J . Phys. Chem. 1990, 94, 8390-8393
8390
Emission from Ozone Excited Electronic States Jichun Shi and John R. Barker**+ Department of Atmospheric, Oceanic and Space Sciences, Space Physics Research Laboratory, The University of Michigan, Ann Arbor, Michigan 48109-2143 (Received: July 30, 1990)
Time-resolved infrared fluorescence has been observed following the laser photolysis of small amounts of O3diluted in up Besides the 9.6- and 4.7-pm bands, which are well-known, three new emissions have been observed near to 1000 Torr of 02. 1.9, 2.1-2.7, and -3.4 pm. The 1.9-pm band originates from an unknown electronic state, 03(8),which is produced through quenching of O3('B2) by O2 or Xe during O3photolysis in the Hartley band. The other emission bands originate from the 0 + O2recombination reaction, and their kinetics are complex, indicating the formation and subsequent collisional cascade of excited ozone intermediates.
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I. Introduction Theoretical studies have predicted the existence of several low-lying electronic states of and several experiments have provided supporting, but not unambiguous, evidence. In 1975, Swanson and Celotta4 obtained an electron scattering spectrum that shows one or more unresolved ozone electronic states at energies near the dissociation limit of the ground state. More recently, McGrath et aLs observed a transient absorption band near 320 nm produced by direct photoexcitation of O3 in the Chappius band; they attributed the absorption to the O3('A2+ 2lB1)transition. Anderson et ale6recently measured the origin of the 'A2 state by studying isotopic shifts in the Wulf absorption bands and concluded that 03('A2) lies only slightly above the dissociation energy to form ground-state O(3P)+ 0,. On the basis of this result and the theoretical calculations,1,2it seems likely that the 3A2,3B2,and 3B, states may be stable with respect to ground-state 0 + 0,. The 3A2,3B2,and 3Bl states are all triplets and therefore cannot be populated by direct absorption from the ground-state O3(lA1), but 03(3A2)and 03()B2)might be formed in the recombination because they are correlated with O(3P)+ 02.Formation of such excited electronic intermediates has been suggested in several previous studies. For example, von Rosenberg and Trainor' studied the infrared chemiluminescence from the recombination reaction, and in addition to the 4.7- and 9.6-pm bands, they observed emission features at -8 and -6.6 pm, which they attributed to excited electronic ozone species. Riley and Cahil18 observed transient UV absorption they attributed to an intermediate from the recombination and found that its spectrum was much simpler than they expected for a vibrational cascade, concluding that it may be due to an electronically excited species. More recently, Bair and c o - w ~ r k e r s ~suggested .'~ that as much as 60% of the recombination reaction products are excited electronic ozone (probably 03('B2)), and because this intermediate is deactivated inefficiently, ground-state O3appears much more slowly than the disappearance of the reactant 0 atom. These studies are summarized in a recent review by Steinfeld et a1.j Excited electronic 0,states may be important for the understanding of atmospheric chemistry. The possible role in the atmosphere of excited O3produced by the 0 + O2recombination reaction has been widely discussed,**-*3especially with regard to the mesosphere and lower thermosphere where current chemical models predict a lower O3density than that ~ b s e r v e d . ' ~ In this study, time-resolved infrared fluorescence was observed following laser flash photolysis of O3at high O2pressures. The expected 9.6- and 4.7-pm emissions were observed, as well as three new emissions near 1.9, 2.1-2.7, and 3.4 pm. Kinetic analyses of these emissions suggest that they originate from excited electronic and vibrational ozone states. In this Letter, we will briefly describe the observations of these emissions, but only the 1.9-pm emission band results will be discussed in detail.
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'And Department of Chemistry.
