Photodissociation and Photoionization Mechanisms in Lanthanide-based Fluorinated β-diketonate MOCVD Precursors Jiangchao CHEN, Robert J. WITTE, Yajuan GONG, Qingguo MENG, P. Stanley MAY, Mary T. BERRY* Department of Chemistry, University of South Dakota, Vermillion, SD  Experiments Understanding the photochemistry of gas-phase metal-organic compounds is fundamental to harnessing the full potential of laser-assisted metal- organic chemical vapor deposition (LCVD). A detailed photodissociation mechanism for the lanthanide-based MOCVD precursors LnL 3 [L=1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6- octanedionate (fod - ), hexafluoroacetylacetonate (hfac - )] and Ln(hfac) 3 diglyme was developed using Nd:YAG or OPO (optical parametric oscillator) laser induced photoionization time-of-flight (TOF) mass spectrometry. The collisionless environment of the molecular beam source revealed a series of unimolecular steps starting with dissociation of an intact  -diketonate ligand. Dissociation steps for the second and third ligands are each potentially associated with deposition of a fluoride on the metal, leading to one of three ultimate products Ln, LnF, or LnF 2.  Introduction  References Pollard, K. D.; Jenkins, H. A.; Puddephatt, R. J.Chem. Mater. 2000, 12, 701. Ow, F.P.; Berry, M.T.; May, P.S.; Zink, J.I. J. Phys. Chem. A 2006, 110, 7751 Meng, Q.G.; Witte, R.J.; May, P.S.; Berry, M.T. Chem. Mater. 2009, 21, 5801  Results of LnL 3  Results of Hhfac  Results of EuL 3  Conclusion In laser-assisted chemical vapor deposition using metal-organic precursors, the nature of deposited materials in strongly influenced by unimolecular gas- phase reactions. The fluorination process proposed during the photodissociation is in competition with production of bare metal Ln(0) which is thought to proceed through a mechanism of three-fold intact-ligand dissociation mediated by photo-excitation to dissociative regions of LMCT states.  Acknowledgements Dr. M.T. Berry and Dr. P.S. May Chemistry Department, Univserisyt of South Dakota NSF-EPSCoR  Results of LnL 3 Diglyme                                                                       Hfod Hhfac Scheme 1. Experimental diagram for photo-ionization time-of- flight mass spectrometer (PI-TOF-MS) Diglyme Figure 1. PI-TOF-Mass spectra for Eu(hfac) 3 (top) and Eu(fod) 3 (bottom) at 532 nm (200 mJ/pulse). The dominant ions are Eu 2+, Eu +, and EuF + in both figures, together with the much weaker feature for EuF 2 +. Figure 2. PI-TOF-Mass spectra of Pr(hfac) 3, Eu(hfac) 3 and Gd(hfac) 3 at 355 nm. Scheme 2. Sequential dissociation of intact neutral ligands following photo-excitation to repulsive regions of a ligand- to-metal charge-transfer state. Scheme 3. A proposed fragmentation mechanism along with annotation regarding observation of the individual fragments in the PI-TOF-mass spectrum. Figure 3 PI-TOF-Mass spectra of Eu(hfac) 3 and H- nm. The peaks at 43, 52, 71 amu are characteristic fragments from Eu(hfac) 3, and are weak or absent in the spectra of Hhfac and Ln(thd) 3. The observation of every charged species in the proposed photofragmentation mechanism, provides a convincing argument in support of the mechanism in Scheme (3), though the fluorination may not occur until after the first hfac ligand has dissociated. Figure 4. PI-TOF-Mass spectra of Pr(hfac) 3 diglyme at 532 nm and 266 nm. Figure 5. PI-TOF-Mass spectra of Gd(hfac) 3 diglyme and Eu(hfac) 3 diglyme at 532 nm and 266 nm respectively. Figure 6 PI-TOF-Mass spectra of Eu(hfac) 3 nm. The Eu species shows a parallel path in which the second hfac ligand dissociates without depositing fluoride. The right panel shows an expanded scale, revealing the parallel paths to EuF and EuF 2. Eu(hfac)diglyme  EuFdiglyme  EuF Eu(hfac) 3 diglyme  Eu(hfac) 2 diglyme EuF(hfac)diglyme  EuF 2 diglyme  EuF 2