j.d. tandy, j.-g. wang and p.f. bernath- high-resolution laser spectroscopy of baoh and baod:...

8/2/2019 J.D. Tandy, J.-G. Wang and P.F. Bernath- High-resolution laser spectroscopy of BaOH and BaOD: Anomalous spin-orbi

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High-resolution laser spectroscopy of BaOH and BaOD: Anomalous spin-orbitcoupling in the ~A2P state

J.D. Tandy, J.-G. Wang, P.F. Bernath*

Department of Chemistry, University of York, Heslington, York YO10 5DD, UK

a r t i c l e i n f o

Article history:

Received 24 February 2009Available online 17 March 2009

Keywords:

High-resolutionLaser excitationBaOHBaOD

a b s t r a c t

The~

A

2

P00 0 ~

X

2

R

0 00 transition of BaOD has been rotationally analyzed using laser excitationspectroscopy. BaOD molecules were synthesized through the reaction of Ba atoms with D2O in a Bro-ida-type oven. Rotational and fine structure parameters determined for the ~A2P0 00 state of BaODyielded a spin-orbit coupling constant A significantly different from the corresponding value for BaOH($605.8 cm1 versus 560.1 cm1). K-doubling constants for the ~A2P state were observed to be in pooragreement with the predictions of the pure precession model for the interaction between the ~A2P andthe ~B2R states in BaOD. Explanations for the unusual spin-orbit splitting and apparent failure of thepure precession and unique perturber models in BaOH and BaOD are discussed, with particular refer-ence to possible global and local perturbations arising from vibrationally excited bands of the ~A02Dstate.

2009 Elsevier Inc. All rights reserved.

1. Introduction

The perturbations between the low-lying ~B2R and ~A2P elec-tronic states in most of the alkaline earth monohalides and mono-hydroxides have been observed to obey the pure precessionmodel [15] and are believed to form a unique perturber pair[6] with:

p~A2P 2ASOBll 1

E~A2P E~B2R; 1

q 2B2ll 1

E~A2P E~B2R; 2

and

p~

A

2P % c

~

B

2R

; 3

where B is the rotational constant, ASO is the spin-orbit constant ofthe 2P state, l is the effective quantum number of atomic orbitalangular momentum, E is the energy, c is the spin-rotation constantof the R state and p and q are the K-doubling constants of the Pstate. For example, Berg et al. reported a generally good agreementbetween the experimental and calculated values of c, p and q inthe B2R and A2P electronic states of CaF and showed that

pA2P % cB2R in this case [1]. Similarly, Steimle et al. found

the B2R and A2P states were well described by the pure preces-

sion model and observed essentially identical p and c values [2].

The~

A

2

P excited state of CaOH was investigated by Hilborn et al.who again found a good agreement between the p value calculatedvia the pure precession model and measured spectroscopically [3].Bernath and Kinsey-Nielsen then reported good agreement be-tween this p value and c in the ~B2R state of CaOH [7]. Similar re-sults have also been observed for the ~B2R and ~A2P states of SrOH[4,8].

We recently reported the first high resolution analysis of the~A2P ~X2R transition of BaOH [9]. It was demonstrated that thepure precession model and unique perturber approximation arenot valid for the ~B2R and ~A2P states. It was proposed that pertur-bations from the closely lying ~A02D state were the source of theproblems. Further investigation is therefore required to ascertainthe degree of mutual global and local perturbations among the~B2R, ~A2P and ~A02D states of BaOH.

The perturbations of BaOD should be somewhat different fromthose of BaOH, due to the difference in vibrational and rotationallevels. Therefore the ~A2P00 0 ~X2R0 00 transition of BaODis expected to provide useful complementary information on theperturbations in the low-lying electronic states. This paper reportsthe results from our high-resolution study of the ~A2P0 0 0~X2R0 0 0 transition of BaOD using laser excitation spectroscopy.Rotational and fine structure constants have been determined forthe ~A2P0 0 0 state through a combined least-squares fit. A sur-prisingly different spin-orbit constant for the ~A2P state of BaODfrom that of BaOH was observed; potential explanations are dis-cussed based on the possible perturbations occurring betweenthe ~B2R, ~A2P and ~A02D states.

