  The Heterodyne Instrument for the Far Infrared (HIFI) onboard the Herschel
Space Observatory allows the first observations of light diatomic molecules at
high spectral resolution and in multiple transitions. Here, we report deep
integrations using HIFI in different lines of hydrides towards the high-mass
star forming region AFGL 2591. Detected are CH, CH+, NH, OH+, H2O+, while NH+
and SH+ have not been detected. All molecules except for CH and CH+ are seen in
absorption with low excitation temperatures and at velocities different from
the systemic velocity of the protostellar envelope. Surprisingly, the CH(JF,P =
3/2_2,- - 1/2_1,+) and CH+(J = 1 - 0, J = 2 - 1) lines are detected in emission
at the systemic velocity. We can assign the absorption features to a foreground
cloud and an outflow lobe, while the CH and CH+ emission stems from the
envelope. The observed abundance and excitation of CH and CH+ can be explained
in the scenario of FUV irradiated outflow walls, where a cavity etched out by
the outflow allows protostellar FUV photons to irradiate and heat the envelope
at larger distances driving the chemical reactions that produce these
molecules.
