Electronic spectra of iron monohydride in the infrared near 1.35 and 1.58 μmWalter J. Balfour, John M. Brown, and Lloyd Wallace Citation: The Journal of Chemical Physics 121, 7735 (2004); doi: 10.1063/1.1797731 View online: http://dx.doi.org/10.1063/1.1797731 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/121/16?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The Pt2 (1,0) band of System VI in the near infrared by intracavity laser absorption spectroscopy J. Chem. Phys. 134, 184304 (2011); 10.1063/1.3589862 High resolution emission Fourier transform infrared spectra of the 4 p - 5 s and 5 p - 6 s bands of ArH J. Chem. Phys. 122, 114314 (2005); 10.1063/1.1862622 Fourier transform infrared emission spectra of MgH and MgD J. Chem. Phys. 120, 10002 (2004); 10.1063/1.1724821 Detection of ClSO with time-resolved Fourier-transform infrared absorption spectroscopy J. Chem. Phys. 120, 3179 (2004); 10.1063/1.1641007 The weakly bound complex CH 4 -H 2 : Observation and analysis of infrared spectra in the 350 and 1311 cm−1regions J. Chem. Phys. 110, 9989 (1999); 10.1063/1.478872

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Electronic spectra of iron monohydride in the infrared near 1.35and 1.58 mm

Walter J. Balfoura)

Department of Chemistry, University of Victoria, Victoria BC V8W 3P6, Canada

John M. Brownb)

Department of Physical and Theoretical Chemistry, University of Oxford, South Parks Road,Oxford OX1 3QZ, United Kingdom

Lloyd Wallacec)

National Optical Astronomy Observatories, Tucson, Arizona 85719

~Received 14 June 2004; accepted 3 August 2004!

The complex, many-line spectrum of FeH, lying in the infrared region between 5500 and 7500cm21, has been shown to consist of two separate electronic transitions:E 4P-X 4D andE 4P-A 4P.High resolution Fourier transform spectra from thermal emission have been rotationally analyzed indetail. Lambda doubling in both theE 4P and A 4P states is considerable. The experimentallydetermined energies and bond lengths of the newly characterized states are in good agreement withtheoretical predictions. ©2004 American Institute of Physics.@DOI: 10.1063/1.1797731#

I. INTRODUCTION

Transition metal monohydrides are almost legendary forthe complex, many-line electronic spectra they exhibit. Ironmonohydride fits the mould. However, with the recognitionin 1983 that the FeH system near 1mm is a 4D-4Dtransition,1 the door was opened to slow but steady progresstowards understanding the molecular structure of thespecies.2–10 Theory predicts11,12 six states to be low lying,with symmetries4,6D, 4,6P, and4,6S1. The lower4D state ofthe 1mm system has been shown to be the molecular groundstate; the correspondinga 6D state lies;2000 cm21 above.Known and predicted electronic states of FeH below 25 000cm21, together with the transitions used to characterize them,are shown in Fig. 1, which is taken from Ref. 10. It may benoted that, of the six low-lying states, all three in the sextetmanifold but only theX 4D state of the quartet manifold havebeen identified. In this paper, with the analysis of transitionsnear 1.35 and 1.58mm, we are able to add two low-lyingquartet states, labeledA 4P andE 4P, to the overall experi-mental picture.

One of the major reasons to study the FeH spectrum indepth is astrophysical. The molecule is known to beresponsible13 for the so-called Wing-Ford bands14 seen inMdwarf andS giant stellar spectra, and its 1mm absorption isa prominent feature of sunspot spectra.15,16 Most recentlyWallace and Hinkle17 have reported the detection of the FeH1.58mm absorption in sunspot umbral spectra and suggestedthat the responsible transition is the 0-0 band of theE 4P-A 4P system, from the agreement between its energyand theoretical predictions. The detailed rotational analysisof that spectrum reported here establishes the4P-4P natureof the transition. Furthermore, the recognition that the 1.35

and 1.58mm systems share a common upper state, confirmsthat these two systems arise from the expectedE 4P-X 4DandE 4P-A 4P transitions, respectively.

II. EXPERIMENTAL PROCEDURE

The spectra used in this study date from 1983. Theywere recorded by Sumner Davis using the Fourier transformspectrometer at the McMath telescope of the National SolarObservatory, Kitt Peak, Arizona and carry the NSO FTS ar-chival code 1983/01/19#2. Spectra taken at that same timewere used by Phillipset al.2 in their analysis of the FeHspectrum near 1mm. Since a detailed description of the ex-periment is given in their paper, only a brief summary of themore salient points need be included here.

