Rydberg excited iodine–argon van der Waals complexes studied by resonance enhanced multiphoton ionization spectroscopy

Several new gerade Rydberg states of the I₂–Ar van der Waals complex are reported in the region 53 000–69 000 cm⁻¹ above the electronic ground state, using mass resolved (2+1) resonance enhanced multiphoton ionization (REMPI) spectroscopy. The spectra all exhibit anharmonic vibrational progressions in the I₂...Ar van der Waals stretching mode, ν₃, having typical ω_e values in the range 49–62 cm⁻¹, depending upon the principal quantum number of the Rydberg state involved. The observed increase in ω_e and spectral red shift which accompanies the increase in principal quantum number is consistent with a progressive strengthening of the van der Waals bond, resulting from less effective shielding of the ion core by the Rydberg electron. The apparent good Franck–Condon overlap in the ν₃ mode between the ground state and the Rydberg states together with the absence of any clear evidence of bending mode progressions in the spectra, indicates that the I2–Ar complex retains a T‐shaped geometry in all the Rydberg excited states observed.