1. 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

2. Previous Experimental Studies:
Review of actinide spectroscopy
Very limited :
Heaven, et al. ; JCPA, 118,pp , 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

3. 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

4. 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 !

5. 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 !

6. 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

7. Monochromator with ICCD for 2D spectroscopy

8. “on-resonance” laser excitation Observation: 2D spectrum DF of ThO
cm-1
DF of Th2
cm-1
2D spectrum
500
1000
1500
2000
500
1000
1500
2000
“on-resonance”
laser excitation
Th2
ThO

9. Narrow excitation scans
No obvious rotational structure !
FWHM 10 cm-1
cw-laser scans also exhibit no features !
FWHM 15 cm-1

10. 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=15256cm-1-A
e=212  2 cm-1
Te=17756 10cm-1
e=169 3cm-1

11. 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

12. 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= 0.75cm-1
exe=0.500.04 cm-1
Ground state vibrational parameters

13. 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) 

14. 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= 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 !