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Jon Tandy – The University of York, UK Investigating the low-lying electronic states of BaOH through high resolution spectroscopy and ab initio calculation.

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Presentation on theme: "Jon Tandy – The University of York, UK Investigating the low-lying electronic states of BaOH through high resolution spectroscopy and ab initio calculation."— Presentation transcript:

1 Jon Tandy – The University of York, UK Investigating the low-lying electronic states of BaOH through high resolution spectroscopy and ab initio calculation

2 The Alkaline Earth Monohydroxides, MOH MOH molecules observed in ‘high energy’ environments MOH flames also utilised in particular industrial applications P. F. Bernath, Advances in Photochemistry, 23, 1 (1997) Stellar Atmospheres Industrial Plasmas

3 P. F. Bernath, Science., 254, 665 (1991) First studied by emission spectroscopy in flames Studies by Bernath group using Broida oven technique  Relatively low temperatures (~ 500 K) and pressures (< 5 Torr)  Large concentration of free radicals (~ molecules cm -3 )  LIF spectra can become congested for large molecules Metal (Ba) Reactant gas (H 2 O 2 ) Carrier Gas (Ar) Alkaline Earth Monohydroxides, MOH Thermodynamically stable free radicals in gas phase

4 Why BaOH? Fewer analyses of BaOH in comparison to CaOH and SrOH Strong perturbations predicted between A 2  and B 2  + states A’ 2  state predicted to be energetically close to A 2  state from comparison with BaF Weak band observed in low- resolution assigned to A’ 2  state High resolution analyses of the A 2  and A’ 2  states are desirable S. Kinsey-Nielsen, C. R. Brazier, P. F. Bernath, J. Chem. Phys., 84, 698 (1986) W. T. M. L. Fernando, M. Douay, P. F. Bernath, J. Mol. Spectrosc., 144, 344 (1990) S. J. Pooley, A. M. Ellis et al., J. Electron Spectrosc. Relat. Phenom., 97, 77 (1998) 

5 LIF Setup Molecules excited using a Ti:Sapphire laser Fluorescence detected using PMT through a monochromator Monochromator used as a band-pass filter (position fixed) H2O2H2O2 Ar Monochromator Lock-in amplifier Ti:Sapphire laser Heated Iodine cell PMT 1 PMT 2 Chopper Computer

6 A 2  – X 2  + transition of BaOH Strong band heads for both (000)-(000) transitions Rotationally assigned by ground state combination differences J.-G. Wang, J. D. Tandy, P. F. Bernath, J. Mol. Spectrosc., 252, 31 (2008)

7 A 2  – X 2  + transition of BaOH P 11 /Q 12 and R 12 /Q 11 (higher J) 1B lines doubled by ground state spin splitting J.-G. Wang, J. D. Tandy, P. F. Bernath, J. Mol. Spectrosc., 252, 31 (2008)

8 A 2  – X 2  + transition of BaOH and BaOD D 2 O used as the reactant gas to produce BaOD spectra J. D. Tandy, J.-G. Wang, P. F. Bernath, J. Mol. Spectrosc., 255, 63 (2009) Isotope shift = 52.7 cm -1 Isotope shift = 6.5 cm -1

9 A 2  – X 2  + transition of BaOH and BaOD Lines modelled using Hund’s case (a) 2  - 2  + Hamiltonian Combined least-squares fit performed J. D. Tandy, J.-G. Wang, P. F. Bernath, J. Mol. Spectrosc., 255, 63 (2009)

10 A 2  – X 2  + transition of BaOH and BaOD High resolution analysis confirms A 2  -X 2  + assignment Higher order parameters required in fit of A 2  state J. D. Tandy, J.-G. Wang, P. F. Bernath, J. Mol. Spectrosc., 255, 63 (2009)

11 A 2  – X 2  + transition of BaOH and BaOD Large difference in spin-orbit splitting (~ 45 cm -1 ) suggests strong global perturbation of BaOH and/or BaOD A 2  states J. D. Tandy, J.-G. Wang, P. F. Bernath, J. Mol. Spectrosc., 255, 63 (2009)

