J . Phys. Chem. 1989, 93, 5195-5203 remaining unobserved molecules may decay by fluorescence or another nonradiative process. One decay possibility is C-C bond cleavage to yield CH, OCOH. The obvious extension of the present work is to look for CH, or C O fragments from '(n,n*) excited acetic acid.
+
Acknowledgment. W e thank C. J. Seliskar for the generous loan of a dye laser. S.S.H. thanks the Procter and Gamble Company and the University of Cincinnati Research Council
5195
(URC) for fellowship support. J.A.G. is grateful to the U R C for a summer faculty fellowship. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the Ohio Board of Regents under the Academic Challenge Program, for partial support of this research. Registry No. O H , 3352-57-6; CH3C0, 3170-69-2; C H 3 C O O H , 6419-7.
Chemiluminescence and Energy Transfer in Mn Energy
+ N,O
Collisions at High Translational
Martin R. Levy Department of Chemical and Life Sciences, Newcastle- upon- Tyne Polytechnic, Ellison Buiding, Ellison Place, Newcastle-upon-Tyne NE1 8 S T , UK (Received: Nouember 14, 1988; In Final Form: February 10, 1989)
Laser vaporization of a solid metal target has been used to generate a pulsed beam of Mn atoms with velocities from -2000 to -20000 m s-'. In addition to ground-state Mn(a6S) atoms, the beam contains a number of low-lying metastable states, as indicated by the long-lived Mn*(z8PJ+apS) emission at -540 nm. Comparison of the time-of-flight spectra for the 8P7/2,s/2 substates has enabled determination of their radiative lifetimes and thus of the velocity distribution of the atoms in the beam. On passing the pulsed beam through a low pressure of N20,MnO*(A6Z++X6Zt) chemiluminescence and Mn* collision-induced emission (z6PJ+a6S and possibly z6DJ+a6DJ) have been detected; and the excitation functions for these processes have thereby been determined. Analysis of the data by the "line-of-centers'' model indicates that, at most, only the a6S and a6D, states of Mn are responsible for either process. For chemiluminescence, the result is consistent with state correlations in the C, point group; but the considerable activation energy observed for this channel appears to derive from a lack of orbital correlations. Collisional excitation of Mn(a6S,a6DJ)to Mn*(z6PJ)shows significant excess thresholds, indicating that interaction between the reagent covalent potential and the ionic curve occurs at low internuclear distances.
-
Introduction Despite the enormous expansion in chemical reaction dynamics over the recent period, the study of transition-metal atom reactions remains largely unexplored. Across the first transition series from Ti to Cu, serious investigations of only 10 reactions have been publi~hed;I-~ and among the remaining d and f block elements, only Sm, Eu, Ho, Yb, and U have received any attention.6 Several factors are responsible for this: the highly refractory nature of many of the elements, which has inhibited the production of intense atomic beams; the high multiplicities of the atomic ground states, and the large number of low-lying metastable states in many cases, due to the partial occupancy of the d and f shells; and the complex electronic structures of many of the potential product molecules, which has rendered their spectroscopy extremely difficult to analyze. However, it is precisely the last two features which make transition-metal atom reactions an intriguing field to study. A large number of electronic potential surfaces of high multiplicity may interact, resulting in interesting dynamical features, including the possible breakdown of adiabatic correlations based on l j coupling. It is therefore especially desirable to investigate the reactivity of different atomic states, and their propensity for product electronic excitation. Among the few papers that have appeared, a number have reported electronic chemiluminescence from reaction of ground-state atoms;'-6 and in one case, Sm + ( I ) Dubois, L. H.;Gole, J. L. J . Chem. Phys. 1977, 66, 779. (2) Parson, J. M.; Geiger, L. C.; Conway, T. J. J . Chem. Phys. 1981, 74. 5595. (3) Schwenz, R. W.; Parson, J. M. J . Chem. Phys. 1980, 73, 2 5 9 . (4) Schwenz, R. W.; Parson, J. M. Chem. Phys. Lett. 1980. 7 / , 524. ( 5 ) Parson, J. M. J. Phys. Chem. 1986, 90, 1811. (6) Levy, M. R. Prog. React. Kinet. 1979, I O , I .
0022-3654/89/2093-5195$01.50/0 , ( ,
F2,' the photon yield appears to be 12%. On the other hand, the relatively high chemiluminescent cross sections for some excited metal reactions, e.g., CU*(*D~,~) + CI,, Br2, imply that correlation rules hold in these cases4 Consideration of reaction exoergicities alone suggests that chemiluminescence should be observed from M + N 2 0 ( M = Ti Ni); but, while this is certainly true for Ti,' very preliminary experiments* with the metals Cr Ni found emission only for Cr, Fe, and Co. One technique which has been used successfully to study reactions of involatile metals is pulsed laser vaporization. Friichtenicht, Wicke, and co-workers have produced pulsed atomic beams of B,9 Pb,l0 Ho,' Zn," and a number of other metals by irradiating thin metal films deposited on microscope slides. Beams of atoms, rather than ions, were obtained by focusing the laser through the glass onto the rear of the metal film. In all cases the beam had a substantial velocity spread, which could be resolved by its time of flight, allowing the measurement of translational excitation functions for chemiluminescent reactions. The abovementioned preliminary experiments on M N 2 0 (M = Cr Ni)8 employed this technique, but the work does not appear to have been pursued to the point where it could be published in the general scientific literature. One disadvantage of the method is that the microscope slide has to be moved for each laser shot.
-
-
-
+
(7) Dickson, C. R.; George, S . M.; Zare. R. N. J . Chem. Phys. 1977, 67, 1024. ( 8 ) Friichtenicht, J. F.; Tang, S. P. In Proceedings ofrhe 2nd Summer Colloquium on Electronic Transition Lasers; Steinfeld, J. I., Ed.; MIT: Boston, M A , 1976; p 36. ( 9 ) Tang, S. P.; Utterbeck, N. G.; Friichtenicht, J. F. J . Chem. Phys. 1977. 64, 1024. ( I O ) Wicke, B. G.;Tang, S. P.; Friichtenicht, J. F. Chem. Phys. Lett. 1978. 53, 304. ( 1 I ) Wicke, B. G.J. Chem. Phys. 1983, 78, 6036.
0 1989 American Chemical Society
5196
The Journal of Physical Chemistry, Vol. 93, No. 13, 1989
Levy
1’1
AVER AGE R
tr’99e‘. DIGITISER
I
PUMP
PUMP
-
-
; ... .
....