The spectra were obtained in thermal emission from aKing-type carbon resistance furnace, operating at 2400 °C,charged with iron filings and a mixture of helium and hydro-gen gas ranging in pressure from 75 to 500 Torr. Atomic ironlines were used for wavelength calibration. Despite limita-tions due to Doppler broadening, lines separated by 0.05 at6000 cm21 are clearly resolved and combination differencesobtained from lines lying several hundred cm21 apart gener-ally agree within 0.02 cm21. This level of experimental pre-cision was crucial to the resolution of ambiguities in theanalysis in dense regions of the spectrum.

The 1.58mm band, spanning 5400–6400 cm21, appearsfree from contaminations from other species; however, the1.35mm band contains moderately strong emission from the~0-1! band of theA 2P-X 2S1 ~red system! of CN. Fortu-nately, the CN lines have been catalogued by Cernyet al.18

III. THE SPECTRUM OF FeH

Two bands attributable to the FeH molecule lie in theinfrared region between 5400 and 7400 cm21. Both bandsare characterized by dense, complex rotational structure that

a!Electronic mail: balfour@uvvm.uvic.cab!Electronic mail: jmb@physchem.ox.ac.ukc!Electronic mail: wallace@noao.edu

JOURNAL OF CHEMICAL PHYSICS VOLUME 121, NUMBER 16 22 OCTOBER 2004

77350021-9606/2004/121(16)/7735/8/$22.00 © 2004 American Institute of Physics

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degrades towards lower energies. Their branch heads on thehigh-energy side lie near 6300 and 7400 cm21. Because ofthe high experimental temperatures many rotational levelsare significantly populated and lines from the 7400 cm21

band run into the 6300 cm21 band region. Of the two, the6300 cm21 feature appears stronger in the King furnacespectra and initially it was assumed that the bands formedpart of a single electronic transition. This has proven not tobe the case. A reproduction of the spectrum, covering 6000–6635 cm21, may be found in Ref. 17. The 7400 cm21 band issimilarly many lined. However, its high-energy side, appear-ing deceptively simple~Fig. 2!, shows a somewhat irregularbut widely spaced series of lines. This wide spacing and thepresence throughout the spectrum of weak companions onthe strongest lines, which can be assigned to54FeH, providedearly reassurance, if any were needed, that the spectral car-rier is indeed FeH. The quality of the raw data may begauged from the portion at high dispersion reproduced inFig. 3.

IV. ANALYSIS OF THE SPECTRUM

A close inspection of the spectra led to the tentativeidentification of a number of potential branches. During theearly stages of the analysis these series of lines containedmany ambiguities caused by irregularities in line spacing.Difficulties were compounded by the facts that, within abranch, consecutive lines can be as much as 20 or 30 cm21

apart and that the line density is such that several possiblecandidates can lie within 0.5 cm21 of a predicted position.When sufficient branch structure had been identified themagnitudes and directions of observed perturbations wereused to make educated guesses as to a matching of corre-sponding R and P branches. This approach eventuallyyielded results when pairings in the 7400 cm21 band pro-ducedD2F(J) second rotational combination differences inprecise agreement with values known for the ground statefrom the analysis of theF 4D-X 4D system.2 With this break-through it proved possible to expand the 7400 cm21 analysisand show that the transition involved is of4P-X 4D type. Welabel the upper stateE 4P following the theoretical work ofLanghoff and Bauschlicher.11

The E-X spectrum containsDV50 and DV521 sub-bands. In theDV50 subbands onlyR andP branches havebeen found; in theDV521 subbandsQ branches are alsopresent.L-doubling is substantial in both electronic statesand all the branches are doubled. Unfortunately, only three ofthe four spin-orbit components of theE state have been iden-tified. Despite considerable effort, transition lines belongingto theE 4P21/2-X

4D21/2 andE 4P21/2-X4D1/2 subbands re-

main unassigned.A similar approach was used to analyze the 6300 cm21

band structure. Combination differences confirmed that thetransition responsible for this spectrum involves theE 4Pstate. The transition is of4P-4P type and readily identifiedfrom theory as theE-A system. TheE-A spectrum consistsof DV50 subbands withL-doubledR and P branches. As-signments in the4P1/2-

4P1/2, 4P3/2-4P3/2, and4P5/2-

4P5/2

components are secured through combination differences.For the4P21/2-

4P21/2 component assignments are tentative.The P-branch lines in theE-A spectrum all carry resolved54FeH companions.

FIG. 1. Electronic states of FeH below 25 000 cm21, after Wilsonet al.~Ref. 10!. Bold lines indicate states accessed experimentally, includingthe two characterized in the present work. Other levels are based on theory~Ref. 11!.

FIG. 2. A small portion of the Fourier transform emission spectrum of FeHin the infrared region near 1.35mm. The head at 7397.659 cm21, which isthe feature to highest energy in theE-X system, belongs to anV855/22V957/2 R branch whose companionL-doublet forms a head at7275.880 cm21. The spectrum is further complicated by the presence of theE 4P3/2-X

4D5/2 subband, giving anR head at 7278.874 cm21 ~see Fig. 3!.

