We describe nanometer-scale feature definition in adsorbed hydrogen layers on Si(001) surfaces by exposure to low energy electrons from a scanning tunneling microscope tip. Feature sizes range from < 5 to > 40 nm as a function of bias voltage (5-30 V) and exposure dose (1-10(4) mu C/cm). We show that the cross section for electron stimulated desorption of hydrogen has a threshold at 6-8 eV and is nearly constant from 10 to 30 eV, so that above threshold the feature profiles are a direct reflection of the electron flux profile at the surface. Radial flux distributions are best fit by a simple exponential function, where the decay length is dependent primarily on the tip-sample separation. Low intensity tails at large radius are also observed for high bias emission. Comparison to field emission simulations shows that our tip has an "effective radius" of approximately 30 nm. Simulations demonstrate that tip geometry and tip-sample separation play the dominant role in defining the electron flux distribution, and that optimum beam diameter at the sample is obtained at small tip-sample separation (low bias) with sharp tips. We show that adsorbed hydrogen is a robust resist that can be used as a mask for selective area deposition of metals by chemical vapor deposition. Fe lines 10 nm wide are deposited by pyrolysis of Fe(CO)(5) in areas where H has been desorbed, with minimal nucleation in the H-passivated areas.