J. Phys. Chem. 1984, 88, 5454-5456
5454
-
Threshold Behavlor for Chemical Reactions: Line-of-Centers Cross Section SiH+ H for SI+(*P) 4- H,
+
J. L. Elkind and P. B. Armentrout*t Department of Chemistry, University of California, Berkeley, California 94720 (Received: July 18, 1984; In Final Form: September 6, 1984)
The total reaction cross section for the title reaction has been measured as a function of reactant kinetic energy with a guided ion beam mass spectrometer. The reaction is endothermic and shows an energy threshold in the vicinity expected from literature thermochemistry. By testing a variety of functions, the most likely threshold form for this reaction is determined to be the line-of-centers model. This function, u = uo( 1 - & / E ) , accurately reproduces the energy dependence of this endothermic reaction with a threshold energy in excellent agreement with that derived from the literature. The results indicate that the bond energy for SiH+ is 3.23 i 0.04 eV.
Introduction The threshold behavior of chemical reactions is important for a variety of reasons. Not only is it of intrinsic interest, but knowledge of this behavior allows thermochemical data to be derived from reaction excitation functions, provides insight into the dynamics of endothermic reactions, and holds the potential for being predictive in nature. Despite this utility, proposed models for threshold dependences of chemical processes, even atomdiatom reactions, remain largely untested. Exceptions include work by Chupka, Berkowitz, and Gupta on anionic charge transfer' and by Parks, Wexler, and co-workers on collision-ind u d ion-pair formation? For some time, we have been interested in designing experiments to elucidate the threshold nature of chemical reactions. In particular, because the translational energy of ions can be easily and systematically changed over a wide range, we have focused these efforts on the examination of ion chemistry. R e ~ e n t l ywe , ~ studied reaction 1 and found that its behavior C+(zP)
+ H2
-
CH+ + H
(1)
a t threshold is consistent with eq 2 where E is the relative translational energy of the reactants, Eois the threshold energy, uo is an energy-independent scaling factor, and n = 1/2 and m = 1. This form has been predicted for endothermic ion-molecule reactions at low energies from microscopic reversibility arguments and the long-range ion-induced dipole potential." Unfortunately, this intrinsic threshold behavior is not evident due to the broad energy distribution in these experiments. Thus, this system does not provide as convincing a demonstration of the true threshold dependence as is desirable. An experimental system which would provide an unequivocal test for threshold models must meet several requirements. The true threshold for reaction should be known accurately, Le., the thermochemistry must be well established. This threshold should not be so low that the energy broadening completely obscures the true threshold behavior as in reaction 1. Activation barriers in excesss of the endothermicity must not be present. We have found that reaction 3, isovalent with process 1, meets these requirements Si+(zP)
-
+ Hz
SiH+
+H
(3)
admirably. The bond dissociation energy of SiH+ has been placed within narrow limits, Doo = 3.22 f 0.03 eV, by spectroscopic studies5of silane discharges. With Doo(H2)= 4.477: the expected threshold for reaction 3 is 1.26 0.03 eV, three times that of reaction 1. Theoretical calculations7 and experiments3 show that reaction 1 proceeds without activation barriers and thus it is reasonable to expect none for the isovalent reaction 3. To our knowledge, this study is the first experimental observation of this
*
Presidential Young Investigator 1984-89.
reaction. Flowing afterglow studies* found no reaction at 300 K and set an upper limit on the rate constant of lo-" cm3 s-l.
