R.S. Ram, P.F. Bernath and J.W. Brault- Fourier Transform Emission Spectroscopy: The...

8/2/2019 R.S. Ram, P.F. Bernath and J.W. Brault- Fourier Transform Emission Spectroscopy: The Vibration-Rotation Spectrum

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JOURNAL OF MOLECULAR SPECTROSCOPY 113,269-274 (1985)

Fourier Transform Emission Spectroscopy: The Vibration-RotationSpectrum of CuH

R.S. RAMANDP.F.BERNATHDepartment of Chemistry, University of Arizona, Tucson, Arizona 85721

ANDJ.W. BRAULT

National Solar Observatory, National Optical Astronomy Observatories. Tucson, Arizona 85726The Fourier transform infrared emission spectrum of CuH was observed. The (I, 0), (2, I),

and (2, 0) vibration-rotation bands of both %tH and %tH were recorded from a copperhollow-cathode discharge in neon and hydrogen. Improved molecular constants for the u = 0, I,and 2 levels of CuH are provided. This work is the first observation of a vibration-rotationspectrum of a metal hydride in emission. 0 1985 Academic press, IIIC.1. INTRODUCTION

The diatomic transition metal hydrides are of considerable experimental and theo-retical interest (I-13). Some transition metal hydrides such as TiH and FeH (14) areastrophysically important. Others such as NiH (10) and PdH are often considered asmodels for the chemisorbed hydrogen atom on catalytic metal surfaces. The elec-tronic spectra of CuH are relatively simple and so CuH has been observed in emission(15, 16) and absorption (I 7-20) from the visible to the vacuum ultraviolet (20).

The most extensive work on CuH has been carried out by Ringstrom, who observedthe AZ+ bA2, B3110f,Cl, cl, and EZ+ states in the region 25 000-40 000 cm-(18. 19). More recently, Brown and Ginter (20) have discovered four new electronicstates (Fl, GO+, HI, and II) in the region 48 000-53 000 cm-. The best groundstate (XZ) molecular constants (previous to our work) are provided by Ring-Strom (18).We have observed the vibration-rotation spectrum of CuH in emission with a Fouriertransform spectrometer. The spectrum of CuH was found at the same time as ourdetection of the infrared emission spectrum of NeH+ (21). The CuH was produced ina neon hollow-cathode discharge containing a small amount of hydrogen. The Cuatoms required to create CuH were provided by sputtering of the copper cathodematerial.There have been only two previous observations of infrared spectra of metal hydrides.James et al. (22) observed the infrared absorption spectrum of LiH present in a hotcell. More recently Haese et al. (23) observed the vibration-rotation absorption spec-trum of KH with a diode laser. The KH was produced in a long discharge cell and

269 0022-2852/85 $3.00Copyright 0 1985 by Academic Press, Inc.All rights of reproduction in any form reserved.


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270 RAM, BERNATH, AND BRAULTdetected by concentration modulation of the KH by an a.c. discharge (23). Our workthus represents the first detection of the vibration-rotation emission spectrum of ametal hydride.

II. EXPERIMENTAL DETAILSThe CuH spectrum was excited in a water-cooled copper hollow-cathode lamp ata current of 330 mA. A continuous flow of 2.0 Torr of neon and 30 mTorr of H2 was

maintained through the lamp. A similar experiment was carried out with Dz insteadof Hz but CUD was not detected.The spectrum was recorded with the Fourier transform spectrometer associatedwith the McMath Solar Telescope of the National Solar Observatory located at KittPeak. The interval 1800-5000 cm- was recorded at 0.05-cm- resolution using liquidnitrogen-cooled InSb detectors with a wedged Ge filter. The lower wavenumber limitis determined by the red limit of the detector while the upper limit is set by the Gefilter. Sixty four scans were coadded in 2 hr of integration.

III. RESULTSThe (1, 0) band of CuH was noted and easily identified during our NeH work

because of its characteristic spectral pattern. A regularly spaced branch with each linedoubled and split by about 0.5 cm- was found. The doublets had a 2: 1 intensity ratiocharacteristic of 65Cu and 63Cu in natural abundance. Ringstroms molecular constants(18) allowed the lines to be unambiguously identified. Once the (1,O) band had beenpicked out and fit, the (2, 1) and (2,O) bands were predicted and found. A section ofthe spectrum showing the R(7) line of both isotopic species is displayed in Fig. 1.

