Observations of the near-infrared spectrum of molecular hydrogen in photo-dissociation regions has become a standard tool for revealing the detailed physical conditions and complex density structures of molecular clouds. Most recently, consideration has been give to the detailed behaviour of the ratio of ortho-to-para excited states, and the information that this ratio may contain regarding the history of the molecular cloud (Draine & Bertoldi 1996, Sternberg & Neufeld 1999). This paper will review NIR observations of the H2 spectrum with particular reference to the ortho-para ratios observed. Recent spectroscopy of both galactic and extragalactic sources provide some interesting constraints on the models.

Introduction

Modelling of the H2 emission from photodissociation regions (PDRs) has reached a very high level of sophistication a decade after the first observations of H2 fluorescent emission, from the planetary nebula NGC2023. The earliest models, which predicted the response of low density H2 gas to a moderate intensity UV field (Black & van Dishoeck 1987, Sternberg & Dalgarno 1998) have been expanded to include the effects of collisional excitation of the lowest H2 energy levels (Burton, Hollenbach & Tielens 1990, Sternberg 1991) and of self-shielding of dense H2 (Draine & Bertoldi 1996). Observations of the H2 far-red and near-infrared spectrum confirm the model results for emission arising in energy levels as high as Ek > 40,000K (Draine 2000). Recently, theoretical attention has turned to the observed ortho-para ratio of H2 and the potential that this measure may hold for furthering our understanding of the past and present physical conditions in the PDR.