Dynamics of the KrF* Laser Multiphoton Dissociation of a Series of Arene Chromium Tricarbonyls
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DYNAMICS OF THE KrF* LASER MULTIPHOTON DISSOCIATION OF A SERIES OF ARENE CHROMIUM TRICARBONYLS
GEORGE W. TYNDALL AND ROBERT L. JACKSON, IBM Almaden Research Center, San Jose, CA 95120
ABSTRACT The KrFa (248 nim) laser multiphoton dissociation (MPD) of a series of (arene)chromium tricarbonyls has been investigated in the gas-phase using emission spectroscopy to detect the excited state photoproducts. In the MPD of all compounds studied, chromium atoms are formed in a variety of electronically excited states via a two-channel dissociation mechanism. The predominant pathway for formation of the ground electronic state and the lowest excited states is by a sequential absorption/ fragmentation process, where the product of the one-photon dissociation of the parent molecule absorbs an additional photon and dissociates to Cr(I). The higher energy Cr(I) states are formed exclusively by a direct dissociation process, where the parent absorbs multiple photons prior to dissociation. The distribution of excited chromium atoms formed in the direct channel is statistical for all compounds studied and is independent of the nature of the arene ligand. In contrast, the distribution of Cr(I) states formed via the sequential dissociation channel is strongly dependent on the vibrational density of states in the arene ligand. INTRODUCTION The gas-phase photodissociation of organometallic compounds plays a key role in the de.position of metals via laser chemical vaqor deposition. In thepresent work we have studied the KrF (248 nm) laser MPD of: a) Cr(CO)3 (71 C6H6), b) Cr(CO) 3(71 CH 3 C6H5 ) c) Cr(CO)3 [s3 (n- C3H7 )C6H5 ], and d) Cr(CO)3[(13 (t - C4H9)C6H5 ]. We present evidence that the formation of Cr(I) occurs via two fragmentation channels: 1) a sequential absorption/fragmentation process involving an intermediate formed in the single photon dissociation of the parent (arene)chromium tricarbonyl, and 2) a direct dissociation process which involves a multiphotnn absorption occuring prior to any dissociation step. EXPERIMENTAL The experimental apparatus has been described in detail in ref. [1]. Formation of Cr(I) is monitored in these experiments by measuring the fluorescence obtained following irradiation of compounds (a) - (d) at 248 rnm. The focused output of an excimer laser (Lambda Physik model 101E) makes a single pass through a photolysis cell containing the vapor of one of the chromium compounds. A buffer gas, either Ar, He or CO, was passed through a reservoir containing the solid organochromium compounds, and the mixture was introduced to the cell through a molecular leak valve. The buffer gas was also passed over the laser entrance window, to minimize the deposition of chromium. Under normal operating conditions, the partial pressure of the (arene)chromium tricarbonyls was of the order of I mtorr, and the total cell pressure was < 1 torn. Emission normal to the laser beam is wavelength resolved with a 1 meter scanning monochromator and detected with a photomultiplier tube. Signal averaging is performed with a variable g
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