Molecular Beam Study of the KrF* Laser Photodissociation of Cr(CO) 6 and Mo(CO) 6
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MOLECULAR BEAM STUDY OF THE KrF* LASER PHOTODISSOCIATION OF Cr(CO) 6 AND Mo(CO) 6 GEORGE W. TYNDALL AND ROBERT L. JACKSON IBM Almaden Research Center, San Jose, California 95120-6099 ABSTRACT The one-photon and sequential two-photon KrF* laser (248 nm) photodissociation of Cr(CO) 6 and Mo(CO) 6 were studied in a molecular beam using mass spectrometry to detect the photoproduct molecular ions. For Cr(CO) 6 , the major one-photon product, Cr(CO) 4 , undergoes secondary photodissociation to give predominantly Cr(CO) 2 . The photodissociation cross section of Cr(CO) 4 is found to be 1.6 x higher than that of Cr(CO) 6 at 248 nm. For Mo(CO) 6 , the major one-photon products, Mo(CO) 5 and Mo(CO) 4 , undergo secondary photodissociation to give predominantly Mo(CO) 3. INTRODUCTION Ultraviolet multiphoton dissociation (UV MPD) of metallic compounds provides a mild technique for producing metal atoms and highly reactive unsaturated metallic species in the gas phase [1]. UV MPID has been used successfully to produce high quality thin films at relatively low substrate temperatures for microelectronic applications [2]. Metal atom formation via UV MPD appears to occur predominantly by a sequential mechanism, where a series of absorption/fragmentation processes eventually results in the loss of all ligands from the metal compound [3]. Unfortunately, the identity of the molecular intermediates involved in the UV MPD of most metallic compounds has not been determined. This is largely because the techniques typically used to study UiV MPI), including multiphoton ionization, emission spectroscopy, and laser-induced fluorescence, are not ameanble to the characterization of unsaturated metallic species. These species typically do not fluoresce, and thus far, they have eluded observation by standard multiphoton ionization techniques. In this paper, we examine the products of KrF* laser MPD of Cr(CO) 6 and Mo(CO) 6 with emphasis on identifying the products formed in the sequential two-photon dissociation process. Photodissociation of Cr(CO) 6 and Mo(CO) 6 are examined in a molecular beam, using mass spectrometry to detect the molecular ions of the photoproducts. One reason for choosing Cr(CO) 6 and Mo(CO) 6 in this study is that each molecule readily yields metal atoms upon MPD at 248 nm[ 1,4, 5]. In addition, the products formed upon single-photon Krlt* laser dissociation of Cr(CO) 6 have been well-characterized [6]. We show that Cr(CO) 2 is the major product formed in the second step of the sequential two-photon dissociation of Cr(CO) 6 . Cr(CO) 2 is produced primarily by photodissociation of the major single-photon product Cr(CO) 4 . We also show that Mo(CO)5 and Mo(CO) 4 are significant products formed in the one-photon dissociation of Mo(CO) 6 , and that these species undergo secondary photodissociation to give predominantly Mo(CO) 3 . EXIPERIMENTAI, The experimental techniques employed in this work have been described in detail elsewhere [7], so only a brief summary will be given here. Photodissociation is carried out in a different
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