%uH

R(7)

CuH

I , I I I1971.0 1972 .O 1973 .o CM-FIG. 1. The R(7) transition for both %uH and 63C~H of the fundamental (1.0) band.


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CuH VIBRATION-ROTATION 271The line positions of the (1, 0), (2, l), and (2, 0) bands of 63C~H and 6sC~H are

provided in Tables I and II. Only R lines were recorded for the (2, 1) band and just afew P lines of the (I, 0) band because of the 1800-cm- cutoff of the InSb detector.The CUD (1,O) band is below 1800 cm- and the (2,O) band was too weak to observe.Since %I has an abundance of 3 190,ewer j5CuH than 63C~H lines were found, andthe observed 65C~H lines have a lower signal-to-noise ratio. The best signal-to-noiseratio was about 20 (see Fig. 1).The spectra of HZ, CO, and OH were present in our records. The CO (24) and OH(25) lines were used to provide an absolute calibration accurate to +O.OO cm-. How-ever, the rather modest signal-to-noise ratio for CuH limits the accuracy of our ob-servations to about 0.005 cm- for the stronger, unblended lines.

IV. ANALYSIS AND DISCUSSIONThe line positions of Tables I and II were fit using the customary rotational energy

level expression for Z states (26):F,(J) = T, + BJJ + 1) - D,J2(J + 1) + H,J3(J + 1)3. (1)

The u = 2. 1, and 0 energy levels were simultaneously fit for each isotopic speciesusing a nonlinear least squares procedure. The molecular constants from these twofits are provided in Table III. Note that the H values of the less abundant 65C~H wereconstrained using the expected p6 isotopic dependence and the 63C~H values (26). Anextra digit is provided in Table III: becuse of correlation (as usual) ABs are betterdetermined than Bs.

TABLE IThe Observed Line Positions for the Vibration-Rotation Spectra of 63C~H (in cm-) [the numbers inparentheses are observed-calculated line positions using the constants of Table III (in 10m3 m-)]

(l,O) (2.1) (290)JR(J) P(J) R(J) R ( J ) P(J)

0 1881.530 (-2) 3673.090 (-5)1 1896.142 (0) 1850.775 (3) 3686.707 (5)2 1910.222 (2) 1834.643 (-3) 1834.393 (3) 3699.276 (-2)1923.759 (3) 1818.042 (0) 1847.436 (-4) 3710.791 (-22) 36Oi.601 t-2)4 1936.739 (1) 1800.968 (-2) 3590.038 (10)5 1949.154 (1)

l87L3013)

373i.699(-8) 3570.491 C-9)

6 1960.988 (-2) 1883.242 (-2) 3739.052 (8) 3550.027 (-4)7 1972.242 (2) 1894.023 (-3) 3746.286 t-10) 3528.636 (2)8 1982.888 (-2) 1904.212 (-2) 3506.321 (0)9 1992.931 (-2) 1913.808 (9)

10 2002.350 (-2) 1922.765 (-4)11 2011.144 (0) 1931.114 (-2)12 2019.300 (3) 1938.830 (1)13 2026.798 (-5) 1945.898 (-3)14 2033.657 (7) 1932.323 (2):i 2039.826045.333 (-3)-4117 2050.151 (4)

a : o b s c u r e d b y a n a t o mi c l i n e .


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212 RAM. BERNATH, AND BRAULTTABLE 11

The Observed Line Positions for the Vibration-Rotation Spectra of 65C~H (in cm-) [the numbers mparentheses are observed-calculated line positions using the constants of Table III (in 10m3 m-)]

(1.0) (2,1) (2,O)J

A( J ) P ( J ) R ( J ) R ( J ) P ( J )

0 1881.098 (3) 3672.290 (19)1 1895.699 (2) 3685.859 (-12)2 1090.760 (-8) 1834a .224 (-9) 1833.984 (41 3698.443 (4)3 1923.299 (2) 1817.631 (-2) 1847.030 (6) 3709.954 (-11) 3607.803 (0)4 1936.271 (-2) 1859.509 (-8) 3720.440 (3)1948.691 (8) 1871.446 (-3) 3729.856 (13) 356i.708 (-2)6 1960.508 (-9) 1882.808 (-3) 3549.242 (4)L 1971.761982.408 (-1)0) 1903.774893.577 (-12)0) 3745.420 (5) 3527.830 (-3)9 1992.454 (7) 1913.362 (6)

