R. J. VanZee and Ahsan U. Khan
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The Phosphorescence of Phosphorus’ Richard J. VanZeet and Ahsan U. Khan* Departmentsof Chemistry and Biophysics, Michigan State University, East Lansing, Michigan 48824 (Received March 3, 1976)
The electronically excited species in the chemiluminescence associated with the oxidation of P4 vapor under reduced pressure and oxygen deficient conditions are markedly different from those of the oxygen sufficient, atmospheric pressure system. P2(C12,+ XIZ,+ and AIII, XIZ,+) transitions are the dominant emission in the near-ultraviolet region, replacing the PO emission found under atmospheric pressure. A comparison of the spectra shows that the visible continuum is red shifted by -2000 cm-l in the reduced pressure, oxygen deficient system compared to atmospheric pressure conditions. Two alternative suggestions are offered to explain the origin of the visible continuum in the reduced pressure system: (1)as in the atmospheric pressure system, the emitting species is the (PO),* excimer, with different vibrational distribution; (ii) an exciplex of P2 and PO is responsible for the emission. The existing mechanism for phosphorus chemiluminescence suggested by Semenov and Dainton fails to interpret these experimental observations.
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Introduction The phosphorescence of phosphorus is the oldest known chemiluminescing system., The major fine band systems in the visible and near-ultraviolet portion of the chemiluminescence under atmospheric conditions are due to the PO y system, the PO 0system, and HP0.3aThe broad continuum emission which is responsible for most of the visible intensity has been identified as an excimer of PO, (P0)2*.3b74In addition the reaction under ambient conditions produces infrared emission corresponding to transitions between vacuum ultraviolet and ultraviolet states of PO, identifying part at least of the previously unidentified vacuum ultraviolet emission from the r e a ~ t i o nIn . ~ this paper we report the effect of reduced pressure, oxygen deficient conditions on the chemiluminescence reaction. These conditions drastically alter the nature of the emitting species;the PO emission disappears and is replaced by P2 emission in the ultraviolet region of the chemiluminescence, and the continuum emission in the visible region also changes. The existing chemiluminescence model for phosphorus oxidation is inadequate in interpreting these observations.6 Experimental Section White phosphorus (Baker) was placed in a 50-ml roundbottom glass flask connected directly to the quartz reaction chamber. Air was used as the source of molecular oxygen and was regulated into a glass tube by a fine needle valve. The tube serving as the air inlet was led into the 50-ml flask containing the phosphorus and terminated at the entrance to the quartz (22 mm 0.d. and 20 cm long) reaction chamber. A vacuum pump, separated by a liquid nitrogen trap, maintained 0.1 Torr total’ pressure inside the quartz tube. A dim chemiluminescence glow filled the entire length of the tube and hose to the trap. A Heath Model EU-700,0.5 m monochromator, with a grating of 1200 lines/mm and blazed a t 250 nm, connected to a Heath Model EU-701-30 photomultiplier module containing a RCA 1P28 photomultiplier was used to monitor the spectra which were recorded with a Heath Model EU703-31 photometric readout unit with a typical sensitivity of A. + Present address: Department of Chemistry, University of Florida, Gainesville, Florida. The Journal of Physical Chemistry, Voi. 80, No. 20, 1976
