Spectroscopy and dynamics of the Rydberg states of C2H2 and their relevance to astrophysical photochemistry

In order to understand the role of the Rydberg states in the photodestruction of acetylene in circumstellar envelopes, we have investigated the spectroscopy of some of the ungerade and gerade states by resonant multiphoton ionization in a magnetic bottle photoelectron spectrometer. The ungerade ns + nd Rydberg supercomplexes of C2H2 have been investigated by one-colour (3 + 1) multiphoton ionization, in the energy region 79 500-87 000 cm(-1). Our spectral data show a very different behaviour of the various components of these supercomplexes towards predissociation. The highly excited gerade nf Rydberg states have been observed for the first time, by two-colour (3 + 1') multiphoton ionization. A value for the adiabatic ionization potential, 91 956 cm(-1), has been obtained from the observed series limit and from earlier high-resolution infrared data on the ground state of the ion. This value is 4 cm(-1) higher than the one measured by ZEKE spectroscopy. The apparent broadening of the observed Rydberg features may originate both from Zeeman 'broadening' of the non-penetrating nf complexes and from predissociation. The np Rydberg states, for which observation is fully allowed in the same spectral region, do not appear in the ionization signal at least in the low-n range, most probably because of their very short lifetime. In the high-energy region, spectral congestion, probably involving p-f mixing, occurs.