  High energy electron beams can now be routinely focused to 1-2 {\AA} and
offer the ability to obtain vibrational information from materials using
monochromated electron energy-loss spectroscopy (EELS) in a scanning
transmission electron microscope (STEM). Here it is shown that long and short
wavelength phonons can be probed simultaneously with on-axis vibrational STEM
EELS. The advantage of using focused electron beams is that the short
wavelength phonons are probed via impact scattering while the long-wavelength
modes are probed via dipole scattering. The localized character of the
short-wavelength modes is demonstrated by scanning the electron beam across the
edge of a hexagonal boron nitride nanoparticle. It is found that the condition
required for high spatial resolution STEM imaging also correlates with the
condition to enhance the short-wavelength phonon contribution to the
vibrational energy-loss spectrum. Probing short-wavelength phonons at high
spatial resolution with on-axis vibrational STEM EELS will help develop a
fundamental connection between vibrational excitations and bonding arrangements
at atomic scale heterogeneities in materials.
