LONG-PATH INTRACAVITY LASER SPECTROMETER FOR THE MEASUREMENT OF ATMOSPHERIC TRACE GASES

WE PROPOSE TO INVESTIGATE A TECHNIQUE FOR INCREASING THE SENSITIVITY OF INTRACAVITY LASER ABSORPTION SPECTROSCOPY, WITH APPLICATION TO THE DETECTION AND MEASUREMENT OF TRACE GASES IN THE ATMOSPHERE. THE TECHNIQUE THAT WE WILL INVESTIGATE IS TO PLACE A MULTIPASS ABSORPTION CELL WITHIN THE RESONANT CAVITY OF A DYE LASER. WITH AN INTRACAVITY MULTIPASS CELL, THE OPTICAL ABSORPTION PATHLENGTH CAN BE INCREASED BY A FACTOR OF TEN TO THIRTY OVER THE TYPICAL VALUES ( 1 METER) OF EXPERIMENTAL SYSTEMS REPORTED IN THE LITERATURE. THUS WE MAY INCREASE THE SENSITIVITY OF INTRACAVITY LASER SPECTROSCOPY BY A FACTOR OF TEN OR MORE OVER THE PREVIOUS STATE OF THE ART. THERE ARE NUMEROUS IMPORTANT ATMOSPHERIC TRACE GASES WITH ABSORPTION LINES IN THE VISIBLE TO NEAR ULTRAVIOLET WHICH MAY BE MEASURED WITH ALONG-CAVITY ILS INSTRUMENT. THESE GASES INCLUDE NO3, NO2, O3, SO2, HNO2, CH2O, AND OH. THE ABSORPTION CROSS SECTIONS FOR THESE GASES ARE MODERATE, BUT THEIR EXTREMELY LOW CONCENTRATIONS (SOME BELOW PART-PER-TRILLION LEVELS) MAKE THEM DIFFICULT TO MEASURE. OUR GENERAL GOAL FOR PHASE I RESEARCH IS TO DEMONSTRATE AND EXPLORE THE BEHAVIOR OF A TUNABLE DYE LASER THAT HAS AN INTRACAVITY LONG-PATH MULTI-PASS ABSORPTION CELL. WE ALSO HAVE THE GOAL OF MAKINGA PRELIMINARY EVALUATION OF THE APPLICABILITY OF SUCH A LASER SYSTEM TO THE PROBLEM OF MEASURING WEAKLY ABSORBING ATMOSPHERIC GASES. PHASE II RESEARCH WILL INVOLVE THE CONSTRUCTION OF A MORE COMPLETE ATMOSPHERIC TRACE GAS MEASUREMENT INSTRUMENT, BASED ON THE BASIC LONG-CAVITY ILS SYSTEM FIRST DEMONSTRATED IN PHASE I. AN ATMOSPHERIC TRACE GAS MEASUREMENT INSTRUMENT BASED ON A LONG-CAVITY ILS SYSTEMWOULD HAVE MANY OF THE PROPERTIES DESIRED IN MODERN ATMOSPHERIC CHEMISTRY RESEARCH. THESE PROPERTIES INCLUDE: HIGH SENSITIVITY, FAST RESPONSE, POINT MEASUREMENT CAPABILITY AND RUGGEDNESS.