Proc. Natl. Acad. Sci. USA Vol. 77, No. 12, pp. 6952-6955, December 1980

Energy transfer from PO excited states to alkali in the phosphorus flame [metastable P0(4jj,)/(P0.P0)* excimerl AHSAN U. KHAN Institute of Molecular Biophysics and Department of Chemistry, State University, Tallahassee, Florida 32306 Communicated by Michael Kasha, August 21, 1980

ABSTRACT Phosphorus chemiluminescence under ambient cence is a visible continuum upon which are superposed a conditions of a phosphorus oxidation flame is found to offer an number of sharp emission bands. In 1938 Rumpf (9) confirmed efficient electronic energy transferring system to Ball regarding the ultraviolet atoms. The lowest resonance lines, 2P312,12-.S1/2, of potas- the observations of Ghosh and sium and sodium are excited by energy transfer when an emission from the reaction and attributed the visible emission stream at 800C carrying potassium or sodium atoms intersects also to PO. In 1957 Walsh (10) made a detailed investigation a phosphorus vapor stream, either at the flame or'in the post- of the ultraviolet bands originating from the A22+ and B2Z* flame region. The lowest electronically excited metastable 4fl; states of PO and suggested that an unknown 2z+ state was re- state of PO or the (P0OP)* excimer is considered to be the sponsible for the visible emission; In 1965 Cordes and Witschel probable energy donor. The (PO.;PO)* excimer resilts from the on the interaction of the "ll; stite of one PO with the ground (11) pointed out that the sharp band systems superposed 211r state of another. Metastability of the donor state is strongly visible continuum arising from the reaction of P4 vapor with indicated by the observation of intense sensitized alkali moist air were identical to the bands obtained by Ludlam (12) fluorescence in the postflame region. in 1935 from phosphorus burning in a hot flame; they attributed these bands to PO2. However, in 1963 Lam Thanh The observation of the cool greenish glow of phosphorus and Peyron (13) showed that the Ludlam bands originated not chemiluminescence has a long and singular history, being one from P02 but from HPO; and in 1974 Van Zee and Khan (14) of the most intriguing reactions studied from alchemy to substituted 2H20 for 'H20 in a moist P4 vapor stream and es- modern times. The element was isolated in 1669 by Henning tablished clearly that the sharp bands in the visible region Brand by reducing residues of ; his experimental generated by the reaction on contact with air originated from conditions were a closely guarded mystery and he was reputed HPO (2HPO). Van Zee and Khan (8) showed also that the to have discovered the secret of youth. However, shortly underlying visible continuum of the flame, previously attrib- thereafter succeeded in duplicating Brand's re- uted to P02 by Davies and Thrush (15), could not be indentified sults, published a scientific method for the isolation of elemental with any'simple electronic transition but exhibited the kinetic phosphorus, and reported that the mysterious glow associated and spectral characteristics of an excimer. In the'excimer state with the substance was an air-dependent chemical reaction (see of a dimeric pair only one member of the molecular pair orig- ref. 1 for a discussion of the early history and a comprehensive, inally is in an ; in the case of phosphorus'chemilu- excellent review). In 18907Thorpe and Tutton (2) made an minescence the excimer is (PO + PO*).'In summary, the unsuccessful attempt to identify chemically the emitting species emission band systems identified in ambient phosphorus in the reaction P4 + 502 = 2P205, by studying the oxidation chemiluminescence are: of lower . Several early spectrographic investigations of I. Ultraviolet