An unexpected pronounced enhancement is observed in sputtering yields per atom for N 1 2 compared to N 1 from a polycrystalline gold target. This effect is seen when the kinetic energy per projectile atom is below 500 eV and increases as projectile energy decreases to near-threshold energies. We report on the first observations of a pronounced enhancement in sputtering yields for molecular ions as compared to atomic ions at low projectile energies near sputtering thresholds (30 -500 eV) [1]. At high energies (* 50 keV) sputtering yield enhancement with molecules has been observed and attributed to nonlinear effects in the collision cascade [2][3][4]5]. In the medium energy (linear cascade) regime (approximately 1-50 keV) it is known that the sputtering yield per atom of a molecular ion equals that of an atomic ion at the same impact velocity. In contrast, few data are available on sputtering yields at energies below 1 keV (the low-energy regime). Furthermore, the literature that is available deals almost exclusively with atomic projectiles [6][7][8][9][10][11][12][13]. This work represents the first measurements of lowenergy sputtering yields by molecules near threshold, and shows an unexpected enhancement in sputtering yields for molecules as compared to atoms. This new effect can be attributed to the fact that the kinetic energy of a molecular projectile is most efficiently transferred when the time of collision is comparable to or larger than the vibrational period which is dependent on the detailed nature of the molecular potential. These experiments provide new and significant insight into fundamental projectile-surface interactions relevant to growth and etching.The experiments were conducted in an ultrahigh vacuum chamber with a base pressure of 3 3 10 29 Torr. Low-energy atomic and molecular ions were delivered by a unique accelerator that produces high fluxes of ions with energies tunable between 1 eV and 2 keV. A schematic of the experiment is shown in the inset of Fig. 1. Ions were produced by a discharge source and focused by an Einzel lens. Mass separation was carried out by use of a Wien filter and neutral particles are eliminated by turning the beam by 1.3 ± and passing it through a small aperture (4.8 mm diameter). The beam was then steered back onto the beam axis and refocused by a second Einzel lens. The beam was well defined spatially (FWHM 0.5 mm at 1 keV) and in energy (FWHM 1 eV at 1 keV). The beam was scanned in a raster pattern over a mask with a 2 mm diameter aperture in order to have a homogeneous current density on the target. The current density was determined via an 80% transmission nickel mesh. The kinetic energy of the primary ions ranged from 10 eV to 2 keV with a current density $100 nA͞cm 2 . Sputtering yields were acquired by monitoring the mass change of a goldcoated quartz-crystal microbalance (QCM). By using an FIG.