We have used a laser heterodyne spectrometer to resolve the emission line profile of the 63 \micron ^ { 3 } P _ { 1 } - ^ { 3 } P _ { 2 } fine-structure transition of \ion O1 at two locations in M42 .
Comparison of the peak antenna temperature with that of the 158 \micron \ion C2 fine-structure line shows that the gas kinetic temperature in the photodissociation region near \theta ^ { 1 } C is 175 - 220 K , the density is greater than { 2 \times 10 ^ { 5 } } cm ^ { -3 } , and the hydrogen column density is about { 1.5 \times 10 ^ { 22 } } cm ^ { -2 } .
A somewhat lower temperature and column density are found in the IRc2 region , most likely reflecting the smaller UV flux .
The observed width of the \ion O1 line is 6.8 km s ^ { -1 } ( FWHM ) at \theta ^ { 1 } C , which is slightly broadened over the intrinsic linewidth by optical depth effects .
No significant other differences between the \ion O1 and \ion C2 line profiles are seen , which shows that the narrow emission from both neutral atomic oxygen and ionized carbon comes from the PDR .
The \ion O1 data do not rule out the possibility of weak broad-velocity emission from shock-excited gas at IRc2 , but the \ion C2 data show no such effect , as expected from non-ionizing shock models .