J. Phys. Chem. 1902, 86, 3730-3745
3738
Single-Collision Chemiluminescence Study of the Sr('S, 'Po) Reactions
+ N20 and Mg(3P0) + N20
John W. Cox and Paul J. Dagdlglan*+ Depem"et of Chemkby, The Johns Hopklns Unhwslfy, BaMmore, Maryland 2 12 18 (Received: May 4, 1982)
A study of the chemiluminescence under single-collisionconditions of the reactions of ground-state Sr('S) and 3P0metastable Sr and Mg atoms with NzO is presented. Chemiluminescence cross sections and photon yields for production of various SrO* band systems are reported for the Sr reactions. The total chemiluminescence croas section was found to be 5.5 f 3.1 A2. No chemiluminescencewas detected for Mg(3Po)+ N20. The results for the Sr reactions are qualitatively similar to those of the corresponding Ca reactions, previously studied in this laboratory. The reaction dynamics of the Mg(3Po),Ca(3Po),W3PO)+ N20 are discussed, and speculations on the causes of the profound differences between the Mg and Ca/Sr reactions are made.
I. Introduction The chemiluminescent reactions of alkaline-earth atoms with various oxidants, especially N20,have been investigated in a number of These reactions have evoked great interest because of the large and pressuredependent photon yields and the surprisingly complex kinetic behavior exhibited in flame^.'^-^^ From a fundamental point of view, the primary oxidation process is particularly worthy of study because of the number of low-lying product oxide electronic states energetically accessible. Thus, these reactions afford the opportunity for testing various models predicting the product states formed. In recent years there has been increasing attention paid to reactions involving multiple potential energy surfaces."-26 The oxidation of alkaline-earth atoms by N20 has been extensively studied under single-collision conditions both by spectral analy~is'-~?'~ of the chemiluminescence from the decay of radiating product states and by laser fluorescence detecti~n'~J~ of nonemitting states. This work has been greatly facilitated by recent spectroscopic studies16Jg~27-3s and ab initio computation^^^^ which have nearly completely fixed the locations of the low-lying electronic states of the oxides. In a series of studies,7-1° we have investigated the reactions of metastable alkaline-earth atoms with N20. This work complements the study of the corresponding ground-state reacti~ns'-~J~ in that these systems are probed from another reactant potential energy surface. We present here an investigation of the chemiluminescencefrom the reaction of ground-state (5s2 IS) and metastable (5s5p 3P0)strontium atoms with N20. This work corrects and extends an earlier s t u d 9 in this laboratory on Sr(3Po)+ N20 through the use of an improved detection system. We have characterized the Sr atomic state distribution from our molecular beam discharge source, as was done with Ca." We also have searched in vain for chemiluminescence from Mg(3Po)+ N20 and have derived an upper bound for the chemiluminescence cross section for this reaction. From this study and earlier we find that the chemiluminescence cross sections and electronic state branching for the Ca(3Po)and Sr(3PO)+ N20 reactions are quite similar. By contrast, no emitting MgO states are found to be formed in Mg(3Po)+ NzO. In an earlier laser fluorescence study,'O one of us detected the dark producta MgO X'Z+, a3n, and A'II from this reaction. In a recent flame study, Bourguignon, Rostas, and TaiebZ3also con'Camille and Henry Dreyfus Teacher-Scholar. 0022-3654/82/2086-3738$01.25/0
