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We present an ab initio model of the photoelectric charging of PAHs and PAH clusters in the ISM. These particles have important effects on the heating of a range of ISM from the cold diffuse ISM to warm, dense photodissociation regions (PDRs). Previous treatments of photoelectric heating in the ISM considered the grain population as being composed of a typical grain at an average charge. Our analysis considers a grain size distribution (3--300\AA) and a charge distribution. The rate of gas--grain heat exchange in the interstellar medium depends to a large extent on grain charge, which is set by the balance between the photoelectric ejection of electrons from and the collisional recombination of electrons and ions with the grain surface. The ejected photoelectron imparts energy to the surrounding gas while the subsequent recombination of the electron and ions with the grain removes thermal energy from the gas. The PAH population is found to be predominantly neutral in a range of ISM and is the most efficient medium for photoelectric heating, providing around half of the gas heating. We have formulated an analytic expression for photoelectric heating efficiency $\varepsilon$ applicable to a wide range of ISM of gas temperature T$_{\rm gas}$, electron density n$_{\rm e}$ and illuminated by FUV fields G$_{\rm o}$ times the standard Habing value such that \begin{displaymath} {\rm \varepsilon = \frac{3.0\times 10^{-2} \;(T_{gas}/4000)^{a}}{1 + 2.0 \times 10^{-4} \left(\frac{G_{o}T_{gas}^{1/2}}{n_{e}} \right)}} \end{displaymath} where a=0 for T$_{\rm gas}$ $\leq$ 4000 K and a=--0.9554 for T$_{\rm gas}$ $>$ 4000 K. Our model gives the total photoelectric heating rate $\Gamma_{\rm pe}$ for PAHs and small grains as \begin{displaymath} {\rm \frac{\Gamma_{pe}}{n_{H}} = 10^{-24} G_{o} \;\varepsilon\;erg \;s^{-1} \;(H\;atom)^{-1}} \end{displaymath} where n$_{\rm H}$ is the hydrogen nucleus density. It compares well with observations for the diffuse ISM and the FIRAS observations on COBE.