0022-2852/$ - see front matter 2009 Elsevier Inc. All rights reserved.doi:10.1016/j.jms.2009.03.002

* Corresponding author. Fax: +44 1904 432516.E-mail address: [email protected] (P.F. Bernath).

Journal of Molecular Spectroscopy 255 (2009) 6367

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Journal of Molecular Spectroscopy

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2. Experimental

BaOD molecules were synthesised via the reaction of D2O vapourwith Ba atoms in a Broida-type oven as previously described by Yuet al. in their analysis of SrOD [10]. The ~A2P ~X2R transitions ofboth BaOH and BaOD were recorded via laser excitation spectros-copy. The output from a cw single-mode titanium:sapphire laser

(Coherent 899-29) was focused vertically into the reaction chamberof the Broidaovenwitha typical laser power of$450 mW.As the la-ser was scanned, the excitation fluorescence from the ~A2P state wascollected using a photomultiplier tube (PMT).

To improve the signal-to-noise ratio of the fluorescence signal, a0.32 m monochromator wasemployedas eithera spatial filter forto-tal fluorescence scans, (monochromator set to zero-order) or as aband-pass filter for recording excitation spectra. With the mono-chromator set at zero-order it was possible to locate a number ofvibrational bands by scanning the laser over a large spectral range.Thelaserwasthenfixedatthebandheadofinterestwhilethemono-chromatorwas tuned until thestrongest fluorescence signalwas ob-tained. This monochromator position was then maintained whilstthe laser was scanned through the required spectral range to obtainthe high-resolutionspectrum.The majority of bands obtained usingthis method were clear and easily assigned.

Phase sensitive detection was employed using a mechanicalchopper to modulate the titanium:sapphire laser beam and alock-in amplifier was used to process the fluorescence signal fromthe PMT. Spectra were recorded in 5 cm1 segments at a scanspeed of 2 GHz per second with a data sampling interval of50 MHz. The absolute frequency of the laser was calibrated bysimultaneously recording the absorption spectrum of a heated I2cell [11].

3. Results and analysis

In the previous low-resolution work, band heads at$11 467 and12 039 cm1 were assigned to the ~A2P1=200 0 ~X2R

0 00 and

~A2P3=20 0 0 ~X2R0 0 0 transitions of BaOD, respectively [12].

The recent high resolution analysis of the ~A2P ~X2R transition ofBaOH [9] yielded band heads for the ~A2P1=20 0 0 ~X2R

0 0 0and ~A2P3=20 0 0 ~X2R

00 0 transitions at 11 484.286 and 12046.933 cm1, respectively. High-resolution scans were performedbased on these assignments to accurately locate the position of thetwo corresponding band heads for BaOD.

Fig. 1 shows an overview of the high-resolution spectra of the~A2P1=20 0 0 ~X2R

0 0 0 (upper panel) and ~A2P3=200 0~X2R0 0 0 (lower panel) transitions, plotted on a relative wave-number scale in a spectral range of $100 cm1. As illustrated,~A2P0 0 0 ~X2R0 0 0 transition bands for both BaOH and BaODwere observed in the spectra. The presence of BaOH molecules canbe explainedby residual sourcesof water vapour in the Broidaoven,originating from both the Ar and D2O gas inlets.

Table 1 summarises the band head positions of the observedtransitions in this investigation and shows that in addition to the~A2P0 0 0 ~X2R0 0 0 bands of BaOH and BaOD, four unassignedbands (B1B4) were observed in the spectral range of the~A2P001 ~X2R0 00 transitions of BaOH and BaOD. The spectraof the two sub-bands in each of the ~A2P000- ~X2R0 0 0 transi-tions exhibit a Hunds case (a) 2P Hunds case (b) 2R+ patternwith a clear 1B Qfe/Rff band head and the Rff/Qfe and Pee/Qef pairedlines [13]. Unfortunately, bands B1B4 were not rotationallyassignable due to calibration difficulties caused by problems inscanning the laser in these regions and therefore could be fromeither BaOH or BaOD. Fig. 1 also highlights some weaker head-likefeatures observed in the spectra (labelled by asterisks), likely orig-inating from excited vibrational states of the ~A02D ~X2R transi-tion [12] of either BaOH or BaOD. The intensity distribution ofthe observed lines in each spectrum is affected by the positionand bandwidth of the monochromator, as well as changes in theoven conditions during the laser scan.