cluded that no chemiluminescence results from Mg(3Po) N20. Yark0ny4~has been engaged in ab initio compu-
+
(1)C. D. Jonah, R. N. Zare, and Ch. Ottinger, J. Chem. Phys., 56,263 (1972). (2)A. Schultz and R. N. Zare, J. Chem. Phys., 60,5120(1974). (3)A. Siege1 and A. Schultz, Chem. Phys., 28,265 (1978). (4)F. Engelke, R.K. Sander, and R. N. Zare, J. Chem.Phys., 65,1416 (1976). (5)D. J. Wren and M. Menzinger, J. Chem. Phys., 63,4557 (1975); 67,97 (1979). Faraday Discuss. Chem. SOC., (6) L. Paetemack and P. J. Dagdigian, Chem. Phys., 33, 1 (1978). (7)P. J. Dagdigian, Chem. Phys. Lett., 65,239 (1978). (8)B. E. Wilcomb and P. J. Dagdigian, J. Chem. Phys., 69, 1779 (1978). (9)J. A. Irvin and P. J. Dagdigian, J. Chem. Phys., 74, 6178 (1981). (10)P. J. Dagdigian, J. Chem. Phys., 76,5375 (1982). (11)J. A. Irvin and P. J. Dagdigian, J. Chem. Phys., 73,176 (1980). (12)C. R. Jones and H. P. Broida, J. Chem. Phys., 59,6677(1973);60, 4369 (1974). (13)R. W. Field, C. R. Jones, and H. P. Broida, J. Chem. Phys., 60, 4377 (1974). (14)C. J. Hsu, W. D. Krugh, and H. B. Palmer, J. Chem. Phys., 60, 5118 (1974). (15)G. A. Capelle, C. R. Jones, J. Zorskie, and H. P. Broida, J. Chem. Phys., 61,4777 (1974). (16)G. A. Capelle, H. P. Broida, and R. W. Field, J. Chem. Phys., 62, 3131 (1975). (17)D. J. Benard, W. D. Slafer, and J. Hecht, J.Chem. Phys., 65,1013 (1977);D. J. Benard and W. D. Slafer, ibid., 66,1017 (1977). (18)D. J. Eckstrom, S. A. Edelstein, D. L. Huestis, B. E. Perry, and S. W. Benson, J. Chem. Phys., 63,3828 (1975);D. J. Eckstrom, J. R. Barker, J. G. Hawley, and J. P. Reilly, Appl. Opt., 16, 2102 (1977). (19)A. Torres-Filho and J. G. Pruett, J. Chem. Phys., 70,1427 (1979); Y.C. Hsu and J. G. Pruett, J. Chem. Phys., 76,5849 (1982). (20)R. W. Field in "Molecular Spectroscopy: Modem b e a r c h " , Vol. 2,K. N. Rao, Ed., Academic Press, New York, 1976,p 261. (21)R. W. Field, R. A. Gottscho, J. G. Pruett, and J. J. Reuther in 'Proceedings of the 14th International Conference on Free Radicals", Association for Science Documents Information, Japan, 1979,p 39. (22)G. Taieb and H. P. Broida, J. Chem. Phys., 65,2914 (1976);G. Taieb, J. Phys. (Orsay, FrJ, 42,537 (1981). (23)B. Bourguignon, J. Rostas, and G. Taieb, J. Chem. Phys., submitted. (24)M. R. Levy, Prog. React. Kinet., 10, 1 (1979). (25)M. Menzinger, Adu. Chem. Phys., 42, 1 (1980). (26)W. H.Breckenridge in 'Reactions of Small Transient Species", M. A. A. Clyne and A. Fontijn, We., in press. (27)J. Schamps and G. Gandara, J.Mol. Spectrosc., 62,80 (1976). (28)T.Ikeda, N. B. Wong, D. 0. Harris, and R. W. Field, J. Mol. Spectrosc., 68,452 (1977). (29)R. W. Field, J. Chem. Phys., 60,2400 (1974). (30)R. W. Field, G. A. Capelle, and C. R. Jones, J. Mol. Spectrosc., 64, 156 (1975). (31)J. G. Pruett and R. N. Zare, J. Chem. Phys., 62, 2050 (1975). (32)Y. C. Hsu, B. Hegemann, and J. G. Pruett, J. Chem. Phys., 72, 6437 (1980). (33)R. W. Field, G. A. Capelle, and M. A. Revelli, J. Chem. Phys., 63, 3228 (1975);R. A. Gottscho, J. B. Koffend, R. W. Field, and J. R. Lombardi, ibid., 68,4110 (1978). (34)R. A. Gottscho, J. Chem. Phys., 70,3554 (1979). (35)R. A.Gottacho, J. B. Koffend, and R. W. Field, J.Mol. Spectrosc., 82,310 (1980). (36)R. A.Gottscho, P. S.Weiss, R. W. Field, and J. G. Pruett, J.Mol. Spectrosc., 82,283 (1980).
