  It is well known that plasmons in bulk metals cannot be excited by direct
photoabsorption, that is, by coupling of volume plasmons to light. Here we
demonstrate that the situation in nanoclusters of the same metals is entirely
different. We have carried out a photodepletion measurement for Na_20 and Na_92
and identified a broad volume plasmon absorption peak centered slightly above 4
eV, revealing the possibility of optical excitation of volume-type collective
electronic modes in a metallic system. The observed phenomenon is related to
different selection rules for finite systems.
