Bonding at the extreme: Detection and characterization of thorium dimer, Th2 Timothy C. Steimle , T. Maa, S. Muscarella, and Damian Kokkin Dept. Chem. & BioChem., Arizona State University, Tempe, AZ,USA The 70th International Symposium on Molecular Spectroscopy, June 2015 U. Illinois, Champagne-Urbana Funded by: NSF a) Visiting from: Chemistry Department , South China U. of Tech., Guangzhou, China
Previous Experimental Studies: Review of actinide spectroscopy Very limited : Heaven, et al. ; JCPA, 118,pp 10267 , 2014 “The Th2 and U2 dimers have been produced in our laser ablation source and detected by mass spectrometry. However, initial attempts to record REMPI spectra have yielded structureless signals. This is probably due to the high densities of electronically excited states..” Previous Theoretical Studies; three most relevant 1. “MBO”= molecular bond order “EBO”= effective bond order M.O. occupation for X3Dg state
Previous Theoretical Studies (cont.): JACS, 128, pp17000 , 2006 CASSCF/CASPT2 ; Scalar Rel. & Spin-orbit 2. Without spin-orbit interaction 13 electronic states With spin-orbit interaction 30 electronic states 3Dr 1S+ 3S+ 1D 183 e (cm-1) 186 185 182
Previous Theoretical Studies (cont.): 3. DFT using RECP and one-electron spin-orbit operator. “ massively parallel NWChem program” (3S+) (5F) (1S+) 143 196 183 e (cm-1) Zhang’s predictions are very different from Roos et al.’s !
Ablation laser Supersonic expansion 0.5’’ Th rod Pulsed valve Experimental Approach 11/12/2018 Ablation laser Generation via laser ablation/supersonic expansion Supersonic expansion Pulsed valve 0.5’’ Th rod Production of Th2: Ablated Th + Ar(98%)/X(2%) X= N2O,SF6, CH4, O2 Small amount of a collision partner helps !
Medium Resolution 2D Spectroscopy ( excitation and dispersed LIF) Experimental Approach (cont.) Medium Resolution 2D Spectroscopy ( excitation and dispersed LIF) Example Grating Mirror Entrance Slit 2D –CCD Monochromator Sq. “white” light on entrance slit Rectangle “rainbow” on 2D CCD Vertical binning of 2D CCD Dispersed LIF spectrum 2D CCD Fluorescence Supersonic Expansion Pulsed dye laser light λex
Monochromator with ICCD for 2D spectroscopy
“on-resonance” laser excitation Observation: 2D spectrum DF of ThO Excit.@18345 cm-1 DF of Th2 Excit.@18545 cm-1 2D spectrum 500 1000 1500 2000 500 1000 1500 2000 “on-resonance” laser excitation Th2 ThO
Narrow excitation scans No obvious rotational structure ! FWHM 10 cm-1 cw-laser scans also exhibit no features ! FWHM 15 cm-1
Observed Bands of Th2 via Laser Excitation System A System B v ’ E(cm-1) Dif.(cm-1) a 15360 -2 a+1 15575 1 a+2 15790 4 a+3 15995 -3 a+4 16210 v ’ E (cm-1) Dif.(cm-1) 17825 -16 1 18006 -4 2 18185 6 3 18365 17 4 18530 12 5 18683 -3 18853 -2 7 19020 8 19193 -1 9 19358 -5 E=Te +we(v+1/2) Model: Narrow DF spectra Radiative Lifetimes Te=15256cm-1-A e=212 2 cm-1 Te=17756 10cm-1 e=169 3cm-1
Dispersed Fluorescence of 18365 cm-1 band Ex. state v=1 v=0 X state v=2 v=3 1 2 15 Excited state Laser 138 cm-1
Ground state vibrational parameters Dispersed Fluorescence Measurements & Analysis Observed and calculated ground state spacings v’ v” X state Ex. state E=we(v+1/2)-wexe(v+1/2)2 Model: Obs. e=134.81 0.75cm-1 exe=0.500.04 cm-1 Ground state vibrational parameters
Radiative Lifetimes Conclusions: The broadening in the excitation spectra does not appear to be associated with excited state dynamics. Transition moments , mi,f , for System A and System B are similar: Assuming branching ratios, bi,f, (sys. A) = bi,f, (sys.B)
Summary Thank you ! Th2 has been detected . Based upon the vibrational spacing the ground state is the Zhang’s predicted =0 state: (3S+) (5F) (1S+) Obs. e=134.81 0.75cm-1 143 DFT using RECP and one-electron spin-orbit operator. “ massively parallel NWChem program” 196 183 Roos et al. predictions, which predicts we of 180 cm-1, are in poor agreement. The lack of rotational fine structure on the electronic excitation spectrum is a mystery. Thank you !