  We report a study of high-resolution microwave spectroscopy of
nitrogen-vacancy centers in diamond crystals at and around zero magnetic field.
We observe characteristic splitting and transition imbalance of the hyperfine
transitions, which originate from level anti-crossings in the presence of a
transverse effective field. We use pulsed electron spin resonance spectroscopy
to measure the zero-field spectral features of single nitrogen-vacancy centers
for clearly resolving such level anti-crossings. To quantitatively analyze the
magnetic resonance behavior of the hyperfine spin transitions in the presence
of the effective field, we present a theoretical model, which describes the
transition strengths under the action of an arbitrarily polarized microwave
magnetic field. Our results are of importance for the optimization of the
experimental conditions for the polarization-selective microwave excitation of
spin-1 systems in zero or weak magnetic fields.
