Spectroscopic studies of highly excited states of short-lived molecules

Boggis, Simon Andrew (1996) Spectroscopic studies of highly excited states of short-lived molecules. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Multiphoton ionisation (MPI) spectroscopy has been used to investigate the electronic structure of short-lived molecules in the gas-phase. Electronic transitions observed in the MPI spectra of the short-lived molecules studied were assigned by analysis of the rotational structure in the observed bands. In the main part of this work, the molecules chosen for study using MPI spectroscopy were NF and NC1, which are of interest as possible energy transfer agents in chemical laser systems. NF was produced by the rapid gas-phase reactions F+HN₃→HF+N₃ and F+N₃→NF+N₂, whilst NC1 was produced in a similar manner by the reactions F+N₃→HF+N₃ and C1+N₃→NC1+N₂.

In the MPI spectrum of NF, two-photon resonant transitions were observed from the low-lying NF(a¹Δ) state to 3dδ ^1,3φ and 4dδ ^1,3φ Rydberg states, in which the appearance of the formally forbidden ³φ ← a¹Δ transition was rationalised in terms of a spin-orbit and S-uncoupling interaction in the excited state. In addition, two-photon transitions from NF(a¹Δ) to a ¹Σ and a perturbed ¹Δ Rydberg state were observed. In the case of the ¹Δ ← a¹Δ transition the assignment of a perturbed ¹Δ Rydberg upper state was tentative.

In the MPI spectrum of NC1, two-photon resonant transitions from NCI(a¹Δ) to ¹Σ, ¹Π, ¹Σ, 3dδ ¹φ, ¹Σ and 4dδ ¹φ Rydberg states were observed, in order of increasing transition energy. Two-photon resonant transitions from the NC1(X³Σ^-) state to ³Π and ³Δ Rydberg states were also observed.

Prior to this work, no states lying higher in energy than the b¹Σ⁺ state for NC1 were known, whilst for NF only two Rydberg states (¹Δ and ¹Σ) were known at energies higher than the NF (b¹Σ⁺) state. The observation of resolved bands involving triplet Rydberg states has allowed the determination of the first experimental values for the spin-orbit splitting in any state of NF and NC1.

Finally, known two-photon resonant MPI transitions of atomic chlorine were used to measure the relative population of the C1(²P_3/2) and C1(²P_1/2) spin-orbit states resulting from the UV photolysis of HOC1, a species of importance in stratospheric ozone depletion. The results indicated a stronger preference for formation of ground state atomic (C1(²P_3/2) than expected from statistical weights, and also that most of the energy available upon photolysis is converted into kinetic energy of the photofragments.

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