Photoionization of Sodium
2171
neglected in the case of the excitation by electron bombardment or generally by high-energy radiation.20 Fluorescence from 1K* was not observed in the crystalloluminescence spect ra.
K
-'K* crybtdllization
-% 3K*
-
crystallization
-
hv(P)
(1)
K * 3K* hv(P) (2) In the presence of the sensitizing additives, the original emission from 3K* is placed by the emission from 3S*, the intensity of which remains about the same as that of 3K*. Therefore, at least, the most emission comes from 3§* which is formed by energy transfer from 3K* a i d the following direct excitation of S may be neglected crystallization s Is*
18C
crystallization
3s*-hv
-
(P)
(3)
s 3s* h v ( P ) (4) Fluorescence from IS* is not observed even for oxazoles. Therefore, the possible mechanism is D.
K S
--+
'K*
1SC +
crystallization
IS* 3K*
--isc
S
* 'K*
3S* ---o
(5)
hv(P)
3S*-+-hv(P)
(6) (7)
crystallization
K
*
3K*
(8)
S
3K* =3S* -hv (P) (9) Process 6 involves singlet energy transfer from K to S, while processes 7 and 9 involve triplet energy transfer. In general, triplet energy transfer is more important.21 Actually in the case of strongly fluorescent oxazoles, no fluo-
rescence emission is observed in the crystalloluminescence spectra, so that the process involving the singlet energy transfer (6) is not the case. However, for ketones, the quantum yield of the intersystem crossing is near unity,15316 so process 6 cannot be excluded simply from the fact that no fluorescence emission is found in the crystalloluminescence spectra. References and Notes V. A. Garten and R. 6.Head, Nature (London), 209, 705 (1966). V. A. Garten and R. B. Head, Phil. Mag., 8, 1793 (1963). C. Racz, C. R. Acad. Sci, 212, 900 (1941). S. Tsuda, T. Takeda, and E. Shibata, J. Chem. SOC. Jap., 60, 157 (1939). (5) G. P. Safonov, V. Ya. Shiyapintokh, and S. G. Entelis. Nature (London), 205, 1203 (1965). (6) G. P. Safonov. V. Ya. Shlyapintokh, and S. G. Entelis. Izv. Akad. Nauk SSSR, Ser. Khim., 1432, (1967). (7) M. Trautz, Z. Phys. Chem., 53, 1 (1905). (8) V. A. Garten and R. E. Head, Phil. Mag., 14, 1243 (1966); J. Cryst. Growth, 6, 349 (1970). (9) N. M. Johnson and F. Daniels, J. Chem. Phys., 34, 1434 (1961). (10) H. Longchambon, Bull. SOC.Fr. Mineral., 48, 130 (1925). (11) K. Takeda and F. Williams, Mol. Phys., 17, 677 (1969). (12) M. A. Bonin, K. Takeda, and F. Williams, J. Chem. Phys., 50, 5423 (1969). (13) H. Garcia-Fernandez, Bull. SOC.Chim. Fr., 1599 (1969). (14) R. Borkman and D. Kearns, J. Chem. Phys., 44,945 (1966). (15) R. S. Becker, "Theory and Interpretation of Fluorescence and Phosphorescence," Wiiey, New York, N. Y., 1969, pp 157-159. (16) R. S . Engei and B. M. Monroe, Advan. Photochem., 8, 245 (1971). (17) R. C. Sangster and J. W. Irvine, Jr., J. Chem. Phys., 24, 670 (1956). (18) R. Dehl and G. K. Fraenkel, J. Chem. Phys., 39, 1793 (1963); G. A. Russell and R. D. Stephens, J. Phys. Chem., 70, 1320 (1966). (19) W. H. Hamill in "Radical Ions," E. T. Kaiser and L. Kevan, Ed.. Interscience, New York, N. Y., 1968, p 322. (20) R. A. Hoiroyd and C, Capelios, J. Phys. Chem., 76, 2485 (1972). (21) A. A. Lamoia and N. J. Turro, "Energy Transfer and Organic Photochemistry," Interscience, New York, N. Y., 1969, p 7. (1) (2) (3) (4)
oduction of Trapped Electrons in Glassy 3-Methylpentane Photoiolnization of Sodium S. C. Srinivasan and J . E. Willard* Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706 (Received April 3, 1973) Publication costs assisted by the U. S. Atomic h e r g y Commission
Photoionization of sodium metal dispersed in 3-methylpentane by deposition at 77°K from vapor mixtures yields trapped electrons with esr line width and half-life similar to electrons produced by y irradiation of 3MP glass a t 77°K. An apparatus for transferring samples condensed on a cold finger at 77°K to an esr sample tube at 77°K under vacuum is described. A narrow singlet signal a t g near 2.0000, observed earlier during illumination of 3MP-metal matrices, and ascribed to weakly trapped electrons, is now known to be an electron cyclotron resonance signal from electrons ejected into the evacuated tube.