region: PO the reaction were attempted, but the identification of the transient emitters awaited the theoretical work of Hund (3) and Major Mulliken (4) on the interpretation of band spectra,' published A2Y2 + X211: 228-270 nm; 'y-system (5, 9) in the late 1920s. Then in 1931 Ghosh and Ball (5), comparing B22z+ XX211: 325-337 nm; (very weak) (6, 8) the emission from an electric discharge tube containing P205 fl-system to the chemiluminescence of the P4 reaction, identified by band Minor analysis the ultraviolet A2z2+ X211 transition of the PO C2+ *X211: 199 nrn and longer wavelengths (8, 16) -y-system in both reactions. In 1938 Curry, Herzberg, and Herzberg (6) analyzed the other prominent ultraviolet band C'2A -.X21: 222.7 nm and longer wavelengths (8, 16) of PO generated in similar gas discharge experiments as a B22+ II. Infrared region: PO (17, 18) X211 transition, known as the PO fl-system. Ensuing high- G2,+ AB2zz+; F2,+ 22 + resolution studies have led to the characterization of the elec- tronic transitions of many small polyatomic phosphorus-con- F27,+ B2,+ ; A2z+ B2,+ taining species, and these investigations are an essential back- III. Visible region ground to the current understanding of the chemiluminescence r'eaction (7, 8). Minor HPO: A('A'") - X('A'); 450-650 nm (13, 14) In addition to the fine band systems in the ultraviolet region, Major Excimer: (PO...PO)* - (PO..PO); the major spectral feature of the atmospheric chemilumines- t--335-800 nm (8, 14). In this communication we report that at approximately room The publication costs of this article were defrayed in part by page and atmospheric , the products of the charge payment. This article must therefore be hereby marked "ad- molecule with moist'air sensitize vertisement" in accordance with 18 U. S. C. §1734 solely to indicate reaction of the phosphorus P4 this fact. the fluorescence emission of alkali metal atoms (Na and K) in 6952 Downloaded by guest on September 29, 2021 Chemistry: Khan Proc. Natl. Acad. Sci. USA 77 (1980) 6953 an argon stream at 80'C impinging on the phosphorus chem- iluminescence flame or the postflame region. The sensitized fluorescence from the metal vapors is unexpectedly intense, suggesting that the electronically excited metastable 4fli state of PO, the electronically excited (PO-..PO)* excimer states involved in the phosphorus oxidation reaction P4 + 502 [P0(4ll) + P0(X2ll) (PO...PO)*] P4010, or both can transfer electronic energy efficiently to excite alkali metal atoms at surprisingly low .t EXPERIMENTAL An intersecting stream chemiluminescent reactor was designed to study the phosphorus chemiluminescence energy transfer capabilities. Fig. 1 is a schematic drawing of the gas re- actor. The reactor consists of a 50-ml round-bottom flask containing white phosphorus (Alfa-Ventron, Danvers, MA), warmed (or cooled) by a glass jacket in which warm (or cooled) water circulates. A side inlet in the flask permits the entry of water-saturated gas (Airco, Montvale, NJ), which carries the P4 vapor out of the flask. The chemiluminescence luminosity is increased by the presence of H20 in the P4 vapor stream. A ventricular arrangement surrounds the P4 vapor exit to ensure adequate and efficient mixing with air. At the mixing zone the chemiluminescence flame appears spontaneously. The entire apparatus including the flame was enclosed in a glass tube with a water aspirator connected at the top to ensure a flow and to remove the reaction product from the interior. The source of alkali metal vapor is-a sample of alkali metal within a stainless cartridge wrapped in heating coils to vaporize the alkali metal inside. The metal vapor is transported out of the cartridge by argon (Airco) used as a carrier gas and the 'vapor stream in-

