An electronic spectrum of xenon hydride

Volume 129, number 1

AN ELECTRONIC

CHEMICAL PHYSICS LETTERS

SPECTRUM

15 August 1986

OF XENON HYDRIDE

R.H. LIPSON Herzberg Institute of Astrophysics, National Research Council of Canada, Ottawa, Ontano, Canada KIA OR6

Received 15 May 1986;in final form 12 June 1986

Electronic emission spectra of XeH and XeD have been recorded for the fist time. The bands, found at h = 6600 A, have been analyzed as 2 ZZ:*-~Z’ transitions and accurate molecular constants were obtained. While the XeD band was well behaved, a smrdl perturbation was found in the XeH band.

1. Introduction

The electronic structure of the rare gas hydrides has been the subject of various theoretical calculations [l] since the first observation of one band of argon hydride by Johns in 1970 [2]. Only recently have emission spectra of HeH and NeH been analyzed [3] while spectra of KrH and XeH have not yet been reported. Calculations indicate that the rare gas hydrides (RgH) are Rydberg molecules; that is, they have repulsive ground states and bound Rydberg excited states. In an excited Rydberg state, these molecules are expected to consist of an RgIl+ (_X1ZZ+)core and an outer nlX electron. Since the Rydberg electron does not contribute significantly to the bonding of the neutral, the excited states should closely resemble those of the strongly bound ion core [4]. In this Letter, an electronic transition of xenon hydride is reported for the first time from which spectroscopic constants for the molecule have been obtained.

Xe

I

Xenon hydride molecules were formed in -a Penning excitation source following the design of Cossart, Cossart-Magos, Gandara and Robbe [S], which is reproduced in fig. 1. Typically 20 Torr of Xe was discharged in the high-pressure region of the cell (320 82

_-_

_-_

hallow cothade let H2 or D2

I woter

2. Experimental

_-_ --_

water -r_-

1

--I---

I_-_-_- - -

p---e

-I

c- - - -/ Fig. 1. Penning excitation source after Cossart for the formation of XeH and XeD molecules. was collected through a window, indicated by along an axis perpendicular to the luminescent

et al. [S] used Fluorescence dashed lines, jet.

VDC, 150 mA) and pumped through a capillary (diameter = 2 mm). The resulting luminescent jet in the lowpressure region of the cell (ml .O Torr) was crossed by

CHEMICAL PHYSICS LETTERS

Volume 129, number 1

a stream of H2 or D2 (4.2 Torr) to produce XeH or XeD. Fluorescence, collected 90” to the axis of the jet, was recorded in first order on Kodak 103-F plates using a 21 foot Eagle-type spectrograph equipped with a 1200 lines/mm grating blazed for 5500 A. The dispersion was 1.2 A/mm and the slit width was 50 pm corresponding to a resolution of 0.13 cm-l. The absolute wavenumber measurement is good to +O.OS cm-l while the precision of the stronger unblended lines is estimated to be i70.01 cm-l. Calibration was provided by Fe and Ne hollow cathode lines in first and second order. Exposure times of 4-5 h were required to obtain a strong spectrum for both XeH and XeD. No molecular hydrogen emission was observed using this cell resulting in clean spectra of XeD and XeH. Strong emission from the known band of ArD at h FS:7670 A [2] was recorded in a separate experiment, showing that rare gas hydrides are easily formed in this type of discharge.

3. Results and discussion Fig. 2a is a reproduction of the spectrum of XeH

II@98765

4

3

2

I

0

15 August 1986

observed near 6600 A. The analogous spectrum of XeD is presented in fig. 2b. Both bands have the appearance of a 2Z--2L: transition, exhibiting strong P and R branches, no Q branch and resolvable spin doubling at higher N. Since 2Z:- states are not expected for rare gas hydrides [2] the bands were assigned to 22+-2B+ transitions. The isotope shift upon substitution of D for H is small (Ml cm-l) indicating that the bands are (u’ = 0, u” = 0) transitions. Splitting of the rotational lines due to the many isotopes of Xe was not resolved. There was an uncertainty in determining which member of each doublet in the P and R branches was theF1(J=ZVt~)orF2(J=N--a)component.For low values of N, PI and RI should be more intense than P2 and R2 [6]. However, in these bands, the intensities of the spin components in the P branch are opposite to those found in the R branch. A slightly better tit of the line frequencies could be obtained if the higher-frequency spin component for each N was assigned to R, and P, . Molecular constants were obtained using the least-squares diatomic fitting program of Johns and Lepard [7]. Since the 2Z+ matrix elements are well known, they are not repeated here.

N

P; ??

II I I I

I

b)

Fii. 2. (a) Spectxum of XeH. (b)

Spectrumof XeD. All unassigned lines in the spectra are known atomic Xe transitions. 83

15 August 1986

CHEMICAL PHYSICS LETTERS

Volume 129, number 1 Table 1 Vacuum wavenumbers (cm-‘)

and rotational assignments of lines of XeD

J

RI(J)

Ohs.-talc.