Fig. 2 shows the band heads for the ~A2P3=20 0 0 ~X2R0 0 0

(upper panel) and ~A2P1=2000- ~X2R00 0 (lower panel) transi-

tions of BaOD with their rotational (J) assignments presented

0 10 20 30 40 50 60 70 80 90 100 110

* * * *

**

**

*

*

~~

~ ~

~ ~A

2

3/2-X

2

+head of BaOH

A2

1/2-X

2

+head of BaOD

A2

1/2-X

2

+head of BaOH

relative wavenumber / cm-1

~~A

2

3/2-X

2

+head of BaOD

Fig. 1. The high-resolution laser excitation spectra of the ~A2P1=20 0 0 ~X2R0 0 0 (upper panel) and ~A2P3=20 0 0 ~X2R

0 0 0 (lower panel) transitions of BaOH and BaODon a relative wavenumber scale in the spectral range of $100 cm1. The wavenumber zero for the ~A2P1=20 0 0 ~X2R

0 0 0 scan is 11 380.506 cm1 while the~A2P3=20 0 0 ~X2R

0 0 0 scan starts at 11 985.021 cm1. The spectra were recorded by scanning a single-mode ring titanium:sapphire laser and monitoring the

fluorescence through a 0.32 m monochromator. The two spectra are aligned on the respective BaOH band heads of each transition. All 6 branches of the BaOD transitions wereobserved and assigned for each transition. Other head-like features (marked by asterisks) are likely to arise from excited vibrational bands of the ~A02D ~X2R transition.

64 J.D. Tandy et al. / Journal of Molecular Spectroscopy 255 (2009) 6367


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above. As the figure illustrates, the shape of the two band heads areclearly different with the R22 turning point approximately 0.1 cm

1

from the Q21 turning position, producing a clear R22 sub-head of the~A2P3=200 0 ~X2R

0 0 0 transition. However, for the~A2P1=200 0 ~X2R

0 0 0 transition spectrum, a small shoulderis formed by the R12 branch turning point due to the relativelylow J00 value.

The rotational assignments for the observed bands of BaOD weremade using lower state combination differences in the ~X2R state

[14]. About 839 lines in 12 branches of the~

A

2

P0 0 0 ~

X

2

R

00 0transition were assigned and the measured lines modelled usingstandard Hunds case (a) 2P Hunds case (b) 2R+ Hamiltonians[15]. A combined least-squares fit was performed, which includedthe 839 lines of the ~A2P0 0 0 ~X2R00 0 transition (this work)and 24 pure rotational lines of the ~X2R0 0 0 state [14]. Each linein the fit was weighted according to its experimental uncertainty.For the millimeter-wave data [14] an estimated uncertainty of106 cm1 wasused,whilefor theoptical data of this work an uncer-tainty of 0.005 cm1 was applied. The effective higher order con-stants AH and pD were required to obtain a good fit of the measuredspectral lines for the ~A2P0 0 0 ~X2R0 00 transition. The spec-troscopic constants generated from the combined least-squares fitforthe ~X2R0 0 0 and ~A2P0 0 0 electronic statesof BaOD are givenin Table 2.

