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[Abridged] We present high-resolution optical HST/STIS observations made withtwo slits crossing four of the brightest starburst clumps in the vicinity ofthe nucleus of M82. These provide H_alpha kinematics, extinction, electrondensity and emission measures. From the radial velocity curves we confirm thepresence of a stellar bar, and find that the super star cluster M82-A1 has aposition and radial velocity consistent with it being at the end of one of theunique x2 bar orbits formed by an inner Lindblad resonance. We derive a newmodel for the orientation of the bar and disc with respect to the mainstarburst clumps, and propose that clump A has formed within the bar region asa result of gas interactions between the bar orbits, whereas region C lies atthe edge of the bar and regions D and E are located further out from thenucleus but heavily obscured. We derive extremely high interstellar densities,corresponding to ISM pressures of P/k ~ 0.5-1.0 x 10^7 cm^-3 K, and discuss theimplications of the measured gas properties surrounding the nuclear starclusters on the production and evolution of the galactic wind. Despite varyingpressures, the ionization parameter is uniform down to parsec-scales, and wediscuss why this might be so. Where the S/N of our spectra are high enough, weidentify multiple emission-line components. Through detailed Gaussianline-fitting, we identify a ubiquitous broad (200-300 km/s) underlyingcomponent to the bright H_alpha line, and discuss the physical mechanism(s)that could be responsible for such widths. We conclude that the evaporationand/or ablation of material from interstellar gas clouds caused by the impactof the high-energy photons and fast-flowing cluster winds produces a highlyturbulent layer on the surface of the clouds from which the emission arises.

HST/STIS spectroscopy of the environment in the starburst core of M82