J. Phys. Chem. 1981, 85, 629-635
629
ARTICLES Isotope and Temperature Effects on the Optical Absorption Spectrum of Solvated Electrons in Liquid Ammonia Fang-Yuan Jou and Gordon R. Freeman' Department of Chemistry, University of Alberta, Edmonton, Alberfa, Canada TBG 202 (Received: Januay 15. 1980; In Final Form: December 3, 1980)
Substitution of D and H in ammonia increases the energy of the optical absorption maximum EA, of solvated electrons by 0.04 eV at 200-240 K. The temperature dependence of EA, may also be slightly larger in ND3 (-2.6 meV/K) than in NH3 (-2.4 meV/K). The bandwidth at half-height Wl is unaltered by isotopic substitution. The low-energy side of the absorption band has a Gaussian shape. The widtk parameter of the Gausaian increased slightly with temperature, being gl = 0.010p/2 eV in both NH3and NDB. The high-energy side of the absorption band is neither a Lorentzian nor a simple power function of E . If it is expressed by AIA- 0: E-", the value of CY decreases from 6 in the vicinity of A / A , = 0.4 to 3 near A / A , = 0.03. The band asymmetry parameter is W,/ W, = 1.35 at 200-240 K, similar to that for electrons in water at 300 K. The temperature coefficient -(aE/anA,Amyof the low-energy side of the band is greater than that of the high-energy side in ND3and water, and probably also in NH3. The optical absorption transitions are different on the two sides of the band.
Introduction The width and the asymmetry of the optical absorption band of solvated electrons in water are both smaller in DzO than in H20; the band shift upon deuteration is not uniform throughout the whole band.' One might expect similar behavior for electrons solvated in ammonia. However, the two reports of the band shift and change of shape caused by deuterating ammonia are in conflict with each ~ t h e r . ~The , ~ results were obtained from metal-ammonia solutions. The present article clarifies the band characteristics in NH, and ND,, through a pulse radiolysis study. The optical absorption spectrum of solvated electrons in metal-ammonia solutions has been obtained either from measurements on dilute solution^^^^-^^ or by extrapolation to infinite d i l ~ t i o n . ~The J ~ shape of the measured band and ita temperature coefficient at the absorption maximum vary with the degree of due to association between solvated electrons and the metal ions. This complexity can be reduced by pulse radiolysis of neat ammonia.12 The possible contribution of "*,,+,e; ion pairs will be treated in the Discussion. The temperature coefficient -(aE/aT'lAIA, for solvated
electrons in water was larger on the low- than on the high-energy side of the band.' This is consistent with the suggestion that the types of transition that occur are different on the two sides. Analogous information for the spectra in NH3 and ND3 is reported here.
2478. (6) Douthit, R. C.; Dye, J. L. J. Am. Chem. SOC.1960,82, 4472. (7) Gold, M.; Jolly, W. L. J. Inorg. Chem. 1962, I, 818. (8) Quinn, R. K.; Logowski, J. J J. Phys. Chem. 1969, 73,2326. (9) Belloni, J.; Cordier, P.; Delaire, J. Chem. Phys. Lett. 1974,27, 241. (10) (a) Belloni, J.; Billiau, F.; Saito, E. N o w . J. Chzm. 1979, 3, 157.
Experimental Section The optical system and irradiation facilities were similar to those described earlier.'J3J4 The xenon arc lamp (Osram XBO 9OOW) was run at 450 W and pulsed to a 9-kW peak for a half cycle, 8 ms. The signal was flat enough to use for 100 ps. The circuit will be included in a later publication. The arc hM a high light output in the ultraviolet and visible, but is weaker in the IR region. Therefore, in the IR measurements light leakage due to higher order diffraction of the monochromator grating must be stopped. For measurements at X > 1100 nm, a Coming filter CS no. 7-56 was placed in front of the optical cell. In addition to the filters cited in ref 13 and 14, CS no. 7-56 and 4-76 were used for 2000-2500 nm. The latter two filters absorb all light at wavelengths less than 1250 nm. The band-pass of the visible-light monochromator was 4 f 1nm for X < 1000 nm. For the IR monochromators the band-pass increased gradually from 9 nm at 1000 nm to 17 nm at 1400 nm, 24 nm at 1800 nm, and 50 nm at X > 2000 nm. If one takes half of the band-pass as the variation of the monochromators, the uncertainty of the experimental points is 5 f 1 meV. The visible and IR monochromators were calibrated against xenon emission lines, a neon-helium laser, and their higher order lines. The dose of each pulse was measured by secondary emission monitor (SEM) which was calibrated by (CNS)2absorption16 and checked for linearity with Cerenkov
77, 2872. (12) Farhataziz; Perkey, L. M. J. Phys. Chem. 1975, 79, 1651.
(13) Jou, F.-Y.; Freeman, G. R. Can. J . Chem. 1976, 54, 3693. (14) Jou, F.-Y.; Freeman, G. R. J. Phys. Chem. 1977, 81, 909. (15) Jha, K. N.; Bolton, G. L.; Freeman, G. R. J. Phys. Chem. 1972, 76, 3876.
(1) Jou, F.-Y.; Freeman, G. R. J. Phys. Chem. 1979,83,2383. (2) Burow, D. F.; Lagowski, J. J. Adu. Chem. Ser. 1965, No. 50,125. (3) Hurley, I.; Golden, S.; Tuttle, T. R., Jr. "Metal-Ammonia Solutions"; Butterworths: London, 1970; p 503. (4) Blades, H.; Hodgins, J. W. Can. J. Chem. 1955, 33, 411. (5) Clark, H. C.; Horsfield, A.; Symons, M. C. R. J . Chem. SOC.1959,
(b) Belloni, J.; Billau, F.; Cordier, P.; Delaire, J.A.; Delcourt, M. 0. J. Phys. Chem. 1978, 82, 532. (c) Belloni, J.; Cordier, P.; Delaire, J. A,; Delcourt, M. 0. ibid. 1978, 82, 537. (11) Rubinstein, G.; Tuttle, T. R., Jr.; Golden, S. J. Phys. Chem. 1973,
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The Journal of Physical Chemistry, Vol. 85,No. 6, 1981
emission. The linearity of the SEM was within 1% error. The electron window of the cold box was warmed on the outside by gently blowing dry, room-temperature air on i t t o inhibit water condensation. The extent of condensation during a run was checked by making the last measurement at the same wavelength as the first. The SEM normalized first and last readings were usually within 3% of each other, but sometimes differed by as much as 5%. The spectrum was measured in a random wavelength sequence to avoid a systematic error, and an average line was drawn between the points. In each run there were at least three wavelengths a t which the absorbance was measured twice in the random sequence. The variation a t a given wavelength was 1% . Problems associated with optical (IR) absorption by the solvent were reduced with the pulsed lamp, and by using a smaller optical path length. When the transmitted light was so weak that the signal was noisy, the point was discarded. For measurements of spectrum shape, the radiation pulses were 1.0 ps of 2.1-MeV electrons (20 X 10le eV/g). No correction was made for signal decay during the pulse, as the first half-time was always longer than 3 ps and did not change throughout the experiment. The sample temperature (Fluke 2100A digital thermometer) was maintained within *0.3 K for each spectrum. The product of the solvated electron yield G (e;/100 eV) and the decadic extinction coefficient t (M-l cm-l) was measured a t 950 nm with a silicon photodiode detector. The radiation pulse length was 100 ns (6 X 10l6eV/g). The Gem= value was then obtained by multiplying the ratio of Amax/A950for the spectrum. The dosimeter was triple distilled water bubbled with ultrahigh-purity argon, taking Gem= = 5.0 X lo4 (e;/(lOO eV M cm)) at 715 nm.le The same monochromator and detector were used in the dosimetry as in measuring Gtg50in ammonia, to reduce the number of possible sources of error. The density of liquid NH3 was taken from ref 17, and that of ND, was obtained by multiplying the former by 1.184.18 NH3 (99.99% pure) was obtained from Matheson Co. ND3 (99%) was the product of Merck Sharp and Dohme Canada Ltd. Further purification was done in a vacuum system which had been thoroughly evacuated while heating with a flame. The ammonia gas was passed through a tube filled with molecular sieves 4A and condensed in a bulb immersed in an ethanol slush bath, while pumping slowly on it. The ammonia was then transferred to a degassed bulb containing a mirror of triple-distilled potassium, which was cooled by liquid nitrogen. This bulb was then warmed to -60 “ C ; the liquified ammonia dissolved the potassium. After 4 h the potassium solution was frozen and degassed. The ammonia was then distilled into a trap, from which it was finally transferred to an evacuated irradiation cell. The cell optical path length was 3 mm for IR work and 1.5 cm for visible and UV work in N H , but 3 mm for both IR and visible work in ND,.
