The experimental and theoretical investigations of weakly-bound species

The research work presented in this thesis concentrates on the experimental and theoretical investigations of van der Waals molecules. The cavity ring-down spectroscopic (CRDS) technique has been used on test symptoms to demonstrate its reliability and sensitivity on firstly, the extremely weak hyperfine transitions of iodine atoms, I(2P1/2) ↠I(2P 3/2) near 7603 cm-1 and also on the b 1Sigma+g[v ' = 2] -- X3Sigma -g [v' ' = 0] transition of molecular oxygen in the 15865 cm-1 region. The CRD system was then coupled to a molecular beam source and the van der Waals complex, Ar-I2 was used to test the system. The jet-cooled spectrum of HF dimer (in the 1.3 microm overtone region) is also presented in this thesis to demonstrate that the CRDS technique is reliable and sensitive enough to detect weakly bound species. Finally, a novel method in probing transient species in 'cool flames' is presented here and has been used to successfully measure the 41 0 band and the 401 hot band of formaldehyde using CRDS with a vertical tube reactor (VTR) for the first time. For the theoretical work on weakly bound species, accurate potential energy surfaces (PES) for both Rg-N2 and Rg-C2 van der Waals complexes (where Rg = He, Ne and Ar) have been predicted using ab initio methods. Furthermore, a basis set extrapolation scheme has been applied to these complexes in order to generate accurate rovibrational states supported by the potential energy surface. Finally, two characterisation experiments have been carried out: the gas expansion from a General valve series 9 slit valves has also been characterised while the Raman shifter used in the CRDS experiments to generate 2nd Stokes radiation has been characterised using different lens configurations and hydrogen gas pressure to find the optimum conversion power.