To further confirm the identification of the sub-states~A2P3=20 0 0 and ~A2P1=20 0 0, least-squares fits of the bands usingthe Hunds case (c) expression:

FJ BJJ 1 DJJ 12HJJ 1

3

LJJ 14 1=2J 1=2ppDJ 1=2

2

pHJ 1=24pLJ 1=2

6; 4

(where B, D, Hand L are rotational constants and p, pD, pH and pL areX-doubling constants), were performed for the excited states. The sign in Eq. (4) refers to the e- (+) andf-parity () levels, respectively.The analysis was carried out using the same method utilised whenfitting the three unidentified bands (U1U3) in our previous analy-sis of the ~A2P ~X2R transition of BaOH [9]. Table 3 illustrates thespectroscopic constants generated from the Hunds case (c) com-bined fits for the ~A2P3=20 00, ~A2P1=20 00 and ~X2R

000 statesof BaOD. In addition, the (previously unreported) correspondingHunds case (c) spectroscopic constants for the ~A2P3=20 00,~A2P1=200 0 and ~X2R

00 0 electronic states of BaOH are also pre-sented for comparison. The measured lines for all observed bands in

the~

A

2

P00 0 ~

X

2

R

00 0 transitions of BaOD are provided assupplementary material from this journal.

4. Discussion

The assignment of the various band heads observed in this workto specific electronic transitions requires some consideration. Thetwo band head positions previously assigned to the ~A2P0 0 0~X2R0 0 0 transitions of BaOH [9] are a useful starting point in thesearch for the corresponding ~A2P0 0 0 ~X2R00 0 bands ofBaOD.The isotope shift causedby deuteration ofBaOH wasexpectedto be relatively small (e.g. $6 cm1 for SrOH/SrOD [8,10]). Interest-ingly, as can be seen in Fig. 1, the two band heads assigned to the~A2P0 0 0 ~X2R0 0 0 transition of BaOD are separated from theircorresponding BaOH band heads by considerably differing amounts.

11430.3 11431.0 11431.7

12039.0 12039.5 12040.0 12040.5

34.5

30.5 27.5 24.5 21.5 18.5

wavenumber / cm-1

30.5 27.5 24.5 21.5 18.5

9.56.53.50.5

30.5 27.5 24.5 21.5 17.5

7.53.50.5

62.5

33.521.518.515.5

64.5 55.5 52.5

22.5

~ ~

~

A2

3/2-X

2 +

A2

1/2-X

2 +~

Q11J=

57.5 54.5 48.5

27.524.5

51.5

30.5

16.5

R22J=

34.5 30.5 27.5 24.5 21.5 17.5

7.53.50.5R12 J=

61.5 58.5

25.5 31.5

49.5

35.528.519.5Q21J=

Fig. 2. An expanded portion of the laser excitation spectrum corresponding to the R/Q branch heads of the ~A2P3=20 0 0 ~X2R0 0 0 (upper panel) and

~A2P1=20 0 0 ~X2R0 0 0 (lower panel) transitions of BaOD. J00 assignments of the R22, Q21, R12 and Q11 branches are presented above the spectrum. The R12 branch

forms a branch head at J00 $ 15.5 in the~

A2P1=2000- ~X2R

0 0 0 transition, as compared to the R22 branch of the~

A2P3=2000- ~X2R

0 0 0 transition that forms a branch headat J00 $ 41.5. This results in the appearance of a large shoulder $0.1 cm1 from the Q21 branch head of the ~A2P3=2000- ~X2R

0 0 0 transition.

Table 1

Observed band heads (in cm1).

Molecule Band head Transition or band

BaOH 11 484.286 ~A2P1=20 0 0 ~X2R0 0 0

12 046.933 ~A2P3=20 0 0 ~X2R0 0 0

BaOD 11 431.577 ~A2P1=20 0 0 ~X2R0 0 0

12 040.464 ~A2P3=20 0 0 ~X2R0 0 0

Unassigned 11 896.331 B112 505.159 B211 877.839 B312 490.569 B4

J.D. Tandy et al. / Journal of Molecular Spectroscopy 255 (2009) 6367 65
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The observed isotope shift for the ~A2P1=20 0 0 ~X2R0 0 0 spin

component is approx. 52.7 cm1 in comparison to approx.6.5 cm1 (Table 1) for the ~A2P3=20 0 0 ~X2R

