Title: Impact of Charge Conversion on NV-Center Relaxometry

Abstract: Relaxometry schemes employing nitrogen-vacancy (NV) centers in diamonds are essential in biology and physics to detect a reduction of the color centers' characteristic spin relaxation ($T_1$) time caused by, e.g., paramagnetic molecules in proximity. However, while only the negatively-charged NV center is to be probed in these pulsed-laser measurements, an inevitable consequence of the laser excitation is the conversion to the neutrally-charged NV state, interfering with the result for the negatively-charged NV centers' $T_1$ time or even dominating the response signal. In this work, we perform relaxometry measurements on an NV ensemble in nanodiamond combining a $520\,$nm excitation laser and microwave excitation while simultaneously recording the fluorescence signals of both charge states via independent beam paths. Correlating the fluorescence intensity ratios to the fluorescence spectra at each laser power, we monitor the ratios of both charge states during the $T_1$-time measurement and systematically disclose the excitation-power-dependent charge conversion. Even at laser intensities below saturation, we observe charge conversion, while at higher intensities, charge conversion outweighs spin relaxation. These results underline the necessity of low excitation power and fluorescence normalization before the relaxation time to accurately determine the $T_1$ time and characterize paramagnetic species close to the sensing diamond.