Edinburgh, United Kingdom I Time resolved atomic absorption spectroscopy

Malcolm G. Stock, David J. Little,

ond Robert J. Donovan University of Edinburgh

Two reports giving details of a flash photolysis appara- tus 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 photoly- sis apparatus has become available and is extremely use- ful 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 substan- tial modifications. Thus, in view of the fundamental im- portance of atomic reactions, we have developed an appa- ratus which is suitable for the study of electronically ex- cited iodine atoms.

Direct Studies of Atomic Reactions

in the Undergraduate Laboratory

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 ex- amples 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 in- serts directly into hydrocarbon bonds with a rate ap- proaching the gas kinetic collision frequency (i.e., the reaction has effectively zero activation energy).

M O(2'4) + RH -+ ROH* + ROH (1)

(+ denotes internal excitation)

In contrast, the ground state O(2 3 P ~ ) abstracts a hydro- gen 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 ex- cited 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 Photochemis- try," 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 sup- plied 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 oppo- site 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 oh- servation 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 pbotoly- sis 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 ex- periments.

Experimental Arrangement .

The basic arrangement is shown schematically in Figure 2. The quartz flash lampS ( I = 18 cm) and reaction vessel (1 = 25 cm) 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 effi- ciency 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 / 51

and higher capacitance could also be used as the flash du- ration need only be <1M) psec). The lamp was fued by pushing a brass disc mounted on a Teflon plunger across two copper electrodes, one connected to the lamp and the other to earth, as shown in Figure 2 (HS) . Masking the ends of the reaction vessel with black paper reduced the scattered light from the flash, and thus helped to prevent possible overloading of the photomultiplier; this is proba- bly not essential and no problems with scattered light were encountered. The "collimator" in Figure 2 (C) was simply a short tube lined with black paper.

Probably the most important feature of the apparatus is the use of an inexpensive atomic emission lamp. This is excited by a simple radio frequency power supply (Ther- mal Syndicate, "Thermaline Probe" unit6) and consists of a U-shaped quartz tube with a capillary section in the central limb, to constrict the d i~charge .~ The lamps have a limited life, and typically run for 50 hr. Efforts are being made however to extend this lifetime.a The atomic lamp was simply placed a t one end of the reaction vessel and gave rise to sufficient light levels a t the photomulti- plier such that a lens system was not essential. Clearly the use of a lens system, to collect more of the emission from the lamp and focus it onto the slit of the monochro- mator, will improve the signal to noise ratio and reduce the effect of scattered light, but involves additional ex- pense.

At the opposite end of the reaction vessel was situated a simple and cheap monochromator (Hilger and Watts D292 with grating blazed for 320 nm), the main function of which was to reduce scattered light reaching the photo- multiplier. We have in fact been able to observe the for- mation and decay of I(52P~p) atoms without using the monochromator, as over one third of the total emission from the atomic lamp ( A >I90 nm) is in the atomic reso- nance line a t 206.2 nm. The use of a monochromator or suitable filter does however reduce the difficulties associ- ated with high levels of scattered light, and is thus recom- mended.

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 refer- ence 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 under- graduates in our laboratory we have chosen one to de- scribe 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 io- dide as the source of I(SZPIlz) and the diluent gas, used to main-

Figure 3. Oscilloscope trace showing the decay of l(52P,,2). P(CH31] = 40 Nm->: P(N2) = 4 k N r r 2 : P(CH4) = 200 N m ' (quenching by meth- ane was being investigated when this trace was taken]. Vertical scale = 10 mVldivision: horizontal scale = 0.2 msec/division.

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 <0.1% ot 02: H O.C . Ltd.,, taken directly from thr cvlinder wthuut further puriticetlon. Methyl i o d h ~ a i lht)roughl? degasred bv rrpcat~d trapping at I l P K , using a conventional high vacuum line. Mixtures of CHII and nitrogen (typically 1% CHd in N d were made up in a one liter bulb and allowed to mix for 2 hr. A few minutes before the main experiment was due to start, the atomic lamp was switched on and allowed to stabilize. A sample of the mixture was then introduced into the reaction vessel (PtOtar -- 4 kNm-2)9 and flashed. Figure 3 illustrates a typical trace ah- served on the oscilloscope under such conditions. If the Beer- Lambert relationship holds, (see below for further discussion) then the atomic concentration should be proportional to In (411) (where 10 is the intensity of light transmitted in the absence of an absorbing atomic species, and I, the intensity in the presence of such species, see Figure 3), and a plot of In In (ZolD against time will yield a straight line if the decay is exponential (see Figure 4). Under the conditions described, the decay is essentially first order and the slope of such a plat yields the first-order rate coefficient for the decay of I(52P1i2).

