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We present a semi-analytical description of the relative HD/H2 abundance inthe diffuse interstellar medium. We found three asymptotics of the relativeHD/H$_2$ abundance for different parts of the medium and their dependence onthe physical parameters, namely, number density, intensity of the ultravioletfield, cosmic-ray ionization rate and metallicity. Our calculations are in agood agreement with the full network calculations using Meudon PDR code. Wefound that in the case of low metallicity and/or higher cosmic ray ionizationrate, HD formation rate is significantly enhanced, HD/H2 ratio increases, andthe DI/HD transition occurs at a lower penetration depth of UV radiation thanthe HI/H$_2$ transition. This can explain the observed difference in theHD/H$_2$ abundance between the local and high-redshift measurements.

HD/H$_2$ ratio in the diffuse interstellar medium