1. D.L. KOKKIN, N.J. REILLY, J.A. JOESTER, M. NAKAJIMA, K. NAUTA, S.H. KABLE and T.W. SCHMIDT Direct Observation of the c State of C 2 School of Chemistry, University of Sydney 62 nd International Symposium on Molecular Spectroscopy, June 21, 2007 RD03

2. Introduction ground meta-stable Low-Lying Electronic State of C 2

3. Previous Studies Experimental Studies of the state #Perturbation partner of the state E.A. Ballik and D.A. Ramsay, Astrophys. J. 137, 84 (1963). A.E. Bragg et al., Chem. Phys. Lett. 376, 767 (2003). #Photoelectron spectroscopy of C 2 – c(neutral) − X or B(anion) [vibrationally resolved] L.J. Brug and M.C. Heaven, J. Chem. Phys. 87, 4235 (1987). #LIF excitation spectroscopy (tentative assignment) The A state is perturbed by the state. transition of C 2 (lower state is )?

4. Introduction Theoretical Studies of the state D.L. Kokkin et al., J. Chem. Phys. 126, 084302 (2007). Based on this high-level calculation, we surveyed an excitation spectrum of the d–c system! Oscillator strength of the d←c transition is much smaller (~1/10) than that of d←a (Swan). MR-CISD+Q/aug-cc-pV6Z [incl. core and core-valence correlation energies] # Molecular constants in # Oscillator strength of the transition

5. Experimental # Generation of C 2 Pre-mixed gas of 1 % Acetylene in Ar Pulsed discharge by a pulsed-discharge nozzle The state of C 2 was produced in a jet # Excitation light source A pulsed dye laser (~5 ns pulse, ~0.05 cm -1 ) # Experimental Conditions Stagnation pressure: ~5 bar background pressure (chamber): 2×10 -4 torr # Detection method Laser-induced fluorescence (LIF) technique

6. Experimental #Experimental Scheme of LIF Excitation Spectrum (Most fluorescence is observed as Swan emission.) Swan (d−a) emission, caused by d←c excitation, was detected through a monochrometor. Theoretical transition moment of d-c is ~1/10 of that of d-a. Only a few percents of emission from the d-state terminates to the c-state. Difficult to detect the d→c emission! In LIF excitation measurements of d←c…

7. Survey Spectrum of jet-cooled C 2 monitoring @~470 nm Swan emission 7←37←3 3←03←0 4←14←1 5←25←2 6←36←3

8. Excitation/Emission 2D Image Swan (4→5) Swan (5→6) & overlapping region We can separate “Swan” and “d-c” bands in emission! even if they are overlapping in an excitation spectrum. Monochrometor slit ~1 mm 60 cm -1 (~2 nm) bandpass { Selective detection of the Swan (4-5) emission through a monochrometor d-c (4←1) Swan (d-a: 5←8)

9. Rotationally Resolved d-c Bands v ' - v "

10. Rotationally Resolved 3-0 Band Sim. Obs.

11. Molecular Constants in the State Form lower state combination differences… v"v"B" v /cm -1 " v /cm -1  " v /cm -1 01230123 1.92220(32) 1.89751(32) 1.88668(16) 1.86616(14) −0.3330(26) −0.3042(27) −0.3216(16) −0.3151(17) 0.0107(12) −0.01433(97) 0.01553(68) 0.00323(59) Rotational levels in the v ' = 4 level of the d-state have been reported to be perturbed even at low J levels. Rotational analyses: 3-0, 4-1, 5-2, and 7-3 bands of the transition. A. Tanabashi et al. Astrophys. J. Suppl. Ser. 169, 472 (2007). 3  Hamiltonian:

12. T e (c-X) = 9171.84(49) cm -1  e = 2061.56(58) cm -1  e x e = 14.73(14) cm -1 Molecular Constants in the State A. Tanabashi et al. Astrophys. J. Suppl. Ser. 169, 472 (2007). S.P. Davis et al. J. Opt. Soc. Am. B 5, 1838 (1838). Using… # Determined 3-0, 4-1, 5-2, & 7-3 band origins of the transition # Known term values of v = 3, 4, 5, & 7 levels in the d 3  g state B e = 1.9319(17)  e = 0.01855(68) cm -1 Using determined B v values…

13. Comparison with Ab Initio Values *Ab initio work: D.L. Kokkin et al., J. Chem. Phys. 126, 084302 (2007). Full-valence MR-CISD(+Q)/aug-cc-pV6Z [incl. core and core-valence correlations] C 2 molecular constants in the state Exp. / cm -1 *Theory / cm -1  TeeexeBeeTeeexeBee 9171.84(49) 2061.56(58) 14.73(14) 1.9319(17) 0.01855(68) 9315 2061.3 13.54 1.9289 0.01695 –143 cm -1 +0.01% +8.8% +0.16% +9.4% Good Agreements!

14. Swan & … “Swan” system New band system Duck Bands!

15. Observation of Emission Pumping: Swan (d←a) 3←2 band @469.183nm

16. Conclusion We observed an excitation spectrum of a new band system of dicarbon,, based on the high-level ab initio calculation. # The experimentally determined molecular constants show good agreements with the theoretical calculation. # #We also observed emissions terminating to the state, from the state.

17. Molecular constants in the v = 1 level of the state are deviate from the smoothly changing curves. Plots of B v and v values

18. Possible Reasons 1. Mis-assignments of rotational lines in the 4-1 band? Leading mis-assignments? The upper vibrational level of the 4-1 band ( ) has been known to be perturbed by the state at low-J levels. 2. Perturbation between ? The state molecular constants, which are similar to our values, have been derived from the state perturbations. J. Chauville et al., J. Mol. Spectrosc. 68, 399 (1977). E.A. Ballik & D.A. Ramsay, Astrophys. J. 137, 84 (1963). S.P. Davis et al., J. Opt. Soc. Am. B 5, 2280 (1988).

19. Perturbation with the State Only the v = 1 level of the c state crosses the A state levels at low J value! However… Corresponding perturbations in the state have not been observed for J<10 levels.