... Optical mm abs. mm abs. ... 2004) Molecular composition of diffuse clouds 1. ... 2002) XC -- H molecular XD -- H atomic Model H-atomic 0.3 0.2 0.1 0.0 103 10-4 10-5 10-6 10-7 10-8 10-9 10-1 0 TMC-1 H atomic H molec ular 104 n, cm 105 ­3 106 CO 10-6 10-7 103 10-7 10-8 10-9 Initial abundances Hydrogen is totally atomic; other elements are atoms H- molecular Hydrogen is totally molecular ; other elements are atoms TMC-1 Complex molecular composition, but hydrogen is still totally ...