Raman spectra of diamond powders with size less than 2 mum have been m
easured as a function of the particle size. The Raman line was found t
o become more asymmetric with some tailing towards lower Raman shifts,
broader, and weaker with decreasing particle size. The observed resul
t can be explained by a phonon confinement effect rather than by a str
ain effect. This work predicts that it is very difficult to detect Ram
an spectra of diamond particles with size less than approximately 50 a
ngstrom. A broad Raman band, whose intensity becomes stronger with dec
reasing particle size, was observed around 600 cm-1 in the spectra of
diamond powders with particle size less than 2 mum. We hypothesize tha
t the broad band arises from transverse acoustic phonons near the Bril
louin zone boundary because of the relaxation in the wave vector selec
tion rule.