A. Shayesteh et al- Infrared emission spectra of BeH and BeD

8/2/2019 A. Shayesteh et al- Infrared emission spectra of BeH and BeD

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Infrared emission spectra of BeH and BeD

A. Shayesteh, K. Tereszchuk, and P. F. Bernatha)

Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

R. Colin Laboratoire de Chimie Physique Moleculaire, Universite Libre de Bruxelles,C.P. 160/09, 50 av. F.D. Roosevelt, 1050 Brussels, Belgium

Received 8 August 2002; accepted 22 October 2002

High resolution infrared emission spectra of beryllium monohydride and monodeuteride have beenrecorded. The molecules were generated in a furnace-discharge source, at 1500 C and 333 mAdischarge current, with beryllium metal and a mixture of helium and hydrogen or deuterium gases.Approximately 160 BeH lines and 167 BeD lines for the vibrational bands v10 to v43were observed in the spectra and spectroscopic constants were determined. The Dunham constants(Yl,m) and BornOppenheimer breakdown constants were obtained in a combined fit of the BeHand BeD data. The equilibrium rotational constants (B e) for BeH and BeD were found to be10.319 593 cm1 and 5.688 292 cm1, respectively, while the equilibrium vibrational constants(e) are 2061.4163 and 1529.9563 cm

1. The equilibrium distance (Re) was determined to be1.342 4362 for BeH. 2003 American Institute of Physics. DOI: 10.1063/1.1528606

INTRODUCTION

The BeH molecule is one of the lightest diatomics withonly five electrons. BeH is, therefore, a popular target for abinitio calculations and is often used to test new methods foropen-shell systems.1,2 Because beryllium-containing com-pounds are highly toxic,3 BeH has been less studied experi-mentally. The previous work on BeH and BeD was recentlyreviewed by Focsa et al.,4,5 who measured new high resolu-tion emission spectra of the A 2X2 transitions of BeHand BeD with a Fourier transform spectrometer. Since thiswork was published in 1998, no new experimental resultshave been reported, but a number of theoretical papers have

been published. For example, a new ground state potentialenergy curve of BeH was calculated by Martin6,7 using highlevel ab initio methods. Machado et al.8 have calculatedtransition dipole moment functions and radiative transitionprobabilities for A 2X 2 and C2X2 systemsof BeH, and some calculations on the nature of the Rydbergstates have been carried out.9,10 In spite of this considerableinterest in BeH, no vibrationrotation or pure rotation spec-trum has been recorded. The existing infrared spectrum isbased on BeH trapped in an argon matrix.11 We have nowrecorded the first vibrationrotation spectrum of gaseousBeH and BeD. The spectra were recorded in emission usinga new molecular source that combines a high temperature

furnace with an electrical discharge.

EXPERIMENTAL DETAILS

The high resolution infrared emission spectra of BeHand BeD were recorded with a Bruker IFS 120 HR Fouriertransform spectrometer. A new emission source with an elec-trical discharge inside a high temperature furnace was used

to make the molecules. Powdered beryllium metal about 5g was placed inside a zirconia boat in the center of an alu-mina tube 5 cm120 cm. The central part of the tube washeated to 1500 C by a CM Rapid Temp furnace, and the endparts were cooled by water and sealed with CaF2 windows. Aslow flow of helium about 20 Torr and hydrogen or deute-rium a few Torr was passed through the cell. Two stainlesssteel tube electrodes were placed inside the cool ends of thetube, and a dc discharge 2.5 kV, 333 mA was struck be-tween them. A CaF2 lens was used to focus the emitted lightfrom the source into the entrance aperture of the spectrom-eter.

Two spectra for BeH and BeD were recorded in the1600032000 cm1 visible region using a visible quartzbeamsplitter and a photomultiplier tube detector. In thesespectra we observed the A 2X2 transitions, and weoptimized the experimental conditions to obtain the best sig-nal. We then switched to a CaF2 beamsplitter, evacuated thespectrometer and changed the detector to either InSb orHgCdTe.

