Wavelength calibration of the lasers was established using a Bristol Instruments model 821 wavemeter and lines of atomic Th that were frequently observed in the LIF spectra. The line centers for Th were taken from reference . RE2PI measurements were carried out in a differentially-pumped vacuum chamber that was equipped for time-of-flight mass spectrometry . RE2PI spectra were recorded with massresolved ion detection. All two-color measurements used spatially overlapped, counterpropagating laser beams. Digital delay generators (Stanford Research Systems, DG 535) controlled the synchronization of light pulses.