Raman Spectroscopic Investigation of Matrix Isolated Rubidium and Cesium Molecules: Rb 2 , Rb 3 , Cs 2 , and Cs 3 †,1 Andreas Kornath* and Anja Zoermer Anorganische Chemie, Fachbereich Chemie der Universita ¨t Dortmund, 44221 Dortmund, Germany Ralf Ludwig Physikalische Chemie, Fachbereich Chemie der Universita ¨t Dortmund, 44221 Dortmund, Germany ReceiVed May 10, 1999 The rubidium molecules Rb 2 and Rb 3 and the cesium molecules Cs 2 and Cs 3 were isolated in argon matrixes and characterized for the first time by Raman spectroscopy. The fundamental frequencies of the dimers were observed at 59.1 cm -1 for the rubidium dimer and 45.8 cm -1 for the cesium dimer. The Raman lines of the rubidium trimer appeared at 38.3 and 53.9 cm -1 , and the lines of the cesium trimer, at 24.4 and 39.5 cm -1 . The vibrational frequencies were compared with gas-phase frequencies and with density functional theory (DFT) calculations. Furthermore, the lowest vibrational levels of the 1 Π u state of Cs 2 isolated in solid argon were observed by Raman matrix spectroscopy. Introduction Dimers of alkali metals, especially cesium, have been a topic of investigation since the early decades of this century. 2 The interest in alkali metal dimers first arose because of the special properties of these small molecules. While the electronic struc- ture of these molecules is rather simple, the density of excited states leads to rich absorption and emission spectra which have been extensively studied. The amount of theoretical work done on alkali metal dimers is also immense because several computational methods were developed and refined during the past decade which permit accurate calculations on the electronic and energetic properties. While cesium dimers were an important topic for the first researchers, partly because cesium is used in atomic clocks, nowadays the interest in alkali metal dimers is renewed because of their potential use in quasitunable lasers, the first prototypes of which have already been built. 3 In comparison to the amount of research done on alkali metal dimers, the study of alkali metal trimers, especially trimers of cesium and rubidium, is very small. 4-18 This may originate from the fact that electron excitation and fluorescence spectra of these clusters are very complex and difficult to interpret. Another problem is the low concentration of larger clusters in the gas phase compared with the concentrations of dimers and single atoms. An attractive alternative is therefore a spectroscopic technique that allows the accumulation of the desired species, yielding less complex spectra. A promising solution for these problems is the isolation of the molecules in a host material of solid noble gas and a subsequent study of the matrix by vibrational spectroscopy. The study of small alkali metal clusters trapped in a solid host was not extensively attempted since the most suitable method for the investigation of matrix-isolated homonuclear molecules is Raman spectroscopy. This technique suffered from the inherent difficulties of Raman matrix mea- surements. Recently, these problems were largely solved by us. 19 In this paper, we report our investigation on cesium and rubidium trapped in solid argon. Experimental Section Matrix Isolation. The cryostat and the laser irradiation geometry are described elsewhere. 19 The spectra were recorded with an Instru- ments SA T64000 Raman spectrometer equipped with a Spectra Physics Ar + laser. Rubidium (purity 99.6%) and cesium (purity 99.95%) (both from Aldrich) were used without further purification. Argon was dried by passing it through a column filled with P4O10. Vapor phases of rubidium and cesium were prepared using a Knudsen cell heated externally with a heater jacket. The samples were heated to ca. 85 °C (rubidium) and ca. 55 °C (cesium). The metal vapors were co-condensed with argon on the copper cold tip of a cryostat. The average thickness of the samples was 100 μm. † Dedicated to Prof. R. Schmutzler on the occasion of his 65th birthday. (1) Raman Matrix Isolation Spectroscopy. 9. Part 8: Kornath, A.; Zoermer, A.; Ko ¨per, I. Spectrochim. Acta, Part A, in press. (2) Walter, J. M.; Barrat, S. Proc. R. Soc. London, Ser A. 1928, 119, 257. (3) Moeller, S.; Gu ¨rtler, P. J. Chem. Phys. 1997, 106, 3920. (4) Kusch, P.; Hessel, M. M. J. Mol. Spectrosc. 1968, 25, 205. (5) Kusch, P.; Hessel, M. M. J. Mol. Spectrosc. 1969, 32, 181. (6) McClintock, M.; Balling, L. C. J. Quant. Spectrosc. Radiat. Transfer 1969, 9, 1209. (7) Gupta, R.; Happer, W.; Wagner, J.; Wennmyr, E. J. Chem. Phys. 1978, 68, 799. (8) Kobyliansky, A. I.; Kulikov, A. N.; Gurvich, L. V. Chem. Phys. Lett. 1979, 62, 198. (9) Kraulinya, E. K.; Papernov, S. M.; Janson, M. L. Chem. Phys. Lett. 1979, 63, 531. (10) Kato, H.; Yoshihara, K. J. Chem. Phys. 1979, 71, 1585. (11) Ho ¨ning, G.; Czajkowski, M.; Stock, M.; Demtro ¨der, W. J. Chem. Phys. 1979, 71, 2138. (12) Zouboulis, E.; Bhaskar, N. D.; Vasilakis, A.; Happer, W. J. Chem. Phys. 1980, 72, 2356. (13) Raab, M.; Ho ¨ning, G.; Demtro ¨der, W. J. Chem. Phys. 1982, 76, 4370. (14) Amiot, C.; Crepin, C.; Verges, J. Chem. Phys. Lett. 1984, 106, 162. (15) Amiot, C.; Demtro ¨der, W.; Vidal, C. R. J. Chem. Phys. 1988, 88, 5265. (16) Amiot, C. J. Chem. Phys. 1988, 89, 3993. (17) Rodiguez, G.; John, P. C.; Eden. J. G. J. Chem. Phys. 1995, 103, 10473. (18) Kasahara, S.; Hasui, Y.; Otsuka, K.; Baba, M.; Demtro ¨der, W.; Kato, H. J. Chem. Phys. 1997, 106, 4869. (19) Kornath, A. J. Raman Spectrosc. 1997, 28, 9. 4696 Inorg. Chem. 1999, 38, 4696-4699 10.1021/ic990506m CCC: $18.00 © 1999 American Chemical Society Published on Web 09/23/1999