Jeffrey I. Steinfeld I A Molecular Fluorescence Experiment Harvard University

Cambridge. Mossach~serrs I for Undergraduate Physical Chemistry

A well-defined problem in electronic spec- tra of molecules is an important part of an uodergradu- ate physical chemistry laboratory course. I t is also desirable that, in addition to demonstrating some a s pects of molecular quantum mechanics, the experiment should expose the student to practice in modern spec- troscopic techniques. This has generally taken the form of an arc spectrum in air (1) or an iodine vapor absorption spectrum (3). These experiments have the disadvantage of confronting the student with a larger amount of data than can be conveniently ana- lyzed within the scope of a brief laboratory exercise. We have recently developed a laboratory experiment based on the resonance fluorescence of molecular iodine, capable of yielding quite precise results, for use in lab- oratory work accompanying the third year physical chemistry course at Harvard College. The experiment consists of exciting a single rotation-vibration level1 in the B3&,+ state of iodine by means of the mercury

' It should he noted that the level excited has recently been shown (3) to be v' = 25, J' = 34; previous authors have identi- fied it ass' = 26, J' = 34.

green line a t 5460.75 &, and observing the vibrational band progression in fluorescence.

The sample and excitation source are quite inex- pensive and, once assembled and mounted, require no further attention from the teaching staff. The spec- trum should be taken with a grating spectrograph of moderate resolving power, since prism instruments have poor dispersion in the region of interest for this experiment. The most accurate results are obtained by analyzing the data with the aid of an electronic computer, but the data may equally well be analyzed manually, with some sacrifice in speed and accuracy.

Since most textbooks do not go into sufficient detail for the purpose of understanding this experiment, we have found it useful to assign additional reading. Por- tions of the texts by Barrow (4) and Herzberg (6) cover all the theoretical material required. Very read- able, complete, and well-documented accounts of this particular system are given by Wood (6) and Pring- sheim (7). Information about experimental principles is given by Harrison et al. (8). Allowance of time for this library work should he made when scheduling the experiment.

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Apparatus

The fluorescence cell is shown in Figure 1. The front window is a circular Pyrex optical flat, obtainable from the A. D. Jones Company, Cantbridge, Mass. The cell body is a 20-cm length of Pyrex tubing on which are glassblown a sample well, a seal-off tube, and a "Wood's Horn." The end is painted black to reduce scattered light. Reagent grade iodine is distilled into the sample well and the cell sealed off under a vacuum of Torr or better. The iodine well is maintained a t room tem- perature.

Figure 1. Diagram of fluorarcence cell,

The excitation source is a General Electric UA-3 medium-pressure quartz mercury arc. The arc, com- plete with reflector, stand, and plug-in supply, is avail- able from the George W. Gates Con~pany, Franklin Square, New York, for about $200. The fluorescence cell is mounted at the focus of a reflector similar to the one in which the arc lamp is mounted, having holes bored in the end plates. This is then mounted con- focally with the lamp reflector for most efficient illumi- nation. A filter of didyminm glass, available from Corning Glass Company for about $40, should be inter- posed between the lamp and the cell to cut out the interfering yellow lines of mercury. I t may also be worth while to cool the reflectors with a squirrel-cage fan. The complete opt,ical system is shown in Figure 2.

Figure 2. Diagram of opficol system, taken from labomtory mamud.

For a standard spectrum, a Cenco neon discharge tube, operated from a sign transformer and a Variac variable autotransformer, may be used. The tube should be mounted so that it may be easily removed and replaced on the optical bench. For best results, the discharge should be operated a t as low a voltage as possible-1000 v ac, or less.

The spectrograph we have used is a Jarrell-Ash 1.5- meter Wadsworth instrument, giving a film dispersion of 10 A/mm, and priced a t $4000. The spectra are taken on Eastman Kodak Royal-X Pan Recording 35-mm film. Also suitable would be the Applied Re- search Laboratories' "Spectrographic Analyzer," priced

a t about $3000. This instrument has the advantage of including a built-in film developing apparatus, but does not have an adjustable entrance slit. Either of these instruments could profitably be shared with an analyti- cal chemistry laboratory course. The grating spec- trometer described by Schoenbeck and Tabbutt (9) in THIS JOURNAL, which can be built for a t,ot,al cost of $80, would also be quite 6at.isfactor.y.

Procedure, Calculations, and Discussion

First, a neon spectrum is taken by a 5-minute ex- posure with narrow entrance slits. The image is then displaced vertically by means of either a film rackmg device or a variable diaphragm, the entrance slit opened, and an approximately '/2-hour exposure taken of the iodine fluorescence. A typical spectrum is shown in Figure 3. The line positions should he read, if possible, on a traveling-microscope comparator. .4lternat,ively, positions may be read with a millimeter scale from a good enlargement. It is most convenient to tabulate differences from the mercury "green" line position. The teaching staff should give some help to the students with line identification, t,o avoid time-consuming m i s takes.

Figure 3. Typical fluorescence spectrum taken by undergraduate student. Upper line3 -re neon emission, lower bonds are iodine emirrioq with band numbering shown. Mercury liner are also identifled. Apparent curvature i s on ortifact of the enlarging process.

The neon lines may be identified from Plate XI1 in Pearse and Gaydon ( l o ) , and the exact wavelengths obtained from a recent edition of the Handbook of Chemistry and Physics (11). A dispersion curve is obtained either graphically, or by using a least-squares polynomial fitting program, available as 7.0.002 in the IBM 1620 SHARE Library. The curve is used to obtain the air wavelengths of the Iz emission bands, which are then corrected to vacuum wavenumbers by using the N.B.S. "Table of Wavenumhers" (12). Fi- nally, these data are used to obtain the ground state vibrational constants of I2 from the expression

" = "0 - (w."(u + 'I*) - w.z."(v + '!dP + wsy.Yu + '/dl + . . . )

either by using the polynomial fitting program or by successive numerical differences. By either method, it is possible to obtain w," and w,x," but no higher coeffi- cients. A typical set of data is given in the accompany- ing table. Distributions of results obtained by students in the spring term of 1964 are shown in Figure 4. These may he of use in determining grades for results sub- mitted by the students. The accepted values, a," = 214.519 em-', and a&." = 0.6074 cn-', are from Rank's latest determination (18).

86 / Journol of Chemical Educofion

Typical Data Obtained in an 12 Fluorescence Experiment

With w." = 215.14 em-I, w.zeU = 0.645 om-'. a From Rank and Rao (IS); first number is R(33) line, second

number is PG5I l ine . obscuredi;; 5790 6 A h e . Not observed due to Frenck-Candon effect

A number of topics can be probed in discussion ques- tions. Although the exciting line employed is too broad to produce a simple doublet spectrum, the bands are still sharp enough to indicate the importance or rotational selection rules, and the student can be asked how these selection rules make possible the determina- tion of ground state rotational constants. These con- siderations lead to a discussion of the Franck-Condon principle (3, 14, 16), which is dramatically demon- strated by the intensity alternation of the fluorescence bands in this spectrum. A third interesting question deals with the "Wood Effect," or energy-transfer bands

Figure 41.1. Histogram of distribution of students' rerultr for wen during the spring term of 1964. Ibl Similar hi3togmm for w.xeN distribution.

produced by the addition of foreign gases to the sample (6, 7). A particularly subtle point deals with the ex- planation of the AJ' = * 2n selection rule inmolecular collisions, which would be demonstrated by these bands.

Special Projects

This apparatus is easily adaptable to a number of special laboratory projects. Raman spectra may be observed by replacing the I2 vapor cell with a liquid- filled cell of similar design. The ultraviolet-excited fluorescence spectra of aromatic hydrocarbons may be studied if the Pyrex cell is replaced by one made of quartz. In both these cases, the mercury arc itself is a convenient reference spectrum. Finally, the p h e nomenon of sensitized fluorescence might be investi- gated (16). It might also perhaps be mentioned that the mercury lamp employed in this experiment would find good use in an organic teaching or research lab- oratory, for the purpose of studying photochemical reactions.

Acknowledgments

The development of this experiment was made pos- sible by an equipment grant from the National Science Foundation. I would also l i e to thank Professors J. D. Baldeschwieler and A. H. Maki for supervising the construction and instruction phases, respectively, of this project, and Mr. J. A. van Zee for his expert technical assistance throughout the assembly and use of the laboratory facilities.

Literature Cited

(1) SHOEMAKER, D. P., AND GARLAND, C. W., "Experiments in Physical Chemistry," McGraw-Hill Book Co., New York, 1962, p. 321.

(2) S T ~ R D , F. E., J. CHEM. EDUO. 39, 626 (1962). (3) STEINFELD, J. I., ZARE, R. N., JONES, L., LESK, M., AND

KLEMPERER, W., J . Chm. Phys., in press (1964). (4) BARROW, G. M., "Introduction to Molecular Spectroscopy,"

MuGraw-Hill Book Co., New York, 1962, Chaps. 1, 2, 4, 1" a".

(5) HERZBERO, G., "Spectra of Distamie Molecules," 2nd ed., D. Van Nostrand Co., Inc., Princeton, N. J., 1950, Chaps. 2, 3, 4, 5.

(6) WOOD, R. W., "Physical Optics," 3rd ed., Maemillan Co., New York, 1934, pp. 61648.

(7) PRINGSHEIM, P., "Flu~re~cence and Phosphorescence," Interscience Publishers (John Wiley & Sons, Inc.), New York, 1949, pp. 15147, 19%201.

(8) HARRISON, G. R., LORD, R. C., AND Loo~Bou~ow, J. R., "Practical Spectroscopy," Prentice-Hall, Inc., Engle wood Cliffs, N. J., 1948, Chaps. 2, 4, 5, 6, 7, 11.

(9) SCHOENRECIC, R., AND TABBUTT, F. D., J. CHEM. EDUC. 40, 452 (1963).

(10) PEARSE, R. W. B., AND GAYDON, A. G., "Identification of Molecular Spectra." Chapman and Hall Ltd.. London, 1950.

(111 "Handbook of Chemistrv and Phvsies." 44th ed.. Chemical Rubber Puhlishine c;.. levela and. bhio. 1962: o. 2966. ~~ ~~~ - ~ ~ - - ~ , ~ -~ ~, ~ , ~ . .

(12) COLEMAN, C. D., BOZMAN, W. R., AND IMEOGERS, W. F., "Table of Wavenumbers," Vol. I, National Bureau of Standards Monograph 3, Washington, D. C., 1960.

(13) RANK, D. H., AND h 0 , B. S., J . M d . S p e d . 13,34(1965). (14) CONDON, E. U., Am. J . Phys. 15, 365 (1947). (15) ZARE, R. N., J. Chem. Phys. 40, 1934 (1964). 116) MITCHELL. A. C. G.. AND ZEMANSKY. M. W.. "Rewnance

~ad i s t i dn and ~xc i t ed Atoms," dambridge University Press, New Yark, 1961, pp. 59-89.

+ + +

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