Nd YAG 10 Hz
LASER 1064 r m
M = mirror T
=
target
D
=
ton deflection
plates
Figure 2. Schematic of experimental arrangement. I
THE LOWEST TERMS OF
o
L0
655,2
Figure 1. Low-lying electronic states of M n
-
is -228 kJ mol-I exoergic, and that production of MnO*(A6Z+) from ground state atoms is 14 kJ mo1-l exoergic. Thus it should be fairly straightforward to observe MnO* A-X chemiluminescence even a t low collision energies, if that reaction channel is open. The MnO(A6Z+-X6Z+) spectrum from -450 to -750 nm is well-known, with regular vibrational structure,20,21and a sniali part of it has in recent years been extensively a n a l y ~ e d . ~ ~ . ~ ~
A more recent approach is laser ablation of a solid metal sample Experimental Section in the throat of a pulsed supersonic expansion of a suitable carrier gas. This technique, originally developed by Smalley’s group,I2 The experimental arrangement, which involves a “beam-spray” has now been applied by Costes, Dorthe, and c o - ~ o r k e r sto ’ ~ ~ ~ ~configuration, is shown in Figure 2. The vacuum system consists the reactions of C and AI atoms, using laser-induced fluorescence of two connected cylindrical chambers: the scattering chamber, detection of products. However, the range of translational energies 298 mm diameter X 5 15 mm high, evacuated by a baffled 12411. that can be achieved is much less than that found in the vapordiffusion pump; and the beam chamber, 305 mm diameter X 405 ization of thin films; and, in addition, it seems likely that most mm high, evacuated by a baffled IO-in. diffusion pump. There of the metal atoms entrained in the expansion will be relaxed to is no cryogenic trapping and the limiting background pressure is their ground electronic state. -5 x Pa. A third laser ablation technique has recently been developed Following the procedure of Kang and Beauchamp,ls mentioned by Kang and Beauchamp.l5 Essentially, this falls between the above, the metal atom beam is produced by laser vaporization of two others in that the laser is focused onto a solid metal sample. a solid metal target, the laser being focused onto the target by but without any carrier gas. The result is a rapidly expanding a n on-axis concave mirror, focal length 25 mm, with a 1.5-3 mm plasma of atoms and ions, which is collimated into a beam from diameter aperture at the center.24 Instead of the C 0 2TEA laser which the ions may, if desired, be removed by electric deflection. employed by Kang and Beauchamp, however, a Q-switched The simplicity of the technique, the wide energy spread of the Nd:YAC laser at 1.064 pm (JK2000) has been used in the present resulting pulsed atomic beam, and the undoubted presence in the work. The laser is operated a t 10 Hz, with pulse energy 170 beam of metastable electronic states make it a particularly useful mJ; but the energy arriving a t the target is expected to be conapproach for comparing state reactivities and for measuring exsiderably less than this owing to (a) reflection losses, particularly citation functions. at the focusing mirror, and (b) obstruction by the target. The It is this third approach that has been adopted in the present focusing mirror initially presents a gold-coated surface to the laser work, a preliminary report of which has already been presented.I6 beam; but, during the course of experiments, this coating becomes Although in principle any one of the transition elements could damaged and covered with evaporated metal, eventually requiring have been chosen, it was decided to start with reactions of manthe mirror to be refurbished. The target consists of a 1.5 mm ganese because of the relative simplicity of the Mn atomic cross section strip of electrolytic manganese (Specpure, Johnson s t r u ~ t u r e , ~as ’ . indicated ~~ in Figure 1. The 3d54s2configuration Matthey). mounted vertically, and which may be translated in gives the a6S5/2ground state added stability compared with other a vertical direction from outside the beam chamber. Horizontal transition-metal atoms, and the first excited state (a6DJ multiplet) translation of the concave mirror, in order to optimize the focusing, is comparatively high, a t an energy of -208 kJ mol-’. Unforis also possible during experiments. Once set, the target position tunately, this extra stability also has the effect of reducing the remains satisfactory for several hundred laser shots, providing the AD,,values for Mn reactions. However, a recent redetermination focusing mirror is in good condition; however, because of target of the MnO bond energy, Do 390 kJ mol-’,I9 indicates that the erosion, fine adjustment is eventually required in order to restore rcaction the atomic beam intensity and velocity distribution. The above procedure generates a high-temperature plasma out Mn(a6SSi2)+ N 2 0 ( X ’ Z + ) MnO(X6Z+) + N2(X’Bg+) of which the mirror aperture selects a narrow beam of atoms and ions. Kang and Beauchampls have shown that the atoms produced (12) Powers. D. E.; Hansen. S . G.TGeusic, M7E.;Pulu. A . C.; Hopkins. i n the ablation process are constrained fairly close to the laser J. B.; Dietz, T. G.; Duncan, M .A.; Langridge-Smith, P. R . R.; Smalley. R E. J . Phys. Chem. 1982, 86, 2556. beam axis, while the ions can follow a much wider range of (13) Dorthe, G.; Costes, M.; Naulin, C.; Joussot-Dubien, J.: Vaucamps. trajectories. I n common with that work, a dc electric field has C . ; Nouchi, G . J . Chem. Phys. 1985, 83, 3 1 7 1 (14) Costes, M.; Naulin, C.; Dorthe, G.; Vaucamps, C.; Nouchi, G . Faraday Discuss. Chem. S O C .1987, 84, 15. (19) Levy, M. R..to be published.
-
-
-
-
( 1 5 ) Kang, H . : Beauchamp, J. L. J . Phys. Chem. 1985, 89. 3364. (16) Levy, M. R. Faraday Discuss. Chem. SOC.1987, 84. 120. (17) Sugar. J.: Corliss, C . J . Phys. Chem. Ref. Dura 1985. 1 4 . Suppl. 2.
338ff. ( 1 8 ) Younger, S . M.; Fuhr, J. R.; Martin, G . A,; Wiese, W . L. J . Phys C h r m Re/ Data 1978, 7. 591ff.
(20) Das Sarma, J . M . Z. Phys. 1959, 145, 98. (21) Joshi, K. C. Spectrochim. Acta 1962, 18, 625. (22) Pinchemel, B.: Schamps, J. Can. J . Phys. 1975, 53, 431. (23) Gordon, R. M.; Merer. A . J. Can. J . Phys. 1980, 58, 642. (24) Sinden Optical Co Ltd, Stella, Haugh Lane, Addison Industrial Estate. RSton. YE21 ITE, L K .
Chemiluminescence in Mn
+ N20
The Journal of Physical Chemistry, Vol. 93, No. 13, 1989 5197
been employed in the present experiments to deflect away both the ionic component and any electrons expelled from the plasma. As shown below, the pulsed atomic beam has a velocity range from 20000 m s-', the precise form depending on the laser focusing; and it also contains a number of metastable states. The beam passes into the scattering chamber via a IO mm diameter aperture, 235 mm from the target; and it is detected at a point 283 mm from the target by the z 8 P r a 6 Smetastable emission at -540 nm. The detection system employsf/l collection optics and an R924 photomultiplier (Hamamatsu); wavelength selection is by cut-on filters or an f / 4 monochromator (Spex Minimate). The same optical system is used to monitor the emission from the reaction of Mn atoms with N 2 0 . The latter, taken from a lecture bottle (BDH, 99.6%), enters the chamber through a 100-pm aperture situated 45 mm at right angles from the point of intersection of the atomic beam and the axis of the optical collection system; however, no attempt is made to collimate the N 2 0 "spray" or to pump it differentially, so that, essentially, the gas fills the whole chamber fairly uniformly. The N,O pressure is controlled by means of a needle valve and measured by an ion gauge situated some distance from the intersection point of the beam, spray, and optical axes. Data are time-of-flight spectra of the beam number density, and the corresponding time-dependent emissions produced when N 2 0 is admitted to the scattering chamber. The single-photon pulses from the photomultiplier are amplified, then processed by a transient digitizer/signal averager system (Biomation/Trachor Northern) using a 10-MHz sampling rate, and triggered from the laser firing pulse. Between 64 and 1024 laser shots are averaged, depending on the magnitude of the signal observed. The homebuilt amplifier,25with time constant T = 1.5 ps, slows down the photon pulses sufficiently to "bracket" the transient digitizer sampling gate. Although this results in some smoothing of the time profiles, the loss of resolution is not severe, and in fact much less than used in previous time-of-flight chemiluminescence measurements.26 While in principle every data point could be recorded, the necessity for doing this by hand means that in practice only appropriate points a t -1- or -2-ps intervals are read off.
