We have constructed a high-pressure fast-pulse dosing system for use with Scanning Tunneling Microscopy (STM). For 1-bromopentane on Si(111)-7 x 7 at low temperature (100 K) two physisorbed phases were found to co-exist; spaced-out molecules above corner silicon-adatoms in a one-per-corner-hole (OPCH) pattern, and circles of molecules above middle-adatoms. By tuning the parameters of high-pressure fast-pulse dosing, we can choose which of these two patterns, OPCH or circles, to chemically imprint on room temperature silicon. (C) 2012 Elsevier B. V. All rights reserved.