The fluorescence excitation spectrum of the HeI2 van der Waals complex

Abstract

The HeI₂ van der Waals complex was prepared from a dilute mixture of iodine in helium at a pressure of 100 atm by supersonic expansion through a nozzle into a vacuum. Laser‐induced fluorescence excitation spectra were recorded for the B̃←X̃ transition of HeI₂ as well as corresponding spectra for the He₂I₂ and I₂ molecules in the expanding gas. I₂ was found to be cooled by the expansion to a rotational temperature of 0.4 K, and a vibrational temperature of 50 K. Similarly, cold internal temperatures were attained by the van der Waals complexes. Evidence was found for vibrational predissociation of the HeI₂ complex in both the X̃ and B̃ electronic states. The vibrational predissociation rate was found to depend weakly upon the degree of excitation of the I–I stretching mode, ν₁. For ν₁=1 in the X̃ state the predissociation rate was found to be greater than 5×10⁶ s⁻¹. In the B̃ state the vibrational predissociation rate is ∼5×10¹⁰ s⁻¹ for ν₁=27, decreasing to 9 s⁻¹ for ν₁⩽7. The small (3.4–4.0 cm⁻¹) blue shifts of the vibronic bands of the HeI₂ spectrum relative to corresponding bands of I₂ indicate (1) the van der Waals complex is slightly more strongly bound in the X̃ state than it is in the B̃ state, and (2) the I–I bonding in both the X̃ and B̃ states of iodine is largely unaffected by the formation of the van der Waals bond with helium.