Resonant multi-photon ionisation studies of high-energy states in small molecules

Abstract: This thesis describes separate studies of high-energy Rydberg and ion-pair states in small molecules using resonant multi-photon ionisation spectroscopy.' OODR/REMPI study of ungerade Rydberg states in O2The energy region between 75000 and 99900 cm-1 of O2 has been studied using optical-optical double resonance (OODR) with resonance enhanced multi-photon ionisation (REMPI). Three-photon excitation via single rotational levels of the b(1?+g ) valence state was used to study np (n=3–10) and nf (n=4–9) Rydberg states converging on O2+X(2?1/2,g) and X(2?3/2,g). The excitation pathway picks out non-predissociated states that are singlet in the (?,S) coupling scheme – suitable for np states for n ?8, or are linear combinations of equal weight of singlet and triplet-spin states in the (?,?) coupling scheme which was found to be suitable for nf states for all values of n.' OTR/RI study of Rydberg and ion-pair states in I2 Optical triple resonance (OTR) together with resonance ionisation (RI) has been used to study the ungerade manifold of molecular iodine between 57000 and 73000 cm-1 via the E 0g+(3P2) ion-pair state. Two 6s Rydberg states with 0 +u symmetry based on excited states of the ion core have been observed.During this study we also observed electric-field-induced mixing of nearly isoenergetic rovibrational levels of the E0gP+(3P2) and D0u+(3P2) ion-pair states. Detectable mixing with an applied field of 1kV/cm occurs over a range of energy level separations of ?0.3 cm-1.' (2+1)and (3+1) REMPI study of pyrrole (C4H5N) The two- and three-photon REMPI spectra of Rydberg states of pyrrole (C4H5N) have been recorded using circularly and linearly polarised light. The spectra were compared to the single-photon absorption spectrum of pyrrole and a similar study of furan (C4H4O). Although the ionisation spectra of pyrrole were considerably weaker than corresponding furan spectra, a strong propensity for excitation of d series with two photons and p and f series with three photons was observed for both molecules. The totally symmetric nd-Rydberg series associated with the first ionisation energy, [A2]nd;A1, was identified from n=3 to n=15 and extrapolated to obtain IE1.