ME04		15min2:39

B. GILES, C. A. ARRINGTON, F. C. HAGEMEISTER AND T. S. ZWIER, Department of Chemistry, Purdue University, West Lafayette, IN 47907-1393.

The nominally ³ _u <- X¹ _g⁺ transition in room-temperature diacetylene has been observed by direct absorption for the first time using the cavity ring-down technique pioneered by O'Keefe and Deacon . Using mirrors with high reflectivity (99.85%) the wavelength range 396 nm - 342 nm was investigated. Two bands at 27090 cm^-1 and 27430 cm^-1 are observed which show a red shading consistent with a ^* <- transition. Band contours also suggest a linear to bent electronic excitation. The largest spectral feature has a measured molar absorption coefficient of 1.5x10^-4 Lmol^-1cm^-1. The ³ _u state is one of the states thought to be responsible for photochemical reactivity.