The Role of Collisions in the Fate of Excited States A Simple Experiment L. Lain and A. Torre Universidad del Pais Vasco, Apdo. 644, Bilbao, Spain Collisions between atoms or molecules are important in the explanation of most enrrgy-transfw prorrssrs. In thr rlrcrronicallv excited states the molecules HIP suhiccted to cnllisions with their environment (e.g., with solvent molecules), and their thermal energy is dissipated as thermal motion of the surroundines " (,I .) . This mechanism exnlains that the fluorescence radiation has a lower energy ihau the radiation which excites the molecules. Electronicallv excited states of a system are usually populated by absorption of electromagnetic radiation. and some selection rules must be obeved: however, sometimes, under appropriate cunditinns, it is p o sihie to transfrr, by mrans of colliams, from onr rlectronirnllg exrited state to nnothw onr. S1a.h ~ransfermn populate stitu,s that ant forhiddm hv ahsorotion of ;I dioton from the ground state. The importance of ~ollisions'iuchemical kinetics is obvious ( 2 ) . Collisions induce different kinds of processes depending on the nature of atoms or molecules involved, and on the conditions in which they take place, e.g., pressure, temperature, quantum state of the systems, etc. In this report we describes very simple experiment, suitable for the ungraduate level. The experiment shows how the presence of certain gases can change, in a qualitative and quantitative way, the fluorescence spectra of a pure molecular system. Several inexpensive gases having different molecular sizes, geometrical shapes, and reactivities can be used to carry out various collisional processes, and each process can be individually detected. Molecular iodine has a strong absorption band between 1800 and 2000 A with a maximum at 1825 A; the D('X:) state is populated in this absorption (3).From this state a fluorescence signal is obtained starting around 200 nm and ending near 500 nm; the strongest signals are a characteristic group of peaks near 325 nm as illustrated in Figure 1. The equations that describe this excitation-deexcitation process are (4)
populated by absorbing a photon from the ground state X'X:; the simplest way to populate i t is by inelastic collisions from upper states. Equipment and Experhenis . .
The haair arrangrmcnt is shown schematically in Figure 3. I, crystal? were intnxl~~ced in a Spertrosil quartz cylindrical
Figure 1. Fluorescence spectrum of pure l2 at room temperature, between 315 and 350 nm. The scattered Hg peak at 334 nm is indicated by 4.
When the same fluorescence spectrum is scanned in the presence of inert foreign gases such as Nz, Ar, etc., it is possible to observe how the intensity of the peaks near 325 nm decreases and a new band near 340 nm appears, as shown in Figure 2. The band a t 342 nm has been assigned (5)to the transition
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Id311zr) 12(3112u)+ hu(-340 nm) The ' n a g state has a lower energy than does the D'2: state (3) and is populated only when foreign gases are present; it is a clear example of inelastic collisions between the system Ia(DIX:) and the foreign gas M (4)
Note that, since the Iz molecule has a center of symmetry, the only transitions allowed are those involving a change of parity; that is, g u or u g, so the state 3II2, cannot he
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Journal of Chemical Education
Figure 2. Fluorescence spectrum of I2 in the presence of fweign in& gases. between 315 and 350 nm. The scattered Hg peak at 334 nm is indicated by
Figure 3. Experimental arrangement. M, monachromator;PM, photomultiplier: PS, p o w supply; RP, rotary wmp: CR, chaR recader: C, cells: L, mercury lamp: G, gas bulb: MM. mercury manometer: DP, diffusionpump.
cell (length 10 cm and diameter 2 em). The cell was connected to a conventional vacuum line to de-gas 1% crystals and to introduce foreign gases. 12 vapor, in equilibrium with I2 solid ), with the mercury (vapor presstke6.25 mm ~ i waseicited line at 184.9 nm using a radio frequency low pressure menury Inmo (Thermal -~~~~~ . - ~ SvndirateL ~, Fluorescence was obsewrd at. rieht angles to the mercury lamp through a McKee-Pederson monochromator (MP1018 B) and an EM1 9661 B photomnltiplier, and spectra were drawn by a chart recorder. The first step in the experiments is to check certain conditions so that a good fluorescence spectrum of pure 1%can be obtained. This offers a good opportunity for students t o control the voltage of a photomultiplier power supply, the slit of a monochromator, the sensitivity of a chart recorder, etc. The alignment of themercury lamp; cell, and monochromator is also important: since the 0 2 in the air absorbs a t 184.9 nm, the mrr&y lamp must be close to the crll window to get a aood sirnnl. The mercury lamp also emitri at other wavelengihs i e . ~ .the , strong line at 253.6 nm), but 11vapor is transparent in this region; it is possible to distinguish hetween the fluorescence signal andthe scattered mekury peaks by freezing the cell with a bit of cotton soaked in liquid nitrogen: only the mercurv neaks remain. The waveleneth of the scattered ~ e a k s can br ;lied LO check the calibrationof the monochromatur. The next steD will be the addine of controlled amounts uf different gases the cell. From thrs, one can observe how the neaks a t 325 nm decrease and the hand a t 340 nm increases. Pressures can be controlled hy a mercury manometer or by a capacitance manometer if available. Let us consider some inert gases like He and Ar. Their reometrical s h a ~ is e the same (both are monoatomic gases). Their only impdrtant difference is their size. ~ollisio