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Atmospheric Pressure, Oxygen Sufficient Figure 1 is the visible and ultraviolet chemiluminescence spectrum of the reaction of water saturated P4 vapor with air at atmospheric pressure and room temperature. The discrete band structure in the 228-272-nm region is the PO y system, PO(AaB+) PO(X211).The weak bands a t 450-650 nm superimposed on the broad continuum were identified, by substitution of D2O for HzO in the reaction, as the HPO transition A(IA”) %(1At).3aThe broad visible continuum is due to an excimer of PO, (PO),*, involving the lowest excited state of PO, the 411 state. Because radiative transition from the 411 state to the ground X211state is spin forbidden, and since the 411 state is close enough energetically for thermal population of the B28+ state of the PO (3 system to occur, the excimer has the following equilibrium: (P0)2* i=? PO(X2ll) t PO(411)zll PO(X211) PO(B2Z+).Increasing the temperature of the reaction or dilution of the P4 vapor stream a t higher temperatures with inert gas results in the appearance of PO /3 system transitions, PO(B22+) PO(X211),in the spectrum at 325-337 nm.3b Reduced pressure and oxygen sufficient conditions also produce PO ,8 system emission in the chemiluminescence. These results are analyzed in detail el~ewhere.~ The chemiluminescence spectrum of the oxidation of P4 vapor by moist air a t atmospheric pressure also contains infrared emissions, some of which correspond to transitions between vacuum ultraviolet and ultraviolet states of PO.5 Transitions from the vacuum ultraviolet states, G22+ and F22+,PO(G2Z+) PO(A22+),PO(F22+) PO(A22+),and PO(F22+) PO(B22+);and between the ultraviolet states PO(A28+) PO(B2Z+) have been identified in the spectrum.5 The presence of the G W and F22+states of PO in the phosphorus flame can partially account for the unidentified vacuum ultraviolet emission from the reaction, first observed by Downey in 1924.7 A summary of the PO transitions so far identified in phosphorus chemiluminescence is given in Figure 2.
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Reduced Pressure, Oxygen Deficient Figure 3 is the visible and ultraviolet chemiluminescence spectrum of the reaction of P4 vapor with air at approximately 0.1 Torr total pressure and room temperature. When compared to the ambient chemiluminescence spectrum, there are three striking differences in the spectrum of the reduced
The Phosphorescence of Phosphorus:
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TABLE I: Wavelengths of Band Heads of PI CIZ,+-XIZg+and A1n,XIZg+ Emission Literature Values Intensity
(u',u")
Transition
4 5 5 5
7,15 11,18
c-x c-x c-x c-x c-x A-X c-x c-x A-X c-x A-X c-x A-X c-x c-x A-X c-x c-x
5,15
9,18 6,16 3,O 4,17 5,19 OJ 7,24 02 8,25 1,3 9,26 10,27 0,3 8,26 10,29
4
6 4
4 5
4
Present Work nm
Ref
256.56 258.66 262.55 264.52 264.21 275.82 275.71 283.0 296.984 302.87 303.929 304.64 305.37 306.42 308.20 311.243 310.54 320.19
14 14 14 14 14 15 14 14 16 14 16 14
hair,
15
14 14 16 14 14 E(c6')
hair,
nm
Intensity
257.0 259.5 262.5 265.5 266.5 275.0
5 8 10 8 8 9
283.5 296.0 302.5 304.0
7 2 8 8
305.5
8
308.5 310.5
7 6
320.0
2
I
lo3 B3E;-
50A
40-
A32i30 -
L 200
I
I
400
I
I
600 WAVELENGTH inmi
I
I 800
Figure 2. Relative energies of the electronically excited states of
Figure 1. Chemiluminescence spectrum of the room temperature reaction, on contact with air, under atmospheric pressure, of P4 vapor carried in a nitrogen stream saturated with water vapor. The spectrum was recorded with a 0.3-m McPherson Model 218 monochromator with an EM1 9558QB photomultiplier (VanZee and Khan).4
pressure system: (1) the PO y system emission has disappeared, (2) the visible continuum onsets a t a longer wavelength and has a smaller spectral bandwidth, and (3) the overall luminescence intensity is reduced. Emission from P2 replaces the PO y emission in the 250-300-nm region. In contrast, no obvious Pz emission has been observed under atmospheric pressure of air in studies of phosphorus chemiluminescence. Figure 3 contains emission from the CIZ,+ XIZg+ and AIIIg XIZg+ transitions of P2. Table I is a comparison of literature values with a medium resolution phosphorus chemiluminescence spectrum under reduced pressure and oxygen deficient conditions. Figure 2 shows the relative energies of the P2 states. The continuum emission in Figure 3 starts at about 375 nm; whereas in the ambient chemiluminescence (Figure 1) the continuum emission starts a t about 350 nm. There is thus a red shift of about 2000 cm-l in the onset of the continuum in
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P2,14-16PO,4 HP0,4 and 02. Arrows indicate transitions identified to date in phosphorus chemiluminescence spectra under various conditions.
