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. [2000]
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.
Acknowledgements
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.