10 2001.864 (-1) b11 2010.645 (-8)12 2018.810 (9) 19386.384 (6)13 2026.296 (-3) b14 2033.146 (8) 1951.857 (-3)15 2039.300 (-6)1617 204i.593 (2)

a : o b s c u r e d b y a n a t o mi c l i n e .b : o b s c u r e d b y C O2 a b s o r p t i o n .c : b a d l y b l e n d e d .

Equilibrium molecular parameters were derived for each isotopic species using theequationsB, = B, - c+(v f 1) + rdu + j)

andD, = D, + pc(u + 4).

In addition, the vibrational constants were determined from (26)

(2)(3)

T,.o = w, - 2W& (4)

TABLE IIIMolecular Constants (in cm-) for the XZ+ States of the 63CuH and 6SC~H MoleculesMo l e c u l a r v = o v = l " = 2C On S t a n t s a

6 3 C " H 6 5 C " H 6 3 Cu H %H 6 3 CU H 6 5 C " H

T 0.0 0.0 186fi.4048(19) 1865.9757(23) 3 6 5 8 . 4 6 8 2 ( 2 5 ) 3 6 5 7 . 6 5 3 1 ( 3 4 )0 " 7 . 8 1 7 4 5 ( 2 2 ) 7 . 8 1 3 5 6 ( 2 5 ) 7 . 5 6 4 7 2 ( 2 4 ) 7 . 5 6 0 7 5 ( 2 5 ) 7 . 3 1 4 2 5 ( 2 5 ) 7 . 3 1 0 2 1 ( 2 3 )

1 0 4 0 " 5 . 3 4 1 ( 4 0 ) 5 . 1 6 6 ( 3 0 ) 5 . 2 7 6 ( 4 5 ) 5 . 1 3 3 ( Z ) 5 . 2 1 7 ( 4 2 ) 5 . 0 9 7 ( 1 9 )l Oa H" 8 . 8 ( 3 7 ) t u b 7 . 4 ( 3 1 ) 7 . 4 b 6 . 5 ( 2 3 ) 6 S b

a : T h e n u mb e r s i n t h e p a r e n t h e s e s a r e o n e s t a n d a r d d e v i a t i o n i n t h e l a s t d i g i t .b : F i x e d v a l u e s ( s e e t e x t ) .


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CuH VIBRATION-ROTATION 273and

T,, = 2w , - 6w ,xc. (5)The equilibrium constants are reported in Table IV. Note that all of our determinationsare essentially exact fits so that the uncertainties of Table IV were computed by usingthe uncertainties of Table III and the customary rules for error propagation. The actualuncertainties for w, and w& are expected to be much higher than the numbers quotedin Table IV because of the neglect of the w,y, term. With only a AG,,2 and a AG3,?,o,y, cannot be determined from our data but is certainly not negligible. The usualisotopic relationships between the 63C~H and 65CuH equilibrium constants are obeyedwithin the quoted errors.

The molecular constants of Tables III and IV are in reasonable agreement with,but more accurate than, the previous determinations. Ringstroms (19) parametervalues, Be = 7.8163(3), BI = 7.5631(3), B2 = 7.3130(3), T,_o = 1866.46(2), and T2_0= 3658.50(2) are slightly, but systematically, different from those reported in TableIII. The customary Pekeris and Kratzer relationships were used to test the self-con-sistency of the molecular constants of Table IV. The Pekeris relation (26) predicts ancr, = 0.22695 compared to the observed 0.25499(55) cm- for 63C~H. This disagreementis likely a reflection of the inadequacy of the Morse potential curve (from which thePekeris relationship is derived) and of our neglect of W,JJ~. he Kratzer relation predictsa D, = 5.325 X 10e4 cm- for 63C~H, in good agreement with the observed 5.372X IOh cm-.

