dominant. This results in a decrease of the p.t.p, height of the composite AI,<LL peak, and thus a negative-going profile. This phenomena leaves the analyst in a dilemma, especially if he or sire is interested in more than a qualita- tive profile map.

One way to overcome the problem of the N'(E) peak shape-related profile artifact is to use N(E) data for pro- filing, Grant et al. 9 have shown the advantages of integrat- ing NP(E) data in terms of N'(E) peak shape changes and quantitative use of N(E) peaks. Using a digital recording and computer reduction scheme similar to that described by Solomon and Baun, '° construction of profiles from N(E) spectra is a comparatively simple technique. The entire digitized Auger N'(E) spectrum is repetitively re- corded on magnetic tape while sputter etching, and after- wards it is integrated and computer plotted. Fig. 4 shows a typical N'(E) spectrum from an aluminum oxide surface along with the corresponding computer-integrated N(E) spectrum. From these spectra, energy regions of peaks of interest are selected and input to the computer for peak height determinations. Fig. 5 shows two computer plotted A1KLL profiles on the same aluminum oxide specimen Auger analyzed in Fig. 4. The lower profile, constructed from A1KLL N'(E) p.t.p, heights, shows the previously discussed artifact at the oxide-metal interface. The upper profile, constructed from the A1KLL N(E) peak, does not exhibit an artifact. In addition, since the N(E) peak is truly quantitative, this profile is more sensitive to differ- ences in the A1 concentration in the metal vs the oxide. Although using N(E) peak heights removes the artifact in the case of AIKLL, in general, more accurate quantita- tive profile contours can be obtained using doubly inte- grated N'(E) data or the area under the N(E) peaks, since the profile should be entirely independent of peak shapes to be truly quantitative. 1'

The apparent data scatter in the Fig. 5 profiles is not completely understood. A partial contribution is the exag- geration of small differences when the data are expanded and normalized without being smoothed. Another possible

cause is that no time constant was used when recording the N'(E) data. If the scatter is real, a possible contribu- tion is the nature of the particular surface during ion etching and the effects on the Auger yield.

I1. CONCLUSIONS

In conclusion, a number of factors that can introduce erroneous artifacts into Auger profile contours must be considered if one is interested in more than a qualitative depth profile map. In the case of A1KLL, a profile contour artifact at the oxide-metal interface, introduced by the change in peak shaped A1KLL, is misleading in terms of A1 concentration. Other peaks that were found to exhibit the same phenomena were MgKLL and TiLMM, the latter being very subtle. An optimistic note concerning the A1KLL artifact is that it offers a reference point for defining the interface. However, whenever profile artifacts are sus- pected, precautions are in order in interpretations and possible alternate means, such as N(E) peak heights, should be considered to construct profiles.

ACKNOWDLEGMENT

Research was sponsored by the Air Force Materials Laboratory, Air Force Systems Command, United States Air Force, Contract F33615- 73-C-5099.

1. L. A. Harris, J. Appl. Phys. 39, 1419 (1968). 2. N. J. Taylor, Rev. Sci. Instr . 40, 792 (1969). 3. P. W, Palmberg, R. E. Reach, R. E. Weber, and N. C. MacDonald,

Handbook of Auger Electron Spectroscopy, (Physical Electronics In- dustries, Inc., Edina, Minn., 1972), p. 5.

4. W. M. Mularie and T. W. ILush, Surf. Sei. 19, 469 (1970). 5. E. J. Scheibner and L. N. Tharp, Surf. Sci. 8, 247 (1968). 6. G. W. Simmons, J. Colloid Interface Sci. 34, 343 (1970). 7. L. H. Jenkins and M. F. Chung, Surf. Sci. 28, 409 (1971). 8. A. P. Janseen, R. C. Schoonmaker, A. Chambers, and M. Prut ton,

Surf. Sci. 45, 45 (1974). 9. J. T. Grant, T. W. Haas, and J. E. Houston, Phys. Letters 45A,

309 (1973). 10. J. S. Solomon and W. L. Baun, J. Vac. Sci. Technol. 12, 375 (1975). 11. J. T. Grant, T. W. Haas, and J. E. Houston, J. Vac. Sci. Teehnol.

11,227 (1974).

