Desorption of molecules from interstellar grains and the heating of dust clouds

Temperature fluctuations induced in very small (r10^–3 ) interstellar grains by the absorption of photons from the ultra-violet radiation field or by energy released on molecule formation are shown to lead to significant gas heating due to thermal desorption of condensed atoms or molecules. For clouds with N(H)=1–10×10²⁰ cm^–2, heating rates due to this process are comparable to direct heating by cosmic rays or grain photoelectrons.