tersects with the phosphorus stream through glass inlets: at the T flame (A) and 10 cm above the tip of the flame (B).- Thermo- AIR I couples were placed at the intersection sites. The entire appa- MOLTEN WHITE PHOSPHORUS ratus is displaced vertically to align positions A and B with the / - CIRCULATING optical axis of the spectrograph. add=\_ WATER BATH The luminescence was photographed with a Steinheil Uni- 70 'C versal GH three-prism spectrograph with f3.9 glass optics in an infrared optical alignment, giving a dispersion of 108 A per FIG. 1. Phosphorus chemiluminescence-alkali metal energy mm in the region of 7000 A. Eastman Kodak 1-N plates were transfer apparatus. T, threaded screw; V, voltage regulator. used; exposure times ranged from 1 min to 1 hr at various slit settings. Sodium and discharge lamps were used for cali- bration. Fig. 2 A and B and Fig. 3 are the densitometer tracings Fig. 3 is the spectrum taken at observation port A with so- of the photographic plates, covering the spectral region dium (Mallinckrodt, analytical reagent), contaminated with 5700-9000 A. a trace amount of potassium, vapor intersecting the phosphorus Fig. 2A is the spectrum taken at the flame level (observation chemiluminescence flame. We observe (i) the extremely intense port A) with potassium (Mallinckrodt) vapor intersecting the unresolved 2P3/2,1/2 - 2S1/2 sodium D lines at 5890 A, super- phosphorus reaction. We observe (i) a strong emission band posed on (ii) the tail of the (PO---PO)* excimer, and (iii) the extending from the visible to the near infrared (7500 A); this resolved potassium doublet at 7667 A and 7701 A, observed to is the tail of the (PO...PO)* excimer luminescencet; (ii) the be much weaker than the sodium D lines. resolved atomic potassium doublet with 2P3/2 -'2S1/2 at 7667 In all these experiments, the slit of the spectrograph is set at A and 2P1/2 __ 2S1/2 at 7701 A. Fig. 2B is the spectrum taken 50 /im, and the exposure time is 5 min. During these experi- as the potassium vapor intersects the stream of reaction products ments the phosphorus vessel is maintained at 70'C by circu- 10 cm above the tip of the chemiluminescence flame (obser- lating hot water. The temperature of the metal vapor as re- vation port B). In this spectrum we observe only the 7667 A and corded by the thermocouple is 800C. 7701 A doublet of potassium. At temperatures up to about 100'C saturated sodium and potassium vapors consist mainly of atoms. At higher tempera- tOne referee pointed out that there are numerous examples of transfer tures dimerization is observed, which increases with increasing of molecular vibrational energy to atomic electronic excitation, and temperature. Potassium vapor at 80'C is composed of atomic that this might be the origin of the "postflame" atomic fluorescence K (99.996%) and diatomic K2 with vapor pressure of 2.744 X sensitization. However, the molecular vibrational excitation required 10-6 mm Hg for K and 2.744 X 10-10 mm Hg for K2 (19) (1 occurs only in high-temperature flames, so this mechanism seems mm Hg = 133KPa). Sodium vapor at 800C is composed mainly unlikely here. of atomic Na 1.388 X 10-8 mm Hg), with only The apparent maximum at 6500 A is a false one resulting from the (vapor pressure spectral characteristics of Kodak 1-N plates; the true maximum for a small trace-of the diatomic Na2 (2.617 X 10-'2 mm Hg). The the broad continuum due to the excimer luminescence is around 500 saturation vapor pressure of the intermetallic NaK is nm and has been reported earlier (8). similar to that of potassium (20), but exact values of the NaK Downloaded by guest on September 29, 2021 6954let% Chemistry: Khan Proc. Natl. Acad. Sci. USA 77 (1980)

