SubDoppler Spectroscopy of the A 2 X 2

Sub-Doppler Spectroscopy of the A 2Σ+ – X 2Π and B 2Π – X 2Π Transitions of NCO Nicola L. Elliott and Colin M. Western School of Chemistry, University of Bristol, Bristol BS 8 1 TS, UK Email : C. M. Western@bristol. ac. uk

NCO - Introduction • Ground X 2Π state is a textbook example of the Renner. Teller effect • Many studies of the near UV A 2Σ+ – X 2Π transition as a consequence: – R N Dixon, Phil Trans. . R. Soc. London. Series A. , 252, 165 (1960); a classic study of the Renner-Teller effect – SA Wright and P. J. Dagdigian, J. Chem. Phys. , 104, 8279 (1996) ; A detailed molecular beam study • Sub Doppler spectroscopy of the origin band of the A 2Σ+ – X 2Π transition indicates large hyperfine structure • Mid UV B 2Π – X 2Π transition strongly perturbed (by A 2Σ+ state) and predissociated, but two low vibrational states have been rotationally analysed

Injection Seeded Optical Parametric Oscillator • Transform limited output ~ 0. 003 cm-1 • Net resolution 0. 01 cm-1 in UV

Experimental • Prepare NCO in a molecular beam using a pulsed electric discharge in a mixture of 2% Br. CN and 10% O 2 • Laser induced fluorescence using a pulsed dye laser initially, then injection seeded OPO system. • NCO vibrationally cold, with bands mainly Σ+ – Π A 2Σ+ (0, 11, 0) – X 2 Π (0, 0, 0) 23472 23474 23476 23478 23480 23482 23484 23486 23488 23490 23492 23494 23496 23498 Wavenumber/cm-1

Assignment of 2Π – 2Π bands Pulsed dye laser spectrum NCO A 2Σ+ (0, 11, 2) – X 2Π (0, 0, 0) Simulation as Σ - Π Simulation as Π - Π 28100 28102 28104 28106 28108 28110 28112 Extra Line 28114 28116 28118 Wavenumber/cm-1 • Band is (vibronic) Π – Π, unlike most A-X bands • Difference is only 1 line, obscured at normal dye laser resolution

Sub Doppler Spectroscopy of NCO A 2Σ+ (0, 11, 2) – X 2Π (0, 0, 0) OPO spectrum – resolution ~ 0. 015 cm-1 Simulation as Π – Π including hyperfine "Missing" P 1(3/2) 28108. 2 28108. 3 28108. 4 28108. 5 28108. 6 28108. 7 • Missing line clear at enhanced resolution 28108. 8 28108. 9 Wavenumber/cm-1

Hyperfine Structure in the A 2Σ+ state of NCO • • Most rotational lines split by ~ 0. 02 cm-1 Dominated by b. F, the Fermi Contact term Constants determined from line contour fit For Σ – Σ bands, splitting independent of J 24090. 20 24090. 25 24090. 30 24090. 35 24090. 40 24090. 45 24090. 50 24090. 55 Wavenumber/cm-1

Interpretation of Fermi Contact Term • Weak dependence on vibration • … (1π)4 (7σ)1 (2π)4 • b. F implies ~ 30% s character, so 7σ orbital is sp hybrid on N b / cm-1 0, 2, 0 0. 016 1, 0, 1 1, 1, 0 0. 015 1, 0, 0 0. 014 0, 2, 1 2, 0, 1 1, 0, 2 1, 2, 1 0, 4, 1 0, 1, 2 0, 2, 2 0, 0, 3 22000 23000 24000 25000 26000 27000 28000 29000 30000 State Energy / cm-1

Hyperfine Structure in Π vibronic states of the A 2Σ+ state • While Σ vibronic states show a constant splitting, Π vibronic states show a splitting that varies with J • Key additional constant is (notionally) electron spin orbit interaction, A • Value of 0. 04 cm-1 is very small, consistent with "orbital" angular momentum from vibration, rather than electronic motion A 2Σ+ Tv B A q b=b. F-c/3 (1, 11, 0) 24706. 160(2) 0. 4020(1) 0. 041(1) 0. 000865(5) 0. 0157(1) (0, 11, 2) 28061. 8862 (8) 0. 39721(6) 0. 045(1) 0. 000615 (5) 0. 0144(2)

Origin band of B 2Π – X 2Π transition (Energy - 0. 37 J(J+1)) / cm-1 Band is known to be strongly mixed with a level of the A state, and more weakly mixed with others B 2Π(000) 31720 A 2Σ+(high v, π vibronic) 31700 A 2Σ+(high v, π vibronic) 0 5 10 15 A 2Σ+(high v) 31740 31745 31750 20 25 30 J 35 B 2Π(000) 31755 31760 31765 31770 31775 Wavenumber/cm-1

Hyperfine Coupling Scheme in NCO I • Splitting in the A state is typically relatively large, and independent of F=N+G J or N. • Follows a case bβS coupling scheme, as essentially the splitting between different nuclear spin orientations (G = 0 or 1) is bigger than the two J = N±½ levels. • When mixed with the B state, or at F=N+I high J, N±½ splitting is large and more typical bβJ scheme followed, so hyperfine splitting quenched J=N+S S N G=S+I I S N

Acknowledgements James A. J. Fitzpatrick Keith Rosser Bristol Laser Chemistry Group