Optical transitions at the single substitutional nitrogen centre (P1) in sy
nthetic diamond are discussed. It is shown that the 4.6 eV absorption band
is not due to P1, but due to some other centre, tentatively suggested as a
nitrogen-boron complex. The 4.5 eV photocurrent threshold is assigned to th
e transition from the valence band to the ground state of the P1 centre.