Two-colour bound-free-bound spectroscopy of the [(2)E(1/2)](c)6S Rydberg states of CH3I and CD3I

In this resonantly enhanced multiphoton ionization (REMPI) experiment, an extended vibrational progression in the C-I stretching mode (nu(3)) of methyl iodide (-h(3) and -d(3)) is observed in the 1 + 1' excitation of the [1/2] 6s; 0 Rydberg state when the pump photon wavelength lies in the bound --> free absorption continuum. This is in contrast with one-colour coherent (non-resonant) two-photon excitation, where the nu(3) mode is not excited. By working at several different fixed probe wavelengths and scanning the pump frequency, the relative contributions from the three intermediate repulsive states can be explored through changes in the relative strengths of the Omega=0 and 1 components of the final Rydberg states. Extensive predissociation in the Rydberg states curtails the vibrational progression.