Y. HAAS,G. STEIN,AND M. TOMKIEWICZ
2558
underlying rela'tionships between the various solution theory parameters used in this treatment. The success of this work is that the choice of segments based on equal segment molar volume considerations has at least been consistent. The possibility of being able t o predict very accurately the activity coefficient of a particular solute in a multicomponent stationary phase is now very real. Experimental work on this problem is in progress.
Finally, the success of the quasilattice theory equations does not necessarily vindicate the model but rather serves to pinpoint the type of factors that should be considered when a more sophisticated, possibly more realistic, solution model is proposed.
Acknowledgment. B. W. G. acknowledges gratefully the award of a postdoctoral fellowship from the University of California at Los Angeles.
Fluorescence and Photochemistry of the Charge-Transfer Band in Aqueous Europium(111) Solutions by Yehuda Haas, Gabriel Stein, and Micha Tomkiewicz Department of Physical Chemistry, T h e Hebrew Unisersity, Jerusalem, Israel
(Received March IO, 1970)
Aqueous solutions of Eua+ show fluorescence upon excitation a t the charge-transfer band. This emission
appears to consist of two distinct bands. Evidence is given to support the following assignments: one band is due to charge-transfer emission, the other to emission from an excited state of Eu2+. These assignments, as well as the formation of EuZ4in the system, are further supported by photochemical experiments.
Introduction The broad absorption band found in aqueous solutions of Eu3+salts has been assigned by Jgrgensen' to a charge-transfer transition. This assignment has been substantiated by J@rgensen'and by Barnes and Day2 by observing the influence of different ligands on the wavelength of maximum absorption. We have recently reported3 the observation of new fluorescence bands in aqueous and acetonitrile solutions appearing on excitation in this absorption band. I n this paper we report further details and give possible assignment to the band appearing in aqueous solutions. We also report some experiments on photochemical processes which occur on light absorption in the C T band.
Experimental Section Solutions of Eu(C104)X were prepared by dissolving Euz0399.97% (Fluka) in perchloric acid (Analar grade). Eu(C104)2solutions were prepared from EuC03 (kindly supplied to us by Dr. 3Iayer of the Department of Inorganic Chemistry) by dissolving it in oxygen free perchloric acid. Absorption spectra were obtained with a Cary Model 14 recording spectrophotometer and fluorescence spectra with a spectrofluorimeter described by F e i t e l ~ o n . ~A xenon arc served as a light source, thus limiting the excitation wavelength to 2 250 mp. Flash The Journal of Physical Chemistry, Vol. 74?No. 12, 1070
technique wa8 used to follow the decay of Eu2+. The flash-photolysis setup was described by Ottolenghi and RabanL6 Photochemical Experiments. These were performed with a special apparatus, designed to measure products gas chromatographically. A quartz vessel was connected to the gas chromatograph; the solution in it was flushed with argon and then illuminated with the appropriate lamp. After illumination, argon was again passed through the solution and into the gas chromatograph. We used a Varian Aerograph Model 90-P with active carbon column, enabling separation of hydrogen from air. Calibration was performed using aqueous solutions saturated with purified hydrogen (Matheson). Actinometry was done with uranyl oxalate-oxalic acid solution.
Results Absorption spectra in the region 250-330 mp were found to follow Beer's law when the concentration was JZrgensen, Mol. Phys., 5, 271 (1963). J. C. Barnes and P. Day, J . Chem. Soc., 3886 (1964). Y. Haas and G. Stein, Chem. Phys. Lett., 3, 313 (1969). J. Feitelson, J . Phya. Chem., 68, 391 (1964). M . Ottolenghi and J. Rabani, ibid., 72, 593 (1968).
(1) C. K.
(2) (3) (4) (5)
CHARGE-TRANSFER BANDIN AQUEOUSEUROPIUM(III) SOLUTIONS
4
"-
,.------.\
.,......,.,,A. .....
2
*%. '1"~.11.
-
0
EXCITATION
...,.,.. % --.
4 EXCITATION
2 n
trLl EXCITATION
270 rn)]
EXCITATION
260
'*.I..
20
320
my
480
360
400
440
1 mJJ
Figure 1. CT emission spectra of aqueous europium perchlorate solutions upon excitation a t various wavelengths: --, total luminescence normalized to same arbitrary scale; , , . , , ,, band I, excited at 260 mp, normalized as above; - - - - - -, band 11, obtained by subtracting band I from total luminescence.
2 0 . 1 M . There is some increase of the molar absorption with increasing pH; however, in the p H range 0-2 no effect on fluorescence was observed. At lower concentrations (
