ON THE ULTRA-VIOLET ABSORPTION SPECTRUM O F DIBORANE' G. HERZBERG Department of Physics, University of Saskatchewan, Saskatoon, Saskatchewan E. BLUM
AND
Received October 5, 1996
Much attention has been paid recently to the structure of the BzHs (diborane) molecule. Particularly Mulliken (2) has discussed in considerable detail the electronic structure of this molecule. By the courtesy of Professor Stock of the Technische Hochschule in Karlsruhe we obtained a very pure sample of this gas and have investigated its ultra-violet absorption spectrum down to about 1550 A.U. EXPERIMENTAL
For experiments in the ordinary ultra-violet a quartz tube 15 cm. long with flat windows fused on to each end was used. In the Schumann region this was replaced by a quartz tube of 20 em. length with extremely thin hemispherical quartz windotvs (4)fused on to it, which are transmittant down to 1550 A.U. The first tube was sealed off after it had been filled in the Karlsruhe laboratory with the pure gas at approximately atmospheric pressure. For comparison spectrograms the gas could be frozen out in a side tube. The second tube was arranged in such a way that the gas could be continuously streamed through it during the exposure, going from one trap to another of suitable temperatures. The pressure used in this case varied from 0 to 40 mm. Precautions were taken to avoid contamination of the pure sample obtained from Professor Stock. Mercury valves wereused to avoid contact with stopcock grease, and the quartz tube was joined to the glass apparatus by way of long ground joints not greased but only covered with picein outside. A hydrogen discharge tube served as source of continuous background for the absorption experiments. The ordinary ultra-violet spectrum down to 1900 A.U. was investigated with high dispersion (3rd order of the Darmstadt 3m grating, dis1 Presented at the Symposium on Molecular Structure, held a t Princeton University, Princeton, New Jersey, December 31,1936 to January 2,1937, under the auspices of the Division of Physical and Inorganic Chemistry of the American Chemical Society. 91
92
E . BLTM . 4 S D G . HERZBERG
persion 1.7 A.U. per millimeter). The Schumann region could only be investigated with the small disperbioii of n Cario-Schmidt-Ott fluorite spectrograph (1) (dispersion at 1800 A.U. equal t o 20 A.U. per millimeter; at 1600 A.V., 12 h . U . per millimeter). RESULTS
The spectrograms taken in the ordinary ultra-violet showed an increasing continuous absorption with decreasing wave lengths starting from about 2200 A.U. at atmospheric pressure and 15 cni. absorbing length. There is no sharp wave-length limit. On the contrary the absorption fades out very slowly to longer wave lengths and would certainly extend t o wave lengths > 2200 with longer absorbing lengths or higher pressures. Even with the high dispersion used, above 1900 A.U. there is no indication of discrete absorption bands. The absorption is entirely continuous.
is00
1903
ex
17.0
1430
1500
L IN A -
FIG.1. Absorption coefficient of diborane in the Schumann region. The dotted part is more uncertain than the rest of the curve. Because of a numerical error the ordinates should be multiplied by 0.875.
I n the Schumann region the Continuous absorption further increases until it comes to a maximum at about 1820 A.U. and then diminishes. Owing to the much smaller dispersion used in this region we cannot be quite so sure about its continuous character, but in view of the lack of any indication of discrete bands at the long wave-length limit it seems highly improbable that the continuous spectrum observed in the Schumann region is not a true continuum. After a minimum at about 1700 A.U. the absorption rises again, now much steeper, until about 1550 A.U., the limit of the region observed. Also this absorption appears to be continuous. But of course below 1650 this cannot he stated with certainty, because here the many-line-spectrum of hydrogen sets in, which makes a decision more difficult. KO irregularities in the absorption of the various H Plines have
ULTRA-VIOLET ABSORPTION SPECTRUM OF DIBORANE
93
however been observed, so that if there is discrete absorption it is at any rate not very conspicuous.? One spectrogram taken with the small fluorite spectrograph a t 10 nim. diborane pressure was photometered Jvith a Koch-Goos registering microphotometer and compared with a Spectrogram taken at zero diborane pressure on the same plate. The resulting curve for the absorption coefficient, k , of diborane is given in figure 1. As no intensity marks were available the curve does not claim any great accuracy, but is intended only to give a rough idea of the absorption coefficient and of its order of magnitude. However, the curve shows clearly that there are two distinct regions of continuous absorption, the short wave-length one being much stronger than the long mave-length one. The absorption is given according to the formula
where I o and I are the intensities before and after passing the tube, 1 the length of the tube, and c the concentration in moles per liter. I t is seen that for the first continuous absorption k is about 25, whereas for the second it is about 50 or larger, the maximum being outside the region investigated. It was observed that the windows of the absorption tube became more and more opaque to light of short wave lengths in the course of the investigation of diborane, evidently owing to some film of a solid product of decomposition of the diborane under the action of light. The occurrence of this photodissociation of diborane is in harmony with the continuous character of the absorption spectrum. DISCUSSIOX
The two continuous absorption spectra found for diborane obviously prove the existence of a t least tn-o different excited electronic levels of the molecule. The first of these is unstable; the second probably is too, but might also be a stable state. According to Mulliken the electronic ground state of diborane is a 'Al, state, i.e., the electronic eigenfunction is symmetrical with respect to all symmetry elements of the molecule (assuming symmetry D3d). The electron configuration is given (omitting the carbon 1s electrons) by
For an explanation of the symbols the reader is referred to Mulliken's paper (2). There are a number of other electronic states arising from the With the same spectrograph and light source we could easily detect an H2S band near 1580 A . U . , in spite of the discontinuous background in t h a t region.
