Laserinduced fluorescence study of the reactions Cu+

Laserinduced fluorescence study of the reactions Cu+X2→CuX+X (X=F, Cl, Br, and I) C. C. Fang and J. M. Parson Citation: J. Chem. Phys. 95, 6413 (1991); doi: 10.1063/1.461561 View online: http://dx.doi.org/10.1063/1.461561 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v95/i9 Published by the AIP Publishing LLC.

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Laser-induced fluorescence study of the reactions Cu + X 2 (X=F, CI, Br, and I) c. C.

....

CuX + X

Fang a ) and J. M. Parson

Department of Chemistry, Ohio State University, Columbus, Ohio 43210

(Received 14 June 1991; accepted 19 July 1991) Rea.c~ions o~ a beaI? of Cu with halogen gases at low pressure have been studied in the singlecolliston r~gtme usmg a pulsed tunable dye laser to excite the ground electronic state products to fluore~cmg s.tate~. CuF vi?rational state populations could be estimated up to v = 25, but for t~e he~lV1er h~h~es l?formatI~n ?n only v = ~vP(v)(Erot)v

E

.

~vP(v)

rot

The average vibrational energy is calculated using (EVib

)

=

I.vP(V)Evib (v) I.vP(v) .

It should be noted that all the calculations are done only up to the highest vibrational state observed, even though this value is much less than the highest accessible one. The results are listed in Table III. The fractions of product energy appearing in vibration, rotation, and translation are also included, given as (/.,), (f.. and (/'), respectively. The corresponding prior predictions are given with a superscript "0" in the table.

>,

VI. DISCUSSION

A comparison of product energy disposal with the prior model can be used to explore the possibility of unusual dynamical effects in these reactions. We note that the vibrational distributions of CuX obtained in the present study are qualitatively similar to the prior distributions shown in Figs. 10-13. Also, we find general agreement between experiment and prior predictions for the fraction of energy disposed in vibration. A similar extent of agreement has been reported earlier in comparisons of prior predictions with vibrational state distributions found for the A, B, and C states of CuF formed in the CL reaction of CueS) with F 2 •3 It was suggested that the agreement there might be due to the reactions proceeding by way of the FCuF ground state of the collision complex, which would be expected to live long enough for energy randomization to occur. The results were in sharp

TABLE III. Characteristics of product energy distributions for the vibrational states included in the spectral simulations. The last three rows give the prior model results.

E,OI' kl/mol (EJ. kl/mol (E,). kl/mol (f.,) (f,) (J,) (f?,)

(r,) (r,)

CuF

CuCI

CuBr

CuI

285 46 28 0.16 0.098 0.74 0.245 0.297 0.458

157 2.4 13 0.Ql5 0.084 0.90 0.0305 0.381 0.588

163 3.6 3.5 0.022 0.021 0.96 0.0333 0.382 0.587

192 2.5 3.1 0.013 0.016 0.97 0.0241 0.387 0.589

6419

contrast to the highly inverted vibrational distributions formed when the 2D states of Cu react with F 2 • On close inspection one sees that the experimental X state vibrational distributions generally show higher populations at low vibrational levels than the prior model. The correction from number density to flux density, if it could be done accurately, is not expected to improve the agreement since lower v levels are likely to have higher velocities and hence even larger densities after correction. Since the rotational distributions are only crudely determined in the simulations, two alternative forms for the rotational distribution were chosen to see whether the CuF vibrational distribution depended strongly on the choice of rotational distribution. In Fig. 11 the crosses indicate the CuF vibrational distribution that was determined using the prior form given above for the rotational distribution, and the circles indicate the results found using the form Pv (J)

0::

(2J

+ 1) (1 -

J IJmax,v )n,

with n a variable parameter. All three results for vibrational distribution are seen to be nearly the same. The rotational temperatures found for CuF and CuCI were quite high, 3387 and 1575 K, respectively. Table III shows, though, that rotational excitation is less than the prior model predicts for both reactions. Considerably colder rotational distributions were found for CuBr (420 K) and CuI (413 K), giving much larger deviations from prior distributions. The rotational distributions for these heavier halides are possibly less accurately known because increased mixing of singlet radiative states with nearby triplet states makes the simple singlet-singlet Honl-London factors less appropriate. Clearly, a simple application of the prior model, or more elaborate statistical calculations such as phase-space theory, will not be adequate for explaining all the results presented here. The tendency to form cold rotational distributions may be related to limitations on high J states imposed by the reactivity dependence on impact parameter. If only small impact parameters lead to reaction, then angular momentum conservation may limit the final J value unless the final orbital and rotational angular momentum tend to be oriented in opposite directions. One does not expect these two momenta to oppose each other for separation of a near collinear complex (Coo v or C2v symmetry). Low rotational and vibrational excitation ofthe products could also be attributed to a potential energy surface that directs the energy of an exit channel barrier into product translation. If the lowest adiabatic surface is followed throughout, one might expect a barrier in the region where the intermediate must change from the doubly ionic XCuX structure to the singly ionic ground state molecule plus halogen atom. Although this study presents only a coarse picture of the internal state distributions, it points toward the need for potential energy surfaces and dynamical models in order to characterize these reactions. In comparison with earlier work, the most significant changes in vibrational distributions appear to arise when different atomic states are reacted rather than merely different product electronic states are

formed. J. Chem. Phys .• Vol. 95. No.9, 1 November 1991 Downloaded 02 Oct 2013 to 202.116.1.149. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissions

6420

C. C. Fang and J. M. Parson: Cu reactions with halogens

ACKNOWLEDGMENTS

Support of the National Science Foundation is gratefully acknowledged. We would like to recognize the contributions of Dr. William Rosano in measuring and analyzing cw LIF spectra from reactions of Cu with C12 , Br2 , and 12 ,

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J. Chem. Phys., Vol. 95, No.9, 1 November 1991 Downloaded 02 Oct 2013 to 202.116.1.149. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissions