UV Multiphoton Dissociation of Group VIB Hexacarbonyls and Derivatives
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UV MULTIPHOTON DISSOCIATION OF GROUP VIB HEXACARBONYLS AND DERIVATIVES
GEORGE W. TYNDALL and ROBERT L. JACKSON, California 95120
IBM Almaden Research Center, San Jose,
ABSTRACT Formation of electronically excited metal atoms via excimer laser multiphoton dissociation of Cr(CO) 6 , C6H6 Cr(CO) 3 , and Mo(CO) 6 has been examined in the gas phase. The dissociation mechanism was studied by determining the laserfluence-dependence and buffer-gas-pressure dependence of the metal atom emission intensity. Each of these species was found to form metal atoms via two-photon and three-photon dissociation processes. The data suggest that dissociation occurs by both direct and sequential processes. INTRODUCTION Gas phase photodissociation reactions play a key role in laser-induced photodeposition from volatile organometallic compounds [1]. Studies of the mechanisms of these reactions thus constitute an important part of research in laser deposition. We have undertaken a study of the mechanisms of excimer-laser multiphoton dissociation (MPD) of the group VIB carbonyls and derivatives [2-6], due to the role these species have played in a number of laser deposition processes [7-10]. In this work, we propose that dissociation of these species to metal atoms can occur by two processes: 1)a direct process, where the metal complex is excited into a dissociative continuum to form the metal atom directly, and 2), a sequential process, where a product of a one-photon dissociation reaction absorbs additional photons to form the metal atom. EXPERIMENTAL We monitored the formation of metal atoms in excited electronic states by detecting the metal atom emission obtained upon excimer laser irradiation of Cr(CO) 6 , C6H6 Cr(CO) 3 , and Mo(CO) 6 . The vapor from each species was contained in a flow cell at a pressure well below its room temperature vapor pressure. A buffer gas, normally argon, was flowed over the cell windows to minimize deposition on the window surface. Pressures were controlled by varying the opening of a throttle valve connecting the cell to a diffusion pump. MPD of each species was obtained using the unfocused output of a Lambda-Physik EMG-101 excimer laser operated on the ArF* (1?3 nm), KrF* (248 nm), or XeF* (351 nm) lines at fluences ranging from 3-40 mJ-cm- . Emission normal to the laser beam was dispersed by a 1 mmonochromator (resolution 0.5-2 ;) and was detected using a photomultiplier tube in conjunction with a boxcar integrator. The risetime of the photomultiplier tube was 4 ns. Standard lamps were used to calibrate the wavelength response of the system. RESULTS AND DISCUSSION We will focus initially on KrF* laser-induced MPD of Cr(CO) 6 , since the behavior of this system is typical of that observed in the other systems. KrF* laser dissociation of Cr(CO) 6 under our experimental conditions produces a rich Cr emission spectrum. Over 200 transitions have been assigned from more that 40 excited states of Cr, lying between 2.9 and 6.7 eV above the ground state [11]. Emission is not quenched (except for two states, se
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