Results and Analysis 1 . Beam Characterization. Time-resolved emission a t 539.5 and 543.3 nm, from Mn* atoms in the beam in the z8P7/, and z8P5/2states, respectively, was clearly identified by using the monochromator. A typical time profile of both emissions together, I ( t ) , is shown in Figure 3. As remarked above, the intensity of the emission and the shape of I ( t ) could be altered by adjusting the focusing of the laser onto the target. In the absence of any direct measure of other states present in the beam, it was assumed that all degrees of freedom in the plasma are equilibrated and that I ( t ) could therefore be used to monitor the total beam flux distribution. I n view of the temperature of the plasma found below, this seems likely to be a reasonable as~umption.~' It is first necessary to establish whether I ( t ) measures number density or flux. The probability P(t) that a metastable atom with radiative lifetime T will emit, on passing by the detector, depends on the time At that it spends in the viewing zone, i.e., P ( t ) { I - exp(-At/r)) (1) I f I,'([ is) the flux of metastables in state J , passing the detector at time t , u the atomic velocity corresponding to that time, and 6 the effective path length over which the atom can be detected, then I J ( t ) , the emission rate from state J, is given by a N,'(t) P ( t ) 0:
N,'(t){ 1 - exp(-b/uT,))
= N,'(t){ 1 - exp(-t6/r,x))
(2) ~~~~
(25) Gorry, P. A., private communication. (26) Watson, T. A.; Mangir, M.; Wittig, C.; Levy, M. R. J . Chem. Phys.
i
5
c ,'I
\\
?LLi I
\
50
O OO
\ 100 .
150
-
Figure 3. Typical Mn*(z8P a%) emission time profile, I([) (-), observed at 283 mm from the target, together with the corresponding flux time profile, N'(t) for the whole Mn atomic beam. (-.-e),
I " " I " " I ' I " ' ' " " ' J ' 1
0
50
100
0
50 t / p s
100
Figure 4. Comparison of typical time profiles, I,(t), observed for each of the emitting Mn*(z*P,) states, J = 7 / 2 and 5 / 2 , at 283 mm from the target (data at 1-ps intervals).
where x is the distance from the target to the observation point. In the present case, T~ = 82 f IO ps or 122 14 pus, as shown below, and x = 283 mm; and because of the vignetting of the optical detection system, 6 is likely to be less than 10 mm. This means that, providing t 5 250 ps, t8/s,x 25%. On the other hand, the ratio (&n5/2/(&f)7/2 has been determined quite precisely28 and is in good agreement with theoretical predictions. This value has been used, in conjunction with present measurements of the time profiles of Mn*(8PJ) emission from the separate J levels, to determine the more precise values of 7 7 1 2 and 7 5 / 2 given above. Figure 4 shows typical examples of the separate 8P, time profiles. Because such measurements require high spectral resolution, there is substantial noise in the data; and, therefore, several such measurements were made, and in each case a best curve was drawn to give a "smooth fit", as indicated. The intensity ratio p ( t ) = I 7 / 2 ( t ) / I 5 / 2 ( t ) was then obtained, as a function of time, for
1980, 75, 3789.
(27) Boumans, P. W. J . M. Theory of Spectrochemical Excitation; Hilger and Watts: London, 1966.
(28) Blackwell, D. E.; Collins, B. S. Mon. Not. R. Astron. Soe. 1972, 157, 255.
5198
The Journal of Physical Chemistry, Vol. 93, No. 13, 1989
Levy 1.5
, , ,
, , , ,
I
, , , ,
, , , ,
:
,
,
,
i
O a t [no
L i
-
.
I T I
A
i
-
t
0 2:
t 'Ob
Ib
2b
3b
i'0
5b
6b t,,s
7b
/D
Figure 5. Plot of In p against t , where p ( t ) is the Mn*(zsPJ) emission profile ratio f 7 , 2 ( t ) / I S , Z ( t ) .The straight line is the linear regression fit to the data
Figure 6. Variation with time of the factor r(r)r,used in converting the beam emission intensity time profile I ( t ) to the flux time distribution " ( t ) (see text). The graph is arbitrarily normalized at t = 50 ps. Error bars and dashed lines indicate uncertainty limits. 25.0
the consecutive pairs of profiles, due allowance being taken for any general falloff in signal intensity. If N(t) is the total number density time distribution of the atomic beam, and 4, is the initial fraction of the beam in level J, then IJ(t) is given by [At)
Nt)4,A, exP(-AJt)
,
,
,
,
1
'
1
t4..
1
i
(4)
where Aj = ( T ~ ) - ' .Thus p(?) =
(47/2A7/2/45/2A5/2) exp(-tIA7/2 - A 5 / 2 1 )
(5)
A plot of In p against t should therefore be linear with slope -[A7/2 - A5,*] and intercept (47/2A7/2/45/2A5/2). Such a plot is shown
in Figure 5; only data between 28 and 74 ps are included as, outside that range, the uncertainty in In p becomes unacceptably high. Although the error bars in the figure are still relatively large, reflecting the scatter in the primary data, linear regression gives a good fit, Le., intercept = 0.697 f 0.020 and slope = -(3.96 f 0.38) X IO3 s-'. Since29 A, = 8a2e2vJ2(gf),/(mc3gJ)
(6)
*
and (&f)5/2/(&7/2 = 0.5 13 0.010,28we have A7/2/A512= 1.48 f 0.03; and combination of this with the present value of ( A 7 / * - A5/2) yields ~ 7 / 2= 82 f I O p s , T ~ =/ 122 ~ f 14 hs. In addition 97/2/4512 = 1.357 f 0.028, quite close to the degeneracy ratio g7/?/gSl2 = 1.333. The differences from the previous "best estimates" of the lifetimes are within the uncertainties of those measurements, though here the errors are considerably less as a proportion. In practice, it is much more efficacious to monitor the beam number density by observing emission from both the *PJstates together rather than from one, as the lower resolution allows a much higher signal. I ( t ) , as in Figure 3, is then related to iV'(?) by I ( t ) a N(t) r ( t )= N'(t) r ( t ) t (7) where r ( t ) ,the joint radiative decay function for both emitting states together, is given by
r(2)= 47/2A7/2 exp(-A7/2r) + 45/2A5/2 exp(-A5/2t) Figure 6 shows a plot of r(t)t,normalized to unity at 50 ps since the MnO* chemiluminescence reaches a maximum around that delay time. The significant uncertainty in r ( t ) r , and hence in N ' ( t ) , at long delay times, is not a severe problem in the present case (or in any other so far studiedi9) since the chemiluminescence excitation function falls off rapidly as t increases. The derived iV'(t), for the typical I ( ? ) displayed in Figure 3. is also shown in that figure. The corresponding velocity flux distribution, N'(c). has not been displayed here since, as shown below, it is not necessary to know this explicitly in order to calculate translational excitation functions. However, comparison of the flight distance (283 mm) w i t h the range of flight times in Figure 3 shows im(29) Kirkbright, G . F.; Sargent, M. Atomir Absorpfion und Fluorescence Spec,rrr~.\cop)~, Academic Press: London, 1974: p 29.