Low Temperature Singlet Quenching Experiments by Heavy Atom- Containing Molecules: Anomalies in the Amount of Triplet.Triplet Absorption

G A B R I E L LORD a n d GILLES D U R O C H E R

D@artement de chimie, Universitd de Montreal, C.P. 6210, Suet. A, Montreal, Quebec HSC S V1, Canada

A d d i t i o n of s i n g l e t q u e n c h e r m o l e c u l e s l ike b r o m o b e n z e n e , e t h y l i od ide , and d i m e t h y l mercury to degas s ed s o l u t i o n s of a n t h r a c e n e and p h e n a n t h r e n e in EPA at 77°K leads n o t

Received 30 June 1975; revision received 10 September 1975.

on ly to a decrease in f luorescence m e a s u r e d in s m a l l g lassy s a m p l e s , b u t also to an a b n o r m a l decrease in t h e a m o u n t of t r ip l e t s t a t e a b s o r p t i o n observed u p o n f lash e x c i t a t i o n of t h e s e s o l u t i o n s in large v o l u m e - c o n t a i n i n g low t e m p e r a - t u r e ce l l s . It is s u g g e s t e d t h a t great care s h o u l d be t a k e n

Volume 30, Number 1, 1976 APPLIED SPECTROSCOPY 49

in order to be sure of t h e per fec t h o m o g e n e i t y in t h e d i s - t r i b u t i o n o f t h e q u e n c h e r m o l e c u l e s t h r o u g h o u t t h e s a m p l e i n v e s t i g a t e d at l ow t e m p e r a t u r e . It is s h o w n t h a t t h e r a n - d o m d i s t r i b u t i o n c a n h a r d l y be a t t a i n e d , in v iew of t h e h i g h c o n c e n t r a t i o n o f q u e n c h i n g m o l e c u l e s needed in t h e s e ex- p e r i m e n t s , w h e n t h e c o o l i n g process is s l o w as is t h e case in a large v o l u m e - c o n t a i n i n g low t e m p e r a t u r e ce l l .

Index Headings: E m i s s i o n s pec tros copy; S i n g l c t - t r i p l e t en - h a n c e m e n t ; F l a s h p h o t o l y s i s spec troscopy; F luorescence ; l n h o m o g e n e i t y ; L u m i n e s c e n c e .

I N T R O D U C T I O N

Since the appearance of the flash photolysis technique in the 1950's, 1' 2 many applications related to triplet- triplet absorption spectra of organic molecules have been reported/. 4 Medinger et a l / , 6 and Hadley and Keller 7 followed by Vander Donckt et al . s, 9 used the flash tech- nique to promote triplet-triplet absorption spectra in or- der to obtain the triplet quantum yield of aromatic hy- drocarbons in fluid solutions at room temperature. This procedure involved the diffusion quenching of the first singlet excited state of the aromatic molecules by a heavy atom quencher molecule having the property of increasing the intersystem crossing probability from the S~ state to the T1 state of the aromatic solute under irradiation.

In the course of our photophysical studies on aromatic amines molecules ~°' it and on some pyranne and thio- pyranne derivatives, ~2 it became obvious that experiments like those performed by Medinger et al . and Vander Donckt et a l . , but at low temperature in glassy matrices, would be worth doing. In other words, it appeared to us interesting to study the intramolecular electronic energy transfer at low temperatures in glassy systems perturbed by molecules containing heavy atoms.

In this paper we want to put emphasis on the non-ran-

F

4.00

3.00

2.00

1.00

I I I I ~oo 2.00 &O0 4..o0

¢~DT

FiG. 1. Variation of the relative fluorescence intensities with the relative amount of triplet state absorption with different bromo- benzene concentrations.

TABLE I. Fluorescence q u e n c h i n g and tr ip le t - tr ip le t opt i - cal dens i ty o f anthracene (3.0 > 10 -6 M) measured at 420 n m in EPA at 77°K.,

[Et I] F ° / F D r (M) (=t=0.05) (~0.009)

0 1.00 0.202 0.17 2.25 0.207 0.33 3.49 0.193 0.50 5.36 0.167

dom distribution of quenching molecules across the path of low temperature cells containing large volumes of solu- tion, and its consequences for the triplet-triplet absorption optical density.

