Diffusion engineering has been proposed as an approach to increase the collected charge and energy resolution of a single-photon superconducting tunnel junction (STJ) spectrometer. We present new experimental results confirming this approach. When a photon of energy E is absorbed in a superconductor with energy gap Delta, it creates N initial quasiparticles, with N approximate to 0.6 (E/Delta). Their charge, upon tunneling, is equal to Q = peN, with p = 1 for single tunneling across the voltage biased STJ. The output charge can be amplified by backtunneling, with p > 1, if the quasiparticles are confined around the junction. This charge multiplication is proportional to the confinement time. Previous work used higher gap superconductors for confinement. In this work, the counterelectrode is terminated by a long, narrow wire made of the same material. We find p > 1 due to the slow out-diffusion of the quasiparticles down the wire. The wire dimensions and diffusion constant were chosen to engineer the backtunneling multiplication. For large backtunneling, the signal-to-noise of our spectrometer is increased.