1. MODE SPECIFIC DYNAMICS IN THE PREDISSOCIATED, QUASILINEAR
STATE OF CHF AND CDF PROBED BY OPTICAL-OPTICAL DOUBLE RESONANCE SPECTROSCOPY
Chong Tao, Calvin Mukarakate, Scott A. Reid Marquette University Craig Richmond, Timothy W. Schmidt, Scott H. Kable University of Sydney 63rd International Symposium on Molecular Spectroscopy June 17, 2008 TD08

2. The electronic structure of carbenes
Carbenes contain a divalent
carbon and feature low-lying
singlet and triplet states
The spectroscopy of carbenes
is complicated by spin-orbit
interactions and Renner- Teller effects, as the two
lowest lying singlets are
components of an RT pair
Recent studies have explored
these couplings in the mono-
and dihalogenated carbenes
(Sears, Chang, Kable, Reid,…) C C C
S T S1
Ã1A
ã3A
Energy
X1A
XCY
180°

3. The photochemistry of halocarbenes
is largely unexplored
Halocarbons (e.g., CFCs) are implicated in stratospheric ozone depletion
Recent experiments have shown that halocarbenes are produced in the photodecomposition of halocarbons:
CF2Br2 + hv → CFBr + 2Br (or Br2)
Kable and co-workers, JCP, 110 (1999)
To date, almost nothing is known about the ultraviolet photochemistry of halocarbenes. Recently, Dagdigian and co-workers have observed CF fragments from CFCl
and CFBr, CH fragments from CHCl, and CCl fragments from CCl2, all excited at 193 nm

4. Singlet potential energy surfaces of
CHF (CASSCF/cc-pVTZ)
B1A’ C C
A1A”
X1A’ C

5. Optical-optical double resonance
spectroscopy
Probe
Pump

6. Optical-optical double resonance
Spectroscopy of CHF
SEP transitions
All B state levels are broadened by dissociation
B state is quasi-linear, levels labeled by (v1,v2ℓ,v3)

7. Mode specificity: adding CF stretch enhances dissociation rate

8. Compilation of linewidth data for CHF
In all regions, CF stretching excitation in combination with bending excitation increases the dissociation rate
Contrary to our expectations, CH stretching excitation does not enhance the dissociation rate

9. Spectra of the deuterated isotopomer, CDF

10. Compilation of linewidth data for CDF
In CDF, the mode specificity is essentially absent!
The linewidths for pure bending states are around ½ that of the corresponding states in CHF.

11. Further evidence for strong stretch-bend
coupling in CHF
We fit the observed (0,n2ℓ, n3) levels to a model
quasilinear effective Hamiltonian:
For CDF, a fit of the (0,n2ℓ, 0) levels gave a barrier height of ~ 280 cm-1. The fit standard deviation was 4.1 cm-1 (~35 cm-1 for CHF).
For CHF, the bending frequency increases dramatically upon addition of C-F stretch. Adding one quantum of C-F stretch lowers the barrier by a factor of 3.
In contrast, a very small (~ 8%) decrease in the barrier height is found in CDF when fitting the (0,n2ℓ, 1) levels.

12. What are the products of
the dissociation?
Calculations show that two channels are accessible in this energy region:
C(3P) + HF

13. Extension to CHCl and CDCl
We have recently extended these
studies to CHCl and CDCl,
finding many state levels
The origin (near ~ cm-1)
lies below the lowest dissociation
channel:

14. Preliminary linewidth data for CHCl
Preliminary linewidth data may indicate the opening of the CCl + H channel around cm-1

15. Summary of findings
The state is quasilinear with a ~ 280 cm-1 barrier to linearity
All state levels are predissociated, as evidenced by
broad Lorentzian lineshapes, indicating a fast nonradiative decay
The decay is mode specific for CHF– in particular, a combination of bending and CF stretching excitation enhances the rate (CH stretching excitation has no effect on rate)
In CDF, the mode specificity disappears
Analysis of the vibrational term energies shows evidence for strong stretch-bend coupling in CHF which is not present in CDF
Further calculations are needed to pinpoint to important curve crossings, further experiments to identify the products of the dissociation

16. Acknowledgements People: Yulia Mishchenko Danielle Brusse Funding:
National Science
Foundation
(CHE ,
CHE )