Jou and Freeman I
003
B 5
(16) Hart, E. J.; Anbar, M. “The Hydrated Electron”; Wiley-Interscience: New York, 1970; p 40. (17) Jolly, W. L. “The Inorganic Chemistry of Nitrogen”;W. A. Benjamin: New York, 1964; p 24. (18) Hutchison, C. A. Jr.; O’Reilly,D. E. J. Chem. Phys. 1961, 34,163.
002
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-
Results Optical absorption spectra of solvated electrons in liquid NH3 a t different temperatures are shown in Figure 1. Those of electrons in luqid ND3 are in Figure 2. The spectra have been normalized to the maximum absorbance
,
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00
0.6
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220
‘1
’
I
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1.5
1.0
200
2.0
25
3.0
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Flgure 1. Optical absorption spectra of solvated electrons in NH, at different temperatures. The points represent experimental data. The solid lines are empirical curves. The dashed lines are Gaussian curves. The downward arrows mark EA,, and the upward arrows indicate El. Successive spectra are dlsplaced vertically by 0.2 units. Inset: (A) end of 1.0-ps pulse; (0)0.8 ps after the end of 1.0-ps pulse. 14 12 10
2 . 2
08
0.8, were caused by an apparent interaction between the extra electron and the (overtone of the) symmetric stretching vibration of ammonia; the points were discarded. In ND3 this abnormality occurs a t 0.59 eV, which is in the low-energy tail, for example, A / A - = 0.03 a t 200 K and 0.08 a t 220 K. Both are barely noticeable. However, it becomes discernible at 240 K, at which A / A = 0.20. This point was removed also. The numerical spectral data upon which Figures 1and 2 are based are listed in Table I. The spectrum parameters are summarized in Table 11, and their temperature dependences are shown in Figure 3. The temperature coefficient of each spectrum parameter is listed in Table 111. The spectrum parameters are the same at 40 ps as at 1ps in ND3, except that Gt decreases by a factor of 4.1 at 200 K and 6.7 at 240 K during the time interval. The relatively rapid initial portion of the decay is caused mainly by reaction of e; with NH2 and NH radicals,lobtCfollowed by slower reactions with other intermediates and products. The absorption spectrum in the UV was only measured in NH3 a t 200 K. There is a small band centered a t 3.54
The Journal of Physical Chemistry, Vol. 85,No. 6, 1981 831
Optical Absorption Spectrum of Solvated Electrons
I '
-
-h
I
I
I
I
I
0.8
c
4
1 200
220
240
260
T(K)
Flgure 3. Temperature dependence of spectrum parameters. NH3: (0) present work; ( 0 )ref 6; (A) ref 11: (0)ref 12. ND3: (0)present work; (+) ref 3.
