-ray scattering, photoabsorption and photoionization can also be used for structure determination as we have explored in recent work.
Click the links below to see some experimental results related to our investigations. If you are interested in joining our group or in collaboration contact Hans Ågren.
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