The internal energy distribution of product OH(A) was measured for the photo-initiated intra-cluster reaction of HBr-N2O prepared using either Ar or He seeding gas. We find that rotation of OH(A) produced from HBr-N2O seeded in Ar is more excited than that from HBr-N2O seeded in He. This difference could be ascribed to the difference in relative abundance of HBr-N2O geometric isomers in the cluster beam. The present result is consistent with the previous experimental finding in the focusing experiment of HBr-N2O Using the electrostatic hexapole field, namely the preferential formation of the linear isomer under Ar seeding condition. (C) 2003 Elsevier Science B.V. All rights reserved.