We have performed ab initio calculations to examine the potential energy along the normal modes
of ground-state HCHO and along the reaction coordinates for loss of H2 and atomic hydrogen,
respectively. This exploration showed that there are no specific features that will lead to reaction
on the excited-state surfaces for excitations that are relevant to the troposphere and stratosphere.
The calculations did however lead to the localization of a conical intersection point through
which a specific loss of H2 could take place. However, the conical intersection lies at 5.4 eV
relative to the ground state molecule at equilibrium and is thus inaccessible via single photon
excitation at tropospheric and stratospheric wavelengths. In addition to the ab initio investigation
we have carried out a femtosecond pump–probe experiment using a 266/400 nm excitation. The
results show that the timescale for the internal conversion from the initially prepared high-lying
Rydberg states is on the order of a picosecond. This process populates the n - p* first excited
singlet state which then survives for a substantially longer time before it is depopulated to form
hot ground state or triplet-excited molecules that can then decompose.