T
200
250
300
350 400 WAVELENGTH
450
500
550
600
(nm)
Figure 3. Chemiluminescence spectrum of the reaction at room temperature and 0.1 Torr total pressure of P4 vapor on contact with air. Because of reduced intensity of the luminescence compared to Figure 1, the signal-to-noise ratio in this spectrum is lower. The spectrum was recorded with a 0.5-m Heath Model EU-700 monochromator with an RCA 1P28 photomultiplier. The Journal of Physical Chemistry, Voi. SO,No. 20, 1976
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R. J. VanZee and Ahsan U. Khan
going from ambient conditions to reduced pressure, oxygen deficient conditions. The emission maxima is not greatly affected, however. The bandwidth of the spectrum is narrower in Figure 3 when compared to the ambient chemiluminescence spectrum. There are two alternative interpretations to explain these observations: (i) emission from the (PO)z* excimer is modified by altering the vibrational distribution on going from atmospheric to reduced pressure conditions,l7 or (ii) the species responsible is not the (PO)z* excimer but is an exciplex formed between PZand PO. The lowest metastable state of Pz, the 3Zustate (see Figure 2 ) , is slightly lower in energy than the lowest metastable state of PO, the 411state. Thus exciplex emission involving the Pz(3ZU-)and PO(X211)states would be red shifted with respect to the (PO)z* excimer where the complexing is between the PO(411)and PO(X211)states. This would be consistent with the observed experimental results. The absence of PO emission in the spectrum under reduced pressure conditions could be due to electronic energy transfer from PO* to Pz, accounting for at least a portion of the Pz emission. There are also suggestions of weak HPO bands over the continuum in Figure 3, similar to the stronger HPO bands seen in Figure 1.
Discussion From a kinetic study of the chemiluminescence of phosphorus oxidation, Semenov proposed a branch chain reaction mechanism, suggesting that oxygen atoms are the chain carriers of the branch chain reaction, and that they are inserted into the P4 tetrahedron one by one.6 Dainton and Kimberleys have expanded this model and suggested that the luminescent process in the reaction P4(g) 502(,) P4010(~) (AH= -722.3 kcd m ~ l - is~ actually )~ a side reaction of the oxidation process. However, the oxygen atom insertion mechanism is based upon monitoring the luminescence of the reaction. Moreover, now that the transient emitters have been identified as small diand triatomic species, it is clear that the P4 tetrahedron is dissociated in the reaction.18 The oxygen atom insertion mechanism also fails to explain the necessity of moisture in the reaction. Baker and Dixon observed that phosphorus and molecular oxygen, if previously dried for several months under very stringent conditions, do not undergo a luminescent reaction in the absence of water vapor, even when the temperature of the reaction chamber is raised to 290 O C . l 0 Furthermore, Verma and Broida have found that the reaction of atomic oxygen and molecular phosphorus requires a trace amount of water for activation.ll Under atmospheric conditions there is also an increase in the total luminosity with increasing concentration of moisture.4 Chemiluminescence from the oxidation of P4 by 0 2 cannot be explained by considering the reaction of atomic phosphorus and atomic oxygen. In studies of the reaction of atomic phosphorus and atomic oxygen in a gas discharge, Walsh has found a precise cutoff in the PO emission a t the nearly isoenergetic B2Zf (u' = 10) and A28+ (u' = 2) states at 43 500 cm-l, reflecting the dissociation energy De of the ground state PO molecule.12 The presence in the phosphorus flame a t atmospheric pressure of vacuum ultraviolet and ultraviolet PO states of energies above 0 , 4 3 5 eliminates the possibility that the chemiluminescence proceeds by stepwise combination of atomic phosphorus and atomic oxygen. For energetic reasons
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The Journal of Physical Chemistry, Vol. 80,
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No. 20, 1976
it is likely that PO is initially generated in these high energy states, since the alternative means of exciting PO to these states would involve bimolecular energy summation processes. Further investigation is necessary to clarify the details of these processes under atmospheric conditions. In summary, the essential features to be accounted for in the chemiluminescent mechanism are: (1) the presence of electronically excited PO in the reaction and the drastic change to Pz emission in response to mild changes in experimental conditions, (2) the absolute requirement of water, and (3) the emission of high energy ultraviolet and vacuum ultraviolet photons from a cool flame.
