  We have used a femtosecond pump-probe impulsive Raman technique to explore
the ultrafast dynamics of micelle suspended single walled carbon nanotubes
(SWNTs) in various pH environments. The structures of coherent phonon spectra
of the radial breathing modes (RBMs) exhibit significant pH dependence, to
which we attribute the effect of the protonation at the surface of SWNTs,
resulting in the modification of electronic properties of semiconductor SWNTs.
Analysis of the time-domain data using a time-frequency transformation uncovers
also a second transient longitudinal breathing mode, which vanishes after 1 ps
of the photoexcitation.