.
Chemiluminescence in Mn
+ N20
The Journal of Physical Chemistry, Vol. 93, No. 13, 1989 5199
1
4
i -
Figure 8. Typical time time profiles, I , ( t ) , of MnO*(A6Z+)(0)and Mn* 403 nm, respectively) from the emission ( 0 ) (A 2 550 nm and X reaction of M n with N 2 0 . The data are normalized to unity at their maxima.
In view of the very high temperature of the plasma, it is not at all surprising that the beam contains a significant population of 8PJ atoms. Initially all atomic states should be populated roughly in proportion to their degeneracies (the Boltzmann factor is -0.3 at the ionization limit). However, once the plasma expands into the collisionless regime, spontaneous emission will result in a beam consisting of only the ground-state (a%) and the various metastable levels (a6DJ, zsPJ, a4DJ, a4GJ, a4PJ, ...). This is likely to be a highly non-Boltzmann population distribution since cascading will populate the different states to different extents. A rough calculation, based on the known spectroscopyl' and radiative lifetimesi8 of Mn, suggests that the a6S and a6DJ states are populated to similar degrees, while many fewer atoms end up in the higher spin z8PJ multiplet. However, such a conclusion must be regarded as extremely tentative since, almost certainly, many states near the ionization limit have not yet been identified. Fortunately, as shown below for Mn N 2 0 , and elsewhereI9 for other Mn reactions, the observed reaction dynamics derive only from a very small number of low-lying atomic states. 2. Reaction with N 2 0 . Time-resolved emission from MnO*(A6Z+), observed when N 2 0 was admitted to the scattering chamber, was identified by means of both cut-on filters and the monochromator. The pressure dependence was linear up to 0.020 Pa (- 1.5 X Torr), indicating first-order kinetics. A low-resolution scan of the spectrum suggested that several vibrational levels were populated. Figure 8 shows a typical time profile, I , ( t ) , of the reactive emission (A 2 550 nm): the peak signal, at -50 ps, occurs well after the peak in the beam flux. The maximum time at which MnO* emission occurs cannot be precisely determined, because of the noise on the data. However, it does appear to be over by I 15 ps, while, in the analogous Mn NO2 r e a ~ t i o n , emission '~ was detected beyond 180 ps. Collision-induced Mn* atomic emission at -403 nm was also observed, a typical time profile likewise being shown in Figure a6S 8. As discussed below, at least a part of this must be z6P, (A = 403.08, 403.31, and 403.45 nm for J = 712, 512, and 312, a6D, transitions are in the same respectively); but the z6D, region ( A = 401.81 408.30 nm) with similar oscillator strength,I8 and these would not have been excluded by the monochromator resolution. Comparison of the relative magnitudes of the MnO* chemiluminescence and the Mn* 403-nm emission is difficult since the former extends across the whole of the visible region, and the Mn*(z8P,) metastable emissions from the beam fall right in the middle of this. However, crude difference measurements using cut-on filters indicate that the 403-nm emission is stronger than the chemiluminescence only for delay times up to - 3 7 p s , at most. Since the metal atoms have substantial mass and, even at 1 15 ps, are still moving very rapidly, the beam velocity determines the mean collision energy ( E T )in the center-of-mass system; and, for the bulk of the velocity range, the N 2 0 velocity is negligible,
Figure 9. Excitation functions a(&) for MnO*(A6Z+)and Mn* (-403 nm) production from Mn + N20.For symbols see Figure 8. Again, the data are arbitrarily normalized to unity at their maxima.
so that we can write, without loss of accuracy ET = t/zru2= 1/Z{mMnmN20/(mMn + mN20))(x/t)2 (12)
However, as with the beam characterization, we need to consider whether the observed profiles I,(t) represent number density or flux. For Mn*(z6PJ,z6DJ), 7 50 and 10 ns, respectively;18 and, although the MnO* radiative lifetime is not known, we can expect that it must be 5100 ns since the emission bands had strong intensity from both flames2' and arc discharges a t atmospheric pressure.20 In the absence of detailed knowledge about the laboratory velocity distribution of the emitting species, we may assume, to a first approximation, that their average velocity is the same as the center-of-mass velocity of the system, c . Again ignoring any contribution from the N 2 0 velocity, we get, by conservation of reagent momentum
-
+
-
+
-
--
-
c = {mMn/(mMn + mN20))U
(13)
Substituting these values into eq 2, we find the exponential term vanishes, i.e., there is unit probability of detecting the emitting species, and I,(?) measures flux rather than number density. This allows a straightforward conversion of the emission time profiles I , ( t ) to relative cross sections u(ET), as follows. Since d / d t [MnO*] = u ( t ) a(?) [ M n ] [ N 2 0 ]
(14)
where u ( t ) = collision velocity at time t , and since
I,(?) a d / d t [MnO*]
(15)
df)N'(t)[N201
(16)
then IAt)
0:
and
df)a
~I(~)/N'(~)
(17)
As I,(t) and N'(t) are both flux measurements and therefore have the same Jacobian factor for transformation from time t to mean translational energy ET, then g(f)
= dET)
(19)
Identical considerations apply to Mn* production. The derived excitation functions for both processes, up to ET N 1250 kJ mol-', are shown in Figure 9; similar measurements were presented in the preliminary report,16 but at that stage the Mn*data in particular had not been sufficiently refined, nor had the relative magnitudes of the signals from the two processes been determined. The figure shows that the cross section for MnO* production rises sharply from a threshold at -70 kJ mol-' to a peak a t -300 kJ mol-'. falling off rapidly thereafter. By contrast, the Mn* signal increases relatively slowly, from a threshold at 160 kJ mol-',
-
5200
The Journal of Physical Chemistry, Vol. 93, No. 13, I989
Levy
and saturates above -800 kJ mol-’, this saturation continuing until a t least 2500 kJ mol-I. As in Figure 8, the normalization of the two emissions in Figure 9 is arbitrary; but the aforementioned crude intensity comparison indicates that the Mn* 403-nm emission does not exceed the chemiluminescence until the collision energy reaches a t least 800 kJ mol-’
TABLE I: Energetics of Production (kJ mol-’) of Mn0*(A62+) and Mn* (z6P,,z6D,) from Mn(a%,a6D,) + N2017J9~f3*30.3’
Discussion
“Using the recent redetermination of D,(MnO) is: 390 k J mol-’, from the thresholds for Mn0*(A6Z+) production in Mn NO,, S02.19 Huber and Herzberg’s 1979 compilation” gives 357 k J mo1-I. bAverage values: the spin-orbit splittings are f 4 and f 3 k J mol-] for Mn*(a6D,) and Mn*(z6DJ), re~pectively.~’
The most surprising feature of Figure 9, in view of the multiple-state composition of the atomic beam, is the substantial threshold in each excitation function. This indicates that only a few states can be contributing to the observed processes. In fact, considering the reaction energetics (Table the MnO* threshold of -70 kJ mol-’ implies a significant activation barrier, even for ground-state atoms. The threshold of 160 kJ mol-’ for Mn* emission a t -403 nm likewise implies an activation energy, but the magnitude of this depends on which emitting multiplet, z6PJ or z6DJ, is being observed. At low transitional energies, z6PJ atoms seem the more likely since they require less excitation; and, in this case, there is an excess barrier of -70 kJ mol-’ if Mn*(a6DJ) atoms are the species responsible. A second aspect is the dramatic falloff in MnO* production above ET 300 kJ mol-’. This corresponds closely to the Mn(a6S z6PJ) threshold; and, indeed, the increase in Mn* emission from this point initially suggestedI6 that this process may be competing with MnO* production. However, the dependences on ET of Mn* production and MnO* depletion are different, indicating that the behavior is more complex; and, in any case, the present results show that the Mn* emission is far too weak to account for any but a small fraction of the MnO* depletion. In order to gain further insight into the processes responsible for these observations, it is useful to compare the observed excitation functions with the forms predicted by appropriate dynamic models. As a recent review has shown,32there have been many attempts to model translational excitation functions. However, for reactions with a threshold, the most suitable starting point for analysis is the venerable “line-of-centers” mode133s34
uroduct reagent state
MnO*(A6Z+)
Mn*(z6P,)
Mn*(z6D,)
Mn(a6S) Mn*(a6D,)
-14a
297 89b
503b 295b
-222“~~
+
117J9323*30,3’),
-
-
cz 3
-
a(ET)
=
- EO/ET)
Figure 10. Plots of ETU(ET)against ET for the data of Figure 9 (arbitrary normalization; symbols as in Figures 8 and 9). The inset shows the extension of the plot for Mn* emission to 2500 k J mol-I.
+ N2. For a slightly aspherical diatom, to which shape N 2 0 may be considered to approximate, the angle-dependent potential E( z ) ( z = cos y, where y = angle of attack with respect to N,O symmetry axis) is a s ~ u m e dto~ take ~ * ~the ~ form E ( z ) = Eo
(20)
where Eo is the threshold and uo the limiting high-energy cross section. Equation 20 may be written, alternatively, as
+ E:(l -a;
- z ) ; zo I z I1 zolz
(22)
where E’z is a constant. This leads to the result3* that a plot of ETu(ET) against ET shows quadratic curvature from threshold a t Eo, transforming to a linear f o r m a t ET = Eo E:( 1 - zo). Figure I O compares plots of ETg against ET for both excitation functions of Figure 9. The multilinear behavior, expected from the line-of-centers model, is striking. I n addition, the rise parts of the plots show very little evidence of any initial quadratic curvature as predicted by the ADLCM. Although the MnO* plot does show a change in functionality a t ET N 160 kJ mol-’. quadratic curvature below this point would require a much lower threshold than that observed. It is, however, possible within the uncertainty of the data that there is a small degree of curvature a t the initial threshold and at the breakpoint in the rise, Le., E:( 1 - z o ) I 20 kJ mol-’. Unfortunately a precise upper limit to E’z cannot be determined from these data as zo is not independently known, although, by comparison with the analogous Ba + N 2 0 ( J , I , M )reaction ( J = 2, 1),4’ it is likely to be >O.O (i.e.. yo
+
I f the line-of-centers model applies, a plot of ETu(ET)against ET should be linear, with slope uo and intercept Eo on the ET axis. I f more than one atomic state reacts, or if other channels open up with increasing translational energy, then, as shown by Gonzalez Urefia?’ a multilinear plot should be obtained, each threshold being revealed as a “break”. Some caution is required here, however. The line-of-centers model is essentially only a crude approximation, as it ignores the angular dependence of reactivity, a feature which is likely to be quite important in the case of reactions with N 2 0 . An “angledependent line-of-centers model” (ADLCM) has been developed by Smith,36Levine and B e r n ~ t e i nand , ~ ~ Evans, She, and Bern~ t e i n ; ~and ’ . ~ this ~ has been applied by Janssen and StolteWto the BaO* analysis of steric effects in Ba + N,O(J,I,M = l , l , l )
-
(30) Herzberg, G. Electronic Spectra of Polyatomic Molecules; Van Nostrand: New York, 1966. (31) Huber, K.; Herzberg, G. Constants of Diafomic Molecules: Van Nostrand: New York, 1979. (32) Gonzilez Ureiia, A . Ado. Chem. Phys. 1987, 66, 213. (33) Tolman, R. C. Statistical Mechanics with Applications to Physics and Chemistry; Chemical Catalog: New York, 1927. (34) Frost, A . A,; Pearson, R. G. Kinetics and Mechanism: Wiley: New York, 1953. (35) Gonzilez Ureiia, A. Mol. Phys. 1984, 52, 1145. (36) Smith, I . W. M. J . Chem. Educ. 1982, 59, 9. (37) Levine, R. D.; Bernstein, R. B. Chem. Phys. Lett. 1984, 105, 467. (38) Evans, G . T.; She, R. S. C.; Bernstein, R. B. J . Chem. Phys. 1985, 82, 2258. (39) She, R. S. C . ; Evans, G. T.: Bernstein, R . B. J . Chem. Phys. 1986, 84. 2204. (40) Janssen, M. H. M.: Stolte. S. J. Phys. Chem. 1987. 9 / , 5480.
< 90’).
-
I n fact Figure I O indicates that two processes, with line-ofcenters thresholds -70 f 10 and 160 f 10 kJ mol-l, contribute to MnO* production; and the relative slopes imply that the go values (in the arbitrary units of Figure 9) are similar. The simplest explanation of this behavior is that the two thresholds are due to reaction of the two lowest lying atomic states, Mn*(a6DJ) and Mn(a6S), respectively; but it cannot be excluded that only one species yields MnO*(A), with a change in dynamics above 160 kJ mol-’. The latter possibility is favored by the linear falloff i n the plot in Figure I O , for E , N 300-700 kJ mol-’, corresponding
-
(41) Jalink, H.; Janssen, M . H. M.;Geijberts, M.:Stolte, S.; Parker. D. H; Wang, J. Z . W . I n Selectiuify in Chemical Reacfions; Whitehead, J . C . . Ed.: Reidel: Dordrecht. 1988.
Chemiluminescence in Mn
+ N2O
The Journal of Physical Chemistry, Vol. 93, No. 13, 1989 5201
to the reduction of 80% in the MnO* cross section, since this suggests a single depletion process for these energies. However, it is conceivable that a second depletion process with threshold 4D in the range ET 300-360 kJ mol-' is being masked by the uncertainties in the data. Either way, production of MnO*(A) must have a substantial activation energy for all Mn states: if 8P only one state is reactive, then all others must effectively have 6D an infinite activation energy. In light of these results it is useful to consider the minimal state correlation diagram ( I j coupling) in the C, point group for Mn N 2 0 (Figure 11). The positions of all MnO states above A6Z+ are uncertain, having been obtained only by c a l ~ u l a t i o n . ~ ~ Nevertheless, it is clear that, if all curve crossings are avoided, a6S atoms should produce only MnO(X6Z+), while MnO*(A) should be produced by Mn*(a6DJ). This conclusion, however, neglects the fact that the MnO A and X vibrational manifolds overlap, and almost certainly perturb each other;23in addition there could be intersystem crossing in the MnON, reaction intermediate, if it lives long enough. Thus it seems inevitable that reaction of Mn(a6S) atoms will also yield some MnO*(A). With this proviso, the experimental observations of Mn state reactivity are entirely consistent with the correlation diagram. However, for the two states that are predicted to react, the diagram gives no information about potential barriers; and if, as suggested, some MnO*(A) derives from vibronic interaction with the X state, then it is inconceivable that MnO(X) production should not have a high barrier also. Despite the exothermicity of the reaction, this is not an entirely unexpected result: similar behavior has been observed for reactions of N 2 0 with a number of other metal atoms, e.g., Sn,43Sb, Bi," and Na (to produce NaO*(B)),& and has often been attributed to the "closed-shell" nature of the N 2 0 molecule. However, such a lack of reactivity is by no means universal: for the ground-state alkaline earth metal atoms, Mg(lS) has a high Figure 11. Minimal state correlation diagram in the C, point group for activation energy;47 whereas Ca, Sr, and Ba (IS) all have subMn + N20. Although the A62' state is well characterized a s the first stantial cross sections at relatively low collision e n e r g ~ . ~ * - ~ O excited state of Mn0,23942the position of most other MnO states is unIn the latter family, this dichotomy might at first be considered certain and hence the dashed lines on the right. T h e thresholds for the to be due to different ionization potentials: the relatively high collision-induced dissociation channels Mn(a6S) + N2('Xgt)+ O('P,'D) value for Mg would lead, on the crude electron-jump model,51to a r e respectively 162 and 350 kJ mol-' above the ground-state reactant a crossing radius well within the repulsive part of the potential. asymptote. N20 Indeed, a b initio calculations by YarkonyS2on Mg('S) do predict a substantial charge-transfer barrier, in a preferred the vibration lowers the N 2 0 electron affinity, thus increasing the linear geometry. However, Yarkony points out a second significant electron-jump radius;51but Jalink et aLS3have calculated that this difference between M g + N 2 0 and Ca + N 2 0 : in the former would not give a sufficient increase in cross section, and they have there are no accessible open-shell configurations of the surface proposed instead that bending disturbs the cylindrical symmetry, which correlate with excited singlet states in the isolated atom, thereby reducing the barrier to N 2 0dissociation by allowing direct whereas in Ca the excited singlet states lie significantly lower. coupling to N, + O(3P) instead of the spin-allowed N2 O*('D). He therefore suggests that the reaction of Ca with N 2 0 involves Similar considerations must apply in the Mn N 2 0 system. a preliminary avoided crossing with surfaces correlating with In terms of atomic radius and ionization potential, Mn(a6S) is Ca*('D or 'P), followed by charge transfer to N 2 0 from an quite similar to Mg('S), whereas the a6DJ state has a similar open-shell a-oriented orbital (d or p) on Ca. ionization potential to Ca('S). The crude electron-jump model This x-orientation requirement follows from the preferred linear yields crossing radii of 0.19 and 0.27 nm for the a6S and a6DJ geometry of approach, and the necessary production of N20-(211) states, respectively, if one accepts the literature value of -0.1 eVS6 which then bends, thereby coupling with the exit channel. A for the electron affinity of N 2 0 . Although Mn(a6S) is open shell similar argument has been used by Jalink et aLs3 to explain the as far as the 3d orbitals are concerned, electron transfer from a enhancement, by N 2 0 bending mode excitation, of BaO* chem~ ~ ~ ~ ~d a orbital would leave the excited ionic configuration ...3d44s2, iluminescence in Ba N 2 0 . Originally it was c o n c l ~ d e dthat corresponding to a highly energetic state of both the Mn+ON2intermediate and the MnO product42(in addition the ionization (42) Pinchemel, B.;Schamps, J . Chem. Phys. 1976, 18, 481. (43)Wiesenfeld, J. R.;Yuen, J . M. Chem. Phys. Lett. 1976, 42, 293. potential would be significantly higher). On the other hand, (44) Husain, D.; Norris, P. E. J . Chem. Soc., Faraday Trans 2 1977, 73, transfer of a d a electron from Mn*(a6DJ) would leave a 1815. ground-state 'S ion (...3d54s1)provided the d a orbital in question (45)Costes. M.;Naulin, C.; Dorthe, G.; Nouchi, G. In Selectiuiry i n Chemical Reactions; Whitehed, J. C . , Ed.; Reidel: Dordrecht, 1988. is the one which is doubly occupied. The observation of a sub(46) Pfeifer, J.; Cole, J . L. J . Chem. Phys. 1984, 80,565. stantial barrier in this case therefore strongly suggests that, in (47) Breckenridge, W. H.;Umemoto, H. J . Phys. Chem. 1983, 87, 476, this reaction at least, the d orbitals are essentially inert; Le., they 1804. do not overlap significantly with the a* orbital of the N 2 0 . (48) Irvin, J. A.; Dagdigian, P. J. J . Chem. Phys. 1981, 74, 6178. (49)Cox, J. W.; Dagdigian, P. J . J . Phys. Chem. 1982, 86, 3738. In this context, the reaction thresholds of both Mn states can (50)Dickson, C.R.;George, S . M.; Zare, R. N. J . Chem. Phys. 1977,67, be regarded as occurring at an avoided crossing with an attractive 1024. surface correlating with Mn*(z6PJ) (Figure 12). This excited (51) Magee, J. L.J . Chem. Phys. 1940, 8, 687. state, with configuration ...3dS4s'4p', has a singly occupied p orbital (52)Yarkony, D.R.J . Chem. Phys. 1983, 78, 6763. which could be oriented in the a direction. Transfer of this electron (53)Jalink, H.;Harren, F.; Van den Ende, D.; Stolte, S . Chem. Phys.
+
+
+
+
+
1986, 108, 391.
(54)Wren, D. J.; Menzinger, M. J . Chem. Phys. 1975, 63, 4551. (55) Wren, D. J.; Menzinger, M . Faraday Discuss. Chem. SOC.1979,67, 97.
(56)Nalley, S . J.; Compton, R. N.; Schweinler, H . C.; Anderson, V. E. J . Chem. Phys. 1973, 59, 4125.
5202
The Journal of Physical Chemistry, Vol. 93, No. 13, 1989
Levy
I I I
T t
cl a z W
W -.I
/
Q -
+
L -
z
0.0
W
+ 0
a
%+IC+ M n + ON,
Mnt
Figure 12. Schematic of orbital correlations in Mn origin of activation barriers to reaction.
