J. Phys. Chem. 1988, 92, 1803-1807
SO stretching vibration in the ground state is 1013 f 5 cm-I. This is somewhat anomalous since the actual frequency reduction relative to H 0 2 is less than that predicted by the increase in the reduced mass. The X-X bond lengths of the mixed species (RW = 1.481087 8, for SO, 1.54 8, for HSO) are also intermediate between those of the 0-0 and S-S compounds. The splitting between the triplet ground state and the lowest excited singlet state of O2is about 1.0 eV; this splitting is less well known for S2,but is somewhere around 5000 cm-’. For the mixed compound SO the alA X 3 B splitting is 5890 f 20 cm-’. The excited 2A’ electron states have been observed for H 0 2 , HSO, and HS,; it is interesting that this splitting is 14 367 cm-I for HSO, more than 0.5 eV higher than H 0 2 and HS,. We were unable to observe the 2A’ excited state of CH3S2,presumably because its excitation energy puts it beyond the reach of our laser photons. However, C H 3 0 2has a low-lying 2A’ excited state which is less than 1 eV above the ground state;’O if the electron affinity of
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(69) Schurath, U.; Weber, M.; Becker, K. H. J. Chem. Phys. 1977, 67, 110. (70) Hunziker, H. E.; Wendt, H. R. J . Chem. Phys. 1976, 64, 3488.
1803
CH302 is roughly equal to that of H02 (as is the case for CH3S2 and HS2), then it might be possible to detach CH302- into the 2Af excited state of the radical.
Acknowledgment. We thank the Department of Energy for support under Contract DE-AC02-80ER10722. Conversations with Pave1 Rosmus were helpful in identifying CHIS-. RM~tryNO.Sz-, 12185-15-8; HST, 26693-74-3; DSZ-, 113109-54-9; CH&, 97141-28-1; CD,ST, 113109-55-0; CHSCHZS-, 20733-13-5; CH3SCH g" also suggests that it may result from a crystallographic distortion at the Eu2+sites. Further evidence for this interpretation comes from the observation of a simple combination of overlapping Eu2+ hyperfine lines at low temperatures in EuowzCal998(NH3)6, since the body-centered-cubic structure of Ca(NH3)6is maintained at low temperatures? CEPR signals have been observed at much higher spectrometer sensitivities and are in good agreement with previous work on Sr(NH3)6.z1The EPR parameters summarized in Table I1 are in good agreement with those of EuooozCal998(NH3)6, with the exception of the large g shift for the CEPR signal (1 .O X The dramatic increase in g shift relative to that in Ca(NH3)6indicates that Sr is making the dominant contribution to Age, as expected from its large spin-orbit coupling constant. The positive value of Age is noteworthy and has also been observed in Csz73z8and attributed to core polarization, which takes into account the distortion of the ion cores by the valence electrons. The last compound in the alkaline-earth series, Euo 0 ~ 2 Ba, 998(NH3)6,was unstable and decomposed rapidly at ambient (27) Anderson, P. W.; Weiss, R. R. Rev. Mod. Phys. 1953, 25, 269. (28) Walsh, W. M., Jr.; Rupp, L. W., Jr.; Schmidt, P. H. Phys. Rev. Lett. 1966, 16, 181.
J. Phys. Chem. 1988, 92, 1807-1813
1807
to those for Euo,oo2Cao 998(NH3)6. In particular, the simple envelope of I5lEu2+hyperfine components observed at the lowest temperature is consistent with the bcc structure of Yb(NH3)6and the absence of any structural transitions at low temperaturesS6 The Eu2+ EPR parameters for EuooozYbl998(NH3)6 are summarized in Table I1 and are very close to those for the other dilute europous compounds investigated in this work and also to the parameters for E u ~ , ~ ~ Y ~ ~studied , ~ ~ previously. ( N H ~ ) The ~ Eu2+ parameters in Table I1 are so close to one another that any detailed discussion of their slight differences is unwarranted.