I. E X P E R I M E N T A L

Triplet-triplet absorption spectra of anthracene and phenanthrene were obtained using conventional flash photolysis techniques/, 3, 5

The room temperature (12-cm optical path) and low temperature (7-cm optical path) ceils were illuminated by two flash tubes (the photolysis flash) and a 500-W tung- sten lamp to obtain the triplet-triplet absorption spectrum of the solution at various wavelengths fixed by a 0.25-m Jarrell-Ash monochromator coupled to a 1P28 photo- multiplier tube. The output of the photomultiplier was fed to a Tektronix memory oscilloscope model number 564B. The characteristics of the flash lamps are the fol- lowing: capacitor, 6 #F; voltage 9.5 kV; flash duration 40 gsec. A 1-cm path filter solution was used in order to isolate the first absorption band of the aromatic hydro- carbons from the first electronic absorption system of the quencher molecule. It contained potassium nitrate (2 M) when bromobenzene or ethyl iodide was used as quencher and potassium biphtalate (0.3 M) when dimethyl mercury was used as the quencher.

The fluorescence quenching and the phosphorescence lifetime experiments were done with an Aminco-Bowman spectrofluorophosphorimeter as described previously.18, 14 Dissolved oxygen was removed from all the solutions by cycles of freezing and pumping down to less than 10 -4 mm Hg.

Anthracene and phenanthrene were zone-refined 99.999% products from Materials Limited. Ethanol sol- vent was distilled over sulfuric acid and EPA (ether, isopentane and ethanol, 5:5: 2). Spectroquality from Matheson, Coleman and Bell was used as received. Quencher molecules, bromobenzene, ethyl iodide and di- methylmercury were 99.5 % pure and were used as re- ceived.

II . R E S U L T S A N D D I S C U S S I O N

In order to improve our experimental set up, we have reproduced the results obtained by Medinger and Wilkin- son. 5 They measured the triplet quantum yield of anthra- cene in ethanol at room temperature, using xenon as the singlet quencher. 15 The kinetic formulation gives the following equation: 5

( F ° / F - 1) = ( F ° D r / F D r ° - - 1)¢~° (1)

50 Volume 30, Number 1, 1976

where F°/F is the ratio of the fluorescence intensities in the presence (F) and absence (F °) of bromobenzene, and Dr~Dr ° is the ratio of the triplet state absorption optical densities in the presence (Dr) and absence (Dr °) of bromo- benzene. This equation is strictly valid only if kT/k6, the ratio of second order rate constants for deactivation of the first excited singlet to the ground state and to the triplet (T~) state by the quencher molecule, is negligible. This was shown to be the case for the system investigated here. 5

Fig. 1 shows the variation of the ratio of the fluorescence intensities with the quanti ty (F ° D r / F Dr ° - 1). The concentration of the bromobenzene was varied between 0 and 1.5 M. In these limits of concentrations, the quench- ing of the fluorescence intensity obeys a perfect Stern- Volmer relationship. In all these experiments the con- centration of anthracene was kept constant at 3.0 X 10 -5 M. The slope of the graph plotted in Fig. 1 gives us the triplet quantum yield ~r equal to 0.65 ~ 0.05. This is considered to be in good agreement with the results published by Medinger, Wilkinson, and Horrocks, 5, ~5 Approximately the same results have been obtained with ethyl iodide and dimethyl mercury used as quenchers. All these three quenchers have also been used in order to quench the first singlet excited state of anthracene in a low temperature glassy matrix of EPA. A specific example of the results obtained is given in Table I with ethyl iodide as the particular quencher. The fluorescence in- tensity ratio is no longer a linear function of the quencher concentration as observed at room temperature, but varies like the exponential value of the concentration of ethyl

1.0

O.5

d Ln D~ 1 dt

(msec 4 )

......................... XlO

X lO . . . . . . . . . . . . . . . . . . . . . . . . .

I I I I

0,10 0.1'2 0.1'..4//' 0.{0 &½'5 0,3 5 DT

Fro. 2. Variation of the quantity d in D-~T/dt with the triplet- triplet absorption optical density of anthracene in ethanol at room temperature ( ) and in EPA at 77°K containing no ethyl iodide ( . . . . ) and 0.33 M ethyl iodide (.--.).

iodide as expected for static quenching in rigid matrices. TM

Contrary to the room temperature results (Fig. 1), the triplet-triplet absorption optical densities regularly de- crease with the increase in the quencher concentration.

