Formation of NO+ by the Reaction of O+ and Vibrationally Excited N2 in CrossedMolecular BeamsRonald Bruce Cohen Citation: The Journal of Chemical Physics 57, 676 (1972); doi: 10.1063/1.1678299 View online: http://dx.doi.org/10.1063/1.1678299 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/57/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Reactive scattering using pulsed crossed supersonic molecular beams. Example of the C+NO→CN+Oand C+N2O→CN+NO reactions J. Chem. Phys. 83, 3171 (1985); 10.1063/1.449171 Survey of chemiionization reactions in accelerated atom–O2 crossedmolecular beams J. Chem. Phys. 65, 2562 (1976); 10.1063/1.433442 Chemiionization reactions in accelerated uranium–O2 crossed molecular beams J. Chem. Phys. 64, 306 (1976); 10.1063/1.431922 Molecular beam chemiluminescence. VII. Enhancement of Ba+N2O→BaO*+N2O cross section throughN2O bending vibration: Evidence for electron transfer J. Chem. Phys. 63, 4557 (1975); 10.1063/1.431138 Crossed molecular beam study of chemiluminescent reactions of Group IIIb atoms with O2 J. Chem. Phys. 63, 3575 (1975); 10.1063/1.431798

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THE JOURNAL OF CHEMICAL PHYSICS VOLUME 57, NUMBER 2 15 JULY 1972

Formation of NO+ by the Reaction of 0+ and Vibrationally Excited N 2 in Crossed Molecular Beams*

RONALD BRUCE COHENt

Department of Physics, University of Pittsburgh, Pittsburgh, Pennsylvania 15213

(Received 10 January 1972)

The cross sections for the reaction of vibrationally excited N. with 0+ (4S) to form NO+ plus N over the 0+ ion energy range 4--100 eV have been determined in modulated crossed molecular beams. The total effective cross section of N. did not change by more than 15% in the temperature range 300-3000oK. The reaction cross section for N. (v = 1,2, 3) did not vary by much more than a factor of 2 from the cross section for N. (v=O) in this collision energy range.

The cross sections for the reaction of vibrationally excited N2 with 0+ ions to form NO+, Reaction (1), have been determined over the lab collision energy range 4-100 eV in modulated crossed beams;

( 1)

It was found that vibrational energy for N2 (v= 1, 2, 3) has a small effect on the reaction cross section in this energy range, in sharp contrast to the effects found by Schmeltkopfl for the reaction of thermal 0+ with vibrationally excited N2• In these moderate energy experiments the cross sections for v= 1, 2, 3 do not differ from O'(v=O) by more than a factor of 2, while factors of 100,40 were found for 0'(v=3), 0'(v=2) relative to O'(v=O) in the thermal1 collision energy experiments. The energy dependence of the cross sec­tion for N2 (v=O) is in good agreement with the results of Stebbings2 and Geise.3 The results of our experiments agree well with those predicted by O~Malley' using a semiempirical approach with which he calculated the energy dependent cross section for Reaction (1) as a function of N2 vibrational state. The utility of his ap­proach is clearly indicated both by its success in pre­dicting results, and in its successfully reproducing the earlier results mentioned above.

EXPERIMENTAL

phase lag of the product signal on the translational velocity of the N2,5 and determination of the variation in signal intensity of Reaction (2) with source tempera­ture,

(2)

The furnace was operated to give an output of N2 molecules/sec that was independent of furnace tempera­ture. Under these conditions the relationship of signal (S) due to Reaction (2) to temperature is STl/2 = const, as given by Fite and Brackman.6 The precision to which this relationship was followed (5%-15%) for charge exchange to N2 and Ar was one of the major factors limiting the precision of our results. These tests established that the beam had come to translational equilibrium at the wall temperature.

It was important to demonstrate that the beam had also come to vibrational equilibrium at the wall tem­perature. To do this some experiments were run in which the furnace tube was packed full of tungsten wire or foil. With the limits of precision noted above, the cross sections for (1) or (2) were unchanged. In the unpacked furnace it can be estimated that N2 had ",,103 collisions with the walls. For the full packed furnace the number of collisions between gas and wall must be increased considerably. Mentall, Krause, and Fite used a similar furnace arrangement to demon-

The experiments were performed with a two chamber strate excitation transfer between N2 (v~6) and Na.1 modulated cross beam apparatus. The first chamber It is expected that the cross section for (2) should contained the differentially pumped N2 source, a depend upon N2 vibrational state as well as 0+ energy. resistance heated tungsten tube 5 cm by ! cm diam Within the limits of precision noted, charge transfer with a 1 mm hole. The source temperature was meas- from 0+ to N2 and Ar followed the same STI/2 behavior. ured by an optical pyrometer in the range 1000-3000oK. The 0+ beam was generally produced by electron The main chamber contained the electron impact ion impact induced dissociation of O2 using a 100 eV source, ion optics, an 8 cm radius 1800 electromagnet, electron beam. Stebbings2 has shown that an 0+ beam a shielded beam crossing region, faraday cup for produced at 50 eV contains more than 30% excited 0+. monitoring primary ion currents, and a stationary The excited 0+ is the source for Reaction (2). Ruther­quadrupole mass spectrometer used for measuring ford and Vroom8 have shown that Reaction (1), for product ion currents. Proof that the phase modulated 0+ (2D), has a very small cross section. product ion signal was indeed due to reaction with Experiments were performed in which 0+ ion energy the modulated N2 beam coming from the furnace hole was varied with the N2 furnace temperature held was established in each run by the dependence of the constant, and in which the ion energy was held constant

