Dynamical “Fingerprinting” in Formaldehyde Dissociation Steven J. Rowling Scott H. Kable Sridhar A. Lahankar Arthur G. Suits

The Many Fates of Excited Formaldehyde H 2 CO(S 1 )H 2 CO(S 0 ) + h H 2 CO(S 1 )H 2 CO(S 0 *)CO + H 2 H 2 CO(S 1 )H 2 CO(S 0 *) [HCO—H]‡(S 0 *)CO + H 2 H 2 CO(S 1 )H 2 CO(S 0 *)HCO + H H 2 CO(S 1 )H 2 CO(T 1 *)HCO + H kfkf k IC k ISC

Measuring Product State Distributions H 2 CO Fluorescence HCO

Distributions from Low-Level Excitation & E avail = 330 cm  1 E avail = 865 cm  1 E avail = 869 cm  1 E avail = 1114 cm  1

Distributions from Higher Excitation E avail = 1114 cm  1 E avail = 1482 cm  1 E avail = 1206 cm  1

Photofragment Excitation (Phofex) Probe Laser Excimer Dye Nd:YAG Nozzle MC Vac. gauge Pump Laser PMT Fluorescence HCO H 2 CO

Fingerprinting with Phofex Relative Population N (E avail = 2640 cm  1 ) (E avail = 1242 cm  1 ) T 1 mechanism S 0 mechanism

Fingerprinting with Phofex Relative Population N (E avail = 2640 cm  1 ) (E avail = 1242 cm  1 ) T 1 mechanism S 0 mechanism

At High Energy N = 0 N = 10 Energy (cm  1 ) Band H 2 CO Low N (S+T) E High N (S) T Dominant S Dominant Fluctuations Ground

Near T 1 Threshold N = N = Energy (cm  1 ) Band H 2 CO E T 1 Dominant S 0 Dominant Low N (S+T) High N (S) Ground Fluctuations

N = N = Energy (cm  1 ) Band Near T 1 Threshold H 2 CO E T 1 Dominant S 0 Dominant Low N (S+T) High N (S) Ground

N = Energy (cm  1 ) Band CO (J = 15) Indications from Other Channels H 2 CO E T 1 Dominant S 0 Dominant Low N (S+T) High N (S)

Product Vibrational Excitation Energy (cm  1 ) v 3 = 0 v 3 = Band H 2 CO E T 1 Dominant S 0 Dominant No Vibration Vibrational Excitation Ground

Product Vibrational Excitation Energy (cm  1 ) H 2 CO E T 1 Dominant S 0 Dominant No Vibration Vibrational Excitation Ground v 3 = 0 v 3 = Band

Conclusions  “Fingerprint” technique can be used to qualitatively determine S 0 /T 1 branching ratios.  Near T 1 threshold, ISC is high for most, but not all, parent states.  Dominance of T 1 mechanism grows to become uniform at higher energies.  Both reaction channels produce vibrationally excited products, though the S 0 reaction slightly more so.

Remaining Questions & Future Work  Why do we see fluctuations near T 1 threshold?  How does tunneling through the T 1 barrier play a role?  Use D 2 CO to change state densities and tunneling probabilities.

Acknowledgements  Sydney Uni Laser Group Alan Maccarone Hong-Ming Yin Brianna Heazlewood Klaas Nauta  Suits Group of Wayne State  Sydney Uni Chemistry Machine Shop  Australian Research Council ($)