Photochemistry of Metal Solutions. II. Flash Photolysis of Alkali Metals in Ethylamine

Flash-photolysis studies have been made on Li, K, Rb, and Cs solutions in ethylamine, over a range of temperature and concentration. Such solutions typically show three bands near 650 (V), 850 (R), and 1300 (IR) mμ. These are assigned, respectively, to a cation-centered monomer, a dimer, and a solvated electron. In K, Rb, and Cs solutions, flashing at wavelengths shorter than 500 mμ causes initially a bleaching of V and enhancement of IR. This is followed by a growing in of R, but at a rate quite different from that of IR decay. Another precursor X of the R transient is therefore postulated. The R transient finally reverts back to V. The ratio of the yields of the IR and R transients is constant, independent of excitation wavelength, metal concentration and fractional conversion. From this it is inferred that X and IR have a common origin. Direct reversible photolysis of the original R-band also occurs, leading to an infrared transient which decays back to R in a second-order process. The rate constants all decrease, in passing from K to Cs. In Li solutions, photolysis of V results in enhanced IR absorption followed by direct back reaction to V without the intermediate R-transient stage. The nature of the various species and transients is discussed. It is suggested that X may be a long-lived excited state of the monomer V, the metastability being due to Franck—Condon effects. Alternatively, X may be an unstable monomer with an ion-pair structure, similar to that postulated for the monomer in liquid ammonia.