A BeD spectrum was recorded in the spectral region of12002200 cm1 at an instrumental resolution of 0.03 cm1

using a liquid nitrogen-cooled HgCdTe MCT detector, and200 scans were added. The spectral bandpass was set by the

CaF2 beamsplitter and a 2200 cm1 longwave pass filter.Two BeH spectra were recorded 200 scans each: the

first one with the same conditions as for BeD, but for thesecond one we used a liquid nitrogen-cooled InSb detector inthe spectral region of 18002900 cm1. In this case thebandpass was set by the detector and a 2900 cm1 longwavepass filter. Atomic and molecular emission lines were presentin the spectra, as well as absorption lines from atmosphericCO2 and H2O. The signal-to-noise ratio for the strongestBeH lines was about 350.

aAuthor to whom all correspondence should be addressed; electronic mail:[email protected]

JOURNAL OF CHEMICAL PHYSICS VOLUME 118, NUMBER 3 15 JANUARY 2003

11580021-9606/2003/118(3)/1158/4/$20.00 2003 American Institute of Physics

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RESULTS AND DISCUSSION

The strongest emission lines in the overview spectrumrecorded with the InSb detector Fig. 1 are the R branchlines of the BeH v10 band. Nearly all of the emissionlines above 2240 cm1, and many lines in the 19002200cm1 region are from the antisymmetric stretching mode 3and several hot bands of BeH2 .

12 Atmospheric CO2 ab-sorption lines can be seen near 2350 cm1. An expandedportion of the R branch of the fundamental band of BeH isshown in Fig. 2. This spectrum had channeling in the base-line that we could not eliminate in spite of our efforts. Mostof P branch lines fell below the 1800 cm1 cutoff of InSb

detector and had to be recorded with the MCT detector. Theoverview spectrum in the MCT region Fig. 3 containedmany P branch lines, as well as strong atmospheric H2Oabsorption. All of the BeD lines fell in the MCT region. Thestrongest emission lines in the BeD spectrum Fig. 4 are

from BeD v10 band, and nearly all the emission linesabove 1680 cm1 are from the antisymmetric stretchingmode 3 and a few hot bands of BeD2 .

12

Line positions in the spectra were determined using theWSPECTRA program from Carleer Universite Libre de Brux-elles. The BeH spectra were calibrated using impurity COemission lines, and the BeH lines EPAPS13 have an abso-lute accuracy of better than 0.001 cm1. The calibration ofBeD spectrum was based on five common lines in the BeHand BeD spectra in the 12002200 cm1 region. The spectraof both BeH and BeD contained the fundamental v10band, as well as three hot bands: 21, 3 2, and 4 3. Assign-ment of the bands was carried out using a color Loomis

Wood program.The ground state of beryllium monohydride is X2

state, but no spin-splitting was observed in the spectra, andthe lines were fitted using a 1 state energy level expres-sion using Le Roys DSPARFIT program.14 The spectroscopic

FIG. 1. An overview of the infrared emission spectrum of BeH recordedwith the InSb detector. Nearly all emission lines above 2240 cm1 are fromBeH2 . The absorption of atmospheric CO2 can be seen near 2350 cm

1.

FIG. 2. An expanded view of the R branch of BeH v10 band. Theweak lines are from BeH2 .

FIG. 3. An overview of the infrared emission spectrum of BeH recordedwith the HgCdTe detector. The absorption lines are from atmospheric H2O.

FIG. 4. An overview of the infrared emission spectrum of BeD recordedwith the HgCdTe detector. Nearly all emission lines above 1680 cm1 arefrom BeD2 .

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constants in Tables I and II for both molecules were deter-mined by fitting the lines to the customary expression forrotational energy levels:

EvNTvBvNN1DvNN1

2

HvNN1 3L

vNN14. 1

BeH lines were also fitted using the Dunham energy expres-sion,

EvNBeH

l,mYl ,m

BeH v 12 l

NN1 m, 2

to yield the constants given in Table III. In this fit both iso-topomers were included, and BornOppenheimer breakdownconstants were derived. The Dunham constants of two isoto-pomers of the A B molecule are related via14

Yl,m Yl,m1 MA

MA

l,mA

MB

MB

l,mB 1

ml/2

. 3

The s are the BornOppenheimer breakdown parametersfor the atoms A and B with masses MA and MB , and s arereduced masses. The index 1 is for the main reference iso-topomer and is the index for the other isotopomers.