FIG. 3. The 56FeH spectrum near 7270 cm21. Lines belonging to threeDV521 subbands are labeled. The spectrum gets increasingly more con-gested to lower energies.

7736 J. Chem. Phys., Vol. 121, No. 16, 22 October 2004 Balfour, Brown, and Wallace

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TABLE I. Vacuum wave numbers~in cm21! for the lines in the~0,0! band of theE 4P-X 4D transition of56FeH. ‘‘P’’ denotes perturbed level.

V855/22V957/2

J Ree Rf f Qe f Qf e Pee Pf f

3.5 7164.680 7115.392 7115.322?4.5 7179.429 7177.959 7118.165 7117.6155.5 7194.019 7191.132 7121.162 7119.7386.5 7208.850 7203.960 7124.248 7121.5137.5 7223.824 7215.916 7127.306 7122.7498.5 7238.948 7227.110 7130.213 7122.9979.5 7253.543 7237.345 7133.006 7122.411 7025.491

10.5 7268.135 7246.519 7135.025 7120.873 7016.516 7003.89511.5 7282.278 7254.570 7136.793 7118.336 7006.842 6989.53212.5 7295.147P 7261.295P 7137.888 7114.781 6997.002 6973.93013.5 7310.550 7266.992 7137.498P 7110.050P 6986.643 6957.13414.5 7322.963 7271.278 7139.751 7104.466 6974.977P 6938.948P15.5 7334.923 7274.232 7138.240 7097.684 6965.857 6919.74216.5 7345.956 7275.780 7136.423 7089.797 6953.799 6899.18217.5 7356.305 7275.880 7133.553 7080.743 6941.382 6877.39218.5 7365.551 7274.487 7129.898 7070.486 6928.158 6854.33119.5 7374.494 7271.558 7125.064 7058.997 6914.405 6829.99920.5 7381.882 7267.055 7119.878 7046.234 6899.740 6804.37921.5 7390.924P 7260.941 7113.111 7032.147 6884.981 6777.45922.5 7390.728 7253.191 7107.995P 7016.660 6868.894 6749.23123.5 7394.423 7243.807 7093.669 6854.742P 6719.67624.5 7396.402 7232.729 7083.310 6831.612 6688.80825.5 7397.659 7219.710 7071.293 6812.704 6656.63126.5 7396.103 7058.661 6792.400 6623.12627.5 7393.333 7043.340 6771.727 6588.05528.5 7388.966 6748.60729.5 7382.85530.5 7375.00331.5 7365.14932.5 7353.35833.5 7339.59334.5 7323.404

V853/22V955/2

3.5 7121.974 7057.4114.5 7134.795 7051.255?5.5 7147.901 7044.5876.5 7122.997 7065.231 7038.0607.5 7147.907 7128.366 7065.231 7031.9748.5 7188.436 7133.963 7064.948 7026.4999.5 7201.695 7139.751 7064.186 7021.661

10.5 7214.470 7145.648 7062.761 7017.36711.5 7226.579 7151.525 7060.530 7013.48512.5 7237.980 7157.237 7057.410 7009.78813.5 7248.738 7162.581 7053.422 7006.095 6825.24114.5 7257.785P 7170.562P 7048.748 7002.139 6892.981 6806.10615.5 7264.882 7173.123 7042.432P 6998.354 6879.051 6786.80316.5 7270.723 7177.011 7034.258 6994.177 6863.485P 6770.237P17.5 7275.244 7180.341 7025.026 6988.774 6846.04018.5 7278.351 7182.934 7014.729 6982.761 6827.449 6728.25619.5 7279.802 7184.687 7003.308 6975.940 6807.731 6707.69120.5 7279.874 7185.509 6990.541 6968.190 6786.803 6686.73321.5 7278.193 7185.238 6976.790 6959.422 6764.474 6665.11222.5 7274.676 7183.165 6961.652 6949.479 6740.998 6642.87523.5 7269.540 7180.929 6945.053 6937.676 6716.127 6619.87024.5 7262.618 7177.805P 6927.237 6925.645 6689.757 6595.36725.5 7253.949 7172.539 6908.064 6912.606P 6662.097 6571.06526.5 7243.476 7161.050 6887.514 6897.500 6633.025 6546.225P27.5 7232.497? 7158.319 6865.584 6602.606 6519.60828.5 7221.684 7149.000 6570.786 6487.07129.5 7210.934 7139.453

V851/22V953/2

3.5 6980.7084.5 7054.559 6972.7535.5 7063.077 7026.936 6972.753 6945.1706.5 7070.050 7038.523 6965.269 6946.046

