Abstract: A LIF technique that employs a varying view field has been developed to give quantitative measurement for the vibrational relaxation rates of I_2 B(v′=43) state in supersonic jets. The data for Ar, N_2 and CO are obtained for the first time. The results have shown that the measured vibrational relaxation rates in cold jets bear an inverse tempeature dependence. For He and H_2 carrier gases, the dominant relaxation process is the usual △v=-1 process. For Ar, N_2 and CO, however, the most probable relaxation pathway is no longer Δv=-1 process, which apparently violates the energy gap law. These observations are adequately explained by a metastable complex formation mechanism.