0022-3654/90/2094-8390$02.50/0
II. Experimental Section The fluorescence cell used in these experiments is a stainless steel tube 48 cm long with an i.d. of 3.8 cm. The cell is fitted with two Suprasil end windows for entrance and exit of the laser beam, two opposing side windows for monitoring O3concentration, and two other fluorescence viewing windows fitted with IRtransmitting sapphire and/or NaCl windows. After 24 h of passivation with a high partial pressure of 03,the dissociation of O3on the cell walls is negligible. Oxygen atoms are produced by laser photolysis of O3at 308 nm (XeCI excimer laser, Lumonics HyperEx-400), at 600 nm, and at wavelengths between 260 and 305 nm. Tunable visible and ultraviolet laser pulses are generated by a XeCl laser-pumped dye laser (Lumonics Hyperdye-300) whose output can be frequency-doubled ( h a d , Autotracker 11). The laser intensity is -5 mJ/cm2 per pulse at 308 nm and -0.5 mJ/cm2 per pulse a t the other wavelengths, as measured by an absorbing disk power meter (Scientech 38-0102). The gases used in this study (02, N2, and Xe) are of ultrapure grade (Air Products, 99.99% purity, H 2 0 < I ppm) and are used without further purification. Ozone is generated by a silent electric discharge ozonizer and stored in a silica gel trap at dry ice temperature. For experiments with only O3and 02,the mixture from the ozonizer flows directly through the fluorescence cell. For experiments using N 2 or Xe, a static system is used, in which efforts are made to ensure complete mixing of the gases prior to photolysis. The total pressure in the cell is measured with a pressure transducer (MKS Baratron, 0-lo00 Torr), and the partial pressure of O3is measured by the attenuation of 2537-A light from a low-pressure Hg lamp. The absorption cross section (base e ) of O3at this wavelength is assumed to be 1.14 X lo-'' cm2 at room temperature.I5J6 (1) Hay, P. J.; Dunning, T. H.; Goddard, W. A. Chem. Phys. Lerr. 1973, 23, 457. (2) Hay, P. J.; Dunning, T. H. J . Chem. Phys. 1977, 67, 2290. (3) Steinfeld, J. I.; Adler-Golden, S. M.; Gallagher, J. W. J . Phys. Chem. ReJ Dara 1987, 16, 91 1. (4) Swanson, N.; Celotta, R. J. Phys. Rev. Leu. 1975, 35, 783. ( 5 ) (a) M d r a t h , W. D.; Thompson, A.; Trocha-Grimshaw, J. Planer. Space Sci. 1986,34, 1147. (b) McGrath, W. D.; Maguire, J. M.; Thompson, A.; Trocha-Grimshaw, J. Chem. Phys. k i t . 1983,102, 59. (6) Anderson, S. M.; Morton, J.; Mauersberger, K. J . Chem. Phys., in
press. (7) 2442. 5348. (8) (9)
(a) von Rosenberg, C. W.; Trainor, D. W. J . Chem. Phys. 1974,61, (b) von Rosenberg, C. W.; Trainor, D. W. J . Chem. Phys. 1975,63,
Riley, J. F.; Cahill, R. W. J . Chem. Phys. 1970, 52, 3297. Kleindienst, T.; Locker, J. R.; Bair, E.J. J. Phorochem. 1980, 12, 67. (IO) Locker, J. R.; Jocns, J. A.; Bair, E. J. J . Phorochem. 1987, 36, 235. ( I 1) Wraight, P. C. Planer. Space Sci. 1977, 25, 1177. (12) Joens, J. A. J . Geophys. Res. 1986, 91, 14,533. (13) Solomon, S.;Kiehl, J. T.; Kerridge, B. J.; Remsberg, E. E.;Russell, J. M. J . Geophys. Res. 1986, 91, 9865. (14) Allen, M. A. J . Geophys. Res. 1986, 91, 2844. (15) Molina, L. T.; Molina, M. J. J . Geophys. Res. 1986, 91, 14,501. (16) Barnes, J.; Mauersberger, K. J . Geophys. Res. 1987, 92, 14,864.
0 1990 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 94, No. 22, 1990 8391
TABLE I: Summary of the IR Emission Bands Observed in This Study
integrated intensity decay kinetics rise time source assignment
band, pm 3.4
1.9
2.1-2.7
4.7
9.6
3.9 2nd-order dependence on O2 and O3 fast 0,+ hv electronic transition (1)
-0.09 mixed 2nd/3rd-order dependence on O2
0.1 mixed 2nd/3rd-order dependence on O2
mixed 2nd/3rd-order dependence on O2 and 0,
mixed 2nd/3rd-order dependence on O2
fast O+02+M electronic transition (?)
intermediate O+OZ+M (003) type vibrational (?)
intermediate O+02+M (IO!) type vibrational (?)
slow O+02+M (001) type vibrational
1.o
A fast lnSb detector is used to monitor fluorescence between 1.5 and 5.5 pm and a HgCdTe detector between 5.5 and 14 pm. (Both detectors are cooled with liquid N2 to 77 K and were purchased from InfraRed Associates, Inc.) The signal is first amplified by a matched preamplifier (Perry Amplifiers for InSb and infraRed Associates for HgCdTe) and then further amplified (Tektronix AM 502) before being captured by a digital oscilloscope (LeCroy 9400) for data acquisition and signal averaging. About 25 000 laser shots are usually averaged to obtain a good signalto-noise ratio. The infrared emission bands are isolated by band-pass interference filters; a circular variable filter cooled to 77 K was also used to resolve the 4.7-pm band. The transmission spectra of the band-pass filters were measured with an FTIR spectrometer (Nicolet DX V4.56). Fluorescence decay rate constants, rise times, and signal intensities are obtained from nonlinear least-squares fits of the averaged signals.