Introduction When 3-methylpentane (3MP) and sodium are deposited from the vapor phase on a cold finger at 77°K in an evacuated tube and the system is irradiated with uv light in the cavity of an esr spectrometer, a narrow resonance signal (0.1 G) is produced which has a lifetime of less than
0.2 sec when the light is turned offezThis signal has been attributed t o weakly trapped electrons which differ from those produced by y irradiation of 3MP glass (ca. 3 G and >5 min t l l z 3) because of a difference in physical properties of the matrix or differences in the associated cation and method of production. We have now found4 that the The Journaiof Physical Chemistry, Voi. 77, No. 78, 1973
S. C. Srinivasan and J. E. Willard
2172
signal is an electron cyclotron resonance signal of free electrons ejected into the evacuated space surrounding the cold finger by photoionization of a thin deposit of sodium on the walls of the container. The present note describes further studies which show that trapped electrons with characteristics similar to those formed by y irradiation of glassy 3MP can be produced by photoionization of sodium dispersed in 3MP deposited from the vapor. The photoionization of dispersed metals in organic glasses has potential as a method for investigating the effect of cationic differences on the properties of trapped electrons, and, by the use of different wavelengths, for investigating effects of varying the average cation-electron separation distances. A simple apparatus for transferring deposits condensed on a cold finger to a conventional 3-mm i.d. esr tube for irradiation and examination is described. Experimental Section The body of the apparatus for matrix preparation (Figure 1) was made from the outer and inner (A) and (A') portions of a 45/50 standard taper ground glass joint sealed with Apiezon N grease. It was fitted with a stopcock (B)for access to the vacuum line, a 12-mm 0.d. cold finger (C)which could be rotated in the 14/20 ground joint (D)(lubricated with silicone grease) while containing liquid nitrogen u p to a level slightly below the joint, and a 9-mm 0.d. cold finger (E) which could be rotated and moved vertically in the cajon type adapter (F) (sealed into the glass with Picein wax). The cold finger (E)carried a cutting point (G) made from a strip of stainless steel wrapped around a square indentation in the tube and bolted at the point of overlap. A layer of indium metal between the steel and the glass improved thermal contact. An inlet tube (H) connected the deposition chamber to a reservoir of liquid 3MP a t -78' from which the rate of flow of vapor to the cold finger (C)was adjust. ed to 0.1-0.2 ml/hr by a stopcock. When an Na-containing matrix was desired, the vapor was allowed to pass over Na metal placed near the junction of tube (H) with the vessel (A).The tube (H) was heated with resistance wire for several centimeters prior to this point. About 1 ml of 3MP containing ea. 10'9 atoms of Na (as determined by mass spectrometric analysis for the HP evolved when the deposit was treated with water) was deposited on the finger (C). Following the deposition, the finger (E)was filled with liquid nitrogen, the cutting point (G) was forced against the deposit on (C),and (C)was rotated causing the deposit to flake off. A substantial part fell to the bottom of the 3-mm i.d. Suprasil esr tube (I), which was attached to the main vessel by the graded seal (J). During the scraping the entire apparatus was immersed in a large dewar of liquid nitrogen up to about the level of the cutting edge. Following the scraping the dewar was lowered and the Suprasil tube was sealed off a t its upper end. T o prevent flakes of the matrix which melted on the bottom of the large container from draining into the esr tube, a styrofoam cup filled with liquid nitrogen was positioned a t (K) by means of the outer portion of a ground joint cemented to a hole in its base. The pressure in the vessel A-A' was maintained at 10-5 Torr. A stream of He gas bubbled through the liquid nitrogen in the cold finger (E)reduced its temperature to a point where the temperature of the knife was 77'K, whereas it was W'K without this precaution. The Journal 01 Physical Chemistry. Val. 77.
NO.
IS. 1973
--r
ri'
Figure 1. Apparatus for condensation of 3MP vapor and 3MPNa vapor mixtures on a cold finger and lor subsequent removal
and transfer to an esr tube. See text for lettering code and discussion of operation. The 3MP used was Phillips Pure grade, further purified by passage through 5 ft of freshly activated silica gel. storing over Na-K alloy on the vacuum system, and pumping on the liquid to remove COz. Esr measurements were made in the X band with a Varian spectrometer using 105-Hz modulation frequency.
