At Imperial College a mega-ampere generator for plasma implosion experiments has been designed, built, and commissioned. With a final line impedance of 1.25 Ω this terawatt class generator has been designed primarily to drive a maximum current of 1.8 MA with a rise time of 150 ns into high inductance z-pinch loads of interest to radiative collapse studies. This article describes the design and tests of the generator which has a novel configuration of lines and a new design of a magnetically insulated transmission line (MITL). In summary, the generator consists of four Marx generators each of the Hermes III type (2.4 MV, 84 kJ), each connected to 5 Ω pulse forming lines and trigatron gas switches. The power is fed into the matched 1.25 Ω vertical transfer line which feeds a diode stack and a short conical MITL in vacuum which concentrates the power into the z-pinch load. At 80% charge a current rising to 1.4 MA in 150 ns has been measured in a 15 nH inductive short. Similar results are obtained when using a plasma load.

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The complementary techniques of high-resolution vacuum ultraviolet absorption spectroscopy and resonance-enhanced multiphoton ionisation spectroscopy have been employed in a reinvestigation of the vertical electronic spectra of the molecules H2S and D2S. In accord with previous, lower-resolution, absorption studies of H2S the major part of the spectra of both isotopic species may be understood in terms of Rydberg series that arise from promotion of an electron from the highest occupied 2bl molecular orbital. The electronic symmetries of states involved in six Rydberg series have been established unambiguously through observation of the (partially resolved) rotational structure that accompanies their excitations. A number of additional resonances observed only in the multiphoton ionisation spectra are tentatively attributed to excitations involving Rydberg orbitals comprised predominantly o f f and/or g functions. Several of the absorption features (most notably those involving excited sites of B, symmetry) exhibit anomalous intensity distributions between the various rotational branches. L-uncoupling and/or rovibronic level dependent predissociation are suggested as contributory causes.

Two new electronic states of H2O and D2O have been identified in the energy range 84 000–88 000 cm−1 as three-photon resonances in four-photon ionization spectroscopy. Simulations of the rotational intensity distributions using asymmetric top three-photon line strength theory, and rotational analyses, characterize the states as B1 and A2. These Rydberg states are assigned to the excitations 4sa1 ← 1b1[Formula: see text] and 3d2 ← 1b1[Formula: see text] on the basis of equilibrium geometries, quantum defects, and the polarization dependence of their three-photon transition probabilities. The identification of the one-photon forbidden 1A2–1A1 transition, together with published vacuum ultraviolet (VUV) absorption spectra, permits a consistent assignment for all five members of the 3d ← 1b1 complex.The [Formula: see text] and [Formula: see text] states arc predissociatcd via both homogeneous and heterogeneous mechanisms. The homogeneous channel from the [Formula: see text] state shows a dramatic isotope effect, being about two orders of magnitude faster in H2O than from equivalent levels of D2O. The heterogeneous predissociation exhibits irregular vibronic and isotopic dependencies, which can be rationalized in terms of the intercessional role of accidental near resonances with levels of the heavily predissociated [Formula: see text] state. The (000) levels of the [Formula: see text] states of H2O and D2O show contrasting heterogeneous predissociation behaviour, which can be interpreted with a knowledge of the relevant potential energy surfaces and the electronic–rotational Coriolis interactions that couple the states.

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