12 A 2  – X 2  + transition of BaOH and BaOD A 2  state of BaOH doesn’t follow the pure precession model B 2  + and A 2  states don’t form a unique perturber pair J. D. Tandy, J.-G. Wang, P. F. Bernath, J. Mol. Spectrosc., 255, 63 (2009) cm -1 vs cm -1

13 A 2  – X 2  + transition of BaOD Ba-O bond lengths compare relatively well O-H bond length of the A 2  state is unrealistically short Similar observation by Yu et al. for the C 2  state of SrOH Global perturbations gives non-mechanical contributions to the derived B value causing the anomalous bond lengths M. A. Anderson, M. D. Allen, W. L. Barclay Jr., L. M. Ziurys, Chem. Phys. Lett., 205, 415 (1993) J. D. Tandy, J.-G. Wang, P. F. Bernath, J. Mol. Spectrosc., 255, 63 (2009) S. Kinsey-Nielsen, C. R. Brazier, P. F. Bernath, J. Chem. Phys., 84, 698 (1986) S. Yu, J.-G. Wang, P. M. Sheridan, M. J. Dick, P. F. Bernath, J. Mol. Spectrosc., 240, 26 (2006)

14 Global perturbations in BaOH A 2  1/2 A’2A’2 Energy v = 0

15 V-type double resonance spectroscopy Cannot study A’ 2  –X 2  + transition using traditional LIF setup A’ 2  –X 2  + transition is allowed through bending vibration H 2 O 2 inAr in Monochromator Lock-in amplifier Ti:Sapphire laser Heated Iodine cell PMT 1 PMT 2 Chopper Computer Dye laser X 2  + (000) J = 29.5 X 2  + (001) B 2  + (000) PY2 dye laser Fluorescence A' 2  (010) Ti:Sapphire laser

16 A’ 2  – X 2  + transition of BaOH Observed spectrum mimics a 2  – 2  + transition Many rotational transitions observed via energy redistribution Resonant Line Collision Lines

17 A’ 2  – X 2  + transition of BaOH Currently uncertain which A’ 2  spin state was observed A’ 2  bands fit like  states using Hund’s case (c) expression  -doubling is very small in most  states Analysis yields relatively large p values for both isotopologues J. M. Brown, A. S-C. Cheung, A. J. Merer, J. Mol. Spectrosc., 124, 464 (1987)

18 Ab Initio Calculations A CASSCF followed by MRCI approach was chosen to optimise the geometry of the X 2  + state using the MOLPRO software Two different basis sets and four active spaces were trialed Vibrational frequencies of the X 2  + state were also calculated H.-J. Werner, P. J. Knowles et al., MOLPRO, Version M.A. Anderson et al., Chem. Phys. Lett., 205 (1993) S. Kinsey-Nielsen et al., J. Chem. Phys., 84 (1986) Bond lengths and vibrational frequencies for the X 2  + state of BaOH

19 Ab Initio Calculations CASSCF and MRCI vertical term energies were calculated and show a relatively good agreement with experiment Calculations predict the A' 2  state to arise almost purely from a 6s  5d orbital excitation as expected

20 Ab Initio Calculations Population analysis predicts the C 2 , B 2  + and A 2  states to arise from a mixture of 6s  5d and 6s  6p orbital excitation The D' 2  + and D 2  + states also show a mixture of excitations to the barium 5d, 6p and 7s atomic orbitals

21 Ab Initio Calculations – SOMO Molden Plots X 2  + B 2+B 2+ D 2  + A‘ 2  A2A2 C 2 

22 Future Investigations 1, Studying the O-H stretching mode of the MOH molecules using V-type double resonance X 2  + (000) J = 29.5 X 2  + (001) B 2  + (000) PY2 dye laser Fluorescence X 2  + (100) OPO laser 2, Investigating the high-lying excited states of BaOH using step- wise double resonance 3, Spectroscopic studies of the excited electronic states of BaCCH 4, Ab initio studies to extend calculations to higher lying states 5, Calculate anharmonic vibrational frequencies X2+X2+ A2A2 E 2  / D' 2  + PY2 dye laser Fluorescence Ti:S laser

23 Future Investigations 6, Calculate potential energy surfaces for all the electronic states of BaOH currently investigated A2A2

24 Acknowledgments Prof. Peter Bernath Physical Chemistry Graduate Students Dr Jacky Liévin Dr Jin-Guo Wang


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