7737J. Chem. Phys., Vol. 121, No. 16, 22 October 2004 Electronic spectra of iron monohydride

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V. RESULTS

Rotational line assignments in theE 4P-X 4D andE 4P-A 4P systems are listed in Tables I and II, respectively,and term values obtained from these lines in combinationwith ground state data, are given in Tables III and IV. Manysmall irregularities in branch structure are seen. Points ofmajor disturbance are denoted by ‘‘P’’ in the tables of assign-ments; for example, bothe andf levels forJ515.5,E 4P3/2,andJ513.5,E 4P7/2 are displaced. The latter perturbation isevident in Fig. 2. The data indicate that bothE andA statesare inverted4P states, the approximate values of their re-spective spin-orbitA constants being2145 and2120 cm21.

Four additional branches, of comparable intensities to thosealready identified, have been found at higher energies in the6300 cm21 spectrum. These lines~see Table V! possibly con-stitute theE-A, V8521/22V9521/2 subsystem. Their at-tribution and assignments must, however, be considered ten-tative at present.

Earlier studies of theF 4D and X 4D states of FeH re-vealed impressiveL-doubling. Similar observations aremade here for the newly characterizedE 4P andA 4P states.The magnitude of the doubling is plotted as a function ofJfor the variousV components in Figs. 4 and 5.

Wallace and Hinkle17 have previously noted that the 1.57

TABLE I. ~Continued.!

V851/22V953/2

J Ree Rf f Qe f Qf e Pee Pf f

7.5 7075.813 7049.933 6956.278 6946.9718.5 7080.660 7060.834 6946.447 6947.4299.5 7084.226 7071.248 6936.199 6947.182

10.5 7087.361 7081.115 6925.407 6946.046?11.5 7090.834 7090.369 6914.915 6943.75812.5 7092.610 7098.939 6905.532 6940.37813.5 7093.815 7106.485 6895.212 6935.79614.5 7094.908 7113.710 6884.981 6929.73415.5 7095.079 7119.679 6875.320 6922.95916.5 7094.567 7124.579 6865.247 6914.63017.5 7093.413 7128.366 6854.953 6905.03118.5 7091.421 7130.855 6844.384 6894.18919.5 7088.617 7132.118 6833.315 6882.04320.5 7084.903 7131.869 6821.656? 6868.72121.5 7130.145 6853.89722.5 7126.847 6837.90223.5 7121.974 6820.56524.5 7115.392

V855/22V955/2 V853/22V953/2 V851/22V951/2

J Ree Rf f Qe f Pee Pf f Ree Rf f Pee Pf f Ree Rf f Pee Pf f

1.5 6860.346 6856.349 6749.733 6709.055

2.5 6951.229? 6912.815 6869.740 6860.050 6762.968 6715.592 6639.628

3.5 6957.230 6956.537 6907.251 6878.106 6860.945 6778.234 6771.900 6771.584 6722.611 6660.979 6616.332

4.5 6962.096 6960.214 6900.415 6850.978 6885.981 6859.696 6760.738 6747.502 6775.833 6729.751 6643.414 6594.071

5.5 6966.273 6962.476 6892.489 6832.198 6830.673 6893.124 6856.889 6742.308 6720.610 6776.379 6736.627 6621.563 6572.645

6.5 6969.979 6963.319 6883.601 6812.672 6809.336 6899.338 6853.088 6723.376 6691.998 6773.950 6742.882 6595.867 6551.644

7.5 6973.348 6962.476 6792.565 6786.531 6904.414 6848.715 6703.699 6662.314 6769.242 6748.223 6567.086 6530.620

8.5 6976.579 6960.286 6772.017 6762.306 6908.162 6844.061 6683.090 6632.114 6762.883 6752.427 6536.011 6509.201

9.5 6979.124 6956.672 6751.092 6736.374 6910.440 6839.296 6661.392 6601.793 6754.775 6755.320 6503.339 6487.071

10.5 6981.547 6951.686 6729.951 6709.055 6911.147 6834.502 6638.478 6571.583 6746.056 6756.786 6469.679 6464.025

11.5 6983.451 6945.387 6708.012 6680.348 6910.258 6829.684 6614.264 6541.590 6737.675 6756.742 6434.890 6439.912

12.5 6984.015P 6937.676P 6685.869 6650.331 6907.892 6824.788 6588.703 6511.843 6727.768 6755.132 6400.075 6414.650

13.5 6987.019 6928.946 6663.129 6619.086 6904.290 6819.651 6561.809 6482.290 6717.581 6751.718P 6366.150 6388.197

14.5 6987.019 6918.889 6639.005 6586.551P 6898.542 6817.293P 6533.734 6452.839 6707.691 6747.267 6331.240 6360.541

15.5 6986.431 6907.625 6617.351 6553.132 6890.558 6809.668 6504.727 6423.308 6697.431 6740.998 6296.618 6331.460P

16.5 6984.826 6895.150 6592.678 6518.553 6881.168 6803.493 6473.904 6396.716P 6687.253 6733.219 6262.961 6301.774

17.5 6982.494 6881.462 6567.554 6482.974 6870.422 6796.876 6441.185 6365.020 6677.569 6724.045 6229.855 6270.705

