The resonant excitation of 13 vibrational states of the gauche and the trans forms of CH2D–CH2D isolated in a xenon matrix has been performed with the laser radiation provided by the free electron laser CLIO and by an optical parametric oscillator configuration. The determination of the sample composition by infrared spectroscopy gives the energy dependence of the conformational inversion quantum yield in the 1000–3000 cm−1 domain. This reaction proves the coupling of the molecular internal rotation with the laser excited vibrational mode. We observe a global increase of the isomerization efficiency with the excitation energy, but with large fluctuations in the energy dependence. The increase is interpreted as due to the addition of new coupling channels when higher states of the vibrational ladder are excited, whereas the fluctuations are a signature of specific molecular couplings. The influence of the temperature and of the nature of the rare gas matrix on the isomerization rate emphasizes the key role of the intermolecular interactions along the reaction path.
We present here an EXAFS (extended x-ray absorption fine structure) experiment performed at the K edge of the sulfur atom of OCS isolated in an argon matrix. The aim was to elucidate the structure of the trapping site of the molecule. We show that the argon atoms are located at a mean distance of 3.78 Å from the sulfur atom, with a root mean square variation of the S–Ar distances of about 0.08 Å. Our results indicate that the host atoms occupy a statistical set of positions around the molecule, rather than well-defined sites.
We present here results concerning the first attempt of determining the trapping site structure of molecules isolated in inert matrices at low temperature by the EXAFS (Extended X-ray Absorption Fine Structure) method. The experiments have been performed at the K edge of argon, silicon, sulfur, and chlorine for pure solid argon, and for SiH4, OCS, and HCl isolated in different cryogenic matrices. The EXAFS technique is sensitive to the local environment around the absorbing atom, and the spectral features induced by the matrix material (Ar, Xe, N2, and CH4) are clearly evidenced here. The data allow a characterization of the double substitutional site for OCS in argon and xenon, while no structure can be determined for the accommodation of SiH4 in argon. A discussion of the best choice for the guest/host system to obtain a good EXAFS signal is included.
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