41
111. Results and Discussion The emission bands observed in this study are summarized in Table I. The integrated intensities of these bands are normalized to the 4.7-pm band, except for the 9.6-pm band (because the HgCdTe and the InSb detectors have not been cross-calibrated). Since the 2.1-2.7- and 3.4-pm emission bands are isolated by band-pass filters and not further resolved, they may be parts of a continuum. No filter was available to us for the wavelength region between 2.7 and 3.4 pm, and thus the 3.4-pm “band” may be part of continuum and connected with one, or more, of the other emissions. (For convenience, we will still refer to these emissions as “bands”.) in addition to the bands described in Table I, a search was made in the 6- and 8-pm regions, where emissions were reported by von Rosenberg and T r a i n ~ rbut , ~ no such emission features were observed. Most of our experimental studies to date have been on the 1.9-, 3.4-, and 4.7-pm bands, because of the low intensity of the 2.12.7-pm band. Experiments were also carried out in the 9.6-pm band, and the observed time-resolved behavior of this band agrees well with the results of von Rosenberg and T r a i n ~ r .Kinetic ~ analyses of the 9.6-pm band gave an apparent 0 O2 recombination rate constant of (2.9 f 0.2) X em6 s-I (see Figure l), which is only about half as large as the recommended value ((6.2 f 1.4) X cm6 s-I).I7 Similar (but not identical) discrepancies were also found in the 3.4- and 4.7-pm bands as shown in Figure 1, where the recommended rate constant is shown as a dashed line for comparison. Pure third-order reactions will give straight lines in this figure. The observed curvature signals the presence of more complicated kinetics, as does a difference from the dashed line. Absorption bands have been reported at -3.3,’8319 4.7, and 9.6 pm due to vibrational transitions of ozone, and Rawlins et aL20 found that vibrationally excited ozone of up to 03(oo5) is formed in the 0 O2recombination reaction. Therefore, the 3.4-, 4.7-,
0
+
+
(17) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F.; Kerr, J. A.; Troe, J. J . Phys. Chem. Ref. Dura 1989, 18, 881. (18) Smith, M . A. H.; Rinsland, C. P.; Malathy Devi, V.; Flaud, J.-M.; Camy-Peyret, C.; Barbe, A. J. Mol. Spectrosc. 1990, 139, 171. (19) Camy-Peyret, C.; Flaud, J. M.; Smith, M. A. H.; Rinsland, C. P.; Malathy Devi, V.; Plateaux, J. J.; Barbe, A. J . Mol. Specrrosc. 1990, 141, 134. (20) (a) Rawlins, W. T.; Armstrong, R. A. J . Chem. Res. 1987,87,5202. (b) Rawlms, W. T.; Caledonia, G. E.; Armstrong, R. A. J . Chem. Phys. 1987, 87, 5209 and references therein.
2
4
6
8
10-5 P ,$
1
0
1
2
(TO&)
Figure 1. First-order decay rate constants for infrared bands related to the 0 O2 recombination reaction (Po, = 1.2 Torr). The dashed line is based on the recommended rate constant for the recombination reaction (ref 17).
+
1
i i 1.7
1.8
1.9
2.0
21
2.2
2.3
Wavelength (pm) Figure 2. Low-resolution spectrum of the 1.9-pm band obtained with narrow band filters: Po2 = 900 Torr, Po, = 0.8 Torr. The solid line is a cubic spline fit to the data points. Vertical error bars show hu statistical scatter, and the horizontal bars show the fwhm of each filter.
