The Behavior of the 4930A Absorption Band of Uranine Solutions Under High Pressure

WILLIAM JAMES LYONS, Department of Physics, St. Louis University

(Received November 4,1935)

The object of this investigation was to test experiment-ally the assumption of electromagnetic dispersion theorythat the natural period of vibration of the resonators of amedium remains constant with variations in pressure. Tvodilute aqueous solutions of uranine of slightly differentconcentrations were studied in the high pressure spectrom-eter. Refractive indices for the three principal lines of themercury arc, and for the wave-length corresponding to

the absorption band maximum (4390A) were determinedfor pressures ranging up to 915 kg per cm2 in one solution,and up to 1823 kg per cm2 in the other. It was found thatthe absorption band retained its position relative to therest of the spectrum formed by the liquid prism undervarious pressures. It is concluded that the natural frequencyof vibration of the resonators is affected to no measurableextent by the pressure or density of a medium.

INTRODUCTION

THE influence of pressure on the index ofrefraction of liquids has been studied by

several investigators. The results uniformly havebeen an increase in the indices with increasedpressures, and are quantitatively in accord withthe various electromagnetic dispersion formulae.These formulae, such as Drude's:

A2_ I =2q2e2N/xM(vo2-V2)J,

where ,u=refractive index, q=number of elec-trons per molecule, e = electronic charge, N=number of molecules per cm3, M=combinedmass of the vibrating electrons in a molecule,vo=frequency of the absorption band of the re-fracting medium (natural vibration frequency ofthe resonators), and v =frequency of the incidentlight, assume that v remains constant, withpressure changes. This implies that the absorp-tion band of the dispersive medium does not shiftto another position in the spectrum. The objectof the present study was to test this conclusion,with the degree of accuracy the available equip-ment would allow.

The assumption that vo remains constant hasbeen indirectly verified, in the confirmation of theelectromagnetic formulae (applied to liquids) byprevious investigations. Eq. (1) leads directly toNewton's law:

(A2 -)l)d=K, (2)

where d=density of the medium, and K=aconstant, the "specific refraction"; p& being theindex of a particular spectral line. Using the dataof Poindexter and Rosen' for water, the present

* I Poindexter and Rosen, Phys. Rev. 45, 760 (1934).

writer found relation (2) to hold to about onepercent over pressures ranging up to 1800 kg percm2. The analogous Lorenz-Lorentz law,

g 2 - 1- K (3)

2+2 d

which, theory indicates is valid at all pressures,has been found to hold quite accurately byPoulter and his associates,2 and by Hayden,3

working with glycerin. No direct study has yetbeen made, however, of the behavior of anabsorption band with pressure changes.

It would have been of prime interest to observethe effect of changes in pressure on the absorptionband to which dispersion in water is due. But,since this band lies in the ultraviolet region, thisstudy was impossible with the available appa-ratus. However, it was concluded that if an in-crease in pressure affected the free period of theresonators, and was accompanied by an appreci-able shift of the absorption band, this could beobserved in a medium having an absorptionmaximum in the visible region. Accordingly,uranine (sodium salt of fluorescein), having anabsorption maximum at 4930A, was chosen. Theuranine was dissolved in water as a means ofsuspension. The two solutions used were madevery dilute in order to obtain a narrow band, andalso to permit the transmission of sufficient lightfor observations. No anomalous dispersion wasobserved. The absorption of the uranine wasassumed to be independent of its presence inwater. Accordingly, the center of the observed

2 Poulter, Richey and Benz, Phys. Rev. 41, 366 (1932).3 C. K. Hayden, Ph.D. Dissertation, St. Louis University,

1933.

144

APRIL, 1936 J. . S. A. V OL UM E 2 6

HIGH PRESSURE 145

absorption band in the solutions at the variouspressures was taken to be the absorption maxi-mum of uranine. The uranine molecule presum-ably absorbs radiation in the neighborhood of4930A, thus giving rise to a dark line at thecorresponding point of the continuous spectrumformed by the liquid prism. Known pressure anddensity changes are communicated to the uraninethrough the liquid.

PROCEDURE AND RESULTS

The high pressure spectrometer, developed inthis laboratory and described elsewhere,4 wasused in this study. At first, as the simplestmethod, the glass windows were made parallel toform a nondispersive cell into which one of thesolutions was placed, and could be subjected topressure. It was found, however, that the amountof transmitted light was insufficient for analysisin a glass-prism spectrometer of the usual type.

The high pressure apparatus was thereforeused as a spectrometer, the glass windows beingput in at oblique angles to each other, with theuranine solutions serving as the dispersive media.The angle of the liquid prism was determined byfilling the cell with distilled water at a knowntemperature, and obtaining the angles of mini-mum deviation for the three or four prominentmercury arc lines at atmospheric pressure. Byusing the well-established refractive indices ofwater for these lines, values of the prism angle, ingood agreement, were calculated. Glycerin wasused in the cylinder around the rubber sack totransmit pressure to the solution under study.

Besides a mercury vapor arc lamp, giving thefamiliar 5790A, 5461A, and 4358A lines, anEdiswan Pointolite was used to furnish a continu-ous spectrum, against which the uranine ab-sorption band stood out. The Pointolite was usedalternately with the mercury arc through thesuccessive stages of pressure.

Two series of observations were made; theresults of both are incorporated in Fig. 1. Aprism angle of 48° 8' 11"± 18" was used with the84 mg per liter solution, while one of 430 32'

I Lyons and Poindexter, "Spectrometer for Studies atHigh Pressures," this issue.

X

-a

Ca

060

t.Z55_ _ _ _ _ _ _ _ _

1.345

v~ C<^ vsXo incices otained singK K0 / 84mg ura ine per iter H2-°

,.340 ' ~ 7A K. X_ _ X in se 4 *n10C mg uraiineper iter H0

K o zX A incices of high ave-13305 -.- - enth

0 WoO ~ KKK LK

Pressure - k per cm'

FIG. 1

45"--4" was used with the 100 mg per liter solu-tion. Pressures ranged up to 915 kg per cm2 in thefirst series, and up to 1823 kg per cm2 in thesecond.

Because of the dilution of the solutions thecurves are sensibly the same as those for purewater, the effect of the uranine on the dispersionbeing negligible. The parallelism between thelines indicates that there is no measurable shift ofthe absorption band relative to the rest of thespectrum. While the refractive index of themedium, for the absorbed wave-length, changesunder pressure, this can be charged solely to thechange in the dispersion of the water, for at everypressure the index for the band center (as takenfrom the curve) is that appropriate to 4930A. Avariation in the natural vibration frequency ofthe absorbing medium (the uranine, here) withpressure would be represented by a lack ofparallelism between the curve for the absorptionband and those for the other spectral lines. Theresults demonstrate, therefore, that the naturalfrequency of the dispersion resonators, v0, isaffected to no appreciable extent by a change inthe pressure or density of the medium. Thisconclusion is in accord with electromagneticdispersion theory.

The writer is indebted to Professor Franklin E.Poindexter for his helpful advice and his generousassistance in making observations.

ABSORPTION BAND UNDER