0 00 transition.Fig. 1 clearly demonstrates that the band at 11 431.577 cm1

(relative frequency $50 cm1) was the only prominent band headobserved in this region of the spectrum and is therefore the mostlikely to correspond to the ~A2P1=20 0 0 ~X2R

0 0 0 transitionof BaOD. The band head at 12 040.464 cm1 for the~A2P3=20 0 0 ~X2R

00 0 transition seems reasonable based upona similar observation in the previous low-resolution work at $12039 cm1 [12]. However the band head for the~A2P1=20 0 0 ~X2R

00 0 transition of BaOD differs considerably(11 431.577 cm1 compared to 11 467 cm1). Presumably therewas an error in the calibration of the laser in the previous low-res-olution analyses.

To further justify this assignment an investigation was carriedout to observe the ~A2P0 0 0 ~X2R00 0 band heads of BaOHand BaOD whilst varying the reactant vapour between D2O and

H2O2. The experiment showed a considerable decrease in theintensity of the two BaOD ~A2P0 0 0 ~X2R0 0 0 band heads (un-til they were no longer observed), in addition to a significant in-crease in the two ~A2P0 0 0 ~X2R0 0 0 band heads of BaOHwhen the reactant was switched to H2O2. Similarly, when the reac-tant was changed back to D2O, the intensity of the BaOH bandheads decreased and those of BaOD returned to their originalintensity. These observations unambiguously confirmed theassignment of the band heads at 11 431.577 cm1 and 12040.464 cm1 to transitions of BaOD.

It is also interesting to compare the molecular constants gener-ated in the least-squares fit using both the Hunds case (a) andHunds case (c) expressions for the upper states in BaOH (Ref.[9]) and BaOD (this work). Table 3 reveals that when using the

Hunds case (c) expressions in the fit, the B values yielded for the

~A2P1=2 and ~A2P3=2 states differ slightly for both BaOH and BaOD,($0.2111 versus 0.2136 cm1 and $0.1919 versus 0.1935 cm1

respectively). Furthermore, it was shown that the X-doublingparameters p and pD were well defined in the Hunds case (c) fitsfor the bands assigned to the ~A2P1=20 0 0 ~X2R

0 0 0 transitionsin both BaOH and BaOD, but were not present in either of the fitsfor the bands assigned to the ~A2P3=20 0 0 ~X2R

0 0 0 transi-tions. This similarity between the ~

A

2

P1=2case (c) fit constants for

BaOH and BaOD further supports the assignment of the 11 431cm1 band to the ~A2P1=20 0 0 ~X2R

0 0 0 transition of BaOD.Given these assignments it is appropriate to consider why the

~A2P states of BaOH and BaOD exhibit such a large difference intheir spin-orbit splitting ($45 cm1). Theoretically the spin-orbitconstant, A should be identical in both isotopologues. Indeed, thedifference in the observed spin-orbit splittings of the ~A2P statein SrOH [8] and SrOD [10] is only $1 cm1. As Table 2 illustrates,this is clearly not the case in BaOH and BaOD. It is therefore reason-able to conclude, based on this observation, that the ~A2P states ofBaOH and/or BaOD are globally perturbed as previously proposed[9].

Further examination of the K-doubling constants generated intheHundscase(a)fitrevealap value of0.179378 cm1 anda q va-lueof1.44 105 cm1 inthe ~A2P stateofBaOD(Table2). Using thepure precession model, Eq. (1) with an effective l = 1, E~B2R = 13173.469 cm1 [16] and the E~A2P, ASO~A2P and B values deter-mined in this work (Table 2) we can calculate p~A2P =0.3243 cm1. This value differs considerably from both the spin-rotation constant of the ~B2R state (approx. 0.2285 cm1) [16]and our observed value forp~A2P. Furthermore, the observed q va-lue in this investigation is positive, which should clearly not be thecase according to Eq. (2). These trends mimic almost exactly thoseobserved in our previous analysis of the ~A2P state of BaOH [9]. Ittherefore reasonable to conclude that the pure precession model isnot at all appropriate for the ~B2R and ~A2P states of both BaOHand BaOD and that the unique perturber approximation is also notvalid.

