NOVELTY - In a process for screening potential catalyst libraries reactant gas is passed in a passage (71) in a substrate (70) to an addressable catalyst test site (72) to form product gas plumes through which a radiation beam (77) is passed in a product zone (75) to form specified ions and electrons which are collected in real time by a microelectrode (78) in situ in proximity to the addressable site.    USE - In the screening for activity and selectivity of heterogeneous and homogeneous catalyst libraries.    ADVANTAGE - The process is broadly applicable, can be used to simultaneously, rapidly screen an entire catalyst library and can also be used to study operational lifetimes, resistance to poisoning, regeneration and loss of catalysts in tests or in full scale chemical plant processes. Detection methods in situ in the reactor use the high sensitivity, specificity and real-time features of resonance-enhanced multiphoton ionization (REMPI) in which pulsed and tuneable ionizing light sources are used to selectively photoionize desired reaction products without ionizing reactants and/or other background species. When reaction products are solids or liquids they can be ablated using a pulsed laser beam followed by selective photoionization of the products a suitable UV laser. The process can provide information on catalyst selectivity by detecting several reaction product species by using different light frequencies to sequentially generate specific ions of different products and the REMPI signals can then be converted into absolute concentrations by use of calibration standards. Internal calibration standards introduced with the reactant feed can be used to quantify reaction products.    DETAILED DESCRIPTION - An INDEPENDENT claim is also included for a microreactor for radiation activation screening of potential catalyst libraries for catalyst properties comprising a radiation beam passage (76) perpendicular to the product zone and an inert cover wafer bonded to the substrate body to enclose the passages and including a parallel array of microreactors in a single substrate body. The array having temperature and flow control for each microreactor, a power supply for the microlectrodes, an ion or electron detector, a selector to detect ions or electrons in a specific product zone and a removable frame with internal wiring for supplying power to and taking detection signals from the microelectrodes fitted around the array.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic of a single microreactor.    substrate body (70)    reactant gas passage (71)    catalyst site (72)    product zone (75)    radiation beam passage (76)    radiation beam (77)    microelectrode (78)