This result can only be interpreted by a decrease in the concentration of triplet anthracene molecules along the center path of the low temperature cell (7.0 cm long by 2.0 cm diameter), used in the flash photolysis mode, when the ethyl iodide concentrations are increased. The de- crease in the triplet anthracene concentration can result from anthracene pair interaction in the matrix to form stable dimers or excimers, or from any distribution in- homogeneity in the quencher molecules along the width of the low temperature cell thus contributing to a decrease in the transmission of the intensity of the flash lamp. All the fluorescence work has been performed in an Aminco dewar that contains 2-mm i.d. cells that are very rapidly cooled down.

I t is doubtful that anthracene molecules at a concen- tration as low as 3.0 × 10 -4 M could interact with each other in low temperature rigid matrices without Mlowing for the softening of the rigid glass. 17, 18 We looked for any possible triplet-triplet annihilation event by the calcula- tion of the second order triplet-triplet annihilation rate constant kr in ethanol at room temperature and in EPA at 77°K. The decay of the triplet state is described by the well known kinetic equation:

- (d [T~]) /d t = (kp + k,p) [T~] 4- kr[r~] ~ (2)

where ke is the phosphorescence rate constant, kip is the nonradiative T~ - So intersystem crossing rate constant, and kr is the second order triplet-triplet annihilation rate constant. When one follows the triplet-triplet absorption opticM density Dr with time after the flash, Eq. (2) be- comes :s

(g In Dr-1)/dt = kp 4- Ik~e -4- kr Dr/erl (3)

where er is the triplet-triplet molecular absorption co- efficient taken to be equal to 6.5 X 104 M -1 cm -1 19 at 420 nm and 1 is the optical path length, which is 12 cm at room temperature and 7 cm at 77°K. Fig. 2 shows the linear relationships that exist between the quantity d In Dr-1/dt and the optical density Dr. Triplet-triplet an- nihilation clearly contributes to the kinetics of triplet depletion at room temperature where the kinetic is dif- fusion-controlled, s° A rate constant kr of 2.3 X 109 M -1 sec -I has been measured from the slope of the graph. As expected, 21 this result is not far from the diffusion rate constant of 5.4 × 109 M-lsec -1 in ethanol at room temperature. 21 Results obtained from Fig. 2 show that triplet-triplet interactions are not playing a significant role in the low temperature EPA matrix, whether or not it contains ethyl iodide as a quencher molecule.

In order to be able to check for the possible inho- mogeneity in the distribution of the quencher molecules in large low temperature cells used in flash photolysis, compared to the small low temperature cell used when working with the Aminco spectrofluorimeter, we com- pared the triplet lifetime measured from the phospho- rescence decay (r~ b) in small cells to that measured from the decay of the triplet absorption in the large low tem- perature cell (r~). Since anthracene phosphoresces at

APPLIED SPECTROSCOPY 51

TABLE II. P h o s p h o r e s c e n c e l i f e t i m e s o b t a i n e d by m o n i - tor ing t h e t r i p l e t - t r i p l e t a b s o r p t i o n o p t i c a l d e n s i t y (rv a) a n d t h e p h o s p h o r e s c e n c e i n t e n s i t y (rp b) w i t h t i m e for vari- ous q u e n c h e r c o n c e n t r a t i o n s .

T p a T p b

[Et I] D r (see) (see) (M) (±0.009) (±0.2) (±0.2)

0 0.160 3.6 4.2 0.5 0.150 3.6 2.8 1.0 0.130 3.8 2.1 2.0 . . . . . . 1.0

700 nm, a region of very low sensitivity of the Aminco used with an emission grating blazed at 500 nm, phen- anthrene has been used as the solute at a concentration of 3.0 × 10 -5 M in EPA. The phosphorescence intensity centered at 492 nm has been monitored for the lifetime experiments. The triplet-triplet absorption spectrum has been registered and it has been found to be identical with that published by Craig and Ross 22 showing well defined maxima at 492, 460, and 430 nm.

Table II shows unambiguously that the quencher molecules behave normally in small cells, for the phos- phorescence lifetime (%b) is much affected by the external heavy atom. On the other hand, in a large cell, no quench- ing effect could be detected on the decay of the triplet- triplet absorption measured at the center of the cell, showing the existence of a non-random distribution of the quencher ethyl iodide molecules throughout the diameter width of the cell. This result has also been obtained with bromobenzene and dimethylmercury used as quencher molecules.

