Interstellar nanoparticles are important as emitters of near and mid IR radiation, as absorbers of far-UV radiation, as heating agents of the interstellar gas, and possibly as luminescing agents of red light. Our understanding of interstellar nanoparticles obviously remains incomplete. Besides the long-standing problems regarding the carriers of the 2175 Å extinction hump and the ERE, there are many unanswered questions which will be demanding close attention in the future:
The origin and evolution of interstellar PAHs are not very clear. Suggested sources for interstellar PAHs include (1) injection (into the ISM) of PAHs formed in carbon star outflows (Latter 1991); (2) shattering of carbonaceous interstellar dust or of photoprocessed interstellar dust organic mantles (Greenberg et al. 2000) by grain-grain collisions in interstellar shocks (Jones, Tielens, & Hollenbach 1996); (3) in-situ formation through ion-molecule reactions (Herbst 1991). Although interstellar PAHs containing more than ~ 20-30 carbon atoms can survive the UV radiation field (Guhathakurta & Draine 1989; Jochims et al. 1994; Allain, Leach, & Sedlmayr 1996; Le Page, Snow, & Bierbaum 2003), they are still subject to destruction by sputtering in interstellar shock waves (but also see Tielens et al.  who argued that destruction of PAHs by sputtering is unimportant, except in extreme environments such as very young supernova remnants with 200 km s-1 shocks, because they couple very well dynamically to the gas which cools down rapidly), chemical attack by atomic oxygen, and coagulation in dense regions (Draine 1994). Clearly, a detailed study of the evolution of interstellar PAHs would be very valuable.
The physics and astrophysics of the radiative electronic transitions of interstellar PAHs are not fully understood.
Small neutral PAHs with 40 C atoms are expected to emit near-UV and blue photons through fluorescence, phosphorescence, and perhaps also through the recurrent Poincaré fluorescence. Laboratory studies show that the fluorescence quantum yield can be quite high for isolated molecules (e.g., fluorescence quantum yields in the range of 10%-45% have been measured for gas-phase, collision-free naphthalene C10H8 [Reylé & Bréchignac 2000]). However, no PL shortward of = 5000 Å has been seen in the ISM (Rush & Witt 1975; Vijh, Witt, & Gordon 2003). Although PAH ions do not exhibit strong ordinary fluorescence or phosphorescence (see the caption of Fig.2), they can undergo Poincaré fluorescence which may have a quantum yield larger than one. Therefore, it is difficult to explain nondetection of blue PL in the ISM in terms of PAH photoionization. Moreover, for PAHs containing 40 C atoms in the diffuse ISM, the probability of finding them in a nonzero charge state is smaller than ~ 30% (see Fig.7 of Li & Draine 2001b). One may argue that interstellar PAHs are larger (which is true, as shown in Li & Draine 2001b, the average [diffuse ISM] PAH size is ~ 6 Å, corresponding to ~ 100 C atoms) so that their fluorescence mainly occurs at longer wavelengths, say, in the wavelength range over which the ERE is observed.
However, if interstellar PAHs indeed luminesce in the ERE band with the required quantum efficiency (~ 100%; see Section 2), the "UIR" emission bands expected for these luminescing PAHs would be strongly suppressed because a considerable fraction of the excitation energy is released in the form of PL photons (e.g., in the diffuse ISM with a mean energy 2.1 eV for ERE photons [Szomoru & Guhathakurta 1998], only ~ 60% of the original excitation energy is available as heat for PAHs which have a mean absorbed photon energy 5.2 eV [Draine & Li 2001]; this could be even worse since the Poincaré fluorescence may have a quantum yield higher than one). One consequence of this would be that even more carbon atoms in the form of PAHs would be needed to explain the intensity of the "UIR" bands, deteriorating the already tightened carbon budget "crisis" (Snow & Witt 1996).
It is unclear how different the optical and thermal properties of nanometer-sized materials are compared with their bulk counterparts, although it is generally believed that they may be very different.
For a small metallic grain, the imaginary part " of its dielectric function () = ' + i " is expected to be larger compared to that of its bulk counterpart, as a consequence of the so-called electron mean free path limitation effect (e.g. see Bohren & Huffman 1983). This is easier to understand if we decompose into two components b and f, contributed by bound charges ("interband transitions") and free electrons, respectively. The free electron component is well described by the Drude theory = 1 - p2 / (2 + i ) with an imaginary part " = p2 / [ (2 + 2)], where the plasma frequency p is related to the free electron density ne: p2 = ne e2 / me*, the damping constant is related to the average time between collisions: = 1 / . In bulk materials, is mainly determined by the scattering of the electrons with phonons (lattice vibrations), and to a lesser degree, with electrons, lattice defects, or impurities. However, for particles in the nanometer size domain, is increased because of the additional collisions of the conducting electrons with the grain boundary: = bulk + vF / a, where bulk is the bulk metal damping constant, vF is the electron velocity at the Fermi surface, is a dimensionless constant of order unity which depends on the character of the scattering at the boundary: = 1 for classic isotropic scattering, = 4/3 for classic diffusive scattering, = 1.16 or = 1.33 for scattering based on the quantum particle-in-a-box model (see Coronado & Schatz 2003 and references therein). Since 2 >> 2 in metals near the plasma frequency, " can be written as " = "bulk + vF p2 / ( a 3). Clearly, for a metallic grain " increases as the grain becomes smaller.
In contrast, our knowledge regarding the size dependence of the dielectric function of dielectric materials is controversial. For example, some theoretical and experimental studies have concluded that the dielectric function of Si nanoparticles is significantly reduced relative to the bulk value (Koshida et al. 1993; Wang & Zunger 1994; Tsu, Babic, & Ioriatti 1997; Amans et al. 2003), which is generally attributed to the quantum confinement effect. But we find that the absorption and reflectivity measurements for porous silicon appear to be consistent with bulk Si together with voids and SiO2 (see Li & Draine 2002a for details).
It has been reported that the specific heat of small metal particles is strongly enhanced over the bulk value (see Halperin 1986, Meyer et al. 2003 and references therein). For example, a progressive decreasing of Debye temperature with the decrease of grain size has been observed for palladium (Pd): 273, 226, 193, 175 K for bulk Pd and Pd particles of radius a = 42, 33, 15 Å, respectively (Chen et al. 1995 and references therein). This has been attributed to quantum effects on the vibrational spectrum in small particles: as a grain becomes smaller, a larger fraction of atoms occupy surface sites which are weakly bound to the grain; therefore, small grains are expected to have a larger low-frequency mode density due to the weaker bonds of the surface atoms.
However, although an enhancement of the vibrational specific heat is also expected for dielectric grains, the degree of enhancement is unclear due to the differences between the binding and structural properties of dielectrics with metals. For example, based on a lattice dynamical calculation, Hu et al. (2001) have shown that the difference in vibrational specific heat between Si nanocrystals and the bulk is just about a few per cent. The specific heat of nanocrystalline diamond measured by Moelle et al. (1998) shows a close agreement with that of bulk diamond. It is worth noting that the heat capacity of PAHs can be well described by the two 2-dimensional Debye models of bulk graphite (together with contributions from the C-H vibrational modes) (see Fig. 2 of Draine & Li 2001). Clearly, laboratory studies of the optical and thermal properties of interstellar nanoparticle analogues would be very valuable.
I am extremely grateful to B.T. Draine and A.N. Witt for their invaluable advices, comments, and suggestions. I thank the anonymous referee for his/her very helpful comments and suggestions. I also thank A.N. Witt for the great efforts he has put into making the "Astrophysics of Dust Symposium" a real success.