In the BeH/D case, Eq. 3 is simpler because berylliumhas only one naturally occurring isotope 4

9Be:

Yl,mBeD Yl,mBeHMDMHMD l,mH

BeH

BeD

ml/2

. 4

The Dunham constants for BeD in Table III are derived bycalculation from the BeH constants and the BornOppenheimer breakdown constants (

l,m

H ). The technique ofsequential rounding and refitting15 has been applied to theconstants of Table III to minimize the number of digits. Of

course, the derived Dunham constants for BeD Table IIIwill require more digits as determined by the parametersensitivities.15

The new BeH constants are in good agreement with theab initio calculations by Martin.7 In those calculations, heincluded the adiabatic correction diagonal correction to theBornOppenheimer approximation to the energy and ob-tained equilibrium constants close to the experimental ones.The nonadiabatic correction, which is of equal significance,

was not included in his calculations. Our constants for BeHdiffer substantially from the previous experimental constantsof Focsa et al.4 See Table IV. There are three reasons forsuch differences.

1 The precision of our infrared data is better than0.001 cm1, compared to 0.01 cm1 in the previous FTSspectrum of the A 2X2 transition.

2 The vibrational intervals were determined by thev1 bands of the A 2X2 transition in an oldspectrum recorded with a small classical spectrograph withonly moderate resolution. The uncertainty in the relative linepositions was 0.1 cm1, and the absolute uncertainty wasestimated to be 0.2 cm1.

3 In the previous Dunham fit, all data for v0 to v10 of the ground state were fitted with an eighth-orderpolynomial for Yl,0 constants and seventh order for Yl ,1 .Presumably because of the presence of high-order terms inthe Dunham fit, the lower-order terms lost their physical sig-nificance.

In the BeD case, the agreement between the new con-stants and previous experimental constants5 is more reason-able. However, the Dunham constants determined in thepresent work represent only the lower half of the potentialwell. In future work, we will include the previousA 2X24,5 and C2X216 data for both BeH andBeD in a direct potential fit for the ground state. In this workwe will make a grand fit of all available data, discuss theBornOppenheimer breakdown constants, and provide con-

TABLE I. Spectroscopic constants in cm1 for the X 2 ground state of BeH all uncertainties are 2.

Parameter v0 v1 v2 v3 v4

Tv

0.0 1986.41584 3896.87025 5729.26156 7480.340416B

v10.165 61323 9.855 73721 9.541 63521 9.220 12021 8.886 58946

Dv103 1.026 4116 1.016 2414 1.009 8213 1.008 8513 1.016 0534

Hv108 10.08139 9.75635 9.32731 8.75630 7.92167

Lv1011 1.19732 1.25828 1.36824 1.60521 2.0a

aFixed value. This value was determined by extrapolation of Lv

, v03.

TABLE II. Spectroscopic constants in cm1 for the X 2 ground state of BeD all uncertainties are 2.

Parameter v0 v1 v2 v3 v4

Tv

0.0 1488.84265 2936.18977 4341.37589 5703.461825B

v5.625 28531 5.498 87631 5.371 48632 5.242 53933 5.111 21159

Dv104 3.129021 3.104123 3.084524 3.072924 3.068836

Hv108 1.75572 1.71674 1.66372 1.61766 1.51681

Lv1012 2.0698 1.9586 1.8574 1.9062 1.7761

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stants suitable for the whole range of the potential well.Most of the lines in our infrared spectra turned out to be

due to BeH2 or BeD2 . We have recently published a note onthe detection of the 3 band of BeH2 .

17 The analysis ofBeD2 and BeH2 hot bands has just been completed, and willbe published separately.

ACKNOWLEDGMENTS

This work was supported by the Natural Sciences andEngineering Research Council NSERC of Canada and bythe Fonds National de la Recherche Scientifique FNRS ofBelgium. We thank R. J. Le Roy for help with DSPARFITprogram.

