Triplet-Singlet Mixing in Si­ 3 : the 1 A A 2 Transition Ruohan Zhang and Timothy C. Steimle International Symposium on Molecular Spectroscopy 68 th Meeting (OSU) June17-21, 2013

Previous study on Si 3 : The electronic spectrum of Si 3 : Triplet D 3h system Reilly, N. J.; Kokkin, D. L. ; Zhuang, X.; Gupta, V.; Nagarajan, R.; Fortenberry, R. C.; Maier, J. P.; Steimle, T. C.; Stanton, J. F.; McCarthy, M. C., Journal of Chemical Physics, 136(19), / / (2012). Si 3A2'3A2' 1 A 1 ( 1 E'/ 1 A 1 ') D 3h C 2v From experimental measurement [2] 78.10(3) ◦ 2.177(1) Å From ab initio prediction [1] [1]. J.C. Yang; W.G. Xu; W.S. Xiao, Journal of Molecular Structure: Theochem, 719, , (2005). [2] M.C. McCarthy and P. Thaddeus, Phys. Rev. Lett., 90, /1-4, (2003). 60 ◦ Å Si

Figure.3 Laser-induced fluorescence spectrum of Si 3 (left) and dispersed fluorescence spectra (right) obtained from the LIF bands labeled A, B, and D. [1] Previous study on Si 3 : [1]. Reilly, N. J.; Kokkin, D. L. ; Zhuang, X.; Gupta, V.; Nagarajan, R.; Fortenberry, R. C.; Maier, J. P.; Steimle, T. C.; Stanton, J. F.; McCarthy, M. C., Journal of Chemical Physics, 136(19), / / (2012). SiH 2

Previous work on Si 3 -DF spectra Figure.6 The dispersed LIF spectrum resulting from the excitation of the band of the transition in Si 3. The long progression is tentatively assigned as transitions from an unknown excited state in the singlet manifold to the bending mode, u 2, of the ground 1 A 1, C 2v state. ~505 cm -1 ~171 cm -1 Laser Excitation [1] P. Garcia-Fernandez, J.E. Boggs, and J.F. Stanton, J. Chem. Phys. 126, /1-6 (2007). [2]. Reilly, N. J.; Kokkin, D. L. ; Zhuang, X.; Gupta, V.; Nagarajan, R.; Fortenberry, R. C.; Maier, J. P.; Steimle, T. C.; Stanton, J. F.; McCarthy, M. C., Journal of Chemical Physics, 136(19), / / (2012). [3] M.C. McCarthy and P. Thaddeus, Phys. Rev. Lett., 90, /1-4, (2003). intersystem crossing Evidence? ObservationCalculated Results Assignment ~505 cm cm -1 (  1 ) [1] 3 A 2 ’ (D 3h ) 509 cm -1 (  1 ) [2] ~171 cm cm -1 (  2 ) [1] 1 A 1 (C 2v ) 178 cm -1 (  2 ) [3]

Continuous work and the goals: 1. Extend the survey scan to look for more Si 3 signals. (LIF excitation spectra) 2. Looking for the evidence of the triplet-singlet mixing with applied magnetic fields. (Dispersed LIF spectra) 3. Improve the resolution of the DLIF signal to observe the structure of the Si 3 signal. (Dispersed LIF spectra) 4. Looking for the ground state of Si 3. (Dispersed LIF spectra)

Pulse valve 1% SiH 4 & Ar(carrier gas ) Fluorescence Discharge Magnetic Field PMT Pulsed Dye Laser Monochromator Sample: 1% Siline (SiH 4 ) in argon carrier gas(~200 psi) Vacuum maintain at ~10 -5 Torr Experimental Setup: Voltage: 800~1000 V Resister: 6KΩ

 1. Extend the survey scan for the Si 3 signal (LIF excitation spectra) Mass-selected (Si 3 ) REMPI Spectra from McCarthy’s group(unpublished results) Singlet ? Si 3 (D 3h ) LIF Excitation Spectrum from cm -1 – cm -1 with monochromator on resonance. LIF Excitation Spectrum from 18800cm -1 ~ 20100cm -1 with monochromator sitting ~505cm -1 away. LIF Excitation Spectrum from cm -1 – cm -1 with monochromator on resonance. ? ? ? ? ? ? ? Triplet Conclusion: No transitions belong to the singlet system of Si 3 have been observed in the LIF excitation spectra.

