The argon–hydrogen fluoride binary complex: An example of a long lived metastable system

The optothermal laser–molecular beam method has been used to measure the infrared spectrum of Ar–HF [10°0←00°0]. The results show that the vibrational predissociation lifetime of this complex is greater than the flight time of the molecules from the laser crossing region to the bolometer detector. This gives a lower limit on the lifetime of 3×10−4 s! The upper vibrational state dipole moment has also been obtained for the complex (μ1=1.495 D) by carrying out infrared stark spectroscopy. This corresponds to a 12% increase in the dipole moment upon vibrational excitation. This change can be related to a stiffening of the van der Waals bond, and hence a reduction in the amplitude of the bending motion, in the vibrationally excited state.