A new diode laser-based spectrometer operating at wavelengths close to 1 mu m has been constructed for transient absorption measurements in the gas phase. Using a dual beam detection system for noise cancellation, the minimum detectable absorption was found to be 1 x 10(-4) in 0.5 mu sec. The sensitivity of the instrument is illustrated by the observation of previously undetected spectra of singlet methylene (CH2) between 9780 and 10070 cm(-1). Rovibronic transitions involving two K-a = 1 levels, nominally associated with v(2)(bent) = 1 and 2, in the upper, <(b)over tilde B-1(1)> state have been assigned. These levels are the lowest so far observed for methylene in this electronic state and their positions provide new information on the shape of the bending potential for the molecule at energies approaching that at which the two lowest singlet states (<(a)over tilde (1)A(1)> and <(b)over tilde B-1(1)>) of the molecule become degenerate at the linear configuration. (C) 1998 Academic Press.
