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Energy Transfer from PO Excited States to Alkali Metal Atoms in the Phosphorus Chemiluminescence Flame [Metastable P0(4Jj,)/(P0.P0)* Excimerl AHSAN U

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Energy Transfer from PO Excited States to Alkali Metal Atoms in the Phosphorus Chemiluminescence Flame [Metastable P0(4Jj,)/(P0.P0)* Excimerl AHSAN U Proc. Natl. Acad. Sci. USA Vol. 77, No. 12, pp. 6952-6955, December 1980 Chemistry Energy transfer from PO excited states to alkali metal atoms in the phosphorus chemiluminescence flame [metastable P0(4jj,)/(P0.P0)* excimerl AHSAN U. KHAN Institute of Molecular Biophysics and Department of Chemistry, Florida 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 alkali metal 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 argon 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 molecule 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 atom moist air were identical to the bands obtained by Ludlam (12) fluorescence in the postflame region. in 1935 from phosphorus burning in a hot hydrogen 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 solid residues of urine; 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 Robert Boyle 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 excited state; 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 oxides. 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 temperature and atmospheric pressure, 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 temperatures.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 phase re- actor. The reactor consists of a 50-ml round-bottom glass 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 nitrogen 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 steel 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 neon 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.
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