18.5 6978.947 6866.550 6541.590 6446.389 6858.302 6789.611 6407.395 6334.935 6668.862 6713.445 6198.233 6238.614

19.5 6850.389 6514.966 6408.839 6844.673 6781.574 6372.596 6304.603 6701.655 6205.633

20.5 6832.988 6487.402 6370.309 6829.876 6772.654 6336.797 6273.904 6171.802

21.5 6814.292 6459.723 6330.815 6813.566 6762.685 6299.844 6242.619 6137.029

22.5 6794.317 6430.681 6290.355 6795.729 6262.068 6177.857?

23.5 6248.929 6776.608 6223.220

24.5 6755.987 6183.206

25.5 6734.107

26.5 6710.860

7738 J. Chem. Phys., Vol. 121, No. 16, 22 October 2004 Balfour, Brown, and Wallace

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TABLE II. Vacuum wave numbers~in cm21! for the lines in the~0,0! band of theE 4P-A 4P transition of56FeH.

V855/22V955/2 V853/22V953/2 V851/22V951/2

J Ree Rf f Pee Pf f Ree Rf f Pee Pf f Ree Rf f Pee Pf f

2.5 6146.833 6073.3693.5 6150.639 6149.999? 6062.046 6161.556 6137.538 6061.683 6048.492 6227.047 6116.4424.5 6152.890 6151.048 6041.779 6041.219 6173.127 6135.367 6047.888 6023.175 6228.685 6146.467 6096.257 6010.7745.5 6153.776 6149.999 6019.700 6018.214 6183.632 6130.641 6032.809 5994.355 6225.983 6158.046 6071.167 5994.0596.5 6153.510 6146.833 5996.197 5992.846 6192.546 6123.793 6016.584 5962.702 6219.915 6168.577 6041.831 5977.3437.5 6152.277 6141.228 5971.493 5965.271 6199.483 6115.227 5998.770 5928.825 6211.246 6177.655 6009.088 5960.0578.5 6150.345 6133.483 5945.778 5935.512 6204.180 6105.256 5979.110 5893.308 6200.591 6185.008 5973.718 5941.7819.5 6147.226 6123.581 5919.195 5903.287 6206.470 6094.116 5957.421 5856.612 6187.803 6190.445 5936.367 5922.196

10.5 6143.582 6111.568 5891.981 5868.937 6206.270 6082.006 5933.601 5819.085 6173.960 6193.816 5897.581 5901.05711.5 6139.069 6097.524 5863.628 5832.484 6203.586 6069.073 5907.592 5780.979 6159.966 6195.012 5857.181 5878.18612.5 6132.928 6081.388 5834.781 5794.036 6198.583 6055.419 5879.392 5742.473 6143.931 6193.945 5816.234 5853.46313.5 6129.003 6063.577 5805.092 5753.716 6191.538 6041.051 5849.058 5703.690 6127.095 6190.327 5775.661 5826.81114.5 6121.775 6043.835 5773.787 5711.506 6181.599 6029.151P 5816.794 5664.697 6110.149 6184.940 5733.699 5798.21815.5 6113.745 6022.356 5744.679 5667.867 6168.734 6011.829 5782.907 5625.471 6092.509 6177.109 5691.696 5767.57716.5 6104.306 5999.176 5712.152 5622.580 6153.834 5995.958 5746.571 5589.181 6071.119 6167.245 5646.829 5735.79817.5 6093.408 5974.368 5678.486 5575.880 6137.029 5979.813 5707.800 5547.950 6051.954 6151.588 5604.240 5698.24518.5 6080.010 5947.999 5642.609 5527.847 6118.478 5963.444 5667.576 5508.767 6031.626 6137.471 5560.993 5662.64419.5 5920.158 5478.599 6098.589 5947.283 5626.511 5470.314 6010.939 5515.93320.5 5890.979 5428.305 5930.41021.5 5860.725 5377.247

TABLE III. Term values~in cm21! of 56FeH in thev50 level of theE 4P state. Term values are measuredrelative to the lowerL-doublet of theJ53.5 rotational level of theX 4D7/2 ground state. ‘‘P’’ denotes aperturbed level.

