Oriented xenon hydride molecules in the gas phase

Abstract

The production of the xenon hydride molecules HXeX with X = I and Cl in the gas phase is reviewed. These molecules are generated by the photolysis of the hydrogen halide HI and HCl molecules on the surface of large xenon Xe _n clusters. Molecular dynamics simulations show that the flexible H atoms react with the heavy XeX moiety and form the desired molecules with nearly no rotational motion. They are observed by photodissociation with subsequent detection of the kinetic energy of the H atom fragment. During the generating process, the cluster starts to evaporate and the hydride molecule is left essentially free. For further discrimination against the H atom fragments from HX, the HXeX molecules are oriented in a combined pulsed laser field and a weak electrostatic field. The three topics which represent the background of our experiments are briefly reviewed: the nature and generation of rare gas hydrides, the alignment and orientation of molecules in electric fields, and the photodissociation of selected molecules in rare gas clusters. The conditions for detecting them in the gas phase are discussed. This is the trade off between the stability, which requires high electron affinity, and the conditions for orientation, which necessitate large polarizability anisotropies and dipole moments. Finally the prospects of detecting other classes of molecules are discussed.

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Acknowledgments

We dedicate this article to Roger Miller who left us so early. The orientation of molecules in pendular states and the high-resolution spectroscopy of these molecules was one of his central fields of research. His advice in the early stages of this experiment was extremely valuable to us. We thank Bretislav Friedrich for introducing us to the world of oriented molecules and for his help in calculating the degree of orientation. We are grateful to Benny Gerber and Zolt Bihary for useful calculations on the formation and the rotation of the hydride molecules. Last but not least we thank the two doctoral students Reinhard Baumfalk and N. Hendrik Nahler who measured and evaluated most of the data presented here. We acknowledge useful discussions with Mika Pettersson and Leonid Khriachtchev on the behaviour of the new rare gas hydride molecules. This work was supported by the Deutsche Forschungsgemeinschaft in SFB 357. MF acknowledges the Alexander von Humboldt fellowship.