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

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

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

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

Vibrational Normal Modes

Previously Observed Relationship Between Mode and Predissociation Early gate 0-40 ns Late gate 40-160 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

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.

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

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 47000 – 47300 cm- (1132 3462 35); predissociation threshold: 46074 -calibration to tellurium PMT~4% Focusing Lens BBO Half Wave Plate BBO

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

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

ν3 Facilitates S1 – T3 Interaction

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

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

J Dependent Predissociation Rate 0 1 2 3 47202 47204 47206 47208 47210 47212 47214 E/cm-1 H-atom Fluorescence LIF

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

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

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!