Some empirical estimates of the H2 formation rate in photon-dominated regions E. Habart, F. Boulanger, L. Verstraete, C. M. Walmsley and G. Pineau des Forêts A&A, 414 2 (2004) 531-544 Published online: 19 January 2004 DOI: 10.1051/0004-6361:20031659