Investigation of broadband and state-selective photofragmentation of KI using resonance ionisation spectroscopy

In this thesis we report on the photodissociation of KI molecules which was induced either by UV radiation at 355 nm (third harmonic of a Nd:YAG laser) or by tuneable UV radiation. The neutral photofragment, K, was detected using multiphoton ionisation (MPI) or resonance ionisation (RIS). The first goal of the investigation has been to establish sensitive detection of metal atom photofragments, in particular to optimise the excitation schemes for potassium. This was to be used as a test bed to explore the feasibility of isotope specific evaluation of the fragment atoms, exploiting a time-of-flight (TOF) mass tube, in cases where the laser bandwidth does not provide isotope selectivity. The results of this part of the investigation show that indeed the sensitivity is sufficient to allow for the detection of minor isotopes in principle, but that our simple TOF system would need further improvement to realise sufficient mass resolution. The second goal has been to explore the possibility to trace resonance effects in the dissociation continuum which should come about by the interaction of adiabatic and diabatic potential energy curves in the excitation region of the first absorption maximum for KI. The enhancement or depletion in the signal from the fragment atom detection was expected to mirror vibrational-rotational structure in the quasi-bound state, originating from the avoided-crossing between the ground state potential with Ω = 0⁺ and the (repulsive) first excited state potential, also with Ω = 0⁺. The results reveal some structure in the RIS signal, after appropriate background subtraction, and for the first time provide proof that adiabatic/diabatic interaction structure can be detected even for the molecule KI for which the avoided crossing is at unfavourably large internuclear separation. For comparison, we also conducted experiments of the same kind for NaI for which such structure was previously known. Indeed, we confirmed this in our measurements.