We have obtained moderate-resolution ( R \sim 3000 ) spectra of the Orion bar and Orion S regions at J ( 1.25 \micron ) , H ( 1.64 \micron ) , and K ( 2.2 \micron ) .
Towards the bar , the observations reveal a large number of H _ { 2 } emission lines that , when compared to model predictions of Draine & Bertoldi ( 1996 ) , are indicative of a high-density photodissociation region ( PDR ) ( n _ { H } = 10 ^ { 6 } cm ^ { -3 } , \chi = 10 ^ { 5 } , T _ { 0 } = 1000 K ) rather than shocked material .
Behind the bar into the molecular cloud , the H _ { 2 } spectrum again matches well with that predicted for a dense PDR ( n _ { H } = 10 ^ { 6 } cm ^ { -3 } ) but with a lower temperature ( T _ { 0 } = 500 K ) and UV field strength ( \chi = 10 ^ { 4 } ) .
The H _ { 2 } spectrum and stratification of near-IR emission lines ( O I , H I , [ Fe II ] , [ Fe III ] , H _ { 2 } ) near Orion S imply the presence of a dense PDR with an inclined geometry in this region as well ( n _ { H } = 10 ^ { 6 } cm ^ { -3 } , \chi = 10 ^ { 5 } , T _ { 0 } = 1500 K ) .
The extinction measurements towards the bar ( A _ { K } \sim 2.6 ) and Orion S ( A _ { K } \sim 2.1 ) H _ { 2 } emission regions are much larger than expected from either face-on ( A _ { K } \sim 0.1 ) or edge-on ( A _ { K } \sim 1 ) homogeneous PDRs , indicating that clumps may significantly affect the structure of the PDRs .

In addition , we have observed the strongest \sim 30 near-IR He I emission lines , many of which have not been detected previously .
There is good agreement between most observed and theoretical He I line ratios , while a few transitions with upper levels of n ^ { 3 } P ( particularly 4 ^ { 3 } P - 3 ^ { 3 } S 1.2531 \micron ) are enhanced over strengths expected from collisional excitation .
This effect is possibly due to opacity in the UV series n ^ { 3 } P - 2 ^ { 3 } S .
We also detect several near-IR [ Fe II ] and [ Fe III ] transitions with line ratios indicative of low densities ( n _ { e } \sim 10 ^ { 3 } -10 ^ { 4 } cm ^ { -3 } ) , whereas recent observations of optical [ Fe II ] emission imply the presence of high-density gas ( n _ { e } \sim 10 ^ { 6 } cm ^ { -3 } ) .
These results are consistent with a model in which high-density , partially-ionized gas is the source of the iron transitions observed in the optical while low-density , fully-ionized material is responsible for the near-IR emission lines .