 2. Evidence of the triplet-singlet mixing due to the applied magnetic field (Dispersed LIF spectra) ~505 cm -1 ~171 cm -1 Laser Excitation cm -1 Applied magnetic field

Experimental Parameters: 40 ave, 20 Hz, 3 scans of spectra added in total. Triplet Signal (reference line) Singlet Signal Background Field Strength: 0 Gauss  2. Evidence of the triplet-singlet mixing due to the applied magnetic field (Dispersed LIF spectra) Intensity ratio: ~1:3.5

Experimental Parameters: 40 ave, 20 Hz, 5 scans of spectra added in total. Triplet Signal (reference line) Singlet Signal Background Field Strength: ~ 500 Gauss  2. Evidence of the triplet-singlet mixing due to the applied magnetic field (Dispersed LIF spectra) Intensity ratio: ~1:2

Strength of field Singlet signal : Triplet signal (without background) Field Free ~ 1 : 3.5 ~ 500 Gauss ~ 1 : 2 ~ 950 Gauss ~ 1 : 1.4 Summary: Conclusion: The changing happened on the intensity ratio does support our assumption that there might be a triplet-singlet mixing in the excited states of Si 3. Linear relationship!  2. Evidence of the triplet-singlet mixing due to the applied magnetic field (Dispersed LIF spectra)

 3. Improve the resolution of the DLIF signal to observe the structure of the Si 3 signal (Dispersed LIF spectra) Slow scan on the transition at ~18600 cm -1 with slit width of the monochromator narrowing down.. Resolution ~ 1.8 Å (6 cm -1 ) Any structures? Conclusion: With the highest resolution we can go, no rotational structures of Si 3 have been observed. ~ 10.0 Å (33 cm -1 ) ~ 3.0 Å (10 cm -1 ) Parallel transition?

 4. The difference of the energy of the ground state between the triplet and singlet manifolds in Si 3 Energy (cm -1 ) =522 cm -1 =327 cm -1 =569 cm -1 =540 cm -1 =196 cm -1 X1A1X1A1 ~ Spacing? [1]. Garcia-Fernandez, Boggs & Stanton JCP, v126, (2007) Calculation: ~890 cm -1 Observation?

? ? ~505 cm -1 ? ~ 173 cm -1 Resolution: ~10.0 Å (33 cm -1 ) Energy (cm -1 ) ~890 cm -1  4. The difference of the energy of the ground state between the triplet and singlet manifolds in Si 3 Conclusion: Our observation shows the result the triplet system has a lower ground state than the singlet one. ~250 cm -1

Summary No transitions belong to the singlet system of Si 3 have been observed in the LIF excitation spectra. The magnetic field dependence of the relative intensities support the triplet-singlet intersystem crossing hypothesis in Si 3. No structure is observed on the “triplet” bands. DLIF spectra suggest that E triplet < E singlet.

Acknowledgements Arizona State University Timothy Steimle Fang Wang Anh Le

Thank you!

(1,1,0)

Experimental Parameters: 40 ave, 20 Hz, 7 scans of spectra added in total. Triplet Signal (reference line) Singlet Signal Background Field Strength: ~ 950 Gauss  2. Evidence of the triplet-singlet mixing due to the applied magnetic field. (Dispersed LIF spectra) Intensity ratio: ~1:1.4

 3. Improve the resolution of the DLIF signal to observe the structure of the Si 3 signal (Dispersed LIF spectra) Slow scan on the transition at ~18600 cm -1 with slit width of the monochromator narrowing down.. Resolution ~ 1.8 Å (6 cm -1 ) Any structures? Conclusion: With the highest resolution we can go, no rotational structures of Si 3 have been observed. ~ 10.0 Å (33 cm -1 ) ~ 3.0 Å (10 cm -1 ) FWHM: 15 Å Triplet FWHM: 25 Å Singlet < wider Parallel transition?

Rotational Constants (MHz): A = B = C = A = B = C = Predicted using the data from the ab initio calculation [1]. From the experimental data. [2] [1]. J.C. Yang; W.G. Xu; W.S. Xiao, Journal of Molecular Structure: Theochem, 719, , (2005). [2] M.C. McCarthy and P. Thaddeus, Phys. Rev. Lett., 90, /1-4, (2003). Singlet manifold Triplet manifold Perpendicular ? Parallel ? Lead to large rotational spacing. Lead to small rotational spacing. Si Broad band width. Narrow band width.

Reference: [1] Reilly, N. J.; Kokkin, D. L.; Zhuang, X.; Gupta, V.; Nagarajan, R.; Fortenberry, R. C.; Maier, J. P.; Steimle, T. C.; Stanton, J. F.; McCarthy, M. C. Journal of Chemical Physics 136(19), / / (2012). [2] Vladimir I. Makarov; Igor V. Khmelinskii; Advances in Chemical Physics Volume 118, 45-98(2001). [3] M.C. McCarthy, and P. Thaddeus, Phys. Rev. Lett. 90, /1-4, (2003). [4] J. Fulara, P. Freivogel, M. Grutter, and J.P. Maier, J. Phys. Chem. 100, (1996). [5] Arnold C.C., Neumark D.M., J. Chem. Phys. 100(3), 1797, (1994). [6] P. Garcia-Fernandez, J.E. Boggs, and J.F. Stanton, J. Chem. Phys. 126, /1-6, (2007).