676

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REACTION OF 0+ AND VIBRATIONALLY EXCITED N2 677

and the furnace temperature varied. In each case a relative cross section was determined with the current of NO+ formed in Reaction (1) compared with the current of N 2 + formed in Reaction (2).

RESULTS

After verifying the cross sectional dependence of Reaction (2) with 0+ energy, relative cross sections for Reaction (1) were determined from the currents of NO+ and N2+. The relative cross section for NO+ determined in this way was about 50% smaller than the values reported.2 In Fig. 1 we have equated our value and Stebbings' value for (1) at 16 eV. It can be seen that there is excellent agreement between our data and theirs for relative cross sections for N2 beams at 300oK. Stebbings suggests that their values are accurate to within a factor of 2. We believe that their absolute cross sectional values are more accurate than ours. The difference between our values and theirs in small part is due to the difference in electron energy used to form 0+, and in large part due to poor col­lection efficiency in our apparatus for ions (NO+) with initial kinetic energy in the direction of the pro­duct ion mass spectrometer.

In Fig. 1 we have also shown the upper and lower limits to the effective cross section for N2 between 1000 and 30000 K in Reaction (1), normalized to ac­count for STI/2 and phase lag behavior. Within ± 15% the temperature dependence of N2 and NO+ formation were the same at constant ion energy.

Estimates of the upper and lower limit to the cross sections for excited vibrational states of N2 can be made from the following considerations. At 30000 K the percent populations of v=O, 1, 2,3 are 67%, 22%, 7%, and 3%. If u(v=3)"-'u(v= 2)~u(v= 1) = 2u(v=O), then the corrected NO+ signal would have been 35% greater at 30000 K than at 300oK, about twice the upper limit of the variation observed. If u(v=O)~u(v= 1)

4 8 12 16 20 24 28 32

0+ LA8 Energy (eV)

FIG. 1. The measured dependence of the relative cross section for NO+ formation on 0+ energy (LAB) and N2 beam tem­perature. Data of Stebbings, Ref. 2, for N2 beams at 300oK, (-). Our data for N2 beams at 3000 K (- -). Upper and lower limits to relative cross sections for N2 beams between 1000-3000oK, (I).

9.--------------------------------

OJ 7 E u

!D

Q 5

b 3 /

;j / / /

__ ~ ........ ~-- T:300

--- "-v_- _____ '---T: 1500

"-T:3000

4 16 20 24 28 32

0+ LAB Energy (eV)

FIG. 2. The calculated dependence of NO+ formation on 0+ energy (LAB) and N2 beam temperature, from O'Malley, Ref. 4.

a t a particular energy, then a large decrease in u (v = 2, 3) would have not been observable. The weak tempera­ture dependence and the magnitude of experimental error precludes finer analysis of the energy dependence of the cross sections of the excited vibrational states of N2 in this energy range. By contrast, SchmeltekopfI showed that in the thermal collisions N2 has an effec­tive cross section 400% and 900% greater at 2000 and 30000 K than at 300oK, and found u(v=3), u(v=2), u(v= 1), u(v=O) were in the ratios 100, 40, 1, 1.

DISCUSSION

Reaction (1) has been the subject of a number of theoretical studies aimed at explaining the unusually low cross section for N2 (v = 0), thermal 0+ collisions, the cross sections for thermal 0+, excited N2 collisions, and the dependence of N2 (v=O) cross sections on 0+ energy.g A very useful qualitative analysis is that of Kaufman and Koski. tO

The variation in the cross section of Reaction (1) with 0+ energy for N2 beams at 300, 1500, and 30000 K taken from O'Malley! is shown in Fig. 2. Our failure to observe vibrational effects in these experiments is consistent with the results of O'Malley's calculations showing the effect to be small in this collision energy range.

Leventhalll measured the kinetic energy of NO+ formed in Reaction (1) for ground state N2 and found it consistent with a stripping mechanism. He pointed out that his data at 7 eV is inconsistent with O'Malley's model since one of O'Malley's assumptions was that a sum over partial waves could be replaced by a Langevin cross section. It is not apparent that there is an in­consistency, since O'Malley's model predicts the probability of a successful collision occurring as a func­tion of translational and vibrational energy, and is not explicit about the way in which energy is channeled between product internal and kinetic energy. The stripping mechanism, on the other hand, provides a way to proportion reaction exoergicity in a successful

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678 RONALD BRUCE COHEN

collision, without predicting the magnitude of a reaction cross section or its dependence on translational or vibrational energy.

t Current address, Department of Chemistry, Illinois Institute of Technology, Chicago, Ill. 60616.

1 A. L. Schmeltekopf, E. E. Ferguson, and F. C. Fehsenfeld, J. Chem. Phys. 48,2966 (1968).

ACKNOWLEDGMENTS

The author gratefully acknowledges the support and advice of W. L. Fite and R. T. Brackmann, who made this research possible. The author also acknowl­edges the Donors of the Petroleum Research Fund administered by the American Chemical Society, for partial support of this research.

* This research was supported in part by the Defense Nuclear Support Agency and the United States Army Research Office (Durham). .

THE JOURNAL OF CHEMICAL PHYSICS

2 R. F. Stebbings, B. R. Turner, and J. A. Rutherford, J. Geophys. Res. 71,771 (1966).

3 C. F. Geise, Advan. Chem. Ser. 58, 20 (1966). 4 T. F. O'Malley, J. Chem. Phys. 52, 3269 (1970). 5 H. Harrison, D. G. Hummer, and W. L. Fite, J. Chern. Phys.

41,2567 (1964). 6 W. L. Fite and R. T. Brackmann, Phys. Rev. 112, 1141

(1958) . 7 J. E. Mentall, H. F. Krause, and W. L. Fite, Discussions

Faraday Soc. 44,157 (1967). 8 J. A. Rutherford and D. A. Vroom, J. Chern. Phys. 55, 5622

(1971) . 9 F. A. Wolf, J. Chern. Phys. 44, 1619 (1966); D. K. Bohme,

J. B. Hasted, and P. P. Ong, J. Phys. B 1, 879 (1968). 10 J. J. Kaufman and W. S. Koski, J. Chem. Phys. 50, 1942

(1969) . 11 J. J. Leventhal, J. Chern. Phys. 54, 5102 (1971).

VOLUME 57, NUMBER 2 15 JULY 1972

Electronic Structure and Dynamic Jahn-Teller Effect of Cobaltocene from EPR and Optical Studies

J. H. AMMETER* AND J. D. SWALEN

IBM Research Laboratory, San Jose, California 95114

(Received 13 March 1972)

In order to determine quantitative information about chemical bonding in metallocenes, we investigated the electronic structure of the cobaltocene molecule by EPR and magnetic susceptibility measurements at 4.2°K and by optical spectroscopy at 77°K. Diamagnetic ruthenocene, weakly paramagnetic nickelocene, and paramagnetic cobaltocene single crystals as well as polycrystalline ferrocene served as host systems. From the poorly resolved optical spectra of pure cobaltocene, approximate ligand field parameters were determined. The magnetic properties (g tensor, cobalt hfs tensor) of the lowest Kramers doublet are ex­plained in terms of the relative magnitudes of (a) spin-orbit coupling, (b) static orthorhombic distortion, and (c) vibronic coupling (dynamic Jahn-Teller effect) in the orbitally degenerate 2El. ground state. From the analysis of the EPR data of cobaltocene-doped ruthenocene, we conclude that covalency effects and vibronic interactions, ("Ham effect") are of comparable importance resulting in a drastic modification of the magnetic parameters compared to a free COH ion in a static crystal field. In agreement with earlier qualitative and semiquantitative predictions, considerable covalency of the singly occupied el: orbital (42%±5% ligand, 58%±5% cobalt 3d character) was found. The strong change of the EPR parameters going from the ruthenocene to the ferrocene host lattice originates mainly in a strongly enhanced static orthorhombic splitting parameter in the tighter packed ferrocene environment. In cobaltocene single crystal, magnetic dipole-dipole interactions broaden the EPR lines beyond detection even at 2°K. Nickelocene, an S= 1 case with a large positive zero field splitting, behaves as a pseudodiamagnet at liquid helium temperature; exchange interactions with the cobaltocene dopant cause significant modifications of the g values but leaves the cobalt hfs tensor almost unaffected.

INTRODUCTION

Considerable experimental and theoretical work1,2 has been done on metallocenes, especially ferrocene, mainly motivated because of their unusual structure, remark­able stability, aromaticity, and possible utility. Of special interest to the theoretical chemist has been the nature of the chemical bonding of these highly sym­metrical sandwich compounds. Paradoxically metallo­cenes and dibenzene complexes are among the most popular examples for describing and illustrating the procedures and the results on electronic structure and chemical bonding in terms of MO theory but precise

data about covalency, as available for many other complexes, are practically nonexistent. None of the numerous M03-15 and crystal field calculations16 has given quantitatively a completely satisfying picture; only a qualitative picture has emerged-nonetheless, most useful for interpreting experimental results.

Practically every common experimental technique has been enlisted to elucidate some facet or probe some detail of these complexes. For example, x-ray and electron diffraction has been used for the determination of molecular and crystal structures, magnetic sus­ceptibility for the ground state magnetic moments, optical spectroscopy for the location of electronically

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