B Sodium 2P3/2,112 - s/2 1- it


Q C.)cd


Potassium bb ~3I2,321rn 2s 1/2 C._

G) o0 - CdcU I I I 0-

- A 1- 570 600 700 800 900 1ooo Wavelength, nm FIG. 3. Spectrum taken at the intersection region of an argon stream carrying sodium containing a trace amount ofpotassium vapor (800C) with the phosphorus chemiluminescence glow (250C) of P4 vapor with moist air. The spectral region between 570 and 900 nm shows the intense unresolved sodium D lines at 5890 A and the weaker resolved potassium doublet at 7667 A and 7701 A, the phosphorus excimer chemiluminescence, uncorrected for the spectral sensitivity of the Kodak 1-N plate used. MECHANISMS OF ENERGY TRANSFER Much attention has been given in the past to energy transfer studies from excited atoms (e.g., Hg) to polyatomic acceptors (21). In the present study we have an uncommon case of atomic resonance fluorescence excited by a polyatomic species, the (PO...PO)* excimer. Previous investigations of the dynamics of PO excimer for- 0- mation have shown that, once PO is generated in an - I1 I I I I I I I ically excited state, the energy readily degrades to the lowest i . -i excited electronic state, the forbidden 4fl state, possibly through 600 700 800 900 1000 a mechanism of dipole-dipole interaction between two PO Wavelength, nm . Radiative transitions from collision complexes or FIG. 2. Photographic recordings of phosphorus chemilumines- excimer complexes of PO are in direct competition with deg- cence energy transfer to atomic potassium (2P3/2,1/2 - 2S1/2). (A) radation to lower excited states, and the probability of direct Spectrum taken at the intersection region of an argon stream carrying emission from such collision complexes is small, resulting in an potassium vapor (80°C) with the chemiluminescence glow (25°C) of accumulation of energy in the 4fl state. Because of the meta- P4 vapor with moist air. The spectral region of 570-750 nm shows phosphorus excimer luminescence; the maximum around 650 nm is stability of the 4II state, excimer formation is particularly im- a false one resulting from the spectral sensitivity characteristics of portant under ambient atmospheric conditions. Excimer for- the Kodak 1-N plate used. The spectral region of 750-800 nm shows mation is less likely under gas discharge conditions at much the emission due to the potassium doublet. (B) Spectrum taken at the lower and concentrations and may appear only as a intersection region of the argon stream carrying potassium vapor very weak continuum in the visible. (80°C) with the flowing postflame stream 10 cm above the tip of the transition from the 411 state to the flame. The photographic plate shows only emission due to the po- Because radiative ground tassium doublet. X211 state is spin forbidden, and because the 4II state is close enough energetically for thermal population of the B2z2+ state of the PO system, the excimer has the following equilibrium: vapor pressure are not currently available for these low tem- [PO... PO]* ;= [PO(411) + PO(X211)] It is worthwhile that because of its peratures. noting higher ; [PO(B22+) + PO(X21)]. the diatomic NaK species may be the species swept out from the furnace in the sodium experiment. Increasing the temperature of the reaction or dilution of the Downloaded by guest on September 29, 2021 Chemistry: Khan Proc. Nati. Acad. Sci. USA 77 (1980) 6955

