  The interplay between discrete vibrational and electronic degrees of freedom
directly influences the chemical and physical properties of molecular systems.
This coupling is typically studied through optical methods such as
fluorescence, absorption, and Raman spectroscopy. Molecular electronic devices
provide new opportunities for exploring vibration-electronic interactions at
the single molecule level. For example, electrons injected from a scanning
tunneling microscope tip into a metal can excite vibrational excitations of a
molecule in the gap between tip and metal. Here we show how current directly
injected into a freely suspended individual single-wall carbon nanotube can be
used to excite, detect, and control a specific vibrational mode of the
molecule. Electrons inelastically tunneling into the nanotube cause a
non-equilibrium occupation of the radial breathing mode, leading to both
stimulated emission and absorption of phonons by successive electron tunneling
events. We exploit this effect to measure a phonon lifetime on the order of 10
nanoseconds, corresponding to a quality factor well over 10000 for this
nanomechanical oscillator.