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E . BLUM AND G. HERZBERG
same electron configuration. These, as well as the states of the configurations
+
[sa1I2[sa1l2[a a,a~,l[re14[re13
(2)
[sail2[s ~ 1 ] ~ [ ~ e ] ~ [ ~ e ] ~
(3)
must, according to Mulliken's theory, lie rather low, below about 1 volt. Therefore none of them can be identical with the upper states of the two ultra-violet absorption continua observed. For the discussion of the possible upper states of the ultra-violet absorption of diborane it seems useful first to consider the molecules ethane (C2Hs) and ethylene (CZH~), both of which show some similarities to diborane (BzHs). Ethane has in all probability the same symmetry as diborane but has two more electrons, whereas ethylene, though it has not the same symmetry, has the same number of electrons as diborane. The ground state of ethane, according to Mulliken, is (omitting carbon Is electrons)
+
[sa~lz[sa~12[~e14[~e14[~ a,a1,lZ1.410
(4)
i.e., the incomplete shells of diborane are filled. Thus there is only one low-lying state of ethane. In ethylene all degeneracies are removed, owing to its smaller degree of symmetry. Therefore though it has the same number of electrons as diborane, all shells are closed in the ground state and there are no neighboring states of similar energy. I t is described by [a
f s , a i ~ ] ~-[ sS,bit?[y
bzI2[y bzl2[r
+ ~ , a i g ] ~f[ x
5,b3uI2
'Ai,
(5)
The first excited states of ethane arise in all probability from a transfer [u u,al,]or [ r e ] orbital to a three-quantum state [3sal]. This accounts for the fact that ethane starts to absorb only a t 1600 A.U. Similar excited states with a three-quantum electron are possible for ethylene. But, besides, other excited states arise from higher two-quantum orbitals. Mulliken (3) has explained the two observed electronic band systems of ethylene at 2000 and 1750 A.U. as due to transitions to one of each group of excited levels. It seems very probable that the second, short wave-length diborane absorption also corresponds to a transition to a three-quantum state:
of an electron from the
+
+
[sa112[sa~12[a ~ , a ~ , l 2 [ ~ e I 3 [ ~ e l 2 [ 3 ~ ~ l l
(6)
However it is hardly possible to explain the longer wave-length absorption of diborane in the same way as that of ethylene, in spite of the near agreement of the energies of the states above the ground state. The excited
ULTRA-VIOLET ABSORPTION SPECTR‘C‘M O F DIBORANE
95
two-quantum states of ethylene correspond to the various diborane states which lie very near to the ground state, owing to degeneracy of the [re] orbital and the fact that the two [re] orbitals do not appreciably interact in d i b ~ r a n e . ~ Therefore the first ultra-violet absorption of diborane has either to be interpreted as a transition to one of the numerous states arising from configuration 6 other than the upper state of the second absorption or as a transition to a state of the configuration
Le., a transition of an inner electron similar to the well-known transitions and others. This is an additional in diatomic molecules like CN, possibility for diborane which, as can be seen from configurations 4 and 5 , does not exist for ethane or for ethylene. Both states 6 and 7 should be stable because the number of bonding electrons is the same as in the ground state. But predissociation into the various dissociation products BH3 BH3, B2H4 Hz, or others may be so strong as to give a completely continuous spectrum. With the material at present available it is not possible to decide definitely between the two possibilities discussed above.
Art,
+
+
SUMMARY
The ultra-violet absorption spectrum of diborane has been investigated down to 1550 A.V. TWOregions of continuous absorption have been found, extending from 2200 and 1700, respectively, to shorter wave lengths. These correspond to transitions to two different excited states, the possible electron configurations of which are discussed on the basis of Mulliken’s theory. In conclusion we wish to express our sincere thanks to Prof. Stock and his assistants for the supply of the pure diborane used in this investigation. The experiments were carried out at the Physikalisches Institut der Technischen Hochschule, Darmstadt, Germany. REFERENCES (1) CARIO,G., A N D SCHMIDT-OTT, H. D.: Z. Physik 69, 719 (1931). (2) NULLIKEN, R. S.: Phys. Rev. 43, 765 (1933); J. Chem. Physics 3, 635 (1935); cf. also HERZBERG, G. : Leipziger Vortrage 1931, p. 167. References t o earlier papers are given in Mulliken’s second paper. (3) MULLIKEX, R. S.: J. Chem. Physics 3, 517 (1935). (4) SCHMIDT-OTT, H . D.: Z. Physik 69, 724 (1931). The only possibility for a higher two-quantum excited state is [rel-l [u-u, UZ,] which, however, very probably lies above configuration 6 and has also to be conaidered as a three-quantum state (cf. reference 3).
