  The picosecond dynamics of excitons in colloidal CdSe nanorods are directly
measured via their 1s to 2p-like internal transitions by ultra-broadband
terahertz spectroscopy. Broadened absorption peaks from both the longitudinal
and transverse states are observed at 8.5 and 11 THz, respectively. The onset
of exciton-LO phonon coupling appears as a bleach in the optical conductivity
spectra at the LO phonon energy for times > 1 ps after excitation. Simulations
show a suppressed exciton temperature due to thermally excited hole states
being rapidly captured onto ligands or unpassivated surface states. The
relaxation kinetics are manipulated and the longitudinal transition is quenched
by surface ligand exchange with hole capturing pyridine.
