Study of the Rotational Kinetic Energy Dependence of the Reaction CrossSection: Ar++H2→ArH++HNeil Sbar and J. Dubrin Citation: The Journal of Chemical Physics 53, 842 (1970); doi: 10.1063/1.1674070 View online: http://dx.doi.org/10.1063/1.1674070 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/53/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Kinetic energy dependence of partial cross sections for the collisional ionization of H2O, H2S, O2, andAr with He(23 S) metastable atoms J. Chem. Phys. 91, 1618 (1989); 10.1063/1.457121 Monte Carlo trajectory study of Ar+H2 collisions. II. Vibrational and rotational enhancement of crosssections for dissociation J. Chem. Phys. 66, 772 (1977); 10.1063/1.433955 On the energy dependence of reaction cross sections near threshold J. Chem. Phys. 63, 592 (1975); 10.1063/1.431093 Molecular Beam Study of the K+I2 Reaction: Differential Cross Section and Energy Dependence J. Chem. Phys. 54, 2831 (1971); 10.1063/1.1675263 CrossedMolecularBeam Measurements of the Total Cross Sections of Ar–N2, Ar–Ne, Ar–He, andAr–H2 at Thermal Energies J. Chem. Phys. 45, 240 (1966); 10.1063/1.1727317

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842 LETTERS TO THE EDITOR J. CHEM. PHYS., VOL. 53, 1970

TABLE I. Calculated spectroscopic constants.

State

1};.+ (II)

3Ll. (I)

3.06--3.96

0.54--1. 38

r. (1)

1. 63-1. 59

1.80--1. 70

per cent too large. Both effects are due to the fact that these calculations are optimal for large separations and may become poorer representations at close inter­nuclear separations.

The two new molecular states which have been found from these calculations to lie below 12 eV are shown in Fig. 1. The upper dashed curves represent the calculated potential curves from the variational CI wavefunctions. Our previous studiesl.2 would indicate that these represent upper bounds to the true potential curves. The lower solid curves are constructed by subtracting from the upper curves, as a function of internuclear separation, the average of the calculated energy error in our potential curves for the a llIg , a' l~,,-, B' 3~,,-, W l~", and 5~g + (I) states of N2. All of these states lie between 8-10 eV and1are non-Rydberg in character. The calculated error varies only slightly among this set of states. The lower curves are thus representative of lower bounds to the true potential curves.

As indicated on Fig. 1, the molecular symmetries of these two states are l~g+ and 3~g. The l~g+ state was constructed from a wavefunction consisting of 152 configurations. The 3~g state wavefunction consisted of 82 configurations. Both calculations represent a full CI analysis excluding K-shell excitations. An analysis of these rather complicated CI wavefunctions at molecular distances (2-3 a.u.) indicates that the dominant configuration (> 80%) for both symmetries corresponds to the MO assignment, (h,,)2(hg)2(30'g)2. This con­figuration is also found to be the most important assignment for the lowest 5~g+ state. The alternate possibility, (l1r.,,)3(hg) (30'g) (30',,) is found to be unimportant at molecular distances but becomes significant at larger separations. The calculated l~g+ state represents the first excited state of this symmetry. Since this state must correlate with the separated atom limits, 2D+2D, it is strongly bound with a calculated dissociation energy of nearly 4 eV. The 3~ state is the lowest state of this symmetry and is only weakly bound. A vibrational-rotational analysis was carried out for these states, and the results are given in Table I. