Esr Ssectra of Condensed Ammonia
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(13)P. MeakinandP. J. Krusic, J.Amer. Chem. Soc., 95,8185 (1973). (14)K. S.Chen, P. J, Krusic, P.Meakin, and J. K. Kochi, J. Phys. Chem., 78, 2014 (1974). (15)M. C. R. Symon, J. Amer. Chem. SOC., 91,5924 (1969).
(19)P.J. Krusic, manuscript in preparation. (20)R. B. Lloyd and M. T. Rogers, J. Amer. Chem. Soc., 95,1512 (1973). (21)P. J. Krusic and P. Meakin, manuscript in preparation. (22)(a) D. England, J. Amer. Chem. Soc., 83,2205 (1961);(b) D. Phillips, J. (?sa) Note Added in Proo,b This assignment has recently been confirmed by Phys. Chem., 70, 1235 (1966). the detection of radical V’ formed by type I cleavage during the photoly(23)B. S.Smart and P. J. Krusic, manuscript in preparation. sis of CF30C!CF&COF in inert fluorocarbon solvents. (24)(a) P. J. Kruslc and T. A. Rettig, J. Amer. Chem. Soa, 92, 722 (1970); (b) P. J. Krusic, T. A. Rettig, and P. v. R. Schleyer, ibid., 94, 995 (1972). ~ ~ ~ ~ ~2 CF3OC(CF3)2 C ~ l ~ COF F ~ ) ~ C ~ (25) ~ P. J. Krusic and J. K. Kochi, J. Amer. Chem. Soc., 91, 3938 (1969). (26)A. J. Bowles, A. Hudson, and R. A. Jackson, J. Chem. SOC. 8, 1947 We thank Dr. B. 8. RIssnick for suggesting this experiment and a sample (1971);J. Cooper, A. Hudson, and R. A, Jackson, J. Chem. Soc., ferkin of the fluoroacyl fluoride. Esr studies of the radicals involved in the phoTrans. 2, 1933 (1973). tolysis of fluoroacyl halides are in progress. (27)P. J. Krusic, manuscript in preparation. (16) R. C. Bingham and iM. J. S. Dewar, J. Amer. Chem. Soc., 95, 7182 (28)K. S. Chen and J. K. Kochi, Chem. Phys. Lett., 23, 233 (1973);J. Amer. (1973);cf. also A. .I. Dobbs, B. C. Gilbert, and R. 0. C. Norman, J. Chem. Soc., 98,794 (1974). Chem. SOC.AT 124 i1971). (29)(a) M. Iwasaki, Nuorine Chem. Rev., 5, 1 (1971):(b) A Hudson and K. (17)P. J. Krusic, P. ~ e a ~ and ~ n J., P. Jesson, J. Phys. Chem., 75, 3438 D. J. Root, Advan. Mag. Resonance, 5 , l(1971). (1971). (30)J. Cooper, A. Hudson, R. A. Jackson, and M. Townson, Mol. Phys., 23, (18) (a) P. J. Krusic and d. K. Kochi, J. Amer. Chem. Soc., 93, 846 (1971); 1155 (1972). (31)M. T. Rogers and L. D. Klspert, J. Chem. fhys., 48, 3193 (1967). (b) K. S.ishen, P. ,I. Krusic, and J. K. Kochi, J. Phys. Chem., 78,2030 (1974). (32)P. J. Krusic and J. K. Kochi, J. Amer. Chem. Soc., 91,6161 (1969).
+
Electron Spin Resonance Measurement of Ammonia Condensed at 77’K after Reaction F? Products and after High-Frequency Discharge F. HI. Froben’ Max-Planck-lnstitut fur Biophysikalische Chemie Abteiiung Spektroskopie,D 34 Gottingen-Nikoiausberg, Germany (Received March 22, 1974) I”ublication costs assisted by fhe Max-Planck-Gesellschaft
Amino radicals are condensed a t 77OK from the vapor of a fast flow system. The esr spectra are somewhat different for the discharge (where their shape depends on HF discharge energy and pressure) and for the reaction of discharge products from Nz, H2, and Ar with ammonia, added through an inlet between the discharge and the detection system. The differences can be caused by the environment of the radicals and by signals from other radicals.
~ ~ t ~ o ~ u ~ t i o ~ Electron impact excitation of ammonia at 2 x 10-5 to 5 X Torr yield NH radicals as primary products.2a Above lo-$’ Torr i s a small contribution of NH2 radicals, visible by emission of the a bands of ammonia (zA1). In gas-phase pulse radiolysis,3 and in reaction of atoms with ammonia4 NH radicals have been found as well as NIP2 radicals, By condensation of the radicals from a fast flow system and subsequent esr measurement it was possible to obtain further information on this system. For better ident,ification deuterated ammonia is used as well. Esr spectra of ammonia at 77OK have been measured after y-irradiation,G--9 photolysis,5,6J0 and electron imp a ~ t . ~ The J I spectra have been ascribed to amino radicals with one exception, which has been corrected by private communication. NN radicals have been observed only by their uv spectral2 and the lack of esr spectra is explained by broadening of the resonance lines even a t 4OK. The present investigation deals with the measurement of amino radicals a t 77*K. and the variation of the esr spectra found for different experimental conditions.
Experimental Section The apparatus is shown in Figure 1. Radicals are produced in a fast flow system by a microwave discharge (Microtron 200, 2.45 GC) and condensed on a cold finger a t 77OK inside the esr cavity. The position of the discharge and the substrate inlet can be varied along the flow tube. The distance between cold finger and discharge varied from 50 to 100 cm and between substrate inlet and cold finger from 5 to 25 cm. That corresponds at the linear flow velocity of 1000 cmhec (at the pressure used for most experiments and measured by the disappearance of esr signals in the gas phase with different positions of the discharge) to a time of 0.05-0.1 sec for the discharge products to reach the cold finger and for the added materials 0.005-0.025-sec flow time until they are condensed. The t o t d pressure was varied between 5 X and 1 Torr and the discharge energy between 15 and 150 W. The deposit was collected for a constant time of 10 min for all experiments, after checking the linear dependence of esr signal intensity with time of deposition. The discharge in flowing ammonia is a very complex sysThe Journal of Physical Chemistry, Vol. 78. No. 20, 1974
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1
F F Z r g e
substrate
c
gasinlct
T
Figure 1. Experimental arrangement. The position of discharge and
substrate inlet can be moved relative to the cold finger.
I
25G
I
H
Figure 3. Esr spectra of deposition from ND3 discharge (20W) at 0.3 Torr (upper trace) and from the reaction of the N2 discharge (20W)
with added ND3 (lower trace): N2:NH3 = 2:1,total pressure 0.2 Torr, relative intensities 1: 1.
Figure 2. Esr s8pectraof deposition from NH3 discharge at 0.3 Torr and 20-W discharge energy (upper trace) and from N2 dlscharge (20 W)with addition of ammonia (lower trace): N2:NH3 = 2:1,total pressure 0.3 Torr, relative intensities 1:2.5.
tern. End prodi~.ctsl3are, besides ammonia, hydrazine (up to -lo%), H2, and Nz. In the gas phase H and N atoms can be observed by esr. Esr spectra are measured using a Varian X-Band spectrometer with cylinder cavity V-4135 and 100 kHz modulation. NHs (Matheson Anhydrous 99.99%), ND3 (Stohler Isotope Chemicals 99% ID), N2 (L’Air liquide 99.9992%), H2, and Ar (Messer Griesheim >99.998% purity) were used as received.
Results A. Reaction of Discharge Products with Ammonia. The ‘i’7OK esr spectra obtained by discharge in Nz, Hz,or Ar and addition of ammonia to the flowing gas are due to amino radicals. By moving the discharge and the ammonia inlet position it can be shown that the reaction of the N2 discharge products is due to excited IVz molecules (within an experimental error of 10%).For W2 discharge it is due to H atoms and for Ar discharge to excited Ar molecules and electrons. The g value and hyperfine splitting constants of the radicals proThe Journal of Physicai Chemistry, Vol. 78, No. 20, 1974
duced by this method (Figures 2 and 3 bottom) are the same as given in the literature for NH2 (ND2). Their relative concentrations under the same experimental conditions are H2:Nz:Ar = 1:20:20. Variation of pressure and HF energy change only the intensity but not the shape of the spectra. Variation of the position of the discharge relative to the cold finger shows a linear dependence of the intensity, but for the Nz discharge only a 10%change in the 77OK spectra compared to the gas-phase N atom intensities. B. Discharge in Ammonia. At low discharge energy and “medium” pressure the epr spectra (Figures 2 and 3 top) of the deposit are similar to the spectra produced by method A. The difference for NH3 is that the two most intense lines are broadened and for ND3 the outer lines are further apart from the center. The result of discharge energy variation on the spectra is shown in Figures 4 and 5. For energies above 50 (NH3) and 80 W (ND3) respectively the shape of the spectra does not change. Only a t low discharge energy and a pressure below 0.4 Torr can the hyperfine structure be resolved. With increasing power it decreases steadily and vanishes first on the high-field side of the spectrum (Figure 5, two lower recordings). A similar behavior is observed when the pressure is changed. Above 0.5 Torr a t 20-W discharge energy the hyperfine structure disappears. This pressure limit goes down for increasing power. Above 60-W discharge energy the fine structure is not resolved even at the lowest pressure of 5 X Torr at which the discharge burns steadily. Variation of the distance between the discharge and the cold finger causes no effect on the structure but, only on the intensity. For maximum variation the intensity is changed by a factor of 2. C. Reaction of N2 Discharge on the Deposit (for NDs only because of the Fine Structure). At low discharge energy the spectrum is identical with the ammonia discharge. Starting around 40 W some additional features appear on the position of the maximum of the outer lines in the Nz
Esr Spectra of Coridensed Ammonia
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- AI - = 8J
Flgure 4. Esir spectra ot deposition from NH3 discharge at 0.1 Torr: upper spectrum, 50-W discharge energy; lower spectrum, 20 W; relative intensities (50 W):(20 W) = 4: 1.
discharge plu3 ammonia case (compare Figure 3). The center is not affected and the fine structure is not changed even for prolonged treatment with active nitrogen. The intensity increases first linearly with time and shows, for times longer than 20 min, a saturation behavior. D.Discharge i n a Mixture of Nitrogen and Ammonia (for ND3 only). The epr spectra of the deposit from these experiments itre very small (less than 10% of the intensity without nitrogen for the same conditions). Treatment of the condensate with active N2 produces the fine structure of ND2 as in the experiments on N2 discharge with addition of ND3 (part A, Figure 2, bottom). Prolonged treatment causes the fine structure to disappear simultaneously on both sides of the center. E. Discharge i n Ammonia and Addition of N2 Prior to Condensatiorb. The epr spectra intensity is only 1%of the spectra without addition of N2. Reaction of active nitrogen produces ND2 signals with less resolution on the high-field side. The spectra are comparable to the ammonia discharge signals (Figure 2 top) but their final intensity is a factor of 4 smaller.
Biscnssioxi Solids formed by vapor deposition on a cold surface occur often in different modifications from those obtained by cooling the liquid. The ratio of amorphous material to polycrystalline aggregates depends mostly on the deposition speed, which i!3 different €or experiments a t low and high pressure and can cause different structures of the solid. X-Ray diffraction studied4 on condensed ammonia show that there are four different forms at 4OK, including an amorphous modification. The hydrogen bonded structure of ammonia is stable at low temperature5 and affects the shape of the spectra. High pressure condensation can cause a temperature gradient on the surface of the cold finger producing a structure difference perpendicular to the surface. Two effectH found in the experiments need further discussion. First the change in spectra with different discharge energy (Figure 5) especially the difference in the hyperfine spliitting on the low- and high-field side and second the difference in the spectra for discharge and for the reactions. The changc? in spectra with different discharge energy
,
25G H
,
Figure 5. Esr spectra of deposition from NDSdischarge at 0.4 Torr: discharge energy from top to bottom 80, 65, 50, 20 W; relative intensities 2 , 2, 1.5, 1.
can be partly explained by an environment effect. At low discharge enei'gy only -10% of the ammonia is dissociated (depending on the pressure) and a t energies higher than 60 W more than 20% is destroyed in the discharge. This may cause broadening of the lines because of interaction of radicals close to each other.5 But this cannot be the only explanation because the spectra in part C still show fine structure. The difference at low and high field can be explained by the presence of another radical (a singlet or triplet signal with g = 2.002). This could be ND3+.l5 The superposition of the observed ND2 signals a t low discharge energy and the ND3+ spectra reported affect the splitting at the highfield lines more than the low-field side and cause a growing in of a line between the center and the outer lines which is observed in the experiments (Figure 5). The second effect (the difference in the spectra for discharge and reactions) has been observed similarly for samples with large differences in the water c0ntent.62~This cause can be excluded for the experiments reported here since the ammonia is the same and the Nz does not show any epr signal originating from water. However again this difference could be explained by ND3+ lines and by hydrogen bonding effects in the environment of the radicals. The most likely explanation is a different modifications of the solid which has been shown to exhibit such differences in the ~ p e c t r a . ~ The difference in intensity of the signals for varied distance between the production of radicals and condensation is small, compared to the variation in atom intensit,ies and lifetime of radicals under similar conditions. This is important for the ammonia discharge and could be explained by the existence of a long-lived precursor of the amino radical, which can be an excited ammonia moleeule.16 If this is true, the addition of gases could destroy this precursor without generating the radical. The experiments (part E) support this view. In the discharge through ammonia a number of other radicals, such as, NzH3 and more complicated radicals, can be formed, but the spectra for these radicals, which have The Journal of Physicai Chemistry. Vol. 78. No. 20, 1974
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been reported in the 1iterature,l7J8 are different. NH radicals are certainly formed in the gas phase but their stability a t 77OK seems unlikely. It is therefore desirable to extend the measurements to lower temperatures.
Acknowledgment. Thanks are due to Professor A. Weller and Dr. E. Siickmann from the MPI in Gottingen for critical reading of tbe manuscript and valuable comments. References and Notesi (1) Address correspondence to lnstitut fur Molekulphysik, Freie Universitat Berlin, 1 Berlin 33, Germany. (2) (a) H. Bubeft and F. W. Froben, J. Phys. Chem., 75, 769 (1971); (b) K. A. Mantei and E. J. Bair, J. Chem. fhys., 49, 3248 (1968). (3) G. A. Meaburn and S . Gordon, J. fhys. Chem., 72, 1592 (1968).
Renner and P. A. Lyons
(4) D. C. Carbaugh, F. J. Mumo, and J. M. Marchello, J. Chem. Phys., 47 5211 (1967). (5) R. Marx and J. Maruani, J. Chim. Phys., 61, 1604 (1964). (6) B. S. AI-Naimy, P. N. Moorthy, and J. J. Weiss, J. Phys. Chem., 70, 3654 (1966). (7) V. A. Roginskii and A. G. Kotov, Russ. J. fhys. Chem., 40, 88 (1966). (8) D. R. Smith and W. A. Seddon, Can. J. Chem., 48, 1938 (1970). (9) I. S. Ginns and M. C. R, Symons, J. Chem. Soc., faraday Trans. 2, 68, 631 (1972). (10) E. L. Chochran, F. J. Adrian, and V. A. Bowers, J. Chem. Phys., 51, 2759 (1969). (11) G. Beuermann, 2.Phys., 247, 25 (1971). (12) L. F. Keyser and G. W. Robinson, J. Amer. Che" Soc., 82, 5245 (1960). (13) R. Barker, J. Chem. SOC.,Faraday Trans. 1, 66, 315 (1972). (14) H. S. Peiser in A. M. Bass and H. P. Broida, "Formation and Trapping of Free Radicais," Academic Press, New York. N. Y., 1960, p 301 ff. (15) J. K. S. Wan, Ber. Bunsenges. Phys. Chem., 72, 245 (1968). (16) R. Barker, J. Chem. Soc., Faraday Trans. 2,68,421 (1972). (17) K. V. S. Rao and M. C. R. Symons, J. Chem. Soc. A, 2163 (1973). (18) R. Fantechiand G. A. Helckb, J. Chem. Soc., Faraday Trans. 2,66, 924 (1972).
Computer-Recorded Gouy Interferometric Diffusion and the Onsager-Gosting Theory %. A. Renner and P. A. Lyons* Department of Chemistry, Yale University, New Haven, Connecticut 06520 (Received April 22, 1974) Pub/icationcosts assisted by Yale University
Using computer-recorded data, a method for the determination of the intensity distribution in Gouy interference patterns has been devised, and the Onsager-Gosting theory for this distribution has been verified. A procedure for obtaining diffusion coefficients from the complete fit of intensity data has been developed. An improved method for determining diffusion coefficients from intensity measurements at a fixed position in the Gouy focal plane has been described and tested.
Introduction The effect of an index of refraction gradient on slightly convergent light is sketched in Figure 1. If the gradient is produced by diffusion across an initially sharp boundary on the optic axis, sn interference pattern will be produced a t the focal plane. This Gouy phenomenon was recognized to provide a means for the determination of diffusion coefficients. Soon thereafter, theories were developed which made this possible.24 It was quickly established that accurate values for the diffusion coefficients could be obtained from measurements of the positions of the lower fringe minima as a function of time.5 Onsager and Gosting later developed a general theory for the intensity distribution in a Gouy pattern.6 An experimental study verified that the 0-G theory correctly predicted (1) the ratio of intensities of maxima in the fringe pattern, (2) the linear variation of the intensity of a given maximum with time, and (3) the correct values of D from measurement of the positions of maxima as well as minima.7 Instrumental limitations resulted in some questions being left unanswered. Does the 0-G theory coryectly predict intensities at positions other than maxima? Given the intensity data a t all positions, can diffusion coefficients be computed exploiting all the inforThe Journal o f Physical Chemistry. Vol. 78, No. 20. 1974
mation? (The latter question might be pertinent for dealing with skewed boundaries.) To answer these questions, an experiment was devised which involved rapid photomultiplier scanning of the focal plane. High-speed recording of the photomultiplier output voltage with a computer produced the raw data upon which the subsequent analyses were based. During the work, it became apparent that extremely precise diffusion data could be obtained from recording the time dependence of the intensity at a fixed position in the focal plane. This experimental variation was explored. Experimental Section The basic interferometer design used in these experiments has already been described.8~9The equipment for photoelectric scanning was a modification of an earlier s t ~ d yA. ~1P21RCA photomultiplier tube was enclosed in a light-tight brass housing which in turn was fastened to the carriage of a Gaertner traveling microscope. The brass housing was provided with an adjustable slit set a t an opening of about 20 p for all experiments. (Typical fringe widths in the patterns during observations were about 500 p.) The carriage was driven up or down by a reversible, di-
