{HEAD ~ A turnable carbon dioxide gas laser} produces {TAIL ~ laser beams} and both in the infrared region which are highly absorbed by the propane and n butane component gases, respectively, flowing through photoacoustic cell  
produces
{ HEAD ~ A turnable carbon dioxide gas laser } produces { TAIL ~ laser beams } and both in the infrared region which are highly absorbed by the propane and n butane component gases , respectively , flowing through photoacoustic cell
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