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. Here, we treat collision-mediated FST as a fluorescence readout channel under OPM operating conditions. 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 ${\mathrm{\Omega }}_m$, whereas the D2 signal contains a pronounced $2{\mathrm{\Omega }}_m$ component whose amplitude scales quadratically with the modulation-field amplitude. Unpolarized D2 fluorescence from the $P_{3/2}$ level carries a rank-2 alignment term that D1 fluorescence from the $P_{1/2}$ level lacks. This selection rule, the $2{\mathrm{\Omega }}_m$ dominance, and the quadratic scaling together identify the D2 channel as a rank-2-weighted readout opened by FST. From the temperature dependence of the $\mathrm{D}2/\mathrm{D}1$ fluorescence ratio, we obtain an effective Rb-He FST cross section ${\sigma }_{\mathrm{eff}}=(2.7\pm 0.1)\times {10}^{-17}{\mathrm{cm}}^2$ under an effective symmetric-mixing approximation, consistent with reported Rb-He FST values. The same collision conditions occur in spin-exchange-relaxation-free (SERF) magnetometers, NMR gyroscopes, and spin-exchange optical-pumping cells, where FST affects fluorescence readout and optical-pumping models.

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• Fine & hyperfine structure
• Magnetometry
• Optical pumping
• Quantum sensing
• Alkali metals
• Atomic gases
• Fluorescence spectroscopy
• Precision measurements