BThermal Syndicate Ltd.. P.O. BOX NO. 6, Wallsend, Northum- berland, NE28 6DG. Cost of Thermaline probe, C40 + Spectrosil iodine lamp 4525. Mains ripple on the output of the lamp is sometimes observed, the percentage ripple depending on the posi- tion of the lamp in the holder. It is recommended therefore that a smoothed power supply he used (circuit diagrams supplied by Thermal Syndicate; a smoothed supply is also available directly for an extra cost of E10).

'The normal probe lamp holder does not accept the iodine lam0 which was desiened far use with a slizhtlv different nawer - ~. . supply, however, a simple modification is readily carried out (de- tails given by Thermal Syndicate).

8Personal communication from E. Hutchinson, Thermal Syndi- cate.

9 1 kNm-2 = 7.5 mm Hg.

52 / Journal of Chemical Education

The decay of I(5zP11~) is due mainly to quenching by CHJ and Nz, diffusion and spontaneous radiation being negligible under conditions used.j Thus

d[12p,d -- = dt h[NJIIZPml f kzlCH~IIIIZP~~~I

and the measured decay coefficient is equal to kl[Nzl + k2[CH31]. By repeating the experiment, keeping the pres- sure of Nz constant, and varying the partial pressure of CH& both kl and kz can be determined from a plot of the first-order rate coefficients against the partial pressure of CH3I.

In practice the Beer-Lambert relationship for the atom- ic absorption only holds in a modified form oiz.

In f$\ = a(d)' \' I

where a is the extinction coefficient, c the concentration of atoms, and 1 the length of the ahsorbing medium; as we will be dealing with first-order kinetic processes, the abso- lute values and units of a, c and 1 need not be considered. y is a correction factor to the normal relationship which must be determined empirically. The reason for the de- parture from the simple Beer-Lambert relationship may be seen in principle from Figure 5; the emission from the atomic lamp is broad and reversed a t the line center, thus absorption (shaded area) a t the line center will be satu- rated at all but the lowest atomic concentrations. This will clearly lead to a complex relationship between the

frequency - Figure 5. Diagramatic representation of the probable emission (solid line) and absorption (dotted line) line shapes. Shaded area represents light absorbed.

absorption and atomic concentration; however, over a lim- ited range of concentrations the above relationship involv- ing y is found to be adequate. y may be found by carrying out experiments with varying amounts of CHjI in the reaction vessel, but with the same total pressure of nitro- gen. As methyl iodide is a weak absorber and only a few percent of the flash energy is absorbed under the condi- tions used, the concentration of I(52P11z) produced will be propoitional to the initial pressure of CHjI in the reaction vessel. By extrapolating plots of In In (lolo backwards t o t = 0, one obtains a value for y in FPl1zlt=o plus a con- stant. Thus a plot of the intercepts against the pressure of CH31 yields a line whose slope is y. Using this procedure we have determined y to be 0.81 & 0.14 under the condi- tions recommended for this experiment. Thus the values of kl and kZ obtained above should be multiplied by l / y = 1.2 to obtain the true quenching constants. The values we have determined for kl and kz are (2.6 f 0.5) X 10-l6 and (2.8 & 0.6) X 10-l3 ml molecule-' sec-', respectively which compare with literature values of kl = 2.1 X 10-l6 ml molecule-' sec-' and kz = 17 X 10-l3 ml molecule-' sec-I (see footnote 3 and refs. therein).. The discrepancy with the literature value for kz is interesting as our value is close to values observed for other alkyl iodides; methyl iodide was previously thought to he more efficient in quen~hingI(5~P1~z), than the other alkyl iodides.

Other Experiments

We have also carried out a number of experiments with CFJ as the source of I(52P~,Z) atoms, and observed ex- tremelv slow decav rates in ameement with the literature. $uenching by C F ~ I is considerably less efficient than ouenchine bv CHaI and i t is thus possible to measure dif- fkion coefficientsfor I(52PllZ) in the inert gases and even the radiative lifetime of I(52P11Z). The use of extremely pure inert gases is essential in these experiments (less than 1 ppm of 02) and they are thus expensive to carry out, particularly if mistakes are made. With the present apparatus we have only performed a few experiments in this area: the results are in qualitative acreement with published work and demonstraie the flexibifity of such ex- ~eriments. A more detailed description of this type of ex- periment has been given in the literature and reviewed in convenient form3

Clearly, the apparatus described here may be used to study a wide range of reactions and could he used to con- duct some original research. Indeed, i t was the potentially open ended aspect of the experiments which first encour- aged us to design the apparatus. An enterprising under- graduate might thus be able to design his own experi- ment, while less adventurous students may carry out a standard experiment.

We are indebted to Professor C. Kemball and Dr. D. Taylor for their support and encouragement. We also thank those members of the Honors Class (1972-73) who performed the experiments described here.

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