and 9.6-pm emission bands observed here are likely due to vibrational transitions, but the complicated cascade kinetics observed at 3.4 and 4.7 pm may also signal contributions from low-lying electronic states of 03.Detailed analyses of these bands will be reported in future publications. The 1.9-pm band intensity is quite large, with an integrated intensity of -4 times that of the 4.7-pm band (compared at 600 Torr of 02).The low-resolution spectrum shown in Figure 2 was obtained by using narrow band interference filters (fwhm 0.05 pm). The band center is near 1.95 f 0.05 pm, and the fwhm is -0.15 pm. The intensity near the center of the line was isolated with a narrow band-pass filter ,A( = 1.95 pm, fwhm = 0.04 pm) and was measured at laser excitation wavelengths between 260 and 305 nm. The results are shown in Figure 3, together with the absorption cross section of O3.I5Inspection of the figure shows that the 1.9-pm intensity is proportional within f5% to the ab-
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Letters
8392 The Journal of Physical Chemistry, Vol. 94, No. 22, 1990 0.48 r
240
1
260
280
300
1
12
(a)
320
Wavelength (run) Figure 3. Comparison of the 1.9-pm band intensity (filter: ,A, = 1.95 pm, fwhm = 0.05 pm) at Pol = 900 Torr (data points) with the absorption cross section of O3at 298 K (solid line). sorption cross section of ozone throughout much of the Hartley absorption band, Kinetic studies have been carried out in the 1.9-pm band by analyzing its time-resolved behavior. The rise time of this emission band is always limited by the detector response (-1.5 ps). Furthermore, unlike the other bands, the decay in the 1.9-pm band depends linearly on the partial pressures of O,,02,N2, and Xe, with second-order quenching rate constants of (k f l a ) = (1.5 f 0.1) X lo-", (3.1 f 1.2) X lo-", (1.8 f 1.2) X and (4.1 f 0.6) X I 0-I5cm3 s-I, respectively. The kinetics behavior of the 1.9-pm band clearly shows the band to be produced as the result of direct photoexcitation and not due to the recombination reaction 0 + 02,as discussed below. Several excited species other than O3 may emit in the 1.9-pm spectral region, and their contributions must be considered. Vibrationally excited OH(u) radical emits at around 1.9 pm for its Po = -2 transitions, but its contribution can be ruled out because (i) since O(lD) is quenched very efficiently by 02,the formation of OH(o) is not efficient due to the large amount of O2 in the mixture; (ii) only OH(u) radicals with excitation of up to u = 2 can be formed in the O(lD) + H 2 0 reaction, and the quenching rate constant of OH(u=2) by O3 is 3.0 X 10-l2cm3 s - I , ~ ~ which is about one-fifth that obtained here for the 1.9-pm band; and (iii) H 2 0 vapor deliberately added to the photolysis mixture did not affect the results significantly. Possible emission from excited O2 was also considered. The Noxon band of O2(lAg+lZg+) is also found in the 1.9-pm regiomZ2 We have recently studied the quenching kinetics of 0 2 ( l X K + )by monitoring its emission at 762 nm.23 The quenching rate constants of O,('Z,+) by O,,02,and Xe were found to be (k f l a ) = (1.98 f 0.03) X IO-", and (1.26 f 0.17) X cm3 s-l, respectively, significantly different than the quenching rate constants obtained at 1.9 pm in the present work. Moreover, the spectral bandwidth of the 1.9-pm band is far broader than that of the Noxon band (-20 cm-I) as observed by Fink et in a flow discharge system. The most dramatic difference between the kinetics of O2(IZg+) and the 1.9-pm band observed here is the band intensity dependence on O2 and Xe, as shown in Figure 4: the 762-nm intensity was found to be independent of O2pressure and to decrease with the addition of Xe, because of the interception of O(lD) by Xe;23 but the I .9-pm band intensity increases with the partial pressures of 0 2 nnd Xe. The totally different kinetics for these two emission bands rule out O2(IZ,+) as the carrier for the 1.9-pm emission. (21) Teitelbaum, H.; Aker, P.; Sloan, J. J. Chem. Phys. 1988, 119, 79. (22) Fink, E. H.;Kruse, H.; Ramsay, D. A,; Vervloet, M. Can. J . Phys. 1986, 64, 242. (23) Shi. J.; Barker, J. R. Inr. J . Chem. Kiner., accepted for publication.
0.3-
"*
1..
-._ ----_._
02('z;)
Pressure (Torr) Figure 4. The 1.9-pm band intensity as a function of partial pressures. (a) O2 and (b) Xe at Pol = 600 Torr. The dashed line is the expected intensity for the Noxon band under the same conditions (scaled to the intensity observed when Pxs = 0).
