The design and first tests of a new supersonic molecular beam are presented. The instrument is dedicated to study in a controlled way the interaction of atoms and molecules with solid surfaces in combination with high-resolution core level photoemission at the SuperESCA beamline of ELETTRA, the third-generation synchrotron radiation source in Trieste, Italy. The design of the instrument together with the characterization and optimization of the molecular flux and energy resolution are discussed. As a first example of application, we present the results of a time-resolved photoemission study (O-2+H2-->H2O reaction) on a PtRh single-crystal alloy. Time resolution of 100 ms/spectrum has been achieved by using a double-pass electron energy analyzer equipped with a novel 96 channel detector.