Fluorescence Lifetime and Quenching in I2 Vapor

The vibrational energy levels of the B rio electronic state of I2 were studied by absorption spectroscopy in Exp. 39. In the present experiment, selected vibrational-rotational levels of this state will be populated using a pulsed laser. The fluorescence decay of these levels will be measured to determine the lifetime of excited iodine and to see the effect of fluorescence quenching caused by collisions with unexcited I2 molecules and with other molecules. In addition to giving experience with fast lifetime measurements, the experiment will illustrate a Stem-Volmer plot and the determination of quenching cross-sections for iodine. Student results for different quenching molecules will be pooled and the dependence of the cross sections on the molecular properties of the collision parmers will be compared with predictions of two simple models. 

Absorprion Process. The 532-nm doubled output of a pulsed Nd YAG laser is a convenient excitation source for this experiment. Alternatively, a pulsed dye laser can be used in this case the instractor should determine which I2 levels are excited and modify 

Of these absorptions the latter two produce most of the emission intensity so that we are concerned mainly with the v = 32 excited vibrational level. Detailed studies of single vibrational-rotational states show only slow variation of the relaxation constants with upper state J. Thus in this experiment it will be assumed that a single decay constant is sufficient to describe the average relaxation. This assumption has been validated by using a Nd YAG laser that had a single-frequency output, which was tunable to any of the three transitions the decay times vary by less than 10 percent among these three upper states.  

Fluorescence Decay. The fluorescence is mainly from the v = 32 vibrational level of state 5 to various v levels of the ground state, each consisting of a rotational doublet as discussed in Exp. 39. This emission will be to the red (long wavelength or Stokes) side of the 532-nm excitation source thus an orange or red filter is used to block green and pass red light. It is not necessary to resolve the emission into individual transitions since the decay rate of each is assumed to have the same dependence on the upper B state concentration, I2. 

Following excitation, an excited state can relax by radiative and/or nonradiative processes. The latter may or may not require a collision. We can distinguish four processes as follows  

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