J. phys. Chem. lB82, 86, 3585-3591 impurity or interstitial d-band or states
Band model of TO, and Nb206 prepared in this study. The recombination process and the vlslble photoredponse are illustrated. Figure 6.
(probably interstitial Ti ions) bringing about the visible photocurrent will simultaneously produce the recombination states. That is, a natural conclusion based on the reduction effects is that the impurity and interstitial Ti d bands or states bringing about the visible response also act as recombination centers in the elecwdes used in this study. The band model is illustrated i e Figure 6. However, we do not preclude the existence of impurity or defect levels near the 7r valence band; some of these levels may
3585
also possibly contribute to the visible response and the recombination process. The energy position of the impurity band or states agrees with that proposed by Mirlin et al.,29but not with those proposed by Ghosh et al.25and Gautron et al.21The disappearence of the visible photoresponse on strong reduction2lPBwould confirm the existence of the impurity or defect levels near the conduction band proposed by us and Mirlin et aLa These levels will be filled with ele?trons as the Fermi level rises because of the strong reduction, and, therefore, visible photoexcitation will not occur. Especially, Mirlin et al.29observed the disappearance of visible absorption on thermal reduction for Cr-doped single-crystal Ti02,although this disappearance was not observed in the present study, as Figure 4 shows.
Acknowledgment. We thank Professor H. Tamura, Dr. H. Yoneyama, and Dr. 0. Ikeda, at Osaka University, and Mr. Y, Okajima, of Hitachi Research Laboratory, for SIMS measurement. (29) Mirlin, D. N.; Reshina, I. I.; Sochana, L. S. Sou. Phys.-Solid State Engl. Transl. 1979,11, 1995.
Quenching of Singlet and Triplet Excited States of Aromatic Molecules by Europium Ions N. Sabbatlnl,’la M. T. Inddll,lb M. T. GandoH1,la and V. BaIzanlla~c IsHMO ChknlcO “0.Ciamician” dell’UnlversP, Bobgna, Italy: Centro di Studio sulk Fotochimlca e ReattivlrS degii Stati Eccitati dei CompaPtl W coordlnazkns del C M ,Utllv6fsltd di F m m , Italy; and IsHtuto di Fotochimlca e Radiazionl d’Aita Energia del CNR. Bologna, Italy ( R W : Febnvry 24, 1982: I n Flnai F m : M y 13, 1982)
The rate constants of the quenching processes of the lowest singlet excited state of naphthalene, pyrene, anthracene, 9,10-diphenylanthracene,tetracene, coronene, rubrene, and protoporphyrin IX dimethyl ester, and of the lowest triplet excited state of anthracene, tetracene, and coronene by Eu3+in acetonitrile solution have been measured. Singlet excited s t a t a are quenched with practically diffusion-controlledrates, the main quenching products being radical ions and triplets. Triplet excited states are quenched with very low rate constants (
