Real Time Visualization of Atomic Motions in Dense Phases
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It has always been a dream of physicists and chemists to follow temporal variations of molecular geometry during a chemical reaction in real time, to “film” them in a way similar as in the everyday life. Unfortunately, chemical events take place on tiny time scales comprised between 10 fs and 100 ps, approximately. Visualizing atomic motions thus remained a dream over two centuries. This is no longer true today, consequence of an immense instrumental development the last decades. Two methods are particularly important. The first of them is ultrafast optical spectroscopy employing the recently developed laser technology. In his breakthrough work A. Zewail was able to show how can this method be used to follow the photoelectric dissociation of gaseous ICN in real time[1,2]. It has later been applied to several other problems, and particularly so to visualize OH..O motions in liquid water[3,4]. Unfortunately, visible light interacts predominantly with outer shell rather than with deeper lying core electrons that most directly indicate molecular geometry. It is thus difficult to convert spectral data into data on molecular geometry. The second method refers to time resolved x-ray diffraction and absorption. As x-rays interact predominantly with deeply lying core electrons which are tightly bonded to the nuclei, converting x-ray data into data relative to molecular geometry is, in principle at least, much more straightforward than in optical spectroscopy. Unfortunately, pulsed x-rays techniques are technically very demanding.
KeywordsPump Pulse Probe Pulse Molecular Rotation Rotational Anisotropy Probe Electric Field
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