Effect of laser intensity and of lower-state rotational energy transfer upon temperature measurements made with laser-induced predissociative fluorescence

Abstract

Laser-induced predissociative fluorescence (LIPF) is often used to deduce gas temperatures from the relative populations of two rotational states. In this paper we present calculations, as well as a measurement, that show that the ratio of LIPF signals from those two states, and thus the de- duced temperature, is sensitive to laser intensity. Even for an idealized situation without collisions, the deduced tempera- ture can vary by a factor of two or three, although a care- ful calibration procedure will reduce or eliminate this error. However, rotational energy transfer (RET) collisions usual- ly do occur in the lower state, and then the laser spectral intensity dependence of the fluorescence ratio can also de- pend heavily upon the value of the RET coefficients. Physical phenomena involve time-dependent values of the lower-state population, caused by competition between filling by RET and depletion by laser pumping. RET reduces the sensitiv- ity of the observed signal to the laser's spectral intensity; however, the conversion of a measured fluorescence ratio to temperature is particularly difficult, because RET rates can be a function of local conditions and of the rotational state being populated. Furthermore, the spatial alignment of the excited- state molecules decreases at higher laser energies, which can also lead to large changes in the measured fluorescence ratio. We measured the ratio of fluorescence intensities that are in- duced by tunable KrF laser light via the AX, 3 ˇ 0 transitions P2(8) and Q2(11) in OH.