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the present systems, may be superimposed on that of the factor pB'/pA'. This possibility is being investigated.
the American Chemical Society, for support of this research.
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by
Registry NO. 1*p-(C03)2C6D,,83897-68-1;I*p-(CD3)2C6H,, 83897-69-2; 1.p-(CH3)2C&4, 51013-90-2;~-(cD~)~c~D~, 41051-881; p-(CH3),C6H4,106-42-3; p-(CD,),C6H,, 25493-13-4.
Initial Distribution of Vibrational and Rotational Quantum States of MgO(X12') Produced in the Reaction of Mg(3s3p 'P,) with COP W. H. Breckenrldge' and H. Umemoto Department of Chemistty, University of Uah, Sah Lake City, Utah 841 12 (Received August 18, 1982; In Final Form: October 1, 1982)
The reaction of Mg(3s3p 'P1) with C02produces ground-stateMgO(X'Z+). In contrast to the reaction of Mg(3s3p 'PI) with the isoelectronic N20 molecule, no MgO(B'Z+) molecules were detected. The yield of MgO(A'n) was also less than 15% of that of MgO(X'Z+). The initial distribution of MgO(XIZ;+)vibrational and rotational quantum states, determined with a laser pump-and-probetechnique,was "colder" than that predicted by statistical theories. The mechanism of the reaction is discussed in terms of the MgCO, potential surfaces which may be involved.
Introduction There has been considerable interest recently in the chemical reactions of the ground-state and electronically excited group 2 atoms with simple oxidants such as 02, N20, SO2,C102, and C02.1-4 Production of metal monoxide molecules is often considerably exothermic in these processes, and several low-lying excited states of the monoxide molecules are energetically accessible. Besides the practical possibilities of electronic transition chemical laser action, it is of fundamental importance to examine the factors which determine the branching ratios into excited vs. ground-state molecular states. Several studies have shown that large yields of electronically excited products can be produced in the reactions of excited group 2 atoms with O2and N20.1,4-8For example, reaction of Ca(3P) and Sr(3P)with N20 produces >7% yield of fluorescent excited states of both CaO and SrO, respecti~ely.~.~ On the other hand, reaction of the analogous Mg(3P)state with N20 produces predominantly ground-state Mg0.'-4 Reaction of Ca('D2) with O2 produces excited Ca0,8 while the Ca(3P) + O2 reaction produces only ground-state Ca0.7!8Theoretical interpretation of these reactions is difficult, because there are several metal-atom-oxidant potential surfaces involved, many of which have charge-transfer ~haracter.~JO Reactions with C02 have been less studied, partly because COBis less reactive due to its greater bond strength for oxygen atom abstraction. Experiments in the Dagdigian laboratories have shown that Ca('D,) appears to react (1)P. J. Dagdigian, J. Chem. Phys., 7 6 , 5375 (1982),and references therein. (2)W. H. Breckenridge and H. Umemoto, Adu. Chem. Phys., 50,325 (1982). (3)M. Menzinger, Adu. Chem. Phys., 42, 1 (1980). (4)J. W. Cox and P. J. Dagdigian, J. Phys. Chem., 86, 3738 (1982). (5)P. J. Dagdigian, Chem. Phys. Lett., 55,239 (1978). (6)B. E. Wilcomb and P. J. Dagdigian, J. Chem. Phys., 69, 1779 (1978). (7)L. Pasternack and P. J. Dagdigian, Chem. Phys., 33, 1 (1978). (8)J. A. Irvin and P. J. Dagdigian, J. Chem. Phys., 73,176 (1980);74, 6178 (1981). (9)M.H.Alexander and P. J. Dagdigian, Chem. Phys., 33, 13 (1978). (10)M. H.Alexander and P. J. Dagdigian, Faraday Discuss. Chem. Soc., 67, 141 (1979).
with C02 to produce the excited a311 and A"II states of CaO, while Ca(3P)produces ground-state Ca0.798 In this case, production of the excited states by Ca(3P)reaction is thought to be endothermic, although there is still some controversy about the dissociation energy of Ca0.8 Pasternack and Dagdigian7point out that, since production of ground-state CaO(X'B+) + CO(X'Z+) from Ca(3P) + C 0 2 is spin forbidden, potential surface crossings must occur in this case, and suggest that an "electron-jump" mechanism involving an intermediate surface of Ca+C02character may facilitate such crossings. The internal vibrational and rotational quantum-state distributions of product CaO in the Ca(3P)/C02reaction have been determined by laser-induced fluorescence to be quite cold compared to the distribution expected statist i ~ a l l y .Alexander ~ and Dagdigian rationalize this result by postulating a barrier in the exit-channel portion of the CaC02 potential s ~ r f a c e . ~ Presented here are results on the reaction of Mg('P,) with C02. It is found that ground-state MgO(XIZ+)is formed, and that there is much less vibrational and rotational energy in the molecule than expected statistically. The mechanism of the reaction is discussed in terms of the MgC02 potential surfaces which may be involved.
