THE OBJECTIVES OF THIS RESEARCH ARE TO UNDERSTAND THE CHEMICAL OR PHOTOCHEMICAL MECHANISM BY WHICH SULFUR ISOTOPE MASS-INDEPENDENT FRACTIONATION (S-MIF) SIGNATURES WERE CREATED IN THE ANCIENT EARTH ATMOSPHERE. THE RECORD OF S-MIF SIGNATURES IN ARCHEAN ROCKS IS OUR BEST PROXY FOR THE RISE OF O2 ABOUT 2.4 GYR AGO. SINCE THE DISCOVERY OF S-MIF SIGNATURES IN ANCIENT ROCKS IN 2000 MOST RESEARCH INTO THE S-MIF MECHANISM HAS FOCUSED ON SO2 PHOTOLYSIS. IT IS BECOMING APPARENT THAT SO2 IS PROBABLY NOT THE ORIGIN OF S-MIF. HERE WE PROPOSE EXPERIMENTS TO STUDY PHOTOCHEMICAL FRACTIONATION IN OTHER SMALL SULFUR MOLECULES LIKELY TO HAVE BEEN PRESENT IN A PREOXYGENATED EARTH ATMOSPHERE INCLUDING SO S S2 AND S4. THESE SULFUR SPECIES ARE NOT RELEVANT TO THE MODERN EARTH ATMOSPHERE ALTHOUGH THEY ARE PRESENT IN THE MODERN VENUS ATMOSPHERE. WE ARE MEASURING CROSS SECTIONS AND REACTION PRODUCTS FOR THE SULFUR ISOTOPOLOGUES OF THESE MOLECULES IN ORDER TO IDENTIFY POSSIBLE S-MIF PROCESSES. WE USE SEVERAL TECHNIQUES TO ANALYZE SO S S2 AND S4. WE WILL MEASURE SO AND S2 ISOTOPIC ABSORPTION CROSS SECTIONS USING THE SOLEIL SYNCHROTRON IN FRANCE. THIS SYNCHROTRON HAS THE BEST UV FOURIER TRANSFORM SPECTROMETER AVAILABLE TODAY FOR PHOTON WAVELENGTHS<300 NM. WE GENERATE SO AND S2 BY RF DISCHARGE IN SO2 AND H2S RESPECTIVELY AND THEN WE MEASURE THE UV ABSORPTION SPECTRUM IN THE SO AND S2. WE WILL ALSO USE UV PHOTOFRAGMENT INSTRUMENTS AT THE UNIVERSITY OF CALIFORNIA DAVIS (UCD) TO MEASURE ISOTOPE FRACTIONATION IN S ATOMS PRODUCED FROM SO PHOTODISSOCIATION. VELOCITY-MAP IMAGING IN A TIME-OF-FLIGHT MASS SPECTROMETER WILL BE USED TO ELUCIDATE THE EXACT STATE PARAMETERS FOR THE PRODUCT S ATOMS ALLOWING US TO FULLY CHARACTERIZE THE ABUNDANCES OF ISOTOPIC S ATOMS PRODUCED DURING PHOTOLYSIS OF SO A PROCESS THAT WOULD HAVE OCCURRED IN THE ANCIENT ATMOSPHERE. FINALLY WE WILL USE A HEATED QUARTZ FLOW TUBE TO STUDY THE ISOTOPIC PROPERTIES OF S2 SELF-REACTION TO FORM S4. THE PRODUCTS WILL BE IONIZED BY LASERS AND COUNTED BY TIMEOF- FLIGHT MASS SPECTROMETRY. PHOTOCHEMICAL MODELS WILL BE USED TO SYNTHESIZE THESE LABORATORY RESULTS INTO EXISTING EARLY EARTH MODEL ATMOSPHERES. THE PROPOSED WORK LIES WITHIN THE RESEARCH AREA "EARLY EVOLUTION OF LIFE AND THE BIOSPHERE" IN THE EXOBIOLOGY AND EVOLUTIONARY BIOLOGY PROGRAM. THE RISE OF ATMOSPHERIC O2 WAS A MAJOR EVENT IN THE HISTORY OF LIFE AND ENVIRONMENTS ON EARTH. OUR RESEARCH WILL HELP TO ELUCIDATE THE OPERATION OF THE ANCIENT SULFUR CYCLE AND PROVIDE A FRAMEWORK FOR UNDERSTANDING EXACTLY HOW S-MIF SIGNATURES CONSTRAINT ATMOSPHERIC O2 ABUNDANCE.