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Published online by Cambridge University Press: 14 August 2015
A new method for estimating electron fractions in shielded molecular clouds is proposed on the basis of gas phase ion-molecule reactions which involves measuring the quantity . Applied to existing data, it yields upper limits to Xe in the range from 10−8 to 10−7 for a variety of clouds, warm as well as cool. An upper bound to the cosmic ray ionization rate is also obtained.