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

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)

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, 184307 (2007) (HCC angle fixed at 140°) Figure: Ventura et al. JCP 118, 1702 (2004)

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

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

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

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

Vibration and vibration-rotation interaction in the B2 polyad (Ka = 1) Zero-order 42, 62 states close in energy, 50/50 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?

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

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

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

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

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

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)

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)

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)

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

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

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