Conclusions This study has further elucidated the nature of dilute M-NH3 solutions and metallic metal-hexaammine compounds by using low concentrations of lS3Eu2+as the primary paramagnetic probe. In dilute solutions the solvated electron experiences a rather strong magnetic interaction with the Eu2+ moments, whereas in the hexaammine compounds both the Eu2+-conductionelectron and Eu2+-Eu2+ interactions are very weak. Also, the low-temperature EPR spectrum of Eu2+in S T ( N H ~is) suggestive ~ of a structural phase transition. In an analogous fashion, the EPR of Eu2+ as a paramagnetic dopant could be used to study other phenomena in M-NH3 systems, such as the nonmetal-metal transition. Acknowledgment. High-purity metals were kindly supplied by Dr. D. T. Peterson of Ames Laboratory at Iowa State University. The EPR experiments were performed in the Magnetism and Magnetic resonance Facility associated with the Center for Solid temperature, so that it was impossible to obtain reproducible State Science and the Departments of Chemistry and Physics at results. This research was supported by N S F Finally, a sequence of EPR spectra for E u ~ , ~ ~ Y ~ ~ , ~ Arizona ~ ~ ( State N H University. ~ ) ~ Grant DMR-8215315. between 4 and 60 K are shown in Figure 6 . Only the Is1Euz+ raonance is observed, since it has not been possible to detect any Registry No. 1 5 3 E 13982-02-0; ~, NH,, 7664-41-7; Ca(NH,):+, CEPR signals in Yb(NH3)6,even at the highest spectrometer 60086-59-1;Sr(NH,)$+, 20955-09-3;Yb(NH,),'+, 38640-74-3;Eu2+, sensitivities. The 151Eu2+EPR spectra of this alloy are very similar 16910-54-6. Figure 6. EPR spectra of 151E~0.002Yb0.998(NH3)6 at 4 (a), 11 (b), 32 40 (d), and 60 K (e).
(4,
Fluorescence from Diphenylpolyene Vapors Takao Itoh and Bryan E. Kohler* Department of Chemistry, University of California, Riverside, California 92521 (Received: September 23, 1987)
The dependence of fluorescence and fluorescence excitation spectra on buffer gas pressure (perfluorohexane 0-250 Torr) and excited-state decay times has been measured for diphenylbutadiene,diphenylhexatriene,and diphenyloctatetraenevapors. In the case of diphenylbutadiene vapor, fluorescence from l'B, (S,) was observed in addition to the fluorescence from 21Ag (SI),while for diphenylbutadiene seeded in a free jet expansion only the 2IA, fluorescence is observed. The occurrence of the I'B, fluorescence at high total pressure and high vibrational temperature comes from the collisional transfer of population from the 2'Ag state to the I1B, state, while at low pressure and low vibrational temperature there is no mechanism for l'B, repopulation and the fluorescence comes from the primarily 2IA, state. At high buffer gas pressure the I'B, fluorescence yield does not depend significantly on excitation energy. With diphenylhexatriene and diphenyloctatetraene vapors, which have a larger 1'B,-2'Ag separation, only the 2IAg fluorescence was seen. All three molecules showed an increase in fluorescence yield as buffer gas pressure was increased. At the limit of zero buffer gas pressure the fluorescence yield for I'B, excitation decreased strongly with increasing excitation energy for diphenylbutadiene but only weakly for diphenylhexatriene and diphenyloctatraene.
Introduction Diphenylpolyenes have been the subject of a number of spectroscopic investigations,'-I0 not only because these molecules are Hudson, B. S.; Kohler, B. E. Chem. Phys. Lett. 1972, 14, 299. Hudson, B. S.; Kohler, B. E. J . Chem. Phys. 1973, 59, 4984. Hudson, B. S.; Kohler, B. E. Annu. Rev. Phys. Chem. 1974, 25, 437. Heimbrook, L. A.; Kohler, B. E.; Spiglanin, T. A. Proc. Nut/. Acad. Sci: U. S. A . 1983, 80, 4580. (5) Shepanski, J. F.; Keelan, B. W.; Zewail, A. H. Chem. Phys. Letr. 1983, 103. 9. (6) Horwitz, J. S . ; Kohler, B. E.; Spiglanin, T. A . J . Chem. Phys. 1985, 89. 1572.
0022-3654/88/2092-1807$01.50/0
strongly fluorescent and commercially available but also because such studies advance our understanding of polyene electronic structure and the connection between that structure and fundamental photophysical processes. It is well established that for diphenylbutadiene and diphenylhexatriene in the gas phase the one-photon forbidden 2IA, state is S I ,the lowest energy excited singlet state, and that the ( 7 ) Kohler, B. E.; Spiglanin, T.A. J . Chem. Phys. 1984, 80, 5465. (8) Amirav, A.; Sonnenschein, M.; Jortner, J. Chem. Phys. 1986,102, 305. (9) Horwitz, J. S.; Kohler, B. E.; Spiglanin, T. A . J . Phys. (Les Ulis, Fr.) 1985, 46, 10, C7-381. (IO) Itoh, T.; Kohler, B. E. J . Phys. Chem. 1987, 91, 1760.
0 1988 American Chemical Society