0 1982 American Chemical Society
Sr('S, 3P0)
+ N,O and Mg(3Po)i- N20 Reactions
tations at selected geometries of various Mg-N20 potential energy surfaces. With this theoretical information, a model for the reaction dynamics of these reactions is beginning to emerge. For a more complete understanding of the dynamics, the yield of the as yet spectroscopically unobserved low-lying b38+ state will need to be determined, as this state is predicted to be formed in significant yield. Finally, we speculate on the factors governing the electronic state branching in Mg(T"), Ca(3Po),Sr(3Po)+ NzO, in particular on the drastic differences between the Mg and Ca/Sr reactions. 11. Experimental Section The scattering apparatus employed was essentially identical with that used in previous chemiluminescence s t ~ d i e s . ~ JBriefly, ' a beam of metastable atoms was generated in a source housed in a separately pumped vacuum chamber and passed through a collimating slit into a reaction chamber, in which a target gas could be introduced. Chemiluminescence produced in a zone downstream of the collimator was collected and spectrally analyzed. The source of metastable metal atoms is a slightly modified version of that previously described in detail." The improvements included greater superheating of the top part of the crucible containing the metal charge and coating the outside of the crucible around the beam orifice (0.05-cm diameter typically) with a BaO slurry in order to reduce the work function. By increasing the thermionic electron emission from the crucible, these alterations increased the stability and efficiency of the dc discharge in which the metastable atoms were created, particularly for Mg. Typical orifice operating temperatures were approximately 1280 K and 1580 K (corrected pyrometer readings) for Mg and Sr, respectively. Investigations of the conditions for optimum metastable production were not carried out for Sr. For the single-collision studies, the collimator diameter d was 1.6 cm; for scattering gas pressures greater than torr, d was reduced to 0.3 cm. In no case was the pressure rise in the source chamber greater than torr from the leakage of scattering gas through the collimator. In all experiments, the distance 1 from the collimator to the chemiluminescence viewing region was 5.3 f 0.1 cm. The light detection setup was identical with that previously r e p ~ r t e dexcept ,~ that data collection was carried out with the aid of a minicomputer. At each wavelength the photon count rate was accumulated on a counter (Data Precision 5740) for a specified interval and stored, and the spectrometer stepped to the next wavelength. The resulting chemiluminescence spectra were stored on magnetic diskettes for analysis. A photometer monitored the metastable beam intensity by observing the visible emission due to decay of metastable atoms in the beam. With computer-controlled data acquisition, it was straightforward to correct spectra for the detector wavelength response and to remove background contributions. (37)R.F.Marks, R. A. Gottacho, and R. W. Field, Phys. Scr., 25,312 (1982). (38)R.F.Marks, H. S. Schweda, R. A. Gottacho, and R. W. Field, J. Chem. Phys., 76,4689 (1982). (39)J. Schamps and H. Levebre-Brion,J. Chem. Phys.,56,573 (1972). (40)C.W.Bauechlicher, Jr., D. M. Silver, and D. R. Yarkony, J. Chem. Phys.,73,2867 (1980);C.W.Bauschlicher, Jr., B. H. Lengsfield, 111,D. M. Silver, and D. R. Yarkony, ibid., 74,2379(1981);R.N.Diffenderfer and D. R. Yarkony, J. Phys. Chem., submitted for publication. (41)C. W. Bauschlicher, Jr., and D. R. Yarkony, J. Chem. Phys., 68, 3990 (1978);R. Diffenderfer and D. R. Yarkony, ibid., submitted for publication. (42)W.England, Chem. Phys., 53, 1 (1980). (43)D. R. Yarkony, work in progress.
The Journal of phvsical Chemistty, Vol. 86, No. 19, 1982 3739
Beam attenuation measurements on Sr(3PO)and Mg(3PO) for the estimation of total reactive cross sections were 'S carried out by observing the variation of the 3P: emission signal as a function of N20 pressure. The same scattering path length (1 = 5.3 cm) and optical detection arrangement as in the chemiluminescence studies was employed. Because of the short radiative lifetime of Sd3P;), it was deemed inappropriate to employ the defocused optical arrangement used in the previous Ca(3Po) stud9 to reduce the elastic contribution to the attenuation. Because of its radiative decay,44the Sr(3P:) density would vary significantly over the 0.4-cmwide detection zone. For Sr(3P"),a collimator of diameter d = 1.6 cm was employed; for Mg(3Po),d was 0.3 cm. Attenuation of Sr('S) by N20 was also studied by laser fluorescence monitoring of the 5s5p IPo 5s2 'S line at 460.9 nm. The discharge was turned off for these measurements, and the source operated at considerably lower temperature so that the transition employed was optically thin. To minimize the effect of elastic scattering on the attenuation, a large molecular beam and a small laser detection zone were employed. In this way, elastic scattering out of the core of the beam is compensated for by scattering from the outer portions. The same scattering path length (1 = 5.3 cm) was employed with a large collimator (d = 1.6 cm). Sr(lS) atoms were detected with approximately constant efficiency in a zone 0.1 cm in diameter (the size of the laser beam) and 1cm long (defined by the fluorescence detection optics).