J

V55/2 V53/2 V51/2

e f e f e f

1.5 7321.352.5 7076.80 7076.80 7230.26 7226.57 7385.47 7351.643.5 7115.38 7115.21 7273.93 7265.44 7435.37 7397.304.5 7165.24 7164.67 7330.12 7315.62 7496.07 7457.935.5 7226.50 7225.04 7399.16 7377.64 7567.79 7532.976.5 7299.32 7296.47 7480.94 7451.89 7650.90 7621.917.5 7383.81 7379.03 7575.16 7538.72 7745.86 7724.218.5 7480.09 7472.41 7681.67 7638.30 7853.06 7839.509.5 7588.38 7577.01 7800.24 7750.67 7972.73 7967.44

10.5 7708.13 7692.72 7930.71 7875.80 8104.50 8107.7511.5 7839.98 7819.64 8072.91 8013.59 8249.13 8260.2012.5 7983.58 7957.76 8226.71 8163.89 8407.29 8424.5813.5 8138.13P 8106.99P 8392.09 8326.54 8576.82 8600.6914.5 8307.49 8267.62 8569.19 8501.26 8758.72 8788.1015.5 8486.11 8439.32 8756.91P 8690.99P 8953.27 8987.4216.5 8676.56 8622.10 8955.02 8887.64 9159.54 9197.6417.5 8878.28 8815.90 9164.16 9097.76 9377.55 9418.8618.5 9091.49 9020.59 9384.25 9319.48 9607.25 9650.9719.5 9315.66 9236.06 9615.07 9552.44 9848.18 9893.6920.5 9551.57 9462.15 9856.33 9796.43 10100.2821.5 9797.81 9698.75 10108.14 10051.22 10363.1722.5 10057.51P 9945.63 10370.05 10316.5623.5 10319.63 10202.71 10641.78 10591.5124.5 10597.20 10469.76 10923.42 10877.6225.5 10884.42 10746.61 11214.64 11169.94P26.5 11182.16 11032.86 11515.29 11479.0927.5 11488.13 11825.09 11784.77P28.5 11803.77 12145.17P 12117.8029.5 12128.49 12475.99 12446.70

7739J. Chem. Phys., Vol. 121, No. 16, 22 October 2004 Electronic spectra of iron monohydride

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mm FeH spectrum is a prominent feature in the sunspot um-bral spectrum and that some of the lines there show a dou-bling which increases with magnetic field strength. TheseZeeman doublings are most evident within the brancheslisted in Table V, and in theV855/22V955/2 subband. Thelargest splitting noted is 0.20 cm21, centered on 6141.228cm21. Unfortunately, these splittings show irregularities. Theclearest and most regular of the Zeeman observations may beseen at lowJ in the Ree V855/22V955/2 branch wheremeasurements from the high-field strength, low temperature

sunspot spectral recording give the following splittings~incm21!: Ree(2.5)50.177,Ree(5.5)50.100,Ree(6.5)50.101,Ree(7.5)50.095, Ree(9.5)50.071, Ree(10.5)50.064, andRee(11.5)50.045. Polarization studies of some lines haverecently been reported.19 The 6600–7400 cm21 region is lessfavorable for solar spectral observations because of strongtelluric water absorption. Consequently, no useful data areavailable for possible solarE-X lines.

The normal procedure for determining molecular con-stants is to fit either the assigned lines or energy levels tomodels appropriate for the states or state involved. This pro-cedure is straightforward for well-behaved data sets. It is notso useful for the situation that the FeH spectrum presents,where theE 4P and A 4P states are heavily perturbed bynearby electronic states, and where in both instances, reliabledata are available for three spin components only.

The data in Tables I and II, coupled with the tentativeassignments of Table V, were used to determineD2F(J)rotational second combination differences. The values wereaveraged overL-doublets and over the fourV components,

FIG. 4. TheJ dependence of the magnitude of theL-doubling in the rota-tional levels of theE 4P state of56FeH.

TABLE IV. Term values~in cm21! of 56FeH in thev50 level of theA 4P state. Term values are measuredrelative to the lowerL-doublet of theJ53.5 rotational level of theX 4D7/2 ground state.

J

V55/2 V53/2 V51/2

e f e f e f

2.5 968.553.5 1014.68 1014.67 1168.57 1178.08 1269.034.5 1073.61 1073.99 1226.04 1242.27 1339.11 1386.525.5 1145.54 1146.47 1297.31 1321.26 1424.91 1463.876.5 1230.30 1232.20 1382.60 1414.94 1525.95 1555.637.5 1327.82 1331.19 1482.18 1523.07 1641.81 1661.858.5 1438.03 1443.53 1596.06 1645.41 1772.14 1782.439.5 1560.90 1569.13 1724.25 1781.69 1916.70 1917.30

