Excitation spectra of trans-free base isobacteriochlorin in an n-octane Shpol'skii matrix at liquid helium temperatures have been obtained. Upon rapid cooling, the guest isobacteriochlorin molecule inhabits two sites (A and B). In the 0-0 region of the lowest lying optical transition, each site displays a series of spectral peaks, which are investigated by photochemical hole burning. This radiant saturation technique confirms pairings of spectral peaks in site A and higher order connectivity of spectral lines and photophysically induced lines in site B, Application of an electric field to the system, while scanning the excitation spectrum, results in an irreversible reduction in intensity of the photophysically formed lines of site B. Evidence is also presented in which molecules inhabiting site B can interconvert to site A by optical means.