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111. Metastable Beam State Composition Laser fluorescence experiments were conducted in order to characterize the state composition of the metastable atomic beams. The performance of this source with calcium has previously been described in detail.g In the present study, a tunable dye laser (Molectron DL-200) pumped by a nitrogen laser (Molectron UV22) was used to excite atomic fluorescence. Neutral density filters attenuated the dye laser to eliminate optical pumping effects. The J-state distribution of the metastable Sr(5s5p 3Pt,1,2) state in the chemiluminescence observation region was determined by excitation of the 557s 3S1 5s5p 3Pt transitions at 432.8,436.3, and 443.9 nm, respectively.&Vk Laser-induced fluorescence was observed through a narrow-band interference filter centered at 440 nm (10-nm fwhm) in order to block 5s5p 'Po 5s2 'S resonance radiation at 460.9 nm from the source which is scattered into the fluorescence detector by ground-state atoms in the beam. The distribution with no scattering gas present was measured4' to be 1:0.005 f 0.004: 0.13 f 0.09 for J = 2, 1, 0, respectively. With 2 X torr of N20 present, the distribution was found to be slightly altered to 1:0.009 f 0.004:0.19 f 0.09. The depressed 3P1density is a result of its relatively short (21 f 1 ps44) radiative lifetime as compared with the flight time from the source to the observation region (approximately 140 p s ) . The low J = 1 population and the sharp wavelength dependence of the narrow-band filter transmission reduced the accuracy of the distribution measurements. The Mg(3s3p 3PJ)distribution was foundlo to be approximately statistical. The ratio of the densities of the metastable Sr('D) and SI-(~PO) states, n ( l D ) / r ~ ( ~ Pwas ~ ) ,obtained from the ob-
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(44)M. D. Havey, L. C. Balling, and J. J. Wright, Phys. Reu. A, 13, 1269 (1976). (45)C. E. Moore, Natl. Stand. Ref. Data Ser. (U.S., Natl. Bur. Stand.), 35 (1971). (46)Transition probabilities were taken from M. D. Havey, L. C. Balling, and J. J. Wright, J. Opt. SOC., Am., 67,488 (1977). (477All uncertaintres quoted in this paper are 30.
3740
The Journal of Physical Chemistty, Vol. 86, No. 19, 1982
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served 5s4d 'D 5s2 'S and 5s5p 3Py 5s2 'S emission intensities a t 496.3 and 689.4 nm, respectively. While the 3fl 'S transition probability has been measureda to be (4.8 f 0.2) X 104s-', that of the 'D 'S electric quadrupole transition has not been previously determined. The transition rates for the analogous transitions in Ca and Ba have been calculated to be 8148v49and 2 s - ' , ~respectively. 'S quadrupole line strength5' is about the same The 'D for Ca and Ba since these rates approximately follow a u5 frequency dependence. Thus, the Sr 'D 'S transition probability is estimated from the Ca rate to be 50 f 10 s-' by taking into account the u5 factor. Correcting the emission intensities for the detector spectral response (see Figure 1 of ref 9) and employing the determined Sd3P0) J-state distribution, we find o('D)/~(~PO) = 0.018 f 0.010 under typical source operating conditions. This ratio was essentially insensitive to variations of the source parameters. The ratio of metastable to ground-state densities, n(3P0)/n('S), was determined by employing a considerably longer source-to-detector path length since the 5s5p 'Po-5~~ 'S line is optically thick close to the source. The density ratio and velocity distributions were determined by a time-of-flight (TOF) t e c h n i q ~ e ~employing *.