Malcolm G. Stock, David J. Little, ond Robert J. Donovan University of Edinburgh Edinburgh, United Kingdom
Direct Studies of Atomic Reactions
in the Undergraduate Laboratory
I
Time resolved atomic absorption spectroscopy
Two reports giving details of a flash photolysis apparatus suitable for studying iodine atom recombination, in teaching laboratories, have appeared.l In the experiments described in these reports the recombination is monitored by observing the formation of 4, rather than the decay of I. Unfortunately molecular iodine is not a strong absorber in the visible region and the successful operation of this experiment is notoriously difficult. More recently an inexpensive commercial flash photolysis apparatus has become available and is extremely useful for demonstrating a range of photochemical reaction^.^ We have used one such apparatus in our laboratory during the past year with considerable success; however, despite its general versatility, i t is not suited for the study of atomic reactions in the gas phase, without fairly substantial modifications. Thus, in view of the fundamental importance of atomic reactions, we have developed an apparatus which is suitable for the study of electronically excited iodine atoms. Electronically excited atoms are particularly interesting as their chemical behavior may differ markedly from that of the ground electronic state, one of the better known examples being the reactions of O(21D2) with alkanes (the ID state lies -2 eV above the ground electronic state O(2 ~ P J ) ) .The O(21Dz) atom is extremely reactive and inserts directly into hydrocarbon bonds with a rate approaching the gas kinetic collision frequency (i.e., the reaction has effectively zero activation energy).
+
M
O(2'4) RH -+ ROH* + ROH (+ denotes internal excitation)
(1)
In contrast, the ground state O(2 3 P ~ abstracts ) a hydrogen atom yielding two radicals; however, this reaction has an appreciable activation energy and is relatively slow a t mom temperature.
Unfortunately the direct study of reactions involving excited oxygen atoms involves expensive modem research 'Yamanashi, B. S., and Nowak, A. V., J. CHEM. EDUC., 45, 705 (1968); Blake, J. A., Bums, G., and Chang, S. K., J. CHEM. EDUC., 46,745 (1969). %Porter, G., and West, M. A., Educ. in Chemistgv, I , 230 (1970). BDonovan, R J., and Husain, D., "Advances in Photochemistry," Vol. 8, 1971, p. 1. 4Pimentel,G. C., Scientific American, April 1966, p. 32. sA cheap method of constructing a suitable flash lamp is to join two quartz sockets (B14) baek to baek; sufficient quartz is supplied with sockets to yield a lamp -18 crn long. Electrodes can he machined from mild steel, to fit the B14 sockets, and sealed in position with black wax. One of the electrodes should have a hole drilled down the center, and the outer portion (i.e., the end opposite to that forming the electrode) machined to accept a glass socket; the flash lamp may thus he connected to a high vacuum system and filled with krypton (-5 kNm-2=37.5 mm Hg).
Figure 1. Energy levels of atomic iodine relevant to this work. equipment and is thus unlikely to be found in teaching laboratories, a t least not in the near future. The direct ohservation of electronically excited iodine atoms is however much simpler and can be used to illustrate many of the eeneral features of excited atom reactions. When an alkyl iodide is photolyzed (A = 240 nm) a large fraction of the iodine atoms are formed in the first excited state I(52P~/z)3,(this state arises from strong spin-orbit coupling which splits the ground state of the halogens into two states, see Figure 1).In a flash pbotolysis experiment the population of the excited 52Pi/z state may thus greatly exceed that of the ground state, and under suitable conditions light amplification may he achieved4 (i.e., a chemical laser constructed; this will he described in a subsequent part of the series). The decay of the excited atoms is relatively slow under conditions when light amplification is not important, and the kinetics of the 52P11~state may then be monitored using the 206.2 nm absorption line (Fig. 1). We here describe an apparatus for flash photolysis with time resolved atomic absorption spectroscopy which is suitahle for studying the kinetics of I(52P~lz). The essence of the experiment is its simplicity, and the small cost of converting a basic flash photolysis apparatus for such experiments. Experimental Arrangement . The basic arrangement is shown schematically in Figure 2. The quartz flash lampS ( I = 18 cm) and reaction vessel (1 = 25 c m ) were of co&entional design and were simply placed close together with their axes parallel (wrapping the lamp and vessel in aluminum foil increases the efficiency of photolysis considerably). Flash energies of 60 J were found to he quite adequate for these experiments and a 1.5 rF capacitor was used at -9 kV (lower voltages Volume 51, Number 1, January 1974
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and higher capacitance could also be used as the flash duration need only be I90 nm) is in the atomic resonance line a t 206.2 nm. The use of a monochromator or suitable filter does however reduce the difficulties associated with high levels of scattered light, and is thus recommended. An E.M.I. 9781B photomultiplier tube (=R.C.A., IP28) was placed a t the exit slit of the monochromator and a circuit similar to that given previously (see Fig. 2 of reference given in footnote 2) was employed for the dynode chain. The tube was tvoicallv run a t 7M) V. The outout of the multiplier was developed across a 47 kn resistor and displayed on a Telequipment DM64 storage oscilloscope.
Basic Experiment From the numerous experiments carried out by undergraduates in our laboratory we have chosen one to describe in detail, mainly because it employs reagent gases which are cheap and readily available. A number of other experiments will then be indicated more briefly in the next section. The basic experiment to he described here employed methyl iodide as the source of I(SZPIlz)and the diluent gas, used to main52
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Journal of Chemical Education
Figure 3. Oscilloscope trace showing the decay of l(52P,,2). P(CH31] = Nm->:P(N2) = 4 k N r r 2 : P(CH4) = 200 N m ' (quenching by methane was being investigated when this trace was taken]. Vertical scale = 10 mVldivision: horizontal scale = 0.2 msec/division. 40
Figure 4. First-order plot of In In (lo/i] against time. The first paint was taken 400 psec after the initiation of the flash and is arbitrarily shown as t=o. tam inothermal condition*. war "whrre s p d nitrogen 1