10.5 1696.40 1708.07 1866.64 1931.58 2075.16 2066.3811.5 1844.51 1860.24 2023.12 2094.82 2247.32 2229.5712.5 2005.20 2025.60 2193.52 2271.12 2432.90 2406.7413.5 2178.50 2204.04 2377.65 2460.21 2631.63 2597.7714.5 2364.34 2395.48 2575.31 2661.84 2843.12 2802.4815.5 2562.82 2599.75 2786.29 2875.80 3067.03 3020.5316.5 2773.97 2816.73 3010.34 3101.81 3306.44 3251.6217.5 2998.08 3046.22 3247.22 3339.68 3555.30 3499.3918.5 3235.66 3288.06 3496.59 3589.00 3816.56 3756.2219.5 3541.99 3757.74 3849.16 4091.3220.5 3807.76 4120.8121.5 4084.90

TABLE V. Unassigned lies in the FeH 6300 cm21 band region. Branchnumbering, pairing, and alignment is tentative.

E 4P-A 4P V8521/22V9521/2?

J Ree Rf f Pee Pf f

1.5 6229.591 6238.6142.5 6237.013 6219.7173.5 6315.944 6245.215 6196.0324.5 6316.857 6253.782 6170.067 6107.9265.5 6316.613 6262.068 6143.215 6086.0196.5 6314.539 6269.565 6114.773 6064.5797.5 6310.778 6275.677 6084.861 6043.0038.5 6305.389 6280.103 6053.573 6020.8239.5 6298.415 6282.597 6020.963 5997.476

10.5 6289.908 6282.929 5987.095 5972.69211.5 6279.904 6281.291 5952.037 5946.26512.5 6268.491 6277.281 5915.904 5918.03813.5 6255.527 6270.189 5878.561 5887.33214.5 6241.107 6261.914 5840.079 5855.78515.5 6225.506 6250.943 5800.890 5821.98716.5 6208.863 6237.638 5760.735 5786.20717.5 6190.375 6221.906 5719.722 5748.46818.5 6170.922 6203.586 5678.207 5709.19919.5 6150.398 6183.632 5635.507 5668.12820.5 6129.003 6161.556 5589.703 5626.51121.5 6137.471

7740 J. Chem. Phys., Vol. 121, No. 16, 22 October 2004 Balfour, Brown, and Wallace

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and theBv and Dv rotational constants determined graphi-cally from the standard expressionD2F(J)/(4J12)5Bv22Dv(J1 1

2)2. That the linear least squares line shows anR2

correlation coefficient of 0.998 lends some support to thetentative assignments of Table V. The fit givesB0(E 4P)56.39098(60.00021) and D0(E 4P)54.30(60.05)31024 cm21. Similar treatment of theA state also givessatisfactory results: the graph shows slightly more scatter(R250.996) and the resulting rotational constants areB0(A 4P)57.2830(60.00428) andD0(A 4P)55.9(60.1)31024 cm21. When individual V substates ofE 4P aretreated separately, theD2F(J)/(4J12) versus (J1 1

2)2 plots

are not linear. Extrapolation provides the rough estimates ofB0 , 7.42, 6.85, 5.82, and 5.50 cm21, respectively forV521/2, 11/2, 3/2, and 5/2. The corresponding numbersfor the A state are 8.6, 7.48, 6.9, and 6.58 cm21.

An alternative approach to the fitting of theV51/2, 3/2,and 5/2E state term values using an appropriate Hund’s case~a! 4P Hamiltonian was attempted. It was hoped by so doingto obtain parameters with which to locate the missingV521/2 levels. However, the perturbations proved suffi-ciently severe and widespread to preclude anything ap-proaching a good fit, using a single state effective Hamil-tonian. The 140 term values for theE state listed as‘‘unperturbed’’ in Table III were fitted using 13 parameters;the values determined for these parameters are listed in TableVI. The standard deviation of the fit was 1.189 cm21, twoorders of magnitude larger than the experimental uncertainty.

This result is not altogether surprising. Similar observationshave been made for other low-lying FeH electronic states.The largeL-doublings in theE state, even forV55/2, are anindication of strong state mixing. A similar attempt to fit theterm values of theA 4P state was even less successful; thestandard deviation of the fit in that case was 6.210 cm21.

VI. DISCUSSION

We have been able to show that the thermal emissionspectrum of FeH in the infrared region between 5500 and7500 cm21 contains lines attributable to two separate elec-tronic transitions within the quartet manifold of molecularstates. In both cases the spectra are 0-0 bands. These conclu-sions follow from the observations that~i! the lower state inour E-X spectrum is the lower state of the 987.7 nmF-Xsystem, which occurs in absorption;~ii ! measured54FeH-56FeH isotopic shifts in theE-X spectrum are verysmall ~,1 cm21!; and ~iii ! our A state energy levels~TableIV ! are less than an expected FeH vibrational interval. Therotational assignments~;800 in total! account for essentiallyall lines of appreciable intensity in the region, except at theextreme low energy end of the range where it has not beenpossible to extend some perturbed, widely spacedP branchesunambiguously. There remain, nevertheless, generally weaklines unaccounted for, some of which form short brancheswhose characteristics closely resemble the assigned FeHstructure. These unassigned lines are collected in Table VII.Attempts to attribute them to theE 4P21/2-X

4D1/2 transitionhave failed. One possible explanation is that they belong to1-1 transitions.

