(Upper traces) Absorption-display rapid-passage spectra. (Lower traces) digital derivatives. These enhance the tiny signal from adventitious Mn2+. Conditions: Microwave frequency, 35.025 – 35.041 GHz; microwave power, ~ 1 mW; modulation amplitude, 1.3G; sweep rate, 33G/sec; time constant, 128 ms; temperature, 2K.