1. Characterization of CHBrCl2 photolysis by velocity map imaging
Wyatt G. Merrill, Benjamin C. Haenni,
Robert J. McMahon, F. Fleming Crim, and Amanda S. Case
70th Meeting of the International Symposium on Molecular Spectroscopy
June 23, 2015

2. D0(C—H) > D0 (C—Cl) > D0 (C—Br)
System of interest
Less studied than your average halomethane
Found commonly in municipal water, rivers
Prototypical precursor to stratospheric ozone destruction
Tropospheric bromine source
Removed primarily via photolysis (also radical reactions)
Readily purchased
Relative bond dissociation energies
D0(C—H) > D0 (C—Cl) > D0 (C—Br)

3. 2+1 REMPI scheme for atomic bromine
Technique
CHBrCl2 + hν  CHCl2 + Br
State-selective ionization via resonance enhanced multiphoton ionization (REMPI)
Mass selection via TOF
Velocity map imaging (VMI) determines kinetic energy release
2+1 REMPI scheme for atomic bromine
Tang, et al. Chem. Phys. Lett. 2004, 392, 493.

4. Laser setup

5. Chamber and flight tube

6. Photofragment internal energies
Spin-excited bromine probe
Ground state bromine probe
Photolysis wavelength [nm]

7. McGivern, et al. J. Chem. Phys.
Reaction pathways
Adiabatic pathway to spin-excited Br channel
Nonadiabatic pathway to ground state Br channel
Dynamics via the 4A′ excited state of CHBrCl2
Avoided crossing between 4A′ and 3A′ states
Vertical excitation energies found with TD-DFT using UB3LYP/aug-cc-pVTZ
McGivern, et al. J. Chem. Phys.
1999, 111, 5771.

8. Accessible excited state energies
Energy[cm-1]
215 nm
225 nm
235 nm Ψ
C-Br bond distance [Å]

9. All together now
Spin-excited bromine probe
Ground state bromine probe

10. Future work: Br:Br* branching ratios
Relative REPMI transition strengths will be determined via photolysis of brominated species with known [Br*]/[Br] branching (i.e. HBr)
REMPI wavelength (nm)

11. Amanda Case Benjamin Haenni Danny Tabor The Crim Group
Acknowledgements
Amanda Case
Benjamin Haenni
Danny Tabor
The Crim Group
Department of Energy

12. Excited state function
Repulsive electronic states were generated by
Where

13. Vibrational modes found with CCSD, MOs found with CASSCF(5,5)
Calculations
Mode
Freq. (cm-1)
Cl scissor
305
Umbrella
496
Symmetric C-Cl stretch
771
Antisymmetric C-Cl stretch
921
CH wag
1263
CH stretch
3257
CHCl2 LUMO
Vibrational modes found with CCSD, MOs found with CASSCF(5,5)
CIS and CCSD calcs. put first excited electronic state at > 4eV (~30000 cm-1)
CHCl2 SOMO

14. Multiphoton photolysis known
Relative bond dissociation energies
D0(C—H) > D0 (C—Cl) > D0 (C—Br)
Yang, et al. J. Phys. Chem. A 2010, 114, 4785—4790.

15. 2D Imaging Scheme (VMI)
Creation of Newton Spheres via photodissociation
Ionization of photofragments via REMPI
Projection of ions onto 2-D detector
3-D reconstruction 4 1 2 3
Whitaker, B. Imaging in Molecular Dynamics, Cambridge (2003).

16. Known energy vs. pixel calibration
From Image to Energy
Known energy vs. pixel calibration
Case, A. S. PhD. Dissertation, University of Wisconsin - Madison, 2011.