  We use spectral unmixing to determine the number of transient photoproducts
and to track their evolution following the photo- excitation of
1,3-cyclohexadiene (CHD) to form 1,3,5-hexatriene (HT) in the gas phase. The
ring opening is initiated with a 266 nm ultraviolet laser pulse and probed via
fragmentation with a delayed intense infrared 800 nm laser pulse. The ion
time-of-flight (TOF) spectra are analyzed with a simplex-based spectral
unmixing technique. We find that at least three independent spectra are needed
to model the transient TOF spectra. Guided by mathematical and physical
constraints, we decompose the transient TOF spectra into three spectra
associated with the presence of CHD, CHD+, and HT, and show how these three
products appear at different times during the ring opening.