1,
I,
,
, , ,
I
I
eV (350 nm), which grows slightly during the first 0.8 ~s after a 1.0-pus pulse and then decays with a half-life of 6.0 ps. The spectrum is shown in the inset of Figure 1. The band lies -0.2 eV lower (20 nm higher) than that of NH< in metal-ammonia s o l u t i ~ n s , and ~ ~ Jhas ~ been attributed to the NH radical.lobYc Unlike the spectra in ~ a t e r , a~ combined J~ Gaussian and Lorentzian shape function did not fit the spectrum in ammonia a t any temperature. Therefore, the energy at the spectrum maximum, EA,, was obtained empirically. Horizontal lines were drawn a t A/Amax= 0.5-0.9, in 0.1unit intervals. The straight line formed by the midpoints of these five lines crosses the spectrum curve at E A This midpoint line method was then used to extend %e lowenergy side of the spectrum in NH3 at 240 and 255 K to the half-height, in order to obtain its half-width. EApy decreases with increasing temperature. The coefficient is dEA / d T = -2.4 meV/K in NH3, averaged between 200 a n r 2 5 5 K, and -2.6 meV/K in ND3 for temperatures between 200 and 240 K. The energies at half-height on the red side (E,) and the blue side (Eb)of the band change in a similar way: dE,/dT = -2.6 and -2.7 meV/K, while dEb/dT = -2.5 and -2.4 meV/K in NH3 and ND3, respectively. The width at half-height ( W1/2)was separated into portions on either side of the absorption maximum: W, = E h - - E, and wb = Eb- E A - . There is no isotope or temperature effect on the widths at half-height that is larger than the experimental uncertainty (Figure 3 and Table 11),although the averages of the NH, and ND, values provide the hint of small positive temperature coefficients in Wlp and W, (Table 11). The blue shift of EA,, on going from NH3 to ND3 is 0.041 f 0.003 eV at 200-240 K (Table 11). The blue shift for a given AIA,, is also 0.04 f 0.01 eV for E < E b at all temperatures. At E > E b the blue shift gradually decreases to 0.02 eV a t around E = 1.3 eV. Beyond 1.3 eV the blue
.
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(19) Cuthrell, R. E.; Lagowski, J. J. J. Phys. Chem. 1967, 71, 1298.
(20) Lugo, R.; Delahay, P. J. Chem. Phys. 1972, 57, 2122. (21) Jou, F.-Y.; Freeman, G . R. Can. J. Chem. 1979, 57, 591.
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The Journal of Physical Chemistry, Vol. 85, No. 6, 1081
Jou and Freeman
TABLE I: Solvated-Electron Spectral Dataa
NH,,T = 200 K,1000Am,
= 33.2 1850 1800 7.48 10.0 1600b 1460 23.3,23.9 31.7.31.3 1270 1220 31.3 27.7 950 900 7.09 5.64,5.66 630 605 1.28 1.11 425 0.44 NH,,T = 220 K, lOOOA,,, = 40.3 1800 1750 23.0 26.9,26.8 1460 1430 40.0 40.6 1200 1150 27.0 21.2 950 900 7.53 5.76 650 600 1.73 1.46,1.42
1680 17.5 1340 33.2 1050 13.4 705 1.90 480 0.58
1900 6.33 1650 19.2 1310 32.7 1000 9.77,9.73 680 1.62 450 0.48
1750 12.5 1430 32.6 1200 26.7 855 4.29 580 0.97
1730 14.0 1400 33.0 1150 22.5 805 3.14 550 0.84
1900 16.7 1650 34,3 1300 35.7 1050 12.8 750 2.83 450 0.68
1850 18.5 1600 37.1 1250 32.2 1000 9.64 695 2.11 415 0.57
1700 30.9 1400’ 39.7,39.7 1120 18.9 850 4.57,4.55 550 1.16
1350 38.5 1100 17.0 800 3.58 500 0.88
1700 44.2 1440 46.4 1150 19.5 850 4.32 550 1.18
