  A frequency-stabilized ultraviolet laser system, locked to the thallium
resonant transition of 377.5 nm, was demonstrated using a novel bichromatic
spectroscopy technique for tuning the zero-crossing laser-lock point. The
atomic thallium system is a promising candidate in atomic parity violation and
permanent electric dipole moment experiments, and its 377.5 nm 6P1/2->7S1/2
transition is important for thallium laser cooling and trapping experiment. The
pressure shift, owing to the high pressure buffer gas of the hollow-cathode
lamp, was observed using an atomic beam resonance as reference. Such a shift
was corrected by adjusting the peak ratio of the two Doppler-free saturation
profiles resulted from two pumping beams with a 130 MHz frequency difference.
The resulted frequency stability of the ultraviolet laser is ?0.5 MHz at 0.1
sec integration time. This scheme is compact and versatile for stabilizing UV
laser systems, which acquire a sub-MHz stability and frequency tunability.