In earlier work on dispersed fluorescence from thee 6Pstate, Wilson and Brown located several series of energylevels in the anticipated locale of theA 4P state. Using en-ergies of the lowest identified level in each case they labeledthese series ‘‘910,’’ ‘‘1068,’’ and more tentatively ‘‘1155’’and ‘‘1232.’’ The suspicion was that these levels belonged tothe A 4P state. The present research confirms that their‘‘910’’ and ‘‘1068’’ series are levels in theA state,V55/2andV53/2, respectively.

It is satisfying to find that theoretical predictions for thelow-lying quartet states of FeH agree well with the experi-mental results, especially given the number of close, inter-acting states in this molecule. The MRCI1Q calculations of

FIG. 5. TheJ dependence of the magnitude of theL-doubling in the rota-tional levels of theA 4P state of56FeH.

TABLE VI. Possible56FeH branches~unassigned, in cm21!. It is not clear whether a given branch is forward orbackward running.

7032.147 7010.714 6979.575 6972.753 6982.949 6636.8037030.798 7009.119 6978.833 6976.635 6982.434 6619.0867028.422 7006.340 6977.613 6979.575 6981.059 6601.3367025.026 7002.509 6975.739 6981.547 6978.833 6583.1477020.533 6997.513 6972.994 6982.355 6975.739 6564.3547015.153 6991.506 6969.735 6981.990 6971.870 6545.5097009.119 6984.397 6965.857 6980.312 6967.368 6527.4717002.139 6976.314 6961.538 6977.613 6962.096 6509.9846994.120 6967.368 6956.739 6973.930 6956.2796985.107 6957.230 6951.229 6969.1956975.224 6963.4716964.4896952.723

7741J. Chem. Phys., Vol. 121, No. 16, 22 October 2004 Electronic spectra of iron monohydride

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Langhoff and Bauschlicher,11 where MRCI stands for multi-reference configuration interaction, place theE 4P andA 4Pstates at 1200 and 7416 cm21, respectively, in perfect accordwith our findings. They anticipated that theE 4P-A 4P tran-sition would be prominent, which is what we have seen.Their estimates for the equilibrium bond lengthsr e in theX 4D, A 4P, E 4P, and F 4D states, of 0.1588, 0.1553,0.1624, and 0.1718 nm, respectively, compare most favor-ably with the corresponding experimentally determinedr 0

values of 0.1620, 0.1529, 0.1632, and 0.1707 nm.From a situation that was enigmatic a few years ago,

FeH has become, arguably, the best characterized of all thefirst row transition metal monohydrides. Of the six electronicstates that arise from the 3ds13dp23dd3 ground electronconfiguration, five have now been identified. Only theB 4S2

state remains to be found. The quite excellent agreementbetween theoretical predictions and experimental findingsobserved so far gives confidence that thisB 4S2 state mustlie near 5400 cm21, and a search for theB 4S2-A 4P in theinfrared near 4000 cm21 is warranted. Rotational combina-tion differences from theE 4P-A 4P system will assist in its

identification and analysis. Once the final piece of the FeHground configuration puzzle is in place FeH will provide animportant test case for global modeling.

ACKNOWLEDGMENTS

W.J.B. wishes to acknowledge the hospitality received atthe Physical and Theoretical Chemistry Laboratory, Univer-sity of Oxford during a study leave when this researchproject was initiated. Travel and operational funding supporthas been provided by the Natural Sciences and EngineeringCouncil of Canada and the University of Victoria. We arevery grateful to Jeff Ishizuka for assistance in fitting the termvalues.

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TABLE VII. Parameter values~in cm21! determined in a least-squares fit ofthe term values for theE 4P state of FeH to an effective Hamiltonian.

T0 7231.103~358!a

A 2125.291~386!AD 20.532(137)31021

l 2.769~212!B 6.38055~357!D 0.4180(107)31023

H 0.1146(867)31027

o1p1q 14.661~317!p12q 22.5915~499!q 0.25097~665!oD1pD1qD 20.2336(156)31021

pD12qD 0.461(122)31023

qD 0.113(133)31024

Standard deviation of fit 1.189 cm21

aThe numbers in parentheses are 1 standard deviation of the least-squares fit,in units of the last quoted decimal place.

7742 J. Chem. Phys., Vol. 121, No. 16, 22 October 2004 Balfour, Brown, and Wallace

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