Halogen Isotopes and Infra-Red Reflection Spectra

53

PHYSICS: A. H. PFUND

VOL. 11, 1925

HALOGEAN ISOTOPES AND INFRA -RED REFLECTION SPECTRA BY A. H. PFUND

DuPARTM1NT 0*O PHYSICS, JoHNs HOPKINS UNIVIRSITY Communicated December 9, 1924

As a result of an investigation on the selective reflection from inorganic salts, it was shown by the writer' that all sulphates studied showed bands, near 9.05,u in the infra-red. Likewise, all nitrates studied showed a characteristic band near 7.40 u. From these observations it was concluded that the resonating mechanism, responsible for these bands of selective reflection, was localized within the negative ion. Since solutions as well as crystals showed such bands, it is believed that the negative ion retains its identity throughout. Later investigations by Morse,2 Coblentz3 and others have completely verified the above conclusions. Recent work on the structure of crystals and conjectures as to the character of interatomic forces have renewed the interest in the above subject. Of immediate interest is the subject of isotopes. The line of reasoning followed is briefly, as follows: since infra-red reflection maxima are usually complex due to the multi-atomic character of the negative ion and to double refraction, it seemed desirable to limit the considerations to the halogen salts of the alkali metals. Of these, the salts of potassium have been investigated for selective reflection in the region of long wave-lengths by Rubens.4 Now, Aston6 has showed that Cl consists of a mixture of isotopes of atomic weights 35 and 37, Br has isotopes of atomic weights 79 and 81 while I is "pure" with the single atomic weight of 127. Comparing these data with Rubens' results it develops that each halogen salt of potassium has as many bands of selective reflections as there are isotopes, i.e., KCI has 2, KBr has 2 and KI has 1. The numerical values are shown in the following table: BANDS OP

ATOMIC WTS. OF ISOTOPES

35E

SELNCTIVN REPLECTION 1

XX

62.0,U

161

70.3 75.6

142 132

86.5 96.7

115 103

C1

K Cl 37 J 79

K Br

KBr 81J

KI

127 I

Since the wave-lengths of the bands increase with increasing atomic weight of the negative ion it seemed reasonable to associate the lighter isotopes with the band of shorter wave-length. Upon plotting wavelengths against atomic weights, curves of an indecisive character were

54A

PHYSICS: A. H. PFUND

PRoe. N. A. S.

obtained. By plotting, however, wave-numbers (1/X) against atomic weights, the series of straight lines, shown below resulted. (Fig. 1.) While the writer is not familiar with any theoretical work having a bearing on these observations, it does appear that the character of the curves is too striking to be without significance. First, it is evident that the curve for the heavier isotopes is parallel to that for the lighter. Incidentally, iodine lies on the same curve as the lighter Cl and Br isotopes. This might be interpreted as being the configuration of greatest stability. 170 {co

-

'o

X

T1

140 1S0

t:XAtSStttl1~~~~~JJ

.4

E

#to

z loo 6

qo

o

o

to

Jo

40

SO

0

70

s0o

l-o

1o

#As

/36

Atomric-Weignr FIGURE 1

If atomic weight alone came into play here, it might be conceived that points would lie on the same line-such as the upper. In that case, a change in atomic weight from 37 to 35 (Cl) would bring about only a very small change in the wave-length of the reflection maximum. The actual change is, however, large. Granting that all the preceding speculations are true, it would appear that the change in vibrational frequencies brought about by a nuclear loss of two protons and two electrons is much greater than the mere change in atomic weight would seem to justify. Another point of interest is suggested by Rubens' results for NaCl and KC1. His curves are reproduced on the following page. (Fig. 2.) If it be true that the short wave-length maximum is due to the lighter isotope, then it appears that because of the reversal of heights of the two maxima for rocksalt and sylvite, the isotope 37 is most abundant in rock salt while isotope 35 is most abundant in sylvite. Experiments are in progress to find samples of these two crystals which show the above reversal in intensity. Molecular weight determinations of the Cl derived from these two samples ought to show a difference. all

55

PHYSICS: A. H. PFUND

VoL. 11, 1925

It is, of course, entirely possible that these differences in the reflection maxima are to be ascribed to the fact that, while NaCl6 crystallizes as a face center cubic lattice, KC1 crystallizes as a simple cubic lattice.

I I.lX |I lc

10

I

S-

I

7

S. 4

3 2.

4or XO6f1

--1/l--

C

__ T~~--L-7 IXI

0

FIGURE 2

Co/r

70,.&i

It is not without possibility that isotopy might be revealed in still another Morse2 has pointed out from his study of carbonates, that, while their reflection maxima lie very approximately at the same wavelength, there is a slight shift toward longer wave-lengths with increasing atomic weight of the positive ion. If, therefore, salts having an isotopic positive ion are studied, a complexity of the reflection maximum ought to result from this cause, also. Experiments are being undertaken to test this point. manner.

lPfund. Astrophys. J., Nov. 19 (1906). Morse, Ibid., July 26, 225 (1907). 3 Coblentz, Investigations of Infra-red Spectra (1906), Carnegie Inst. of Wash. 4 Rubens and Hollnagel, Verh. D. Phys. Ges., 8,83 (1910). Aston, Isotopes (Edward Arnold and Co.) (1922). B Bragg., X-Rays and Crystal Structure, p. 91 (Bell and Sons) 1924.

2