The apparent failure in the unique perturber approximationsuggests that the ~A2P state of BaOH may have a significant inter-action with a closely lying electronic state other than the ~B2R

state. The previously observed ~A02D state of BaOH [12] could be ex-pected to interact relatively strongly with the closely lying ~A2Pstate. The ~A2P1=20 0 0 spin component is particularly affected,so it is possible that a 2P1=2 vibronic state from the ~A02D state[17], may happen to lie very close in energy to the ~A2P1=2 stateof BaOH. Mixing of these two 2P1=2 states could therefore causesignificant local perturbations and potentially disrupt the uniqueperturber pair approximation of the ~B2R and ~A2P states.K-doubling is known to be very small in D electronic states

[18]. It may therefore be expected that mixing of the ~A2P statewith a vibronic level of a closely lying D state would reduce K-

doubling and consequently yield a smaller observed p value than

Table 2

Spectroscopic constants (in cm1) for BaODa.

Constant ~X2R0 0 0b ~A2P0 0 0b

T 0.0 11 733.58419(19)B 0.195751870(93) 0.19271712(24)D 107 1.26316(63) 1.2268(79)c 103 2.1734(21)

A 605.83684(37)

AD 103 1.42803(46)AH 10

8 0.6909(47)q 105 1.44(42)

p 0.179378(17)pD 10

6 2.2757(48)

a Values in parentheses are 1r standard deviations, in units of the last significantdigits.

b Constants were generated by a combined fit of the ~A2P ~X2R transition dataand the millimeter-wave data of the ~X2R state [14] and a Hunds case (a) 2PHamiltonian.

Table 3

Spectroscopic constants (in cm1) for BaOH and BaOD.a

Constant BaOH BaOD

~A2P1=20 0 0b ~A2P3=20 0 0

b ~A2P1=20 0 0b ~A2P3=20 0 0

b

T 11 483.44476(55) 12 043.13030(40) 11 430.90580(23) 12 036.35757(29)B 0.211068153(79) 0.213636661(51) 0.191941564(25) 0.19349211(36)D 107 1.4844(26) 1.7487(15) 1.15512(72) 1.29808(81)H 1013 1.83(23)

p 0.146118(24) 0.179354(14)pD 10

6 2.5070(50) 2.2650(36)

a Values in parentheses are 1r standard deviation, in units of the last significant digits.b

Constants generated by a combined fit of the~

A2

P~

X2

R

transition data and the millimeter-wave data of the~

X2

R

state [14,19] and a Hunds case (c) expression for the~A2P two spin components.

66 J.D. Tandy et al. / Journal of Molecular Spectroscopy 255 (2009) 6367


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the calculated p value from the pure precession model. This couldat least partially explain the apparent failure of the pure precessionmodel to describe the interactions of the ~B2R and ~A2P states ofBaOH. Furthermore, global perturbations would be expected toshift the observed origin of the ~A2P1=20 0 0 ~X2R

0 0 0 transi-tion, explaining the discrepancy in the spin-orbit coupling param-eters for BaOH and BaOD.

Another indication of the effects of these perturbations is dem-onstrated in the calculation of the bond lengths, r0 for the ~A2Pstate of BaOH. Bond lengths were also calculated for the ~B2R stateof BaOH, using an average of the e and f-parity B values derived by

Kinsey-Nielsen et al. in their spectroscopic analysis of BaOH andBaOD [16]. Table 4 lists the bond lengths for the ~B2R, ~A2P and~X2R states of BaOH. There is a clear discrepancy between theOH bond length of the ~A2P state calculated using the rotationalconstants in this work and those of the ~B2R and ~X2R states. AnOH bond length of 0.758 is unrealistically small for a linear mol-ecule, particularly when compared to the ~B2R and ~X2R state val-ues ($0.9 ). The BaO bond length of the ~A2P state however, isreasonably close to that of the ~B2R state and shows no clear signof discrepancy.

