Introduction

While earlier suggestions had appeared over the years, the importance of large molecules in space was first realized on the basis of the observed strong mid-infrared emission in the ISM. The Infrared Astronomical Satellite (IRAS) discovered widespread emission at 12 μm in the diffuse ISM – the so-called IR cirrus – where the expected temperature of dust in radiative equilibrium with the stellar radiation field is expected to be too cool to emit at such short wavelengths (cf. Section 5.2.3). This problem had actually already been recognized in connection with the observed mid-IR emission from PDRs far from the illuminating stars, which is also much brighter than expected for radiatively heated dust grains (Section 9.4). It was then quickly realized that very small dust grains with 20–100 C atoms – actually, large molecules – can be transiently heated to high temperatures, because of their limited heat capacity. Such hot species will cool through emission in their mid-IR vibrational modes. The observed interstellar IR spectrum is very characteristic of aromatic species and hence the carriers are really large polycyclic aromatic hydrocarbon molecules (PAHs).

In this chapter, we will discuss the physics and chemistry of such large molecules. The emphasis will be on their interaction with radiation. However, the presence of large molecules in space will also have profound influence on other aspects of the ISM and these will be examined as well.