One-color (212-220 nm) Resonantly-enhanced (S1-S0) Multi-photon Dissociation of Acetylene Jun Jiang, Angelar Muthike, and Robert W. Field Massachusetts Institute of Technology Spelman College

The S1 Acetylene System Majority of the trans conformer vibrational levels has been observed and assigned up to 4500 cm-1 above the trans origin level. Cis conformer levels can be observed via tunneling through the cis-trans isomerization barrier.

Difficulties in going up in energy Reduced fluorescence quantum yield due to predissociation Rotational K-staggering, and failure of polyad fit model near the cis-trans isomerization barrier 46074 cm-1 Baraban et al. Science 2015, 350, 1338 Merer et al. JCP 2011, 134, 244310

Long Lifetime Visible-fluorescence from Predissociated S1 acetylene levels (with <10 ns lifetime) >1 µs decay C2H emission ~110 ns decay C2 Swan band Focused excitation beam 400 nm long-pass filter

Photofragments Fluorescence Action Spectroscopy (PFAS) LIF PFAS Significantly improved s/n ratio compared to LIF Relatively easy to implement (only one UV excitation beam is required) Background scatter light essentially eliminated by using a visible long- pass filter.

Dispersed Fluorescence Spectra of the Photofragments Predissociated Flow Cell (3 torr) Focused UV beam: 212-220 nm 200 µJ/pulse Monochromator (Spex 750M) PMT detection (Hamamatsu R928)

Effective Temperatures of the C2 Fragments

Dispersed Fluorescence Spectra of the Photofragments Flow Cell (3 torr) Focused UV beam: 212-220 nm 200 µJ/pulse Monochromator (Spex 750M) PMT detection (Hamamatsu R928)

C2 Fragments in a Supersonic Jet Good agreement between the DF results and the supersonic jet results DF Spectra

Long-lived Photofragments in a Supersonic Jet >1 µs decay

Long-lived Photofragments in a Supersonic Jet This work Literature S. Boyé et al. JCP, 116(20), 8843–8855 (2002)

C2H* Photofragments in a Supersonic Jet This work Literature S. Boyé et al. JCP, 116(20), 8843–8855 (2002)

The CH 𝐴 2 Δ state observed in the DF study is likely generated from secondary chemical reactions 430 nm Band-pass filter (FWHM = 10 nm) Wavelength / nm No sign of CH A-X transition (500 ns decay time)

A summary of the observed photofragments C2 𝑑 3 Π 𝑔 (Swan band, 470 nm) C2 𝐶 1 Π 𝑔 (Deslandres d’Azambuja band, 370 nm) C2H* with unknown electronic assignment (Visible Fluorescence from 400 nm to red) Relative Population C2H*: C2(d) : C2(C) = >40 : 4 : 1

Photodissociation Mechanism

Photodissociation Mechanism C2+H2 cannot be a three photon process, because at two-photon region, c2h2 are highly dissociative, leaving no chance for three photon C2+H2.

Future works Identifying the dissociation precursor states Identifying the electronic transitions associated with the C2H* long-lived visible fluorescence

Acknowledgements Timothy James Barnum Alex Hull David Grimes Clare Keenan Steve Coy John Muenter Trevor Erickson Zhenhui Du

Photodissociation Mechanism Spin-forbidden for C2(d) C2(d)/C2(C) = 4 C2(d)/C2(C) similar from S1 cis and trans conformer levels Previous VUV results C2+H2 cannot be a three photon process, because at two-photon region, c2h2 are highly dissociative, leaving no chance for three photon C2+H2.