0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5 11.5 12.5 13.5 14.5 15.5 16.5 17.5 18.5 19.5 20.5

15026.305 032.552 a) 038.531 a) 044.249 a) 049.727 054.929 059.856 064.507 068.880

-0.012 -0.019 -0.030 -0.037 -0.017 -0.005 0.002 0.005 0.002

076.808 080.354 083.616 086.605 089.305 091.717 093.853 095.709 097.261 098.515 099.463

0.004 0.003 -0.002 0.001 -0.002 -0.008 -0.003 0.011 0.011 0.006 -0.012

PI(J)

Ohs.-talc.

b(J)

Ohs.-talc.

15013.043 006.079 14998.845 991.344 983.619 975.604 967.330 958.810 950.037 941.014 931.743 922.218 912.450 902.439 892.186 881.679 870.932

-0.088 -0.062 -0.047 -0.041 -0.004 -0.003 -0.006 -0.004 -0.003 -0.002 0.000 -0.003 -0.003 -0.000 0.006 0.003 0.002

15032.552 a) 038.531 a) 044.249 a) 049.611 054.718 059.592 064.194 068.523 072.572 076.348 079.850 083.064 086.004 088.654 091.030 093.123 094.916 096.454

0.013 0.049 0.089 0.040 0.004 0.005 0.006 0.006 0.001 -0.001 0.001 -0.005 -0.004 -0.010 -0.005 0.004 0.001 0.034

Ohs.-talc.

P2(J)

15006.000 14998.696

0.015 0.007

983.333 975.267 966.945 958.378 949.555 940.487 931.164 921.596 911.783 901.721 891.414 880.861 870.064

0.006 0.004 -0.001 0.002 -0.000 0.003 0.000 0.000 0.002 0.001 0.004 -0.002 -0.005

a) Unresolved doublet,

Although the rotational assignment of both bands was straightforward, the XeH spectrum was found to be perturbed in the region of N’ = 8 in the upper state. Table 1 lists the observed line frequencies of XeD, their assignments and the residual errors in the final tit. The standard deviation of the fit was ~*O.OOS cm-l. The results of the analysis for XeD are listed in table 2. The observed line frequencies for XeH are listed in table 3. Due to the perturbation in the upper state, lower state rotational constants were determined separately from combination differences [6] and are listed in table 4. As appropriate plots of the upper state term Table 2 Spectroscopic

Bo Do 70

voo

constants for XeD (cm-’ ) a)

Lower state

Upper state

3.37722(31) 5.5596(77) x 1O-5 -0.0853(18)

3.24667(29) 5.724(64) X lo+ -0.0383(53) 15019.8427(61)

a) Quoted errors are *3u standard errors.

84

energies show, in fig. 3, only the F1 component is affected. This indicates that the perturbing state is most likely a 21’1state. However, since only transitions inTable 3 Vacuum wavenumbers (cm-‘) lines of XeH

J

RI(J)

0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5 11.5 12.5 13.5

15011.713 024.059 a) 035.846 a) 047.105 057.895 068.125 077.774 086.784 095.860 103.642 111.035 117.872 130.179

a) Unresolved doublet.

and rotational assignments of

Pi(J)

WJ) 15024.059

14985.498 971.812 957.555 942.762 927.486 911.718 895.456 878.709 861.394 844.231 825.877 807.235

P2(J)

a)

035.846 a) 047.105 a) 057.670 067.756 077.309 086.323 094.782 102.683 110.007 116.761 122.930

14971.638 957.278 942.401 927.016 911.128 894.753 877.913 860.602 842.825 824.587 805.881 786.744

CHEMICAL PHYSICS LETTERS

Volume 129, number 1 Table 4 Spectroscopic constants for XeH (cm-‘) a) Lower state

Upper state

6.6668(10) 2.219(49) x 1o-4 -0.168(39)

6.4089(10) 2.2345(10) X 1O-4 -0.0896(61) 14998.971(29)

Bo Do TO

WI0 a) Quoted

errorsare *3a

standard errors.

volvhrg the 2Z” state were observed, a deperturbation calculation was not attempted. Effective upper state constants were obtained by fixing the ground state constants to the values listed in table 4 and then fitting 06-

00

tc% ,

1 l

-02

-04

.

. l

.

’

. .

.

.

.

L

Fig. 3. Upper state term energies of XeH plotted as Av against N’, where Av = F&W) - 14999.0 - 6.41 N’(N’ + 1) + 2.13 x 10e4 N12(N’+ 1)2 and i = 1 or 2.