Experimental Section The initial quantum-state distributions of MgO(X'Z+) in the reaction of Mg('P1) with C02were determined with a laser "pump-and-probe" technique, the details of which have been described previously." Briefly, the thirdharmonic output of a Molectron MY32 Nd:YAG laser pulse is split, and one portion is used to generate a Molectron DL-200 dye laser pulse which is doubled in frequency by means of an ADP crystal. The resulting 28524 pulse excites Mg(3s3p 'P1) in a He/Mg vapor stream to which COz gas is added. The remaining protion of the Nd:YAG pulse is delayed spatially a few nanoseconds and oscillates a second Molectron DL-200 laser at wavelengths near 5000 8, with which the MgO(X'Z+) product can be (11)W. H.Breckenridge and H. Umemoto, J. Chem. Phys., 77,4464 (1982),and refereces therein.
0022-3654/03/2007-0476$01.50/00 1983 American Chemical Society
Distribution of Quantum States of MgO(X'Z+)
The Journal of Physlcal Chemistry, Vol. 87, No. 3, 1983 477
.-h c
al c
-+
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1
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Wavelength
(i)
Figure 1. Experimental LIF spectrum (XIZ++BIZ+) of MgO produced in the reaction of Mg(lP,) wRh C02.
detected via laser-induced fluorescence of the MgO(B-X) transition. The temperature of the reaction zone was 426 K. Molecular fluorescence was detected with an EM1 6256B photomultiplier. A Schott GG455 cutoff filter was used to block Mg('Pl) fluorescence at 2852 A from the EM1 photomultiplier. Fluorescence from Mg('P1) at 2852 A was monitored separately with an RCA 1P28 photomultiplier and an interference filter. The LIF spectra were obtained by scanning the probe laser and measuring the ratio of molecular fluorescence to Mg('P1) fluorescence intensity, using a PAR 162/164 boxcar integrator in "ratio" mode. This allowed automatic correction for slight variations in Mg('Pl) concentration due to changes in Mg vapor density and absorbed pump laser intensity. At constant concentrations of COz quenching gas, the Mg(lP1) fluorescence intensity should be directly proportional to the Mg(lP,) concentration. The spectra were normally taken at a 10-Hz laser repetition rate. Because the laser bandwidth was not sufficient to resolve the rotational structure of the MgO(X'Z++BIZ+) vibrational sequence bands ( u ' = u'? the initial distribution of MgO(X'Z+) vibrational and rotational states was determined by a trial-and-error procedure in which the deviations between computer-simulated and experimental spectra were minimized. The simulation procedure was similar to that employed previously." The line positions of the B-X system were calculated from the spectroscopic data of Lagerqvist and Uhler.12 Franck-Condon factors employed were the average of the values calculated by Ortenberg et al.13 and by Prasad.14 Typical pressures of gases were He, -17 torr, and COz, -1 torr. Gases (COG Matheson Co., 99.99%; He: US. Welding, 99.998%) were used directly from the cylinders with no further purification.
Results A typical LIF spectrum of the MgO(X'Z++B'Z+) spectrum is shown in Figure 1. The "best-fit" comput(12) A. Lagerqvist and U. Uhler, Ark. Fys., 1, 459 (1949). (13) F. S. Ortenberg, V. B. Glasko, and A. I. Dmitriev, Sou. Astron. (Engl. Trans.),8, 258 (1964). (14) K. Prasad, R o c . Phys. SOC.,86, 810 (1965).
1
I
I
I
4960
4970
4980
4990
1
VI I
I
5000
0
Wavelength ( A ) Flgure 2. "Best-fit'' computer simulation of the spectrum shown in Figure 1. See text.
.. CI
0 . 0
Experimental 0
. 0
a
12 I104 f 0
0
0
50
Phase-spoc
IO0
I50
200
J Flgure 3. Distributionof rotational quantum states Jof MgO(X'Z+,v=O) which yield the "best-fit" computer simulation of the (0,O)band in Figure 2. Also shown are the distributions of Mgoo('Z+,v=O)rotational states for the Mg('P ) 4- C02 reaction predicted by ~urprisal'~ (squares) and phase-space'e-18 (triangles) theories.
er-simulated spectrum is shown in Figure 2. The initial rotational quantum-state distribution of MgO(X'Z+,u=O) which resulted in the "best-fit" spectrum of the (0,O) band is shown in Figure 3 (circles). The "best-fit" distribution is slightly different from that obtained by assuming a rotational "temperature", Le., a B o l t " functional form. Assumption of a Boltzmann distribution gave a best-fit "temperature" of 1600 K. Similar distributions of rotational levels as in Figure 3 were assumed for the other vibrational quantum states of MgO(X'Z+) product. It is important to emphasize that our experimental results are not affected by secondary collision relaxation, i.e., that the spectra and "best-fit" quantum state distributions are indeed those of MgO(X'Z+) produced in single collisions of Mg(lP1) with COP. Changes of the delay time
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The Journal of Physical Chemistry, Vol. 87,No. 3, 1983
Breckenridge and Umemoto
TABLE I : Relative Populations of Vibrational Levels of the Reaction of M g ( l P , ) with CO,, As Compared to the Predictions of Statistical Models
110
-
MgO(X1 2 + ) Formed in Ua
0 1 2
3 4 5
exptl
priorb
1.o 0.8 i 0.6 * 0.5 2 0.5 t
0.1 0.1 0.2 0.2
