THE INTERACTION OF ULTRAVIOLET (UV) RADIATION WITH MOLECULAR MATTER PLAYS A MAJOR ROLE IN THE PHYSICS AND CHEMISTRY OF INTERSTELLAR CLOUDS PROTOPLANETARY DISKS (PPDS) AND PHOTON-DOMINATED REGIONS (PDRS) INFLUENCING THEIR THERMAL STATE AND CHEMICAL EVOLUTION. IN PARTICULAR VACUUM UV (VUV 100-200 NM) RADIATION BREAKS APART SMALL MOLECULES INTO HIGHLY REACTIVE ATOMIC FRAGMENTS SOME OF THEM METASTABLE WITH LARGE AMOUNTS OF INTERNAL ENERGY AND LONG LIFETIMES THAT SPUR FURTHER CHEMICAL REACTIONS AND TRANSFER ENERGY INTO THE SURROUNDING MATERIAL. ACCURATE KNOWLEDGE OF THE RATES OF THESE FRAGMENTATION PROCESSES AND THEIR PRODUCTS IS CRITICAL FOR UNDERSTANDING THE PHYSICAL AND CHEMICAL EVOLUTION OF MATTER IN SPACE PARTICULARLY IN REGIONS ILLUMINATED BY NEARBY BRIGHT STARS SUCH AS PPDS. IN THIS PROPOSAL WE FOCUS ON THREE TRANSIENT DIATOMIC MOLECULES: C2 CH AND CS ALL OF WHICH HAVE BEEN OBSERVED IN THE INTERSTELLAR MEDIUM (ISM) USING VUV SPECTROMETERS ABOARD THE HUBBLE TELESCOPE BUT NONE OF WHICH HAVE SUFFICIENTLY ACCURATE LABORATORY DATA AVAILABLE TO ENABLE QUANTITATIVE INTERPRETATION OF THE ASTRONOMICAL OBSERVATIONS. AS SOURCES OF ATOMIC CARBON HYDROGEN AND SULFUR THEIR PHOTOCHEMISTRY POTENTIALLY PLAYS A MAJOR ROLE IN THE EVOLUTION OF THE ISM. FOR EACH MOLECULE WE HAVE TWO CENTRAL OBJECTIVES: 1.) TO MEASURE STATE-SELECTED VUV PHOTODISSICIATION CROSS SECTIONS AND ATOMIC PRODUCT BRANCHING RATIOS. THESE CHALLENGING MEASUREMENTS ARE MADE POSSIBLE BY THE UNIQUE STATE-OF-THE-ART TWO-COLOR VUV-VUV VELOCITY MAP IMAGING INSTRUMENTATION AVAILABLE AT UC DAVIS. THIS INSTRUMENTATION HAS BEEN SUCCESSFULLY EMPLOYED TO PERFORM SIMILAR MEASUREMENTS ON THE STABLE DIATOMIC MOLECULES CO N2 AND O2 AND IS NOW EQUIPPED TO STUDY TRANSIENT MOLECULES BY USE OF LASER ABLATION/UV PHOTODISSOCIATION PRODUCTION SOURCES. 2.) TO PERFORM NEW AB INITIO CALCULATIONS (AT THE MRCISD+Q LEVEL OF THEORY) OF THE ELECTRONIC STATES INVOLVED IN THESE PROCESSES TOGETHER WITH NONADIABATIC AND SPIN-ORBIT COUPLING PARAMETERS FOR MODELING THE DISSOCIATION. THESE CALCULATIONS WILL AID INTERPRETATION OF THE LABORATORY DATA AND WILL MAKE PREDICTIONS BEYOND THE EXPERIMENTALLY ACCESSIBLE CONDITIONS. EXPECTED OUTCOMES OF THE PROPOSED INVESTIGATION CONSIST OF SPECTROSCOPIC DATA THAT CAN BE DIRECTLY USED TO INTERPRET VUV OBSERVATIONS WITH THE HUBBLE SPACE TELESCOPE AND PHOTODISSOCIATION CROSS SECTIONS AND BRANCHING FRACTIONS THAT CAN BE INCORPORATED TO YIELD MORE POWERFUL AND PREDICTIVE ASTROPHYSICAL MODELS OF THE ISM PDRS AND PPDS. INTERPRETATION OF FUTURE JAMES WEBB SPACE TELESCOPE OBSERVATIONS OF PPDS IN THE MID-IR WILL DEPEND ON ACCURATE PHOTOCHEMICAL MODELS BECAUSE VUV RADIATION FROM THE STAR HEAVILY INFLUENCES THE SPECIES PRESENT IN THE DISK. OTHER NASA MISSIONS SUCH AS HERSCHEL (THZ) AND SPITZER (MID-IR) OBSERVATIONS OF PDRS WILL LIKEWISE BENEFIT FROM IMPROVED PHOTOCHEMICAL MODELS AS THE CONSEQUENCES OF VUV PHOTOCHEMISTRY ARE OBSERVED ACROSS THE ELECTROMAGNETIC SPECTRUM. THE PROPOSED LABORATORY MEASUREMENTS AND AB INITIO CALCULATIONS WILL BE COMPLETED OVER THE COURSE OF FOUR YEARS BY TWO GRADUATE STUDENTS PURSUING A PH.D. DEGREE IN CHEMISTRY AT UC DAVIS. OVER THE COURSE OF THE PROJECT THEY WILL RECEIVE EXTENSIVE TRAINING AND MENTORSHIP FROM THE PI AND CO-IS IN MODERN TECHNIQUES IN LABORATORY ASTROPHYSICS AND AB INITIO QUANTUM CHEMISTRY AS WELL AS THE CONNECTIONS BETWEEN LABORATORY ASTROPHYSICS THEORY AND ASTRONOMICAL OBSERVATIONS.