Volume 4. number 9 CHEMICAL PHYSICS LETTERS 15 Janunry 1370

LUMINESCENCE BANDS OF MOLECULAR OXYGEN FROM A TUNGSTEN LAMP.

THE POSSIBLE ROLE OF SINGLET MOLECULAR 0XYGE.H

IN SURFACE CATALYZED OXIDATION *

AHSAN ULLAH KHAN Instifufe of Molecular Biophysics, Florida State Universify.

Tallahassee, FlonXa, USA

Received 1 December 1969

Sharp emission bands superimposed on ordinary incandescent tungsten lamp output have been observed. These definite spectroscopic bands have been tentatively assigned to Single and Simultaneous Transitions in molecular oxygen. An alternative explanation of a perturbed molecular oxygen, [W.. _ 021. has not been completely ruled out. A possible relation of this novel method of generation of Singlet Molecular Oxygen to heterogeneous catalysis has been pointed out.

We wish to report the generation of electron- ically excited molecular oxygen in ordinary in- candescent tungsten filament lamps. Using these lamps, we have not only observed emission from the singlet metastable states of molecular oxygen, but : I TO from simultaneous transitions in which two 9 *‘, excited singlet species interact to yield a sing,, photon.

Recently McCarroll [l] reported that the chemisorption on a clean tungsten surface at room temperature of molecular oxygen at 5 - 50& pressure generates a very weak luminescence. He studied the lifetime and the kinetics of decay for this luminescence by using a refrigerated 9558QB photomultiplier and narrow band inter- ference filters and found that “most of the lum- inescent radiation is distributed between 520 and 560 run with some emission between 560 and 620 run, approximately”, We have observed these two emissions as definite spectroscopic bands, which appear strongly in our tungsten lamp emission spectra, and have observed in addition several other transitions. We believe that all these transitions originate in molecular oxygen which is present in the incandescent lamp as an impurity and is used cyclically to elicit the ob- served emission. We believe that the two bands recorded by McCarroll are due to simultaneous

* This work was supported by a contract between the Division of Biology and Medicine, 75. S. Atomic Ener- gy Commission and the Florida State University.

transitions in molecular oxygen and are simiLar to the strong bands that appear in the chemiIu- minescence accompanying the decompasition of

E202 [2,31. The experimental setup consists of tungsten

filament lamps (a) GE CPR, L&V, 6A; and (bl GE BVS, 5V, 6.5A; and the detecting systems of the Gary 14 Recording Spectrophotometer. The Gary 14 was used as a single beam monochroma- tar in its reference setting to record the spectrai output of the lamps. Using the PbS detector in the infrared mode we recorded the spectral output (fig. la) of tungsten lamp (a). Similarly we ob- tained the output (fig. lb) of lamp (b) using the visible mode and the IP28 photomuLtiplier of the cary 14.

Fig. la shows the striking emission-spectral features superimposed on the combined blackbody emission of the tungsten lamp. The recorded curve represents the resultant emissivity and response of the momcltuonzato~-PhS detects combination. Mainly because of the very differ- ent spectral sensitivity of the FP28 photomulti- plier, the spectrum obtained In the visible mode is quite different in appearance, but the superim- posed sharp emission-spectral features are dis- tinct and occur at identical wavelengths.

We offer the following tentative assignments for these transitions (vibrational assignments are suggested by the spectraL shifts observed re- lative to room temperature spectra). Band A oc- curring at 12 725A (7 858 cm-l) is the strongest

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Volume 4. number 9 CHEMICAL PHYSICS LETTERS 15 January 1970

observed (half bandwidth approximately .‘7c) cm-l) and represents the transitions: I$( v’=l) --t 3 z$P=l); l$(y’=2) - 3”;(“%2).

