PRESSURE-DEPENDENCE OF LASER-INDUCED FLUORESCENCE FROM ACETONE

The use of laser-induced fluorescence (LIF) from acetone is becoming i ncreasingly widespread as a diagnostic of mixing processes in both rea cting and nonreacting flows. One of the major reasons for its increasi ng use is that the acetone LIF signal is believed to be nearly indepen dent of pressure because of fast intersystem crossing from the first e xcited singlet state, from which the fluorescence signal originates, t o the first excited triplet state, which does not fluoresce. To evalua te the use of acetone LIF at pressures higher than atmospheric, we hav e performed a study of acetone LIF in a flowing gas cell at pressures up to 8 atm. We used four different buffer gases: air, nitrogen, metha ne, and helium. Surprisingly, we find that the acetone fluorescence qu antum efficiency increases slightly (similar to 30%-50%) as the buffer -gas pressure increases from 0.6 to 5 atm for all four buffer gases. W hen the buffer gas is air, we observe a decrease in the acetone fluore scence quantum efficiency as the buffer-gas pressure is increased from 5 to 8 atm; for the other three buffer gases the quantum efficiency i s constant to within experimental error in this pressure regime. The o bserved pressure dependence of the acetone fluorescence signal is expl ained by use of a four-level model. The increase in the fluorescence q uantum efficiency with pressure is probably the result of incomplete v ibrational relaxation coupled with an increase in the intersystem cros sing rate with increasing vibrational excitation in the first excited singlet manifold. (C) 1997 Optical Society of America.