A similar observation was noted by Yu et al. when studying the~C2P state of SrOH [10]. They suggested that global perturbations ofthis state may have given different non-mechanical contributionsto their derived B values resulting in anomalous bond lengths. It

therefore seems reasonable to conclude that the potentially largeperturbations in BaOH and BaOD is likely responsible for theapparently anomalous bond lengths observed in the ~A2P state.The other major factor that could explain the significantly smallerOH bond length is the possibility of large amplitude bending in the~A2P transitions for both BaOH and BaOD. This would cause the cal-culated bond length to correspond to an average of the projectionson the molecular axis, leading to an underestimation of the true OHbond length. Such a large amplitude bending motion however,would imply a low bending frequency and a long progression inthe bending mode that is not observed.

Although the identification of the ~A2P1=2 spin components forBaOH and BaOD is supported by the evidence mentioned above,it is possible that the observed bands may originate from excited

vibrational bands of the~

A

2

P~

X

2

R

or~

A

02

D~

X

2

R

transition ofBaOH and BaOD. These potential misidentifications could also ex-plain the apparent discrepancy between the A values of BaOHand BaOD, as excited vibrational bands of the two isotopologueswould be clearly separated due to their different vibrational fre-quencies. These uncertainties mean that the assignments givenfor the ~A2P ~X2R transitions of BaOH and BaOD (particularlyfor the 2P1=2 spin component) should be, at the current time,viewed with a degree of caution.

A detailed examination of the ~A02D state is now required to fur-ther understand the nature of the interactions between the ~B2R,~A2P and ~A0

2D states of BaOH. To this end, a V-type opticaloptical

double resonance spectroscopy experiment is underway to locatethe lower-lying excited state levels of BaOH. These bands are inac-cessible by direct fluorescence detection with our current photo-multiplier tube because of the low sensitivity in this spectral

region. Preliminary results indicate that there is indeed a band inclose proximity just on the lower wavenumber side of the ~A2P1=2sub-state of BaOH (at $11 306 cm1). Furthermore, initial analysisyielded a relatively large p value, suggesting someP1=2 character inthe excited state of this band, but work is still in progress.

5. Conclusions

The ~A2P0 0 0 ~X2R0 0 0 transition of BaOD has been rota-tionally analyzed using laser excitation spectroscopy. The mea-sured rotational lines of BaOD were fitted in combination withthe millimeter-wave pure rotational data of the ~X2R state [14].Rotational and fine structure constants have been determined forthe ~A2P0 00 state using the standard Hunds case (a) 2PHamilto-nian [15]. The corresponding constants were also derived for the~A2P3=20 0 0 and ~A2P1=20 0 0 spin components of BaOH and BaODindividually, by fitting the excited states using a Hunds case (c) en-ergy level expression. These analyses yield a considerable differ-ence in the spin-orbit constant A, for BaOH and BaOD and thisdifference is believed to result from strong global perturbationsoriginating from the lower-lying ~A02D state. Strong perturbations

are also indicated by the apparent failure of the pure precessionand unique perturber models in both BaOH and BaOD and theunrealistic OH bond length calculated from the B values of the~A2P states. In order to further understand the nature of the inter-actions between the ~B2R, ~A2P and ~A02D states of BaOH a detailedanalysis of the ~A02D state is now required; these experiments arecurrently in progress.

Acknowledgment

This research was supported by funding from EPSRC.

Appendix A. Supplementary data

Supplementary data for this article are available on ScienceDi-rect (www.sciencedirect.com) and as part of the Ohio State Univer-sity Molecular Spectroscopy Archives (http://library.osu.edu/sites/msa/jmsa_hp.htm).

References

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Table 4

Bond lengths () of BaOH.

~X2Ra ~A2Pb ~B2Rc

r0 (BaO) 2.200 2.237 2.231r0 (OH) 0.927 0.758

* 0.909

a Determined from millimeter-wave data of BaOH and BaOD [14].b Determined from optical data of BaOH and BaOD (this work).c

Determined from optical data of BaOH and BaOD [16].* The derived OH bond length is too short for a linear molecule, as discussed in thetext.

J.D. Tandy et al. / Journal of Molecular Spectroscopy 255 (2009) 6367 67
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