1. Singlet-Triplet Coupling and the Non-Symmetric Bending Modes
of Acetylene 1Ãu
SEELEM spectroscopy of metastable states
Kyle Bittinger, Wilton Virgo,
R. W. Field
Massachusetts Institute of Technology

2. Doorway-mediated intersystem crossing in acetylene
Density of states (at 46,000 cm-1)
0.1 / cm-1 for S1, T3
10 / cm-1 for T1, T2
S1 / T3 level crossing 
systematic patterns of intensity for nearby metastable states T3 T2 T1
R(1)
R(1)
R(2)
R(3)
R(2)
R(3)
Figures: Altunata and Field JCP 113, 6640 (2000) Mishra et al. J Mol Spec 228, 565 (2004)

3. Out-of-plane equilibrium geometry of T3 predicted by ab-initio calculations
Calculated HCCH excited state energies as a function of torsional bending angle
T3: global minimum in out-of-plane configuration
S1: minima in planar cis and trans configurations
T3 harmonic vibrational frequencies
and S1/T3 vibrational overlap integrals:
Thom et al. JCP 126, (2007)
(HCC angle fixed at 140°)
Figure: Ventura et al. JCP 118, 1702 (2004)

4. Motivation: Role of bending vibrations in Singlet ~ Triplet coupling
Out-of-plane bend 4 and antisymmetric in-plane bend 6 are near-degenerate
Bending polyads B = (4 + 6 )
Strong Darling-Dennison and A-type Coriolis interactions
Talks this week on C2H2 bending polyads:
Thursday morning in this room
Anthony Merer 9:15am, Adam Steeves 9:30am

5. Target bands in the 2m3nB2 polyads
43,250 cm-1
46,500 cm-1
gerade vibrations  (4 + 6) = B is even
jet spectrum  Ka = 1
Choose a polyad where 2m3n62 and 2m3n42 are not obscured by other bands
2131B2 at 46,100 and 46,200 cm-1
Jet spectrum from Nami Yamakita

6. Simultaneous LIF/SEELEM spectroscopy Surface Electron Ejection by Laser-Excited Metastables
SEELEM detector = grounded metal surface + electron multiplier
Steps for SEELEM detection:
Laser excitation (some singlet character)
‏Must remain in excited state until reaching the surface (180 mm)
Vertical excitation energy must exceed work function of gold (~5eV)
Simultaneous LIF/SEELEM detection
no eigenstate appears in both spectra

7. Simultaneous LIF/SEELEM in the 2131B2 polyad (Ka = 1)
213162
213142

8. Vibration and vibration-rotation interaction in the B2 polyad (Ka = 1)
Zero-order 42, 62 states close in energy, /50 mixed from Darling-Dennison interaction
When Ka=1, constructive and destructive interference with A-type Coriolis matrix elements preferentially couples upper polyad member with 4161
Coupling mediated by 4161 character or proximity of a doorway T3 state?

9. Intensity-weighted average energy of metastable states
Center-of-gravity offset:
Eave(SEELEM) − Eave(LIF)
213142
+1.15 cm-1
-1.13 cm-1
213162

10. A-type Coriolis interaction
Removing vibration and vibration-rotation interactions with IR-UV double resonance
Use ungerade vibration in ground state to access ungerade vibrations in excited state
Darling-Dennison interaction
Use ℓ=1 in the ground state to access bands with K=0
A-type Coriolis interaction
S1 1Au
(4” + 6”)
odd
(4” + 6”)
even
3’ + 4’ P,Q,R
~2.5 μm

11. A-type Coriolis interaction
Removing vibration and vibration-rotation interactions with IR-UV double resonance
Use ungerade vibration in ground state to access ungerade vibrations in excited state
Darling-Dennison interaction
Use ℓ=1 in the ground state to access bands with K=0
A-type Coriolis interaction
S1 1Au
(4” + 6”)
odd
(4” + 6”)
even
3’ + 4’ P,Q,R
~2.5 μm

12. A-type Coriolis interaction
Removing vibration and vibration-rotation interactions with IR-UV double resonance
Use ungerade vibration in ground state to access ungerade vibrations in excited state
Darling-Dennison interaction
Use ℓ=1 in the ground state to access bands with K=0
A-type Coriolis interaction
S1 1Au
(4” + 6”)
odd
(4” + 6”)
even
3’ + 4’ P,Q,R
~2.5 μm

13. IR-UV double resonance spectra of 33B1 polyad: Simultaneous LIF/SEELEM
3361
3341
Second Photon Energy (cm-1)
Second Photon Energy (cm-1)
Intermediate states: 3’ + 4’ Q(1-5) at 3897 cm-1

14. Single rotational levels of 3361 and their “metastable companions”
Q(4)
Q(3)
Q(2)
Au:SEELEM Counts / 100 shots
LIF Intensity (arb.)
Intermediate states: 3’ + 4’ P(3), P(4), R(3)

15. Single rotational levels of 3361 and their “metastable companions”
Au:SEELEM Counts
100 shots
Q(3)
Q(3)
LIF Intensity (arb.)
Second Photon Energy (cm-1)

16. Relative Intensity 33B1 Ka=0 double resonance spectra
Peak SEELEM intensity
(Counts / 100 shots) Ka
Energy (cm-1) 33 1
45,300
15(5)  103
3361
45,935
32(12)  103
3341
46,017
15(2)  103
Comparison to Zeeman quantum beat study of 33B1 K=1 bands*
3361 Ka = 1
Measured g-values in range 0.2 – 0.4, similar to 33 Ka = 1
3341 Ka = 1
No rotational line splitting or Zeeman quantum beats Small singlet-triplet interaction
* Nami Yamakita and Soji Tsuchiya. CPL 348, 53 (2001)

17. Conclusions and future work
SEELEM spectra in 2131B2 polyad
Weak coupling (100x weaker than past studies)
Bands differ in amount of 4161 (bg-symmetry) character
Center of gravity offset  distant doorway?
Double resonance SEELEM spectra in 33B1 polyad
Strong coupling (1-3x that of past studies)
Mode 4 does not suppress singlet-triplet interaction
Intensity patterns in SEELEM spectrum  J, N, Ka assignments
IR-UV investigations at lower energy

18. Acknowledgements Experimental assistance Hans Bechtel Assignments and
perturbation-hunting
Adam Steeeves
Anthony Merer
Soji Tsuchiya
T3 calculations
Bryan Wong
Ryan Thom
Triplet Project
Sam Lipoff
Yukyung Jung
Funding AFOSR

20. Average center-of-gravity offset Eave(SEELEM) − Eave(LIF)
+/- 1 standard deviation
213142
Center of gravity offset (arb.)
213162
Width of spectrum (data points)