~~ pulsed fluorescence excitation with a flight path of 142 cm. The velocity distribution of the metastable atoms is needed for the determination of chemiluminescence crow sections (see section IV). Ground-state Sr(lS) and metastable Sr(3P!) atoms were detected by using the transition 5s5p 'Po 5s2 'S and 537s 3S1 5s5p 3P; at 460.9 and 443.9 nm, respectively. The TOF spectra were fitted by assuming velocity distributions of the form Nu2 exp[-(u - U ~ ) ~ / U ~ For Sr(%), the derived velocity parameters were uo = (9.00 f 0.06)X 104 cm/s and u = (1.75 f 0.06) X 104cm/s, while uo = (8.29 f 0.12) X lo4 cm/s and u = (2.48 f 0.10) X lo4 cm/s for Sr('S) when the source discharge was on. With the discharge off, the ground-state atom distribution was slightly altered: uo = (8.34 f 0.12) X lo4 cm/s and u = (2.43 f 0.10) X 104cm/s. The significantly non-Boltzmann form of the velocity distributions reflects the slightly supersonic expansion of the metal vapor from the small source orifice. The ratio n ( V ) / n ( ' S )was determined by comparing the fluorescence intensities at 460.9 and 443.9 nm, normalizing to constant laser intensity (monitored simultaneously with the fluorescence measurement using a beam splitter and laser power meter, Molectron 53-05). Care was taken to reduce the laser power with neutral density filters so that fluorescence intensities were linear with power. Correcting for the differing TOF distributions, we find the ratio n(3P;)/n(1S)to be 0.011 f 0.002. With this measurement, the atomic state reactant composition in the chemiluminescence experiments can be calculated. With the previously derived Sr(3Po) J-state distribution at the chemiluminescence observation zone and the ratio n('D)/~z(~PO), we find the Sr beam composition to be (98.7 f 0.3)% lS, (1.3 f 0.3)% 3P0, and (0.02 f 0.02)% "D. Therefore, the efficiency a with which the discharge produces metastable Sr atoms is (1.3 f 0.3)%. This conversion efficiency is considerably less than that measuredgfor
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4
A-X 3 2
I.AV
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(48) L. Pastemack, D. M. Silver, D. R. Yarkony, and P. J. Dagdigian, 13, 2231 (1980). (49) R. N. Diffenderfer, P. J. Dagdigian, - - and D. R. Yarkony, J.Phys. B, 14, 21 (1981). (50) P. McCavert and E. Trefftz, J. Phys. B, 7 , 1270 (1974). (51) R.H. Garstane in "Atomic and Molecular Processes". D. R. Bates. Ed.; Academic Press,-New York, 1962, p 1. (52) L. Pasternack and P. J. Dagdigian, Reu. Sci. Instrum., 48, 226 (1977).
J. Phys. B,
Cox and Dagdigian
300
400
500
600
700
800
9OC
wavelength ( n m )
Flgure 1. Singlecolllslon chemiluminescence spectra for the reaction of Sr('S) and Sr(3Po)with N,O at (2.0 f 0.1) X lo-' torr. (a) Background slgnal due to thermal radiation from oven: source discharge off and rx) scattering gas. (b) Spectrum of Sr('S) N,O alone: source discharge off and scattering gas present. (c) Spectrum of Sr(3p0, 'S) N,O source discharge on and scattering gas present. The ordinate is glven by the observed count rate divided by the detector quantum efficiency k, (see Figure 1 of ref 9). The background (a) has been subtracted from spectra b and c, and the latter have been displaced one tick mark upward for clarity. The 3P: 'S and 'D 'S lines are due to the decay of metastable Sr(3P!) and Sr('D) atoms, re's line arises from resonant spectively, in the beam; the 'Po scattering of the source dlscharge light by Sr('S) atoms. The Sr(3P: 'S) intensity is offscale on the figure and equals 1.35 X lo5. Band ] . X'B' and A " I I X'Z' systems are heads of the SrO A'Z' marked.