Experimental Section The guided ion beam apparatus and data reduction procedures used in this work are described in detail el~ewhere.~ Si+ ions are produced by low-energy electron impact ionization of SiH4. The electron impact appearance potential of Si+ from SiH4 is 13.56 f 0.08 eV.Io The first excited state of Si+ is the 4P state at 5.465 eV" which cannot be produced until an electron energy of 19 eV. The results reported here used an electron energy of 18 eV such that Si+ is produced exclusively in the 2Pground state. At higher electron energies (>20 eV), the exothermic reaction of excited states with H2 is clearly evident. We presume that the Si+(2P) ions are produced with a 1:2 statistical population of the J = 1/2 (0 eV) and J = 3 / 2 (0.036 eV) levels.I1 After formation, the ions are mass analyzed, decelerated to the desired kinetic energy, and injected into an rf octopole ion beam trap12 which passes through a gas cell containing H2. The product and unreacted beam ions drift out the cell to the end of the trap where they are focused into a quadrupole mass filter and detected by standard ion-counting techniques. Ion intensities are then converted to absolute cross sections for reactions9 Use of the octopole trap ensures high collection efficiency, removes dynamic biases, and allows low ion kinetic energies. Routine energy analysis of the ion beam is provided by operating the octopole as a retarding potential a n a l y ~ e r . ~Because J~ the interaction region and the energy analysis region are physically the same, ambiguities involving contact potentials and space charge effects are avoided.13 Uncertainties in the ion energy scale are *O. 1 eV lab (*0.007 (1) Chupka, W. A.; Berkowitz, J.; Gutman, D. J. Chem. Phys. 1971,55, 010"
0119
L I L t , ,5133.
(2) Parks, E. K.; Wagner, A.; Wexler, S.J. Chem. Phys. 1973,58,5502. Parks, E. K.; Kuhry, J. G.; Wexler, S.J. Chem. Phys. 1977,67, 3014. Sheen, S.H.; Dimoplon, G.; Parks, E. K.; Wexler, S. J. Chem. Phys. 1978,68,4950. (3) Ervin, K.; Armentrout, P. B. J. Chem. Phys. 1984, 80, 2978. (4) Levine, R. D.; Bernstein, R. B. J. Chem. Phys. 1972, 56, 2281.
(5) Carlson, T. A,; Copley, J.; Duric, N.; Elander, N.; Erman, P.; Larsson, M.; Lyyra, M. Astron. Astrophys. 1980, 83, 238. Douglas, A. E.; Lutz, B. L. Can. J . Phys. 1970, 48, 247. (6) Wagman, D. D. et al. J . Phys. Chem. Ref.Data 1982, 1 1 , Supp. 2. (7) Liskow, D. H.; Bender, C. F.; Schaefer, H. F. J . Chem. Phys. 1974, 61, 2507. Sakai, S.;Kato, S.;Morokuma, K.; Kusunoki, I. J . Chem. Phys. 1981, 75, 5398.
(8) Fahey. D. W.; Fehsenfeld, F. C.; Ferauson, E. E.; Viehland. L. A. J. Chem. Phyi. 1981, 75, 669. (9) Ervin, K.; Loh, S.K.;Aristov, N.; Armentrout, P. B. J . Phys. Chem. 1983, 87, 3593. Ervin, K.; Armentrout, P. B. J . Chem. Phys., submitted for publication. (10) Potzinger, P.; Lampe, F. W. J . Phys. Chem. 1969, 73, 3912. (1 1) Moore, C. E. Natl. Stand. Ref.Data Ser., Natl. Bur. Stand. 1970, 34. (12) Teloy, E.; Gerlich, D. Chem. Phys. 1974, 4 , 417. (13) The efficacy of this procedure is verified by the good agreement in
reaction threshold energies for the H2and D2 systems and between scans having very different nominal ion energy zeros.
0022-3654/84/2088-5454$01.50/00 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 23, 1984 5455
Letters ENERGY
Lab)
(eV.
/ I
d
0. 0
1
0. 0
1
1
I. 0
1
1
1
1
1
1
1
2. 0
1
1
1
1
1
3. 0
ENERGY
(eV.
1
1
4. 0
1
1
1
1
1
5. 0
1
1
I
6.0
CM)
Figure 1. Cross section for reaction 3 as a function of relative translational energy (lower scale) and laboratory energy (upper scale). The data (points) are compared with the line-of-centers model using an average energy threshold (broken line), an explicit state distribution (dotted line), and the convolution of these fits (solid line) described in the text. The inset shows the data and fits expanded by a factor of 10 and offset from zero by 0.5 AZ.The arrow indicates the threshold for process 4 at 4.48 eV.
eV CM). The spread in ion beam energies has a fwhm of 1.5 eV lab (0.1 eV CM) in the axial direction and is much smaller in the radial direction (
