  The collisionally pumped, ground-state 1720 MHz maser line of OH is widely
recognized as a tracer for shocked regions and observed in star forming regions
and supernova remnants. Whereas some lines of excited states of OH have been
detected and studied in star forming regions, the subject of excited-state OH
in supernova remnants -- where high collision rates are to be expected -- is
only recently being addressed. Modeling of collisional excitation of OH
demonstrates that 1720, 4765 and 6049 MHz masers can occur under similar
conditions in regions of shocked gas. In particular, the 6049 and 4765 MHz
masers become more significant at increased OH column densities where the 1720
MHz masers begin to be quenched. In supernova remnants, the detection of
excited-state OH line maser emission could therefore serve as a probe of
regions of higher column densities. Using the Very Large Array, we searched for
excited-state OH in the 4.7, 7.8, 8.2 and 23.8 GHz lines in four well studied
supernova remnants with strong 1720 MHz maser emission (SgrAEast, W28, W44 and
IC443). No detections were made, at typical detection limits of around 10
mJy/beam. The search for the 6 GHz lines were done using Effelsberg since the
VLA receivers did not cover those frequencies, and are reported on in an
accompanying letter (Fish, Sjouwerman & Pihlstrom 2007). We also
cross-correlated the positions of known supernova remnants with the positions
of 1612 MHz maser emission obtained from blind surveys. No probable
associations were found, perhaps except in the SgrAEast region. The lack of
detections of excited-state OH indicates that the OH column densities suffice
for 1720 MHz inversion but not for inversion of excited-state transitions,
consistent with the expected results for C-type shocks.
