D +-X resorance enhxxx-d three-photon excitation spectrum of iodine WBS observed by 5 CI~ intrxavity absorption technique_ W&rational quantum numbers of D + X transitions are gven for every major spectral feature. The corresponding one-photon enhancement from the B state is evident from the one-photon vi3mtional assignment_ The spectroscopic constants t-or the D state are '00 = 30 99s cm-t, ~6 = 113 cm-t and wbX& = 0.045 cm-l_ I. Introduction Recently, several non-linear spectroscopic techniques have been devehped to strtdy high-lying and Rydberg states of gaseous mdecules. Among the few molecular systems investigated to date, a simpIe diatomic moIecuIe, iodine, is invariably chosen by many authors to demonstrate their techniques_ The advantage of this system is that the resonance enhanced absorption through the low Iying excited B state of iodine reduces the high power required in multiphoton processes_ Rousseau and WIliams [I] have used a sequential two-photon excitation technique to study the E s?ate of iodine_ Similar techniques employing two iadependently tunable dye lasers have been developed by Danyluk and King f2]. Sander and WiIson [3] have applied the photofragment spectroscopic technique to study the repulsive excited states prepared after two-photon absorption_ Multiphoton ionization of iodine has been reported by Petty et al. [43 and DaIby et al. [S] . In this paper, we report the first observation of the D + X reso-n&ce enhanced three-photon absorption in iodine by a cw intracavity absorption technique_ * Research supported by the US. Department of Energy, Office of Basic Ener,T Sciences, Division of Chemical Sciences-