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

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 S0 T1 S1 Ã1A ã3A Energy X1A XCY 180°

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) 11789. 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

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

Optical-optical double resonance spectroscopy Probe Pump

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)

Mode specificity: adding CF stretch enhances dissociation rate

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

Spectra of the deuterated isotopomer, CDF

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.

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.

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

Extension to CHCl and CDCl We have recently extended these studies to CHCl and CDCl, finding many state levels The origin (near ~ 22 500 cm-1) lies below the lowest dissociation channel:

Preliminary linewidth data for CHCl Preliminary linewidth data may indicate the opening of the CCl + H channel around 29000 cm-1

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

Acknowledgements People: Yulia Mishchenko Danielle Brusse Funding: National Science Foundation (CHE-0353596, CHE-0717960)