Electronic absorption spectra of the protonated polyacetylenes HC2nH2+ (n=3,4) in the gas phase.

A new approach has been developed for the purpose of measuring the electronic transitions to bound exited states for cations that have been collisionally relaxed to low vibrational and rotational temperatures. This has been used to obtain the first gas phase electronic spectra of the protonated polyacetylenes using a two-color ion-photodissociation approach. Specifically, the origin bands in the B (1)A(1)<-- X(1)A(1) transitions of HC(6)H(2) (+) and HC(8)H(2) (+) (C(2v) geometry) were observed at 26,403.3 and 21,399.8 cm(-1). Data on such cooled systems allow a direct comparison between laboratory and astrophysical measurements.