15 August 1986

the observed line frequencies using Fl assignments up to N’ = 7 and all F2 lines. The results of the analysis are included in table 4. The standard deviation of the fit was =:+0.02 cm-l. The Rydberg states of XeH and XeD are expected to resemble the ground states tX lZ+) of HI and DI respectively, which are isoelectronic with the Xep and XeD+ cores. This comparison is presented in table 5 and the agreement is quite striking. In addition, scattering experiments have deduced an equilibrium internuclear separation of 1.74 A for the X 1Z+ ground state of XeI-I+ [9]. This corresponds to a Bi of 5.57 cm-l, which is in fair agreement with the results of table 4. Much less certain is the correlation of the observed states with the electronic structure of the molecule. While cakulations have been useful in assigning the bands of HeH [3], they did not provide a convincing interpretation of the 7670 A band of ArD. In his paper [2], Johns suggested that the strong predissociation in the corresponding band of ArH was the result of a crossing between the lowest A 2ZZc+ Rydberg state and the repulsive X 2Z+ ground state. However, calculations do not predict this to occur. The observed rotational linewidths of XeH are not significantly broader than those of XeD, which does not allow for any correlation between these bands and the spectrum of an argon hydride. In the united atom approximation [6], the electronic structure of XeH and XeD should be similar to that of Cs. As the observed transitions are in the visible, they must necessarily involve low-lying Rydberg states. Two possible assignments of the 2 Z+-2 Z+ transitions, based solely on the similarity of the observed wavelengths, are to the analogues of the 7d 2D + 6p 2Po lineatA=6723Aorthe9s2S+6p2PolineatX

Table 5 Comparison of the lower 2 L+ state of XeH and XeD with the ground state (X ’ c+) of HI and DI a) Constant

XeH

HI b)

XeD

DI b)

wo

2311 C) 6.6668 2.219 x 1o-4

2269 6.3419 2.069 x 1O-4

1659 C) 3.37722 5.596 x lo-’

1620 3.2231 5.264 X lo-’

Bo Do -1

a) units in cm Constants for HI and DI were taken from ref. [ 81. C)Values of w. were obtained from Kratzer’s relation (61.

b)

85

Volume 129, number 1

CHEMICAL PHYSICS LETTERS

= 6588ii [lo], but no definite interpretation can be put forward until the electronic structure of this molecule is better understood. Another band assigned to xenon hydride has also been observed near h = 9400 A using the Cossart tube, as well as many new transitions of ArD and KrD. The analyses of these spectra are currently underway [ 111. It is hoped that these results will prompt accurate calculations to be done to further the understanding of the electronic structure of the rare gas hydrides.

ported in the Letter. Consequently, all F1 and F2 labels should be reversed and appropriate renumbering of the branches made. The other molecular constants and the discussion remain essentially unchanged. The analysis of the new spectra of XeH and XeD will be reported in a later publication.

References Ill G. Theodorakopoulos,

Acknowledgement The author wishes to thank Dr. G. Herzberg for his strong encouragement in this work and Dr. J.W.C. Johns for many useful discussions concerning his computer program. He also wishes to thank Mr. B. Hurley for his help in obtaining the spectra and to acknowledge Dr. G. Herzberg, Dr. J.W.C. Johns and Dr. Catherine M. Deeley for critical discussions of the manuscript.

Note added Subsequent analysis of spectra of XeD and XeH near 9400 A indicates that the sign of the spin-rotation constants, +ro,are positive, not negative, as re-

86

15 August 1986

S.C. Farantos, R.J. Buenker and S.D. Peyerimhoff, J. Phys. B17 (1984) 1453; M. Berman, U. Kaldor, J. Shmulovich and S. Yatsiv, Chem. Phys. 63 (1981) 163. VI J.W.C. Johns, J. Mol. Spectry. 36 (1970) 408. [31W. Ketterle, H. Figger and H. WaIther, Phys. Rev. Letters 55 (1985) 2941. (41J.W.C. Johns, J. Mol. Spectry. 106 (1984) 124. 151D. Cossart, C. Cossart-Magos, G. Gandara and J.M. Robbe, J. Mol. Spectry. 109 (1985) 166. 161G. Herzberg, Spectra of diatomic molecules (Van Nostrand, Princeton, 1950). 171J.W.C. Johns and D.W. Lepard, J. Mol. Spectry. 55 (1975) 374. PI K.P. Huber and G. Herzberg, Constants of diatomic molecules (Van Nostrand Reinhold, New York, 1978). PI HP. Weise, H.U. Mittman, A. Ding and A. Henglein, Z. Naturforsch. 26a (1971) 1122. [lOIC.E. Moore, Atomic energy levels, Vol. 3, Nat. Std. Ref. Data Ser. (Natl. Bur. Std., Washington, 1958). 1111I. Dabrowski, G. Herzberg, B. Hurley, R.H. Lipson, M. Vervloet and D.C. Wang, work in progress.