1 H. Meissner and J. Paldus, J. Chem. Phys. 113, 2622 2000.2 M. P. Fulscher and L. Serrano-Andres, Mol. Phys. 100, 903 2002.3 S. Budavari, M. J. ONeil, A. Smith, and P. E. Heckelman, The MerckIndex, 11th ed. Merck, New Jersey, 1989, pp. 181183.

4 C. Fosca, S. Firth, P. F. Bernath, and R. Colin, J. Chem. Phys. 109, 57951998.

5 C. Fosca, P. F. Bernath, R. Mitzner, and R. Colin, J. Mol. Spectrosc. 192,348 1998.

6 J. M. L. Martin, Chem. Phys. Lett. 273, 98 1997.7 J. M. L. Martin, Chem. Phys. Lett. 283, 283 1998.8 F. B. C. Machado, O. Roberto-Neto, and F. R. Ornellas, Chem. Phys. Lett.305, 156 1999.

9 F. B. C. Machado, O. Roberto-Neto, and F. R. Ornellas, Chem. Phys. Lett.284, 293 1998.

10 I. D. Petsalakis, D. Papadopoulos, G. Theodorakopoulos, and R. J. Buen-ker, J. Phys. B 32, 3225 1999.

11 T. J. Tague and L. Andrews, J. Am. Chem. Soc. 115, 12111 1993.12 A. Shayesteh, K. Tereszchuk, P. F. Bernath, and R. Colin, J. Chem. Phys.submitted.

13 See EPAPS Document No. E-JCPSA6-117-011303 for the observed linepositions of the v10 to v43 bands of BeH and BeD. A directlink to this document may be found in the online articles HTML referencesection. The document may also be reached via the EPAPS homepagehttp://www.aip.org/pubservs/epaps.html or from ftp.aip.org in thedirectory/epaps. See the EPAPS homepage for more information.

14 R. J. Le Roy, J. Mol. Spectrosc. 194, 189 1999, http://leroy.uwaterloo.ca/.

15 R. J. Le Roy, J. Mol. Spectrosc. 191, 223 1998.16 R. Colin, C. Dreze, and M. Steinhauer, Can. J. Phys. 61, 641 1983.17 P. F. Bernath, A. Shayesteh, K. Tereszchuk, and R. Colin, Science 297,

1323 2002.

TABLE III. Dunham and Born Oppenheimer breakdown constants incm1 for the X 2 ground state of BeH and BeD all uncertainties are2.

BeH BeD

Y1,0 2061.416 26260 1529.955 626Y2,0 37.433 422 20.497 223 2Y3,0 0.032 57 0.018 668 8Y4,0 0.047 848 0.011 505 73

Y0,1 10.319 593 5.688 286 3103 Y1,1 307.474 4330 125.901 363103 Y2,1 0.942 8260 0.163 156103 Y3,1 0.071 784 0.049 292 2103 Y4,1 0.057 99 0.006 663 7106 Y0,2 1032.8420 314.292 09106 Y1,2 13.953150 3.175 25106 Y2,2 2.02290 0.337 579 3106 Y3,2 0.18826 0.023 290 8106 Y4,2 0.0543 0.004 964 25109 Y0,3 102.6451 17.136 07109 Y1,3 4.2334 0.524 044109 Y2,3 0.67886 0.062 328 9109 Y3,3 0.20212 0.013 779 81012 Y0,4 12.14 1.112 361012 Y1,4 0.7130 0.048 4341012 Y2,4 0.526 0.026 322 5

1,0H 0.766 879

2,0H 0.417 983

3,0H

0.156 5344,0

H 0.019 850,1

H 0.021 617103 1,1

H1.31142

103 2,1H 0.814

103 3,1H

0.29518103 4,1

H 0.0363106 0,2

H7.4925

106 1,2H 0.324

TABLE IV. A comparison of equilibrium constants of BeH.

Constant Martina Focsa et al.b This work

e (cm1) 2062.119 2068.86345174 2061.416 26260

ex e (cm1) 36.938 46.145 56352 37.433 422

Re () 1.341 04 1.341 683 1.3424 362

aReference 7.bReference 4.

1161J. Chem. Phys., Vol. 118, No. 3, 15 January 2003 Spectra of BeH and BeD


A. Shayesteh et al- Infrared emission spectra of BeH and BeD

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