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 C2

uΣ3

School of Chemistry, University of Sydney

62nd International Symposium on Molecular Spectroscopy, June 21, 2007

RD03

Introduction

groundmeta-stable

Low-Lying Electronic State of C2

Previous Studies

Experimental Studies of the stateuc Σ3

#Perturbation partner of the state uA Π1

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 C2–

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.uc Σ3

transition of C2 (lower state is )?ΣΣ uc Σ3

Introduction

Theoretical Studies of the stateuc Σ3

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 uc Σ3

# Oscillator strength of the transition ug cd Σ33

Experimental# Generation of C2

Pre-mixed gas of 1 % Acetylene in Ar Pulsed discharge by a pulsed-discharge nozzle

The state of C2 was produced in a jetuc Σ3

# Excitation light sourceA pulsed dye laser (~5 ns pulse, ~0.05 cm-1) Δ

# Experimental ConditionsStagnation pressure: ~5 barbackground pressure (chamber): 2×10-4 torr

# Detection methodLaser-induced fluorescence (LIF) technique

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…

Survey Spectrum of jet-cooled C2

monitoring@~470 nm Swan emission

ug cd ΣΠ 33

7←3

3←0

4←1 5←2

6←3

Excitation/Emission 2D Image

Swan (4→5)

Swan (5→6)

& overlapping region1)(433 ug cd ΣΠ 8)(533 ug ad ΠΠ

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)

Rotationally Resolved d-c Bands

v' -v"

Rotationally Resolved 3-0 Band

0)(333 ug cd ΣΠ

Sim.

Obs.

Molecular Constants in the Stateuc Σ3

Form lower state combination differences…

v" B"v /cm-1 "v /cm-1 "v /cm-1

0123

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. ug cd ΣΠ 33

A. Tanabashi et al. Astrophys. J. Suppl. Ser. 169, 472 (2007).

)/3(32 22 SSSNNB zˆˆˆˆˆ vvv 3 Hamiltonian:

Te (c-X) = 9171.84(49) cm-1

e = 2061.56(58) cm-1

exe = 14.73(14) cm-1

Molecular Constants in the Stateuc Σ3

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 d3g state

ug cd ΣΠ 33

Be = 1.9319(17)

e = 0.01855(68) cm-1

cm-1

Using determined Bv values…

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]

C2 molecular constants in the stateuc Σ3

Exp. / cm-1 *Theory / cm-1 Te

e

exe

Be

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!

Swan & …

“Swan” system

New band system

ug ad ΠΠ 33

ug cd 33Π

Duck Bands!