Band B occurring at 9 ‘76Oi (10 245 cm-l), a

weaker and a slightly broader band (half band- width approximately 190 cm-l), represents the transitions: lA (u’=2) - 3~5(v”=l);

1 +( ~‘=2) * 3 Zg( v”=O). Ban% C occurring at ‘7 750A (12 903 &n-l) is less intense than band A but has a similar half bandwidth (approximately

of the following transitions: 1~,&~=2) - 3~(~=2).

simultaneous transitions for molecular oxygen pair interactions, and ori- ginate from combinations of the single molecul$ states noted above. Band E occurring at 5 570 A (17 953 cm-l), approximate half bandwidth 250 cm-l, has energy almost identical to the sum of the energies of band A plus band B, and can be represented as: [‘Ag+l+] + [3%+8X& Band

D occurring at 5 918A (16 897 cm-l) with approx-

imate half bandwidth 25G cm-l also belongs to the same system as band E but the detailed vi- brational transitions are different and can be re- presented as: [IA~+IA~] 4 [3.Zit3Ci]. Baud F

occurring at 4420A (22624 cm-l) is weak and

appears only as a shoulder. This transition re- presents a vibronlc component in the following simultaneous transition: [ TZ -F 1 & Ls]-[3z~+“cg].

These assignments are based on the assump-

tion that electronically excited singlet molecular oxygen leaves the tungsten surface as a very hot moiecule with associated vibrations and high ro- tational excitation. It is assumed that the exact excitation is dependent on the nature of the sur- face, since the chemisorption of gases on metal surfaces is accompanied by the liberation of large quantities of thermal energy. The temper- ature of the filan:ent determines the rate of pro- duction of the excited species.

A possible alternative interpretation is that these transitions occur in the complex W-02. This interpretation cannot be entirely discarded, but the kinetic studies of McCarroll [l] establish the fact that bands D and E at least do not follow an exponential decay. McCarroll concluded that “independent emission of photons from lumines- cent centers is not the primary mechanism of de- activation of the excited entities. The oxygen/ tungsten luminescence indicates that an interde- pendence of activated centers is involved in the production of photons. ” McCarroll’s analysis qf the s&nificance of the chenzisorption lumirzes- cence hyperboZic decay curve conforms exactly to the description required for observation of simultaneous transitiotrs in nzokxul4zr oxygen Pail-s. Further work has to be done to evaluate the extent of perturbation of the electronic states due to surface interaction. The possibility of a trivial direct optical excitation can be ruled out

simply from consideration of the absolute transi- tion probabilities of these highly forbidden tran-

sitions.

Qooo wm WAVELENGTH, A.U.

Fig. 1. Emission spectra cf tungsten filament lamps - (a) GE CPR 18v 6A lamp with PbS detector, and (b) GE BVS 5V 6.5 A lamp with IP 28 photomultiplier obtained with a Gary 14 Recording Spectrophotometer.

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Volume 4, number 9 CHEMICAL PHYSICS LETTERS 15 January 1970

Owing to the recent surge of activity in the study of physical and chemical properties of sin- glet molecular oxygen, the solid surface sensi- tized generation of this species could prove to be very significant. The observation of Rufov et al. [4] that a vzry weak luminescence in the region of 6000 - 8000A is seen when oxygen molecules are exposed to the solid surfaces of a number of metal oxides indicates the possibility that such a solid surface sensitized generation of singlet oxygen may be a general phenomenon.

The present results clearly establish the par- ticipation of singlet molecular oxygen species in chemisorption luminescence of oxygen on solid surfaces. Since sensitized photo-oxidation studies [5] have demonstrated the role of singlet molec- ular oxygen species in photo-oxidation, it is nat- ural to assume that solid state surface catelyzed oxidation processes may involve singlet molec- ular oxygen as an intermediate. The mechanism of singlet molecular oxygen involvement in such heterogenous systems deserves further study.

I thank Professor bIichael Kasha for his help and encouragement at all stages of this problem.

REFERENCES

[l] B. McCarroll, J. Chem. Phys. 50 (1969) 4758. [2] A. U. Khan and XLKasha, J. Chem. Phys. 39 (L963)

2105: Nature 204 (1964) 241; J. Am. Chem. Sot. 86 (1966) 1574; .

S. J. Arnold, E. A.Ogryzlo and H. Witzke. J. Chem. Phys. 40 (1964) 1769: R. J. Browe and E. A.Ogryzlo. Proc. Chem.Soc. (1964) 117.

[3] A. U. Khan and M.Kasha, submitted for publication. [4] Yu. N. Rufov, A. A.Kadushin and S. 2. Roginskii,

Proc. Acad. Sci. USSR, Phys. Chem. Sec. (English transl.) 171 (1966) 777.

[5] A.U.Khan and D.R.Kearns, AdvanChem. i'i (1568)

143; D. R. Kearns and A. U. Khan, Photochem. Photobiol. 10 (1969) 193.

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