The Noxon band of 0 2 ( 1 X g + ) may be present under the experimental conditions of the present study, but it must be significantly weaker than the observed 1.9-pm band. The kinetics of the 1.9-pm band can be described by the following "minimum" scheme; the actual kinetics may be more complex, but this scheme is the simplest that can still describe all of the observations O3+ hv(Hartley band) 03(a) (1)
-+ + + 0
O,(a)
03(a) + 0
2
0 3 ( a ) Xe
03(P)
-
M
(2)
0 2
OdP) + 0
2
(3)
O,(P) + Xe
(4)
products
(5)
+ hv (A
03(@)0 3 ( y )
-
= -1.9 pm)
(6)
where M represents 03,Oz, N2, and Xe. The electronic states of the 0 atom and O2products in the above mechanism are not known. The direct dependence of the intensity of the 1.9-pm band on O2 and Xe (Figure 4) calls for reactions 2-4; the observed deactivation of the emission by 0 3 , Xe, N2, and O2 calls for reaction 5; and the emission is described by reaction 6 . Because of the close correspondence between the 1.9-pm intensity and the Hartley band absorption as demonstrated in Figure 3, 0 3 ( a )can be identified with the upper dissociative state of the Hartley-Huggins bands (the IB2 statez4). We also carried out photolysis experiments at 600 nm, where O3('B2) is not produced, and no emission was observed at 1.9 pm, although the 3.4- and 4.7-pm bands were observed. (The 2.1-2.7-pm band is too weak to be observed in this experiment.) This observation lends further support to the identification of 0 3 ( a )with OJ('B2). The above chemical mechanism correctly describes the 1.9-pm intensity increase with the partial pressures of O2 and Xe, and the experimental data in Figure 4 indicate that Xe is about 40% more efficient than O2in stabilizing O3(IB2)to O,(p). N2 was also used in the 1.9-pm studies, and the emission intensity was found to be constant within experimental error for N 2 partial pressures a t least up to 600 Torr, indicating that N2 does not (24) Sinha, A,; Imre, D.; Goble, J. H.; Kinsey, J. L. J . Chem. Phys. 1986, 84, 6108.
J . Phys. Chem. 1990, 94, 8393-8396 deactivate 0 3 (IS2) effectively. The relative efficiencies of O2and N 2 in deactivating 03(IB2)suggest that 03(p) may be a triplet, because the 0, may be an efficient quencher due to spin conservation in the following process: 03(’B2)
+ 02(3Z,-)
-
03(3g)+ 0 2 ( l A g oriZg+)
(7)
The effectiveness of Xe quencher also supports this conclusion, because Xe is known to be effective in spin switching due to the heavy atom effect, as found in the quenching of O(’D).23*2SIf reaction 7 is correct, the energy of 03(p) must lie below 32 500 cm-l. In order for 03(p) to be produced from O3(IB2)quenching by collisions with O2and Xe, a significant O3(’B2) lifetime is required. Several estimates of this lifetime have been reported in previous studies: Sinha et al.24found a lifetime of -3.6 ps for excitation at -325 nm; Johnson and Kinsey26estimated the lifetime to be -0.13 ps for excitation near the center of the Hartley band. The fact that the lifetime of O3(IB2) is greater than many vibrational periods is due to the existence of an energy barrier on its potential energy s ~ r f a c e . ~ ’Spectral .~~ analysis and trajectory calculationsz6 suggest that multiple recurrences are important: the excited 03(’B2) may “bounce back” to the Franck-Condon region several times before dissociation. If we assume the dissociative lifetime of O3(IB2)is 1 ps (Le., k2 = 10l2 s-I) and k3 = IO-” cm3 s-l (a reasonable estimate for electronic quenching), the quantum yield for the formation of 0 3 @ ) is -3 X when the Oz pressure is 1000 Torr. These estimates are very uncertain and are based on the assumption of a I-ps lifetime for the 0 3 ( a )state. If k2 is smaller and/or k3 is larger than these estimates, the quantum yield could be much larger and 0 3 ( p ) and states formed by its subsequent further deactivation might then play roles in atmospheric chemistry. Determination of the absolute quantum yield is necessary to determine whether that 0 3 ( a ) is the 03(’B2) state and refine the rate constant estimates. The data shown in Figure 3 indicate that relative quantum yield is nearly the same a t all the wavelengths investigated, and therefore O3(IB2) must have about the same lifetime between 260 and 305 nm (if k3 is independent of energy). This conclusion differs from the results of refs 24 and 26. The lifetime of Oj(’B2) depends on the potential energy surface, and
-
~~~~~~~~
~
(25) Callear, A. B . Spec. Period. Rep. 1978, 3, 82. (26) Johnson, B. R.; Kinsey, J. L. J . Chem. Phys. 1989, 91, 7638. (27) Hay, P.J.; Pack, R. T.; Walker, R. B.;Heller, E. J. J . Phys. Chem. 1982, 86, 862. (28) Sheppard, M.G.; Walker, R. B J . Chem. Phys. 1983, 78, 7191.