References and Notes (1) (a) Presented at the Michael Kasha Symposium on Energy Transfer in Organlc, Inorganic,and Biological Systems, held at Florida State University at Tallahassee, Tallahassee, Fla, Jan 8-10, 1978. (b) This paper is based on the thesis submitted by R.J.V. to the Graduate School of Michigan State University in partial fulfillment of the requirement for the degree of Doctor of Philosophy. (2) E. N. Harvey, "A History of Luminescence", The American Philosophical Society, Philadelphia, Pa., 1957. (3) (a) R. J. VanZee and A. U. Khan, J. Am. Chem. Soc., 96,6805 (1974);(b) R. J. VanZee and A. U. Khan, Chem. Phys. Lett., 36, 123 (1975). (4) R. J. VanZee and A. U. Khan, J. Chem. Phys., in press. (5) R. J. VanZee and A. U. Khan, Chem. Phys. Lett., 41, 180 (1976). (6) N. Semenoff, 2.Phys., 46, 109 (1927); Chem. Rev., 6, 347 (1929). (7) W. E. Downey, J. Chem. SOC.,347 (1924). (8) F. S. Dainton and H. M. Kimberley, Trans. Faraday Soc., 46, 629 (1950). (9) R. C. Weast and S. M. Selby, Ed., "Handbook of Chemistry and Physics", 47th ed, The Chemical Rubber Co., Cleveland, Ohio, 1966, p D25. (10) H. B. Baker, J. Chem. SOC., 47, 349 (1885); Phil. Trans., 179A, 571 (1888). (11) R. D. Verma and H. P. Broida, Can. J. Phys., 48, 2991 (1970). (12) A. D. Walsh in "The Threshold of Space", M. Zeiikoff, Ed., PergemonPress, London, 1957, p 165. (13) R. R. Hart, M. B. Robin, and N. A. Kuebler, J. Chem. Phys., 42,3631 (1965), and references therein. (14) P. N. Skanckeand J. E. Boggs, Chem. Phys. Lett., 21,316(1973). (15) R. D. Verma, Can. J. Phys., 46, 2391 (1970). (16) E. J. Marais, Phys. Rev., 70, 499 (1946). (17) We thank the referee (Dr. Brian Stevens) for drawing our attention to this possibility. (18) In the conference an exploding tetrahedron mechanism was proposed which is not included In the paper, since it is felt further explorationof the role of water is essential. The initiation step in the suggested model is a stable [P4-02] complex exploding on the approach of H20, generating diatomic and triatomic species in highly excited electronic states. In the propagation step a stable [Pp02] complex explodes by absorbing a high energy photon from the initiation step, again generating electronically excited species. The high energy photons act as the chain carriers.
Discussion D. S. MCCLURE.Your exploding tetrahedron model breaks many bonds. This makes me wonder how there can be energy left over for the high degree of electronic excitation implied by your observations.
R. J. VANZEE.The total energy of reaction, 730 kcal/mol, is sufficient to produce, with unit efficiency, photons of wavelengths 400 A. With this potential we need not worry about sufficiency of energy. G . A. CROSBY.Has any one tried to isolate Pq in a matrix and diffuse 0 2 into the matrix? Any associative complex or new species could be monitored by infrared or Raman techniques. R. J. VANZEE.I know of no such attempts.
H. SELIGER. I remember that many years ago a very strange kinetics was observed for this chemiluminescence. Is this the case in your experiments? R. J. VANZEE.Not that we are aware of.
