The wavelength dependent abstraction energy profiles for CH_3 CH_2 CH_3/CH_3 CH_2 CH_2 CH_3+Br are studied using a gas expansion containing Br_2 and either propane or butane abstraction targets. Laser-induced photolysis of cold Br_2 in the visible region produces a narrow, tunable distribution of Br atoms whose collisional kinetic energies can be scanned up to and beyond the thermodynamic threshold of the reaction to yield HBr or DBr as photolytic reaction products. Femtosecond laser ionization allows the reactive products to be efficiently monitored over all product quantum states. Both the observed HBr and DBr reaction thresholds are consistent with thermodynamic and calculated values with no apparent barrier to reaction. Studies addressing the site-specific reaction behavior for selectively deuterated propane and butane molecules is presented and discussed. Future applications of the approach are discussed such as state-dependent detection methods, and ultra-cold nozzle expansions.