Photoisomerization: a new way of thinking about a longstanding problem concerning UV photochemical decomposition of CH$_{\mathrm{2}}$I$_{\mathrm{2}}$ in the gas phase*

Diiodomethane (CH$_{\mathrm{2}}$I$_{\mathrm{2}})$ is a naturally occurring polyhalogenated alkane that plays an important role in atmospheric and environmental chemistry, particularly in the ozone layer decomposition. For a long time it has been assumed that UV excitation of CH$_{\mathrm{2}}$I$_{\mathrm{2}}$ leads to direct photodissociation, i.e. breaking of a C-I bond and separation of polyatomic radical and halogen atom fragments without formation of any other primary product species. In our ultrafast transient absorption work powered by ultra-short (40 fs) laser pulses we show that the UV photochemistry of CH$_{\mathrm{2}}$I$_{\mathrm{2}}$ is more complicate that it was previously thought. The S$_{\mathrm{1}}$ excitation (330, 340 nm light) of CH$_{\mathrm{2}}$I$_{\mathrm{2}}$ in the gas phase leads to ultrafast isomerization of this molecule yielding the isomeric species (CH$_{\mathrm{2}}$I-I), which has been long invoked in solution phase studies as a main, solvent-cage-induced photoproduct. The presence of this isomer is manifested by a broad transient absorption band (550 nm) emerging \textasciitilde 40 fs after excitation, and decaying with a \textasciitilde 70 fs lifetime. In the gas phase, the formation of the isomeric species takes place via the direct isomerization mechanism, i.e. without need of a solvent cage, in a quantum yield \textgreater 20 {\%}. Also, the radical dissociation channel is observed in the \textless 400 nm region.