From these results one can suggest an explanation for the monotonic abnormal decrease of the triplet-triplet optical density with the increase in the quencher con- centration. At a concentration of 2.0 M in ethyl iodide (Table II) the low temperature glassy matrix was no longer visually transparent. One can think that at lower quencher concentration, the microscopic transparency of the glass viewed laterally to the cell by the flash lamps decreases with the quencher concentration increase. It might also be that the quencher molecule absorbs photons

from the flash lamp due to the strong inhomogeneity in the distribution of the quencher molecules.

One has to take care in performing low temperature quenching studies in cells containing large volumes of solutions, for the time required to stabilize the tempera- ture is large enough to allow for the partition of the mixture components.

ACKNOWLEDGMENTS

This work is a part of a project of collaboration on the photochemistry of oxygen- and sulfur-containing chromophorcs, supported by a grant from the Minist~re de l'Edueation de la Province de Qu6bec. We wish to express our sincere thanks for this financial support. We are very indebted to Miss Lucie Baril for valuable help in the experimental work.

1. R. G. W. Norrish and G. Porter, Nature 164, 658 (1949). 2. G. Porter, Proe. Roy. Soc. (London), A200, 284 (1950). 3. G. Porter and M. W. Windsor, Proc. Roy. Soc. (London) A245,

238 (1958). 4. Y. H. Meyer, R. Astier, and J. M. Leclere, J. Chem. Phys. 56, 801

(1972). 5. T. Medinger and F. Wilkinson, Trans. Farad. Soc. 61,620 (1965). 6. A. R. Horroeks, T. Medinger, and F. Wilkinson, Chem. Commun.

19, 452 (1966). 7. S. G. Hadley and R. A. Keller, J. Phys. Chem. 773, 4351 (1969). 8. E. Vander Donckt and J. P. Van Bellinghen, Chem. Phys. Lett. 7,

630 (1970). 9. E. Vander Donckt and D. Lietaer J. Chem. Soc. Trans. Farad. Soc.

1, 68, 112 (1972). 10. D. Muller, M. Ewald, and G. Durocher, Can. J. Chem. 52, 407

(1974). 11. D. Muller, M. Ewald, and G. Durocher, Can. J. Chem. 52, 3707

(1974). 12. G. Lord, L. Baril, and G. Durocher, (to be published) 13. M. Ewald, D. Muller, and G. Durocher, Spectrochim. Acta 29A,

1051 (1973). 14. Y. Beauehamp and G. Duroeher Spectrochim. Acta in press. 15. A. R. Horrocks and F. Wilkinson, Proe. Roy. Soc. (London) A306,

257 (1968). 16. S. Siegel and H. S. Judeikis, J. Chem. Phys. 48, 1613 (1968). 17. E. A. Chandross, J. Ferguson, and E. G. McRae, J. Chem. Phys.

45, 3546 (1966). 18. G. Goumet, M. Martinaud, and G. Nouchi, C. R. Acad. Sci. S6r.

B 268, 1572, 1739 (1969). 19. E. J. Land and A. J. Swallow, Trans. Farad. Soc. 64, 1247 (1968). 20. G. Porter and F. Wilkinson, Proc. Roy. Soe. (London) A264, 1

(1961). 21. C. A. Parker, Photoluminescence of Solutions (Elsevier, New York,

1968), p. 75. 22. D. P. Craig and I. G. Ross, J. Chem. Soc. 1589 (1954).

Analysis for the Platinum Group Metals and Gold by Fire.Assay Emission Spectrography

E L M O F . C O O L E Y , K E N N E T H J . C U R R Y , a n d R O B E R T R. C A R L S O N

United States Geological Survey, Denver, Colorado 80225

P l a t i n u m , p a l l a d i u m , r h o d i u m , r u t h e n i u m , i r i d i u m , and gold are d e t e r m i n e d by d irec t spec trograph ic ana lys i s of a f ire-assay si lver bead. F ire -assay m e t h o d s and s t a n d a r d s

Received 17 March 1975; revision received 29 July 1975.

prepara t ion are descr ibed . T h e f ire .assay bead is p laced i n t o a graph i t e e l ec trode c o n t a i n i n g 3 m g of (NH4)zOsCI6 and arced at 15 A dc in an argon-oxygen a t m o s p h e r e . Wi th t h e m e t h o d s o u t l i n e d , d e t e c t i o n l i m i t s for a 15-g s a m p l e are I ppb for Au and Pd, 2 ppb for Pt and Rh, and 100 ppb for Ru and It.

.52 Volume 30, Number 1, 1976 APPLIED SPECTROSCOPY