+
+
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Ca; however, as discussed in section 11, the source conditions were adjusted for maximum stability of Sr* production. The state composition of the Mg* beam was not measured by laser fluorescence since the 3s3p IPo 3s2 'S resonance line occurs at 285.2 nm, beyond the range of our dye laser (no frequency doubling capability). An indirect rough estimate of n(3Po)/n(1S)was made by comparing with the operation of the source with Ca. First, the ratio Iw/Ica of the total beam intensity with Mg vs. Ca operation was obtained from weight loss measurements of the crucible. The ratio n(Mg(3Po))/n(Ca(3Po)) was calculated from the observed 3Py 'S emission intensities, taken under identical conditions and corrected for differing wavelength sensitivities, Einstein A coefficients, and Jstate distributions. From the previous measurement" of the Ca* conversion efficiency aCa= r~(Ca(~P~))/n(Ca total), the Mg beam conversion efficiency aMgcan be obtained:
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We estimate a- to be approximately 40% under typical operating conditions. It should be noted that, in contrast to the other alkaline earths, Mg has only one metastable state, 3s3p 3P0.As a check on the use of eq 1, the calculation was carried out with Sr. By this means, aSrwas estimated to be (2.7 f 3.0)7%, which differs from the more direct and accurate determination by a factor of 2. IV. Chemiluminescence Spectra and Cross Sections A. Sr + N20.Figure l b presents a typical chemiluminescence spectrum for the reaction of ground-state Sr('S) atoms with N2O under single-collision conditions. With the source discharge turned on and hence with
+ N,O and Mg(3Po)+ N20 Reactions
The Journal of Physical Chemistry, Vol. 86, No. 79, 1982 3741
Sr(’S, 3P0)
TABLE I: Chemiluminescence Cross Sections and Photon Yields reaction
Mg(3P0) + N,O Sr(’S) + N,O S I - ( ~ P ”+) N,O
band systems obsd
< 0.002
none
7 5 %5
> 0 . 1 1 % 0.07d 0.14 0.08 0.38 0.21 > 5 . 0 ? 3.1d > 5 . 5 ? 3.1
SrO A-X and A - X bands, 700-890 nm SrO orange arc bands, 580-600 nm SrO red arc bands, 640-685 nm SrO A-X and A’-X “continuum”, 360-890 nm total chemiluminescence
photon yield,b 5%
utot,(l A ’
"them!
*
33.6 45.9
* 4.6c %
2.9
>0.24
80.6
*
3.1
>6.8
* 0.15
*
* 3.8
Defined as t h e ratio of uchem to the total reactive cross section O R . Entries are rea Total attenuation cross sections. Reference 1 0 . ported as lower limits since t h e yield has been computed by Ochem/Utot, and utot is an upper bound t o O R , d Reported as lower limit since a significant portion of the SrO A-X and A‘-X systems occurs beyond 8 9 0 nm, the l o n g wavelength limit of our detector sensitivity.
metastable Sr atoms being produced, the spectrum is considerably enhanced and altered, as Figure ICshows. The spectra in Figure 1 have been corrected for the detector spectral response (given in Figure l of ref 91, and the background contribution from the source blackbody radiation (shown in Figure la) has been subtracted. The new features in the spectrum which appear when the discharge is turned on are due predominantly to the reaction of Sr(3Po)with N20. As discussed in section 111, the Sr(’D) to Sr(3Po)ratio was measured to be approximately 1.8%, independent of source conditions, so that, even if the chemiluminescencecross section a h for Sr(’D) + N20were twice as large as ache, for M 3 P ) + N20,only 3% of the new features could be ascribed to the former reaction. For the analogous Ca reactions, this ratio of cross sections was determined to be 1.3 f 0.8.9 Given its small contribution, we can ignore the Sr(’D) + N20reaction and ascribe the chemiluminescence spectrum in Figure l a as due to Sr(’S and 3P0)+ N20. Since we have determined the state composition of the Sr* beam, it is possible to subtract from the spectrum in Figure ICthe contribution due to the ground-state reaction. Figure 2 displays the derived Sr(3Po) + N20 chemiluminescence spectrum,obtained by subtracting 0.987 of the spectrum in Figure l b from that in Figure IC. It should be noted that, although the ratio n(3Po)/n(1S)was not determined to high accuracy, the resulting uncertainty in the spectrum in Figure 2 is considerably less because of the low source metastable conversion efficiency a: A 50% error in a leads to only a 5% error in the integrated intensity for the Sr(3Po) N20 reaction. As we have shown previo~sly:~the chemiluminescence cross section for a reaction involving an emitting electronically excited reactant is given by
+
achem
-
= sSr@4
