Context : The north-west photo-dissociation region ( PDR ) in the reflection nebula NGC 7023 displays a complex structure .
Filament-like condensations at the edge of the cloud can be traced via the emission of the main cooling lines , offering a great opportunity to study the link between the morphology and energetics of these regions .

Aims : We study the spatial variation of the far-infrared fine-structure lines of [ C II ] ( 158 \mu m ) , [ O I ] ( 63 and 145 \mu m ) .
These lines trace the local gas conditions across the PDR .
We also compare their emission to molecular tracers including rotational and rovibrational H _ { 2 } , and high-rotational lines of CO .

Methods : We use observations from the Herschel/PACS instrument to map the spatial distribution of these fine-structure lines .
The observed region covers a square area of about 110 ^ { \prime \prime } x110 ^ { \prime \prime } with an angular resolution that varies from 4 ^ { \prime \prime } Â to 11 ^ { \prime \prime } .
We compare this emission to ground-based and Spitzer observations of H _ { 2 } , Herschel/SPIRE observations of CO lines , and Spitzer/IRAC 3.6 \mu m images that trace the emission of polycyclic aromatic hydrocarbons .
We use a PDR code to model the [ O I ] 145 \mu m line , and infer the physical conditions in the region .

Results : The [ C II ] ( 158 \mu m ) and [ O I ] ( 63 and 145 \mu m ) lines arise from the warm cloud surface where the PDR is located and the gas is warm , cooling the region .
We find that although the relative contribution to the cooling budget over the observed region is dominated by [ O I ] 63 \mu m ( > 30 % ) , H _ { 2 } contributes significantly in the PDR ( \sim 35 % ) , as does [ C II ] 158 \mu m outside the PDR ( 30 % ) .
Other species contribute little to the cooling ( [ O I ] 145 \mu m 9 % , and CO 4 % ) .
Enhanced emission of these far-infrared atomic lines trace the presence of condensations , where high excitation CO rotational lines and dust emission in the submillimitre are also detected .
The [ O I ] maps resolve these condensations into two structures , and show that the peak of [ O I ] is slightly displaced from the molecular H _ { 2 } emission .
The size of these structures is around 8 ^ { \prime \prime } ( 0.015Â pc ) , and in surface cover about 9 % of the PDR emission .
We have tested whether the density profile and peak densities derived in previous studies to model the dust and molecular emission , can predict the [ O I ] 145 \mu m emission .
We find that the model with a peak density of 10 ^ { 6 } cm ^ { -3 } , and 2 \times 10 ^ { 4 - 5 } cm ^ { -3 } in the oxygen emitting region , predicts an [ O I ] 145 \mu m line which is just 30 % less than the observed emission .
Finally , we have not detected emission from [ N II ] 122 \mu m , suggesting that the cavity is mostly filled with non-ionised gas .

Conclusions :