1. OBSERVATION OF LEVEL-SPECIFIC PREDISSOCIATION RATES IN S1 ACETYLENE
Catherine Saladrigas, Jun Jiang, Robert Field
Massachusetts Institute of Technology, Department of Chemistry

2. Goal: Study predissociation of S1 Acetylene
Evidence for mode specificity
Evidence for proposed mechanism (S1-T3-T2-T1)
Using new detection scheme
H-atom Fluorescence Action Spectroscopy

3. Predissociation
Adiabatic Dissociation limit
Excited state interacts with a dissociative state, below excited state dissociation limit

4. S1 State Acetylene Predissociation threshold: 46074 cm-1
Well studied: majority of trans vibrational levels assigned in this region

5. Vibrational Normal Modes

6. Previously Observed Relationship Between Mode and Predissociation
Early gate
0-40 ns
Late gate ns
3251
In studying predissociation, we are also interested in studying level dependence. By this, I mean, we are interested in the relationship between vibrational mode and predissociation rate. Based on UV – IR LIF spectra we have taken, we see that mode 2 does not actively help predissociation based on the fact the 2361 band is longer lived. We suspect that mode 3 facilitates predissociation because it is short lived. In our predissociation experiments, we hope to gain evidence to confirm this.
2361

7. Hypothesized Predissociation Mechanism
Cui et al. Chem. Phys. Lett. 1997, 272, 319 S1 T3 T2 T1
Based on calculations done by Cui et al, we are interested in gaining evidence to support the proposed predissociation mechanism of an interaction between S1 quantum bright state and T3 – T2 – T1 dark states, which then dissociates to ground state products.

8. Two photon H Atom Fluorescence
Excite Hydrogen to 3s and 3d levels
Detect fluorescence of relaxation to 2p
3s and 3d 2p
In order to study the predissociation of acetylene, we will take hydrogen action spectra. After acetylene has been excited to predissociative state, the H-atom produced will be excited via a two photon process; we will detect the fluorescence as it relaxes.
Two 205 nm photons 1s

9. H Atom Fluorescence Detection Action Spectroscopy of S1 Acetylene
Nd:YAG
FL3002
FL2002
BBO
Focusing Lens
Vacuum
Chamber
-Use UV light from FL3002 to excited acetylene to predissociative S1 state.
-Use the 205 nm UV light from FL2002 to excite the hydrogen coming off
-filter on PMT?
-BBOs for tripled light
-Supersonic expansion, such that rotationally cool
-acetylene in Argon
-scanning – cm- ( ); predissociation threshold: 46074
-calibration to tellurium
PMT~4%
Focusing Lens
BBO
Half Wave Plate
BBO

10. Comparison of Action Spectra to REMPI
REMPI: Resonance Enhanced Multiphoton Ionization
47155
47150
47145
E / cm -1
47220
47215
47210
47205
47200
47195
47190
E / cm -1
H-atom Fluorescence
H-atom Fluorescence
After developing this new scheme to study predissociation, we can compare it to REMPI is another H-atom detection method. What we find is that our method is not only easier implement than REMPI, but also has superior signal to noise. One hypothesis for why this is the case is that REMPI uses a three photon process to ionize hydrogen, while our action spectra is only two photon process.
No skimmer, higher pressure, REMPI 2+1, H-atom 2
H-atom REMPI
H-atom REMPI

11. Effects of level dependence seen in H Atom Spectra
H-atom Fluorescence
LIF
We have already observed that mode 2 does not play an active role in predissociation. What we find from our H atom spectra is that mode 3 facilitates predissociation. In the bands with more quanta in mode 3, the H-atom spectra shows stronger transitions that the LIF spectra.
1132
3462 and its perturber
35 and its perturber

12. ν3 Facilitates S1 – T3 Interaction

13. Peak Broadening LIF H-atom Fluorescence Unresolved triplet character
47204
47206
47208
47210
47212
E/cm-1
LIF
H-atom Fluorescence
Unresolved triplet character

14. Peak Broadening
47204
47206
47208
47210
47212
E/cm-1 S1 T3 T2 T1

15. J Dependent Predissociation Rate
47202
47204
47206
47208
47210
47212
47214
E/cm-1
H-atom Fluorescence
LIF

16. Future Projects Improvement: Time delay Double resonance
Doppler free: by having two counter-propagating 205 nm beams or using a broader linewidth dye laser
PMT with better quantum efficiency
Pump with two YAGs
Time delay
Double resonance
Torsional Vibration Mode

17. Conclusion Mode 3 promotes predissociation
Data is in agreement with proposed T3 doorway mechanism

18. Acknowledgements
Field Group: Prof. Bob Field Jun Jiang Dr. Carrie Womack Dr. Barratt Park David Grimes Timothy Barnum Prof. John Muenter Alex Hull Trevor Erickson Funding: NSF MIT UROP Department Thanks for your attention!