We demonstrate efficient, pulsed, gas-phase, nonlinear frequency conversion
in a quadruply resonant, double-Lambda system and, simultaneously, verify
theoretical predictions of Rabi-frequency matching unique to absorbing nonl
inear media. This system is used to up-convert ultraviolet light at 233 nm
to the vacuum ultraviolet at 186 nm in atomic Pb vapor with small-signal co
nversion efficiencies exceeding 30% and with modest atomic density-length (
NL) products (scale 10(14) cm(-2)) and optical power densities (10-100 kW/c
m(2)).