Results Characteristics of 3 M P and 3MP-No Mixtures Condensed from the Vapor Phase. Deposits of 3MP condensed from the vapor phase as described above were white and translucent in the absence of sodium. With sodium incorporated they varied from light to deep purple depending on the temperature of the Na reservoir. The four-line esr signal of Na atoms5 was not observed in any sample, implying that all of the Na was present in agglomerates, unless line broadening obscured the spectrum. An esr line of 12 G width (AHms) attributable to conduction electron@ was observed (Figure 2). This implies agglomeration sufficient to give the particles metallic character, while the symmetric nature of the signal implies that the particle diameters are less than the skin thickness of ea. 5 p for the X-band microwave radiation. Effects of y Irradiation. y-Irradiated (lo's eV g-l) samples of 3MP and of 3MP-Na prepared by vapor deposition gave esr spectra similar to those from y-irradiated 3MP glass formed by immersing the liquid in liquid nitrogen. These consist of the spectrum of trapped electrons superimposed on the spectrum of a free radical produced from the matrix (Figure 3a.b). The electron signal decayed with an initial half-life of about 25 min, similar to that in samples of partially annealed 3MP glass formed from the liquid.' Effects of Uu Irradiation. When samples of 3MP-Na condensed from the vapor state were exposed at 77°K to
Photoionization of Sodium
2173
I\ ff " -
24
I!Ii G'
Figure 2. Esr spectrum at 77OK of conduction electrons in 3MP-Na condensate prepared in apparatus of Figure 1. Power, 1.5 mW: modulation amplitude, 2 G ; signal level, 2000: recorder scale, 600 mV. Q
i
Figure 3. Esr spectra from y-irradiated and photolyzed 3MP-Na condensate prepared in apparatus of Figure 1. Power, 0.2 mW; modulation amplitude, 2 G ; signal level, 100; recorder scale, 60 mV. (a) Trapped electron singlet superimposed on six-line free radical signal from y irradiation: (b) trapped electron singlet of spectrum (a), shown at slower scan rate: (c) free radical signal, following illumination of 3MP-Na condensate with A H 4 lamp at 77'K. Small narrow line is residual electron signal.
Figure
4.
Repetitive scans of trapped electron signal produced
by photoionization of Na in 3MP. ( A ) Steady-state illumination with A H 4 lamp and 27 mm of 7-54 filter: (B) light turned off: (C) light turned on with no filter; (D) 3-mm 7-54 filter inserted; (E) filter removed; (F) 9-mm 7-54 filter inserted.
the radiation of a quartz-jacketed AH4 medium-pressure mercury arc through a Corning 7-54 filter, an esr signal attributable to trapped electrons grew to a steady-state concentration in ca. 3 min (Figure 4). When the light was turned off the signal decayed with an initial half-life of ca. 5 min, again in the range observed for electrons formed by y irradiation of 3MP glass7 (for which the t1/2 varies with time following irradiation and time of preirradiation annealing). The signal bleached rapidly in visible light and, consistent with this, was very weak when the AH4 lamp was used without a filter to reduce the intensity of exposure in the region of trapped electron absorption spectrum (Figure 4). A 3-mm thickness of Corning 7-54 filter has a maximum transmission of ca. 85% in a band around 320
nm and removes wavelengths greater than 410 nm except for a relatively weak transmission starting a t 680 nm, passing through a maximum of 40% at 730 nm, and falling to 10% at 1000 nm. Figure 4 shows that the concentration of electrons during continuous illumination increases with increasing filter thickness (3, 9, and 27 mm). This is consistent with the decrease in ratio of transmission of the wavelengths which bleach electrons to that of the wavelengths which form electrons. As observed in the previous work,2 free radicals are formed by a sodium photosensitized reaction during illumination of the 3MP-Na matrices (Figure 3c).
Discussion Annealing of 3MP glass at 77°K changes the physical properties in a way to reduce the rate of decay of trapped electrons.7 Polycrystalline hydrocarbons of low molecular weight are much less efficient in trapping electrons than glassy hydrocarbons.8 In view of such effects of changes in physical state, it would not be surprising if, as suggested earlier,2 electrons were much more weakly trapped in 3MP deposited from the vapor phase than in the glass formed by quenching the liquid a t 77°K. The present work indicates that the properties of trapped electrons are a t least qualitatively similar in the two types of matrices and that trapped electrons with lifetimes convenient for study on the time scale of minutes to hours can be produced in 3MP by photoionization of sodium. This should make possible studies of the relative fates of electrons produced with different energies of activating light, and allow observation of properties of the electrons in the presence of fewer by-product fragments than when y irradiation is used. The free radicals formed during photolysis of the 3MPNa matrix must result from a sodium-photosensitized process utilizing the energy from neutralization of Na+, or energy transferred from an excited state. The initial free radical spectrum (Figure 3c) is different from that following y irradiation (Figure 3a). There is evidence2 that it changes on standing to the type of 3a, suggesting the presence of a second radical, possibly methyl, in addition to the 3-methylpentyl radical. References and Notes U. S . Atomic Energy Commission under Contract No. AT(11-1)-1715and by t h e W. F. Vilas Trust of the University of Wisconsin. (2) F. W. Froben and J. E. Willard. J. Phys. Chern., 75, 35 (1971). (3) For references and discussion, see A. Ekstrom, Radiat. Res. Rev,, 2,381 (1970). (4) S.C.Srinivasan and J. E. Willard, J. Chern. Phys., in press. (5) C. K. Jen, V. A. Bowers, E. L. Cochran, and S. N. Foner, Phys. Rev., 126,1749 (1962). (6) G. Feher and A. F. Kip, Phys. Rev., 98, 343 (1954). (7) D.Shooter and J. E. Willard, J. Phys. Chern., 76, 3167 (1972). (8) N. A. Bonin, J. Lin, K. Tsuji. and F. Williams, Advan. Chem. Ser., No. 82,269 (1968). (1) This work has been supported in part by the
The Journal of Physical Chemistry,
Vol.
77, No. 18, 1973
