Ultrafast photofragment ion spectroscopy of the Wolff rearrangement in 5-diazo Meldrum's acid.

We investigate the gas-phase photochemistry of 5-diazo Meldrum's acid (DMA), a photoactive compound used in lithography, by femtosecond photofragment ion spectroscopy. Transient-absorption studies in solution had revealed an ultrafast intramolecular Wolff rearrangement to a ketene after UV excitation, followed by reactions which also involve the solvent. Due to the absence of solvent molecules in this gas-phase study, we are able to focus purely on the photochemistry of the Wolff rearrangement and subsequent reaction steps. The observation of the time-resolved photofragment ion signals allows us to discriminate the dynamics of ketene and carbene products. By identification of the different possible molecular origins for a certain fragment ion signal, the time scale of the Wolff rearrangement and the lifetime of the ketene product are inferred. We further identified experimental signatures of a second Wolff rearrangement emanating from the carbene product, as had been conjectured indirectly for this molecule from pyrolysis studies.