8393
determination of the absolute quantum yield as a function of excitation wavelength may provide useful information about the dynamics of O3 photodissociation in the Hartley-Huggins bands. The production of 03(p) during ozone photolysis in the Hartley band may be important both in the atmosphere and in laboratory systems. The observation of 03(@ indicates that the quantum yield for photodissociation of O3 is less than unity, when O2 is present, but absolute quantum yield measurements are needed to determine its importance in the atmosphere. As pointed out by Slanger,z9 03(p) may be responsible for the autocatalytic formation of O3 when 02/03 mixtures undergo broad-band 248-nm laser p h o t o l y ~ i s . ~If~03(@ has an excitation energy of more than 10235 cm-I, it can photodissociate into three 0 atoms subsequent to absorption of a 248-nm photon, thereby explaining the net production of odd oxygen. Moreover, the dependence of the autcatalysis on O2 pressure3’ specifically implicates 03(p), rather than 03(a),29 because if only 0 3 ( a )were involved, the autocatalysis would not show the observed3’ dependence on O2 partial pressure. IV. Conclusions Time-resolved fluorescence studies were carried out for the emission bands at 1.9, 3.4, 4.7, and 9.6 pm, following laser O3 photolysis at O2pressures of up to 1000 Torr. Kinetic analyses indicate that excited intermediates are formed in two different ways: (1) by quenching of O3(’B2) in the O! photolysis in the Hartley band; (2) in the 0 O2recombination reaction. The 1.9-pm emission originates from an unknown electronic state of ozone, 0 3 ( p ) ,which is produced by O2 and Xe quenching of 03(’B2) and is likely to be a triplet state. The other bands are from the recombination reaction, and they may originate from vibrationally excited 0 3 , but low-lying electronic state(s) of ozone also may be involved.
+
Acknowledgment. The authors thank J. J. Sloan, T. G. Slanger, and I. C. McDade for helpful discussions and F. W. Bartman for providing several sets of interference filters, which have been of great value in this study. The Department of Energy, Office of Basic Energy Sciences, provided funding for this work. (29) Slanger, T. G. Private communication. (30) Slanger, T. G.; Jusinski, L. E.; Black, G.; Gadd, G. E. Science 1988, 241, 945. (31) Guthrie, J. A.; Wang, X.;Radziemski, L. J. Presented at the International Quantum Electronics Conference (IQEC’90),Anaheim, CA, May 21-25, 1990.
Implications of Hyperfine Anisotropy on the Determination of 14N Quadrupole Interactions from Double-Quantum ESEEM Frequencies Sarah A. Cosgrove and David J. Singel* Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138 (Received: August 7 , 1990)
In a previous investigation of m-DNB (m-dinitrobenzene) adsorbed on activated y-alumina, we determined I4N quadrupole and hyperfine coupling constants from the frequencies of double-”quantum”peaks in the ESEEM (electron spin-echo envelope modulation) spectra. The anisotropic hyperfine interactions neglected in such analyses can, however, corrupt the values of quadrupole coupling constants. We outline the effects of anisotropic hyperfine interactions on I4N ESEEM double-quantum peaks. We present new ESEEM results obtained at electron spin excitation frequenciesof -4 GHz. The “tainted“ quadrupolar peaks that emerge in these spectra provide an independent measure of the quadrupole coupling constants. The values obtained are in excellent agreement with those found previously. We infer that the magnitude of the anisotropic portion of the hyperfine interaction is much smaller than that of the quadrupole interaction.
Recently we reported a multifrequency ESEEM (electron spin-echo envelope modulation) study of the radical species formed 0022-3654/90/2094-8393$02.50/0
by interfacial electron transfer following the adsorption of m-DNB (m-dinitrobenzene) on activated ya1umina.l In that study we
0 1 990 American Chemical Society
