We have conducted a detailed study on the pair of tunable ultraviolet emission lines at 291.5 nm and 311.4 nm in sodium vapor. The excitation source is a pulsed dye laser beam tuned to sodium 3s - 5s two-photon resonance. Numerical analysis indicates that the phase match condition and nonlinear susceptibility chi(3) prefer the 291.5 nm emission rather than the 311.4 nm emission. Experimental results agree with this calculated result. We also find out that the excited state population of the argon buffer gas is one of the key role in contributing to the phase match conditions of these two ultraviolet emissions.
