High-power EPR

The initial high-power controlled experiment (Fig. 2) was performed using a vacuum line setup previously reported (12, 24). X-band EPR measurements were performed on a Bruker ELEXSYS E500 CW EPR spectrometer using a Bruker ER4122 super-high-Q cavity with a liquid nitrogen flow insert (controlled by a Eurotherm 4131VT variable temperature controller, which maintained the sample at 296 K). A Bruker Teslameter (ER036M) and an EIP 548B frequency counter were used to provide calibration of the magnetic field and microwave frequencies, respectively. A spectrum was then taken at 2 mW (the optimal signal to noise for the CCR component) and 585 mW (the highest power output achievable with the power leveler off to saturate the CCR as much as possible). To obtain the high-pH spectrum, we initially exposed the sample under vacuum to an ammonia atmosphere via an attached vial of ammonia (28% concentration), leaving the system to come to equilibrium overnight. Afterward, the sample was evacuated for about 10 min to recreate a vacuum, and the spectrum was taken again at 585 mW.