LABORATORY STUDIES OF DISSOCIATION RECOMBINATION WITH COLD MOLECULAR IONS FOR DIFFUSE CLOUD STUDIES BY NASA ASTROPHYSICS MISSIONS

WE PROPOSE TO ADVANCE OUR UNDERSTANDING OF HOW DIFFUSE ATOMIC CLOUDS TRANSITION TO DIFFUSE MOLECULAR CLOUDS AN IMPORTANT FIRST STEP IN THE STAR FORMATION PROCESS. OBSERVING THIS TRANSITION IS CHALLENGING AS H2 LACKS A PERMANENT DIPOLE MOMENT MAKING DIRECT DETECTIONS EXTREMELY DIFFICULT. ALSO THE COMMONLY USED H2-SURROGATE CO IS READILY PHOTODISSOCIATED IN DIFFUSE CLOUDS. WE WILL IMPROVE THE COMMUNITY'S ABILITY TO USE OH+ HCL+ AND ARH+ TO TRACE OUT THE ASTROPHYSICAL PROPERTIES OF THIS TRANSITION. THESE MOLECULES ENABLE US TO MEASURE IN DIFFUSE CLOUDS THE COSMIC RAY IONIZATION RATE OF ATOMIC H TO INFER THE MOLECULAR HYDROGEN FRACTION TO CONSTRAIN THE FAR-ULTRAVIOLET INTERSTELLAR RADIATION FIELD AND TO USE PROXIES TO TRACE OUT THE DIFFICULT-TO-DETECT H2 MOLECULE AND INFER H2 COLUMN DENSITIES AND MASSES. USING OH+ HCL+ AND ARH+ TO TRACE OUT THE PROPERTIES OF DIFFUSE CLOUDS REQUIRES AN ACCURATE UNDERSTANDING OF THE UNDERLYING GAS-PHASE ASTROCHEMISTRY CONTROLLING THE ABUNDANCES OF THESE MOLECULES. KEY TO THIS ARE RELIABLE RATE COEFFICIENTS FOR DISSOCIATIVE RECOMBINATION (DR) OF ELECTRONS WITH OH+ HCL+ AND ARH+. ACCURATE QUANTUM MECHANICAL CALCULATIONS ARE LACKING DUE TO THE THEORETICAL AND COMPUTATIONAL CHALLENGES OF HANDLING THE MANY-BODY SYSTEMS AND THE INFINITE NUMBER OF INTERMEDIATE STATES INVOLVED. LABORATORY MEASUREMENTS ARE THE ONLY RELIABLE MEANS TO GENERATE THE NEEDED DR DATA. BUT PREVIOUS EXPERIMENTAL WORK IS OF LIMITED USE DUE TO THE MODERATE TO HIGH LEVELS OF INTERNAL EXCITATION OF THE MOLECULAR CATIONS (TYPICALLY ~300-1000 K) WHICH IS MUCH HOTTER THAN DIFFUSE CLOUD TEMPERATURES.