The first emission observation of a metal hydride vibration-rotation spectrum opensup a number of interesting possibilities. Not only might the alkali and alkaline earthhydrides be observed in their Au = 2 sequence, but additional transition metal hydridesmay be detected. The transition metal hydrides have complicated visible transitions,largely because of extensive perturbations among the numerous excited states. Theinfrared vibration-rotation spectra and infrared electronic transitions may be relativelyunperturbed and amenable to analysis.

TABLE IVEquilibrium Molecular Constants (in cm-) for the XZ State of CuHconstants %H %H

we 1 9 4 0 . 7 4 6 2 ( 4 1 ) " 1 9 4 0 . 2 7 4 0 ( 4 7 )We Xe 3 7 . 1 7 0 7 ( 2 3 ) 3 7 . 1 4 9 2 ( 4 1 )

% 7 . 9 4 4 6 7 ( 3 7 ) 7 . 9 4 0 8 2 ( I )a l ? 0 . 2 5 4 9 9 ( 5 6 ) 0 . 2 5 5 0 7 ( 6 0 )y e 0 . 0 0 1 1 3 ( 2 4 ) 0 . 0 0 1 1 3 ( 2 4 )

1 0 4 De 5 . 3 7 2 ( 7 2 ) 5 . 1 8 4 ( 4 9 )

- 0 . 6 2 ( 6 0 ) - 0 . 3 5 ( 3 9 )1 . 4 6 2 5 7 1 . 4 6 2 5 7

a : S e e t e x t f o r d d i s c u s s i o n o f h o w t h e u n c e r t a i n t i e s we r e d e t e r mi n e d .


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274 RAM. BERNATH, AND BRAULTACKNOWLEDGMENTS

We thank Rob Hubbard for expert technical support during our data run. Acknowledgment is made tothe donors of the Petroleum Research Fund. administered by the ACS, for partial support of this research.This research was also supported by Grants NSF CHE-8306504 and ONR NOOO14-84-K-0122.RECEIVED: March 14. 1985

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1875 (1978).6. B. KLEMANAND U. UHLE R.Cunad. J. Phys. 37, 537-549 (1959).7. L. KLYNNING AND M. KRONEKVIST. Phys. Ser. 24, 2 1-22 ( 198 I).8. A. E. STEVENS.C. S. FEIGERLE.AND W. C. LIN EBERGE R.I. Chem. Phj~ 78,5420-5431 (1983).Y. P. S. BAGUSAND C. BJORKMAN.Phys. Rev -123,461-472 (1981).

10. M. R. A. BLOMBERGAND P. E. M. SEIGBAHN.MoI. Ph.w. 47, 127-143 (1982).II. S. P. WALCH AN D C. W. BAUSCHLICHER,R., J . Chem Phys. 78, 4597-460s (1983).l-7 G. DAS. J Chem Phys. 74, 5766-5774 (1981).13. G. H. JEUNGAND J . C. BARTHELAT, . Chem. Phys. 78,2097-2099 (1983).14. R. F. WING. J. COHEN,AND J. W. BRAULT,Astrophys. J. 216,659-664 (1977).15. A. HEIMER AND T. HEIMER, Z. Phtx 84,222-235 (1933).16. B. KLEMAN. Ark. Fys. 6, 17-24 (1952).17. B. GRUN DSTR~M, Z. Phys. 98, 128-132 (1935).IN. U. RINGSTROM, Ark. Fys. 32,2 I I-239 (1966).lo. U. RINGSTROM,Cunad. J. Phys. 46,229 I-2300 (1968).20. C. M. BROWS AND M. L. GINTER. J . Mol. Specfrosc. 80, 145-157 (1980).-71. R. S. RAM. P. F. BERNATH,AND J. BRAULT. J. Mol. Specfrosc.. in press.22. T. C. JAMES,W. G. NORRIS,AND W. KLEMPERER, .I. Chem. PhJs. 32,728-734 (1960).-73. N. N. HAESE.D. LIU. AND R. S. ALTMAN, J. Chem. Phys. 81, 3766-3773 (1984).-74. G. GUELACHVILLI, J. Mol. Spectrosc. 75,25 l-269 (1979).25. T. AMANO. J. Mol. Spectrosc. 103,436-454 (1984).-76. G. HERZBERG. Spectra of Diatomic Molecules, 2nd ed.. Van Nostrand-Reinhold, New York, 1950.

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