1. 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

2. 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.

3. 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,

4. 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

5. 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.

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

7. Effective Temperatures of the C2 Fragments

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

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

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

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

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

13. 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)

14. 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

15. Photodissociation Mechanism

16. 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.

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

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

19. 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.