+ ON;
+ N,O, showing the
to the H* orbital of N 2 0 leaves the ground-state Mn+ ion. As shown in Figure 12, the a6DJstate would be expected to have the lower barrier as the energy gap is less. This implies that both the a6S and a6DJ states indeed do react, and that, as suggested above, the apparent linear falloff above 300 kJ mol-’ in the ETu plot disguises the fact that there are rwo removal processes. At present, one can only speculate on what these removal processes might be. As remarked above, atomic excitation a6S z6P, can only account for a small proportion of the falloff; but one alternative is that, on either the a6S or the a6DJsurface, above -300 kJ mol-‘ an increasing fraction of MnO product molecules are formed with internal energy greater than the dissociation limit, cf Hg lz.57 However, a maximum in the cross section is by no means unique,32and there may be good dynamical reasons for it. Several models have been devised in order to fit the maximum and falloff in such cases as K + CH3132and Ba + N,O(U).~IThe favored explanation in the latter case is “recrossing” of trajectories back to the reactant channel, as a result of reflection from the repulsive wall of the potential surface. The Mn* plot in Figure 10 implies that a t low collision energies there are three processes exciting either z6P, or z6DJ atoms, with thresholds a t 160, -300, and -500 kJ mol-’. The first of these z6P,). The second has already been attributed to Mn*(a6D, could in principle be either or both of Mn(a6S z6P,) and Mn(a6D, z6DJ) (see Table 1); although the former must be preferred since it is hard to see how the latter could account for the magnitude of the increase. The third, with threshold at -500 kJ mol-’, is more problematic. It is true that the Mn(a6S z6D,) thermodynamic threshold is a t -500 kJ mol-’; but the increase in ETu from that energy is rather dramatic, and it is difficult to see why Mn(a6S) + N 2 0 should yield Mn*(z6D,) in preference to the lower lying Mn*(z6P,). A second possibility is that an alternative Mn(a6S z6PJ) excitation mechanism is opening up above -500 kJ mol-’. Of interest in this respect is the considerable body of studies on collisional excitation of alkali-metal atoms.58 These excitation functions have been divided into two groups, depending on whether or not the configurations M + A and M* + A are coupled by the ion-pair configuration M+ A- (M = metal atom, A = collision M* excitation energy is less than partner).59 When the M the difference between the ionization energy of M and the electron affinity of A, then the ionic curve ”crosses” both covalent curves and there is a rapid onset of the excitation cross section near the
-
+
-
- -
-
-
-
-
+
(57) Mayer, T. M.:Wilcomb, B. E.; Bernstein, R. B. J . Chem. Phys. 1977, 67, 3507. (58) Kempter, V . AdG. Chem. Phys. 1975, 30, 417. (59) Herschbach, D. R . In Chemiluminescence and Bioluminescence; Lee, J.. Hercules, D. M., Cormier, M. J . . Eds.; Plenum: New York, 1973.
INTERNUCLEAR DISTANCE
Figure 13. Schematic potential energy curves for nonreactive interaction
between Mn and N20(sextet surfaces only). thermodynamic threshold. In the opposite case, the upper covalent curve, at least, does not interact with the ionic configuration, and there is a substantial excess activation energy. However, for non-alkali-metal atoms with higher ionization energies, excess barriers could also arise in the former energetic case if the ionic curve does not cross the lower covalent curve until the repulsive wall of the potential. I n the present situation, collisional excitation is a minor channel compared with chemical reaction (dark channels as well as chemiluminescence) up to a t least 800 kJ mol-’. It seems likely that most of the collisional excitation must derive from configurations in which chemical reaction is impossible, Le., with the Mn approaching the N z O molecule sideways on or from the N end. In view of the crossing radii quoted above, it would certainly not be surprising if the a6S covalent curve did not interact with the ionic curve until well into the repulsive wall; but, depending on the potential energy of this interaction, there need not be an excess barrier to Mn*(z6PJ) production. However, the -90 kJ mol-’ excess barrier for the a6DJ z6P, process indicates that the a6D, curve must interact with the ionic curve above the z6P, asymptote (Figure 13); the excess barrier for the a6S z6PJ process would therefore be expected to be a t least as high. This implies that the -500 kJ mol-’ threshold in Figure I O does indeed correspond to the onset of one mechanism for the a6S z6P, process. rather z6D,. In fact, it is difficult to see how than (a6S or a6D,) Mn*(z6DJ) could be produced a t all, since its configuration (...3d64p’)will not couple directly with the ground ionic configuration (...3d54s’). The mechanism with threshold a t -300 kJ mol-’ clearly has no barrier, which can only be the case if it is collisional excitation via the reactive approach channel: a small proportion of the “recrossing” trajectories must follow the adiabatic curve to Mn*(z6P,) in Figure 12. Above -700 kJ mol-’, Figure I O suggests that some new process depletes the Mn* emission, while, a t the same time, the MnO* plot falls off less rapidly than expected (although the linear extension of the MnO* plot to 1200 kJ mol-’ is probably not justified). With alkali-metal atoms, a significant depletion in excitation cross sections has been found a t translational energies where collisional ionization, Mt + A-, not only becomes allowed but in fact occurs.s9~60I n the case of Mn + N 2 0 , the best estimate of the adiabatic electron affinity (from collisional ionisation with Cs) is 2-0.1 f 0.1 eV,56which leads to 1 7 2 7 & 10 kJ mol-’ for the Mn(a6S) + N 2 0 Mnt + N20- threshold. This upper limit is not inconsistent with the data of Figure IO, especially when one takes the uncertainties into account; but, without direct observation of collisional ionization, it is impossible to be definitive on the cause
-
-+
-
-
-
-
( 6 0 ) Lacmann,
K.: Herschbach, D. R. Chem. Phys. Lett. 1970. 6.
106.
J. Phys. Chem. 1989, 93, 5203-5209 of the Mn* depletion. However, the coincidence in the MnO* and Mn* break points does suggest an interesting speculation: that a small fraction of the departing Mn+ and N 2 0 - fragments undergo secondary encounters, leading to a slight enhancement of the MnO* cross section. Conclusions
The data show that laser vaporization of solid metal targets is a useful approach for determining excitation functions of transition-metal chemiluminescent reactions and energy-transfer processes. The results have been analyzed in terms of the lineof-centers and they indicate that, in the Mn + N2° system, both the Mn0*(A68+) and Mn*(z6PJ or z6DJ) products derive from only a small number of atomic reagent states. viz., ground-state a6S and first metastable state ash, atoms. The chemiluminescence results are in line with simple state correlations in the C, point group; but the high activation energies indicate a lack of orbital correlations between reactants and products. The sharp falloff in the MnO* cross section above -300 kJ mol-] may be associated with "recrossing" of trajectories due
5203
to reflection from the repulsive wall of the potential. The Mn* emission at -403 nm appears to be from z6PJ atoms rather than the more excited z6DJ state. Excitation from both the a6S and a6DJ atoms has a substantial excess barrier, reflecting a lack of interaction between the lower covalent curve in each case and the Mn+ N 2 0 - curve; however, a proportion of the z6PJ atoms are produced from the a6S thermodynamic threshold, presumably as a result of "recrossing" of potentially reactive trajectories. Radiative lifetimes of 82 i 10 and 122 f 14 ws have been determined for the Mn* z8p,,2 and z8p5,2 levels, respectively, Q-switched Nd:YAG laser power of IO9 W at 1064 nm in vacuo appears to generate a metal plasma with temperature in the region of 80000 K,
+
Acknowledgment. I thank the SERC Laser Support Facility, I. R. Beattie, R , Hill, and E. Lewis for equipment loans, and L. Beauchamp, J , c. Whitehead, and p. A. Gerry for helpful suggestions and J,
Registry No. Mn,7439-96-5; N20,10024-97-2; M n O , 1344-43-0.