Electronic states of tion. Under ambient conditions the chemiluminescence is re- PO species in P chemiluminescence Alkali atom ported to extend all the way to the vacuum ultraviolet (24). electronic energy aod-nfnr Ctftfac Bowen and Pells (25) in 1927 estimated the ratio of the number of visible emitted to the number 30,000 photons of phosphorus mole- cules oxidized to be 1 in 2000, suggesting either that the initially generated electronic excited species are emitting in the non- visible region of the spectrum or that the nascent electronic -I 2\1X\ "\\ '\\ 20,000 I2 -\\ I \\\ states are lost via radiationless processes. The present result of s2h4o\\ alkali metal emission sensitized by the chemiluminescence reaction demonstrates that at a 4 least portion of the reaction energy can be channeled into emission. 10,000 The energy transfer phenomenon is of general interest not |- 1 31r AT only because it indicates the metastable nature of the donor electronic state and is a rare example of energy transfer from polyatomic to atomic x2 2S 2S 2 species, but also because this efficient 0 {X21n+x2n}24I excitation of Na and K atoms opens up the possibility of using PO (PO-PO) Na 2K 2Rb 2Cs Monomer Excimer an extended set of electronic transitions of chosen atoms to obtain a mapping of the nascent electronic excited states gen- FIG. 4. Energy level diagrams for the PO monomer and the erated in the phosphorus chemiluminescence reaction, a (PO...PO)* excimer electronic energy donor and the alkali atom ac- method which could also be extended to other gas phase ceptor states. -, Observed emissions; ---, predicted emissions. chemiluminescence. Just as in solutions, the emission efficiency of sensitized chemiluminescence in gas phase systems may be phosphorus vapor stream at higher temperatures with inert gas very much enhanced compared to the donor luminescence results in PO f3-system emission, PO(B22+) PO(X211), at particularly when the donor luminescence is drastically 325-337 nm, an emission almost totally quenched in the quenched via competitive radiationless processes. phosphorus chemiluminescence spectrum at room temperature and atmospheric pressure (8). This work was supported by a contract between the Division of Fig. 4 is an energy level diagram for the two lowest electronic Biomedical and Environmental Research, Department of Energy, and excited states of monomeric PO and the lowest excited manifold the Florida State University. of the (PO PO)* excimer, and the electronic acceptor states 1. Harvey, E. N. (1957) A History of Luminescence (Americais of the alkali atoms. The solid lines indicate observed phosphorus Philos. Soc., Philadelphia). chemiluminescence sensitized emission. The broken lines are 2. Thorpe, T. E. & Tutton, A. E. (1890) J. Chem. Soc. 57,545-573. expected results of energy transfer to rubidium and cesium, 3. Hund, F. (1926) Z. Phys. 36,657-674. which have not yet been obtained experimentally. 4. Mulliken, R. S. (1927) Phys. Rev. 30, 785-811. For an efficient energy transfer a downhill is 5. Ghosh, P. N. & Ball, G. N. (1931) Z. Phys. 71,362-870. necessary in general, and from Fig. 4 it is clear that this con- 6. Curry, J., Herzberg, L. & Herzberg, G. (1933) Z. Phys. 86, dition is amply satisfied. For excimer formation spin coupling 348-366. between the 411 state of one PO molecule and the X211 ground 7. Van Zee, R. J. (1975) Dissertation (Michigan State Univ., East state of the other PO should give Lansing, MI). 8. Van Zee, R. J. & Khan, A. U. (1976) J. Chem. Phys. 65, 1764- [PO(411) + PO(X211)] Z=t 3,5[po... .PO]* 1,3[po... PO] 1772. 9. Rumpf, K. (1938) Z. Phys. Chem. Abt. B. W± [PO(X211) + PO(X211)] 38,469-473. 10. Walsh, A. D. (1957) in The Threshold of Space, ed. Zelikoff, M. where the spins of the two PO molecules couple by exchange (Pergamon, London), pp. 165-168. interaction producing a triplet, and a quintet state for the ex- 11. Cordes, H. & Witschel, W. (1965) Z. Phys. Chem. 46,35-48. cimer. On the other hand, the spin coupling of the two X211 PO 12. Ludlam, E. B. (1935) J. Chem. Phys. 3,617-620. 13. Lam Thanh, M. & Peyron, M. molecules would give an overall singlet and a (1963) J. Chim. Phys. Phys. Chim. Biot. 60, 1289-1293. triplet and no quintet state. This results in a truly metastable 14. Van Zee, R. J. & Khan, A. U. (1974) J. Am. Chem. Soc. 96, quintet excimer and spin-allowed triplet-triplet transition of 6805-6806. small oscillator strength. The metastability of the 411 state and 15. Davies, P. B. & Thrush, B. A. (1968) Proc. R. Soc. London Ser. of the PO-excimer states are ideally suited for spin-conserved A 302,243-252. collisional energy transfer of the Dexter type (22). At higher 16. Verma, R. D. & Broida, H. P. (1970) Can. J. Phys. 48, 2991- temperature the equilibrium will shift to generate the B22+ 2995. state with its allowed ground state transition; such an electronic 17. Verma, R. D. & Jois, S. S. (1973) Can. J. Phys. 51,322-333. state could participate in the long-range energy transfer of the 18. Van Zee, R. J. & Khan, A. U. (1976) Chem. Phys. Lett. 41, Forster type (23), involving higher excited states of the alkali 180-182. atoms. 19. Nesmeyanov, A. N. (1963) Vapor Pressure of the Chemical El- ements (Elsevier, New York). 20. Foust, 0. J., ed. (1972) Sodium-NaK Handbook CONCLUSION (Gordon and Breach, New York), Vol. 1. Phosphorus chemiluminescence is a highly exothermic oxida- 21. Mitchell, A. C. G. & Zemansky, M. W. (1934) Resonance Ra- tion reaction, diation and Excited Atoms (Cambridge Univ. Press, London). 22. Dexter, D. L. (1953) J. Chem. Phys. 21, 836-850. P4(g) + 502(g) = P4Olo(g), AH = -722.3 kcal (3.02 MJ)/mol, 23. Fbrster, T. (1951) Fluoreszanz Organischer Verbindungen (Vandenhoeck and Ruprecht, Gbttingen). but the flame is cool to the touch, suggesting the generation of 24. Downey, W. E. (1924) J. Chem. Soc. 347-357. electronic excited states as the primary step of chemical reac- 25. Bowen, E. J. & Pells, E. G. (1927) J. Chem. Soc. 1096-1099. Downloaded by guest on September 29, 2021