1900 29.1 1650 45.6,45.6 1400 44.2 1100 15.1 800 3.42 500 0.94,0.94
1750 41.1 1500 47.9 1250 30.4 950 6.98,6.87 650 1.77
1470 47.8 1200 24.6 900 5.42 600 1.42
1730 27.5 1450 28.6 1200 12.5 900 2.90,2.98 600 0.76
25.7 1150 9.91 850 2.32 550 0.62
NH,,T = 240 K, 1000A,,,
1900 21.8 1700 28.4 1400 23.5 1100 7.81,7.54 800 1.86 500 0.52 2400 0.11 1940 1.22 1550 10.7 1300 17.1 1050 8.26 750 1.31,1.31 450 0.25 2480 0.31 2160 1.22,1.18 1780 6.85 1500b
= 48.3 1850 1800 33.5 37.3 1600b 1560 47.4,47.7,48.6 47.9 1350 1300 40.8 35.8 1050 1000 11.6 9.04 750 700 2.73,2.76 2.16 430 0.65 NH,, T = 255 K,1000Am,, = 28.7 1850 1800 1770 24.4,23.3 25.9 26.2 1660 1630 1600’ 28.4 28.7 28.6,28.7,28.7,28.7 1350 1300 1250 21.3 18.7 15.2 1000 950 1050 5.98 4.71 3.68 750 700 650 1.47,1.47 1.18 0.98 450 0.40 ND,,T = 200 K,lOOOAmaX= 17.1 2200 2100 2300 0.54 0.16 0.38 1780 1710 1860 5.04 2.03 3.28 1450 1410 1500b 15.2,15.6 12.9,13.0 14.7 1200 1270 1240 16.8 16.4 15.6 950 900 1000 5.86 4.12,4.07,4.08 2.99 700 650 600 0.77 0.69 0.99
ND,,T = 220 K,1000A,, = 17.5 2400 2340 2280 0.72 0.39,0.41 0.52 2020 1960 2100 2.23 3.02 1.47 1680 1630 1730 8.29 10.2 11.9 1460 1420 1380
2020 0.80 1650 6.94 1370 16.6 1150 13.7 850 2.26 550 0.46
0.36
2220 1.03,1.01 1900 3.86 1580 13.5 1350
1840 5.08 1540 14.7 1330
1600 8.92 1330 17.0 1100 11.0 800 1.70 500
1700 16.2 1370 33.0 1100 17.4 755 2.51 500
0.64
The Journal of Physical Chemlstry, Vol. 85, No. 6, 1981 833
Optical Absorption Spectrum of Solvated Electrons TABLE I (Continued)
lOOOA h(nm) lOOOA h(nm) 1000A h(nm) lOOOA
16.0, 16.0 1300 16.6 1000 4.70 700 0.96
16.8 1250 15.4 950 3.47, 3.44 650 0.75
17.0, 17.4 1200 14.1 900 2.64 60 0 0.60
17.5 1150 11.4 850 2.02 550 0.46
h(nm) lOOOA h(nm) 1000A h(nm) lOOOA h(nm) lOOOA h(nm) lOOOA h(nm) lOOOA h(nm) lOOOA
2480 1.09, 1.10 2020 5.62 1740 13.0 1500b 18.0, 17.9, 18.1, 18.2 1200 11.7 900 2.30, 2.28 600 0.58
ND,, T = 240 K, 1000A,, = 18.1 2250 2380 2300 1.39, 1.46 1.95, 1.93 2.25 1940 1900 1860 6.94 7.93 9.11 1700 1660 1620 14.0 15.1 16.2 1450 1400 1350 18.0 17.6 16.8 1150 1100 1050 7.07, 7.00 5.12 9.07 800 7 50 850 1.06 1.83 1.45 550 500 460 0.47 0.33 0.25
17.5 1100 8.96, 8.97 800 1.57 500 0.34
17.2 1050 6.51 750 1.22, 1.22 450 0.26
2150 3.42 1820 10.1 1580 17.2 1300 15.5 1000 3.91 700 0.91 425 0.20
1780 11.5 1540 17.7 1250 13.6 950 2.97 650 0.70
' The optical absorbances A in a given set have been normalized t o the same electron beam energy absorption. Measurements were taken in random wavelength sequence. The wavelength at which the first and the last measurements were taken in the run o f h 2 900 nm. TABLE 11: Empirical Parameters of Solvated-Electron Spectra in NH, and ND,O
10-3. Gemax,
%-I
W,,eV
a
0.904 0.406 0.236 0.170 NH, 200 0.945 0.400 0.230 0.170 ND, 200 0.850 0.415 0.175 0.240 NH, 220 0.895 0.405 0.170 0.235 ND, 220 0.804 0.409 0.17EJb 0.231 NH, 240 0.841 0.413 0.174 0.239 ND, 240 0.17Eib 0.769 0.414 0.236 NH, 255 The uncertainty in temperature is 0.3 K. The uncertainty of
TABLE 111: Temperature Coefficients o f Parameters of Solvated-Electron Spectra in NH, and ND,' dX/dTb
X
[ i o n ] , mhl
NH,
'EAmax
0.009
