Signal readout in optically pumped magnetometers (OPMs) is usually described in terms of ground-state spin polarization and its Larmor response, while excited-state fine-structure transfer (FST) is absorbed into effective relaxation terms. Using steady-state fluorescence spectroscopy, low-frequency magnetic-field modulation, and temperature-dependent Rb–He measurements, we observe collision-generated D2 fluorescence under single-wavelength D1 pumping without direct D2 excitation. The D1 signal is dominated by the modulation frequency ${\Omega }_m$, whereas the D2 signal contains a pronounced $2{\Omega }_m$ component whose amplitude scales quadratically with the modulation-field amplitude. From the temperature dependence of the D2/D1 fluorescence ratio, we obtain an effective Rb–He FST cross section ${\sigma }_{eff}=(2.7\pm 0.1)\times {10}^{-17}c{m}^2$ under an effective symmetric-mixing approximation.