Kinetics of Oxygen Absorption by a-Zirconium Masahiro Yamamoto,* Shizuo Naito, Mahito Mabuchi, and Tomoyasu Hashino Institute of Atomic Energy, Kyoto University, Uji,Kyoto 61 1, Japan (Received: February 2, 1988; In Final Form: February 1 , 1989)
The rate of oxygen absorption by polycrystalline a-zirconium has been measured at oxygen pressures of 6.7 X and 1.3 X Pa over the temperature range 973-1098 K by using samples prepared under ultrahigh-vacuum conditions, and the mechanism for the early stage of absorption is discussed. The absorption rate is explained by using a model in which the absorption process comprises three successive steps: the dissociative adsorption of oxygen molecules on the zironium surface with an interaction between oxygen adatoms, the transfer of adatoms at the surface sites to the outermost bulk sites, and the diffusion of oxygen atoms into the bulk. The absorption rate is found to be limited by adsorption under the conditions studied. Kinetic parameters in the model have been evaluated. The activation energy for adsorption is (7.4 f 0.6) X J per 0 atom, and the energy of the interaction, which is attractive, is -4 X J per 0-0 pair. The adsorption site has a potential energy lower by (2.2 i 0.1) X J per 0 atom than the site in the bulk.
Introduction Oxidation of zirconium has been experimentally studied over wide ranges of temperature and pressure by a number of aut h o r ~ . ' - ~ $Some of them1-I0 have observed a parabolic relation ( I ) Rosa, C. J. J . Less-Common Mer. 1968, 16, 173. (2) Porte, H. A.; Schnizlein, J. G.; Vogel, R. C.; Fisher, D. F. J . Electrochem. SOC.1960, 107, 506. (3) Sense, K. A. J . Electrochem. SOC.1962, 109, 377. (4) Hussey, R. J.; Smeltzer, W. W. J . Electrochem. Soc. 1964, I l l , 564. (5) Levintan, J.; Draley, J. E.; Van Drunen, C. J. J . Electrochem. SOC. 1967, 114, 1086. (6) Rosa, C. J. J . Less-Common Met. 1968, 15, 35. ( 7 ) Madeyski, A.; Poulton, D. J.; Smeltzer, W. W. Acta Metall. 1969, 17, 579. ( 8 ) Paetz, P.; Sperner, F. In Gase und Kohlenstoff in Metallen; Fromm, E., Gebhardt, E., Eds.; Springer: Berlin, 1976; pp 419-430. (9) Dechamps, M.; Lehr, P. C. R . Seances Acad. Sci. 1970, C270, 169. (IO) Nagasaka, M.; Ueda, E.; Yamashina, T. Vacuum 1973, 23, 51. ( 1 1 ) Veal, B. W.; Lam, D. J.; Westlake, D. G.Phys. Reu. E 1979, 19, 2856. ( I 2) Frandon, J.; Brousseau, B.; Pradal, F. Phys. Status Solidi E 1980, 98, 379. ( I 3) Foord, J. S.;Goddard, P. J.; Lambert, R. M. SurJ Sci. 1980, 94, 339. (14) Valyukhov, D. P.; Golubin, M. A,; Grebenshchikov, D. M.; Shestopalova, V. I . Sou. Phys.-Solid State (Engl. Transl.) 1982, 24, 1594. (IS) Krishnan, G. N.; Wood, B. J.; Cubicciotti, D. J . Electrochem. SOC. 1981, 128, 191. (16) Tapping, R. L. J . Nucl. Mater. 1982, IO, 1 5 1 . (17) Danielson, L. R. J . Vac. Sci. Technol. 1982, 20, 86. (18) Hoflund, G. B.; Cox, D. F.; Gilbert, R. E. J . Vac. Sci. Technol. A 1983, I, 1837. (19) Sen. P.; Sarma, D. D.; Budhani, R. C.; Chopra, K. L.; Rao, C. N. R. J. Phys. F 1984, 14, 565.
0022-3654/89/2093-5203$01.50/0
in the absorption of oxygen by zirconium at high oxygen pressures (105-10-' Pa, T L 673 K)l-' and a t low oxygen pressures (10-'-104 Pa) and low temperatures (e.g., C873 K at lo-* Pa).9J0 The parabolic relation suggests that the absorption rate is limited by the diffusion of oxygen into zirconium, the surface of which is covered with an oxide film immediately after zirconium is (20) Zhou, M. Y.; Milne, R. H.; Karolewski, M. A,; Frost, D. C.; Mitchell, K. A. R. Surf. Sci. 1984, 139, L181. (21) Hoflund, G. B.; Asbury, D. A.; Cox, D. F. Appl. Surf.Sci. 1985, 22/23, 252. (22) Hui, K. C.; Milne, R. H.; Mitchell, K. A. R.; Moore, W. T.; Zhou, M. Y . Solid State Commun. 1985, 56, 8 3 . (23) Sasaki, T. A.; Baba, Y. Phys. Reu. E 1985, 31, 791. (24) Axelsson, K.-0.; Keck, K.-E.; Kasemo, B. Surf.Sci. 1985, 164, 109. (25) Wong, P. C.; Mitchell, K. A. R. Can. J . Chem. 1986, 64, 2409. (26) Wong, P. C.; Hui, K. C.; Zhong, B. K.; Mitchell, K. A. R. Solid State Commun. 1987, 62, 293. (27) Wong, P. C.; Mitchell, K. A. R. Can. J . Phys. 1987, 65, 464. (28) Corallo, G.R.; Asbury, D. A.; Gilbert, R. E.; Hoflund, G.B. Phys. Rev. E 1987, 35, 9451. (29) Hoflund, G. B.; Corallo, G. R.; Asbury, D. A,; Gilbert, R. E. J . Vac. Sci. Technol. A 1987, 5 , 1120. (30) Palacio, C.; Sanz, J . M.; Martinez-Duart, J. M. Surf. Sci. 1987, 189/190, 175. (31) Palacio, C.; Sanz, J. M.; Martinez-Duart, J. M. Surf. Sci. 1987, 191, 385. (32) Sanz, J. M.; Palacio, C.; Casas, Y.; Martinez-Duart, J. M. Surf. Interface Anal. 1987, I O , 177. (33) Aebi, P.; Erbudak, M.; Leonardi, A,; Vanini, F. J . Electron Spectrosc. Relat. Phenom. 1987, 42, 351. (34) West, P. E.; George, P. M. J . Vac. Sci. Technol. A 1987, 5 , 1124. (35) Asbury, D. A.; Hoflund, G. 8 . ; Peterson, W. J.; Gilbert, R. E.; Outlaw, R. A. Surf. Sci. 1987, 185, 213.
0 1989 American Chemical Society
