Context : The Helix Nebula ( NGC 7293 ) is the closest planetary nebulae .
Therefore , it is an ideal template for photochemical studies at small spatial scales in planetary nebulae .

Aims : We aim to study the spatial distribution of the atomic and the molecular gas , and the structure of the photodissociation region along the western rims of the Helix Nebula as seen in the submillimeter range with Herschel .

Methods : We use 5 SPIRE FTS pointing observations to make atomic and molecular spectral maps .
We analyze the molecular gas by modeling the CO rotational lines using a non-local thermodynamic equilibrium ( non-LTE ) radiative transfer model .

Results : For the first time , we have detected extended OH ^ { + } emission in a planetary nebula .
The spectra towards the Helix Nebula also show CO emission lines ( from J = 4 to 8 ) , [ N ii ] at 1461 GHz from ionized gas , and [ C i ] ( ^ { 3 } P _ { 2 } - ^ { 3 } P _ { 1 } ) , which together with the OH ^ { + } lines , trace extended CO photodissociation regions along the rims .
The estimated OH ^ { + } column density is \sim 10 ^ { 12 } -10 ^ { 13 } cm ^ { -2 } .
The CH ^ { + } ( 1-0 ) line was not detected at the sensitivity of our observations .
Non-LTE models of the CO excitation were used to constrain the average gas density ( n { ( H _ { 2 } ) } \sim ( 1 - 5 ) \times 10 ^ { 5 } cm ^ { -3 } ) and the gas temperature ( T _ { k } \sim 20 - 40 K ) .

Conclusions : The SPIRE spectral-maps suggest that CO arises from dense and shielded clumps in the western rims of the Helix Nebula whereas OH ^ { + } and [ C i ] lines trace the diffuse gas and the UV and X-ray illuminated clumps surface where molecules reform after CO photodissociation .
[ N ii ] traces a more diffuse ionized gas componnent in the interclump medium .