Jon Tandy – The University of York, UKJon Tandy – The University of York, UK

Investigating the low-lying electronic Investigating the low-lying electronic states of BaOH through high resolution states of BaOH through high resolution spectroscopy and ab initio calculationspectroscopy and ab initio calculation

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

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 (~ 1013 molecules cm-3)

LIF spectra can become congested for large molecules

Metal (Ba)

Reactant gas (H2O2)

Carrier Gas

(Ar)

Alkaline Earth Monohydroxides, MOH

Thermodynamically stable free radicals in gas phase

Why BaOH?

Fewer analyses of BaOH in comparison to CaOH and SrOH

Strong perturbations predicted between A2 and B 2+ states

A’2 state predicted to be energetically close to A2 state from comparison with BaF

Weak band observed in low-resolution assigned to A’2 state

High resolution analyses of the A2 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)

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)

H2O2Ar

Monochromator

Lock-in amplifier

Ti:Sapphire laser

Heated Iodine cell

PMT 1

PMT 2

Chopper

Computer

A2 – 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)

A2 – X 2+ transition of BaOH

P11/Q12 and R12/Q11 (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)

A2 – X 2+ transition of BaOH and BaOD

D2O 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

A2 – 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)

A2 – X 2+ transition of BaOH and BaOD

High resolution analysis confirms A2-X 2+ assignment

Higher order parameters required in fit of A2 state

J. D. Tandy, J.-G. Wang, P. F. Bernath, J. Mol. Spectrosc., 255, 63 (2009)

A2 – 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 A2 states

J. D. Tandy, J.-G. Wang, P. F. Bernath, J. Mol. Spectrosc., 255, 63 (2009)

A2 – X 2+ transition of BaOH and BaOD

A2 state of BaOH doesn’t follow the pure precession model

B 2+ and A2 states don’t form a unique perturber pair

J. D. Tandy, J.-G. Wang, P. F. Bernath, J. Mol. Spectrosc., 255, 63 (2009)

)~

()~

( 22 BAp - 0.146 cm-1 vs. - 0.101 cm-1

A2 – X 2+ transition of BaOD

Ba-O bond lengths compare relatively well

O-H bond length of the A2 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)

Global perturbations in BaOH

A21/2

A’2

En

erg

y

v = 0

v = 0

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

H2O2 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)

B2+ (000)

PY

2 dye

laser

Fluorescenc

e

A'2 (010)

Ti:S

apph

ire la

ser

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

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)

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 2008.1

M.A. Anderson et al., Chem. Phys. Lett., 205 (1993) 415-422

S. Kinsey-Nielsen et al., J. Chem. Phys., 84 (1986) 698-708.

Bond lengths and vibrational frequencies for the X2+ state of BaOH Ab initio Experiment

r Ba-O / Å 2.270 2.200 r O-H / Å 0.955 0.927 1 O-H stretch / cm-1 3929 2 Ba-O-H bend / cm-1 380 342 3 Ba-O stretch / cm-1 470 492

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

Ab Initio Calculations

Population analysis predicts the C2, B2+ and A2 states to arise from a mixture of 6s 5d and 6s 6p orbital excitation

The D'2+ and D2+ states also show a mixture of excitations to the barium 5d, 6p and 7s atomic orbitals

Ab Initio Calculations – SOMO Molden Plots

X 2+ B 2+ D 2+

A‘ 2 A2 C 2

Future Investigations

1, Studying the O-H stretching mode of the MOH molecules using V-type double resonance

X2+ (000)

J = 29.5

X2+ (001)

B2+ (000)

PY

2 dye

laser

Fluorescenc

e

X2+ (100)

OPO

lase

r

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+

A2

E2 / D'2+

PY

2 dye

laser

Fluorescenc

e

Ti:S

laser

Future Investigations

6, Calculate potential energy surfaces for all the electronic states of BaOH currently investigated

A2

Acknowledgments

Prof. Peter Bernath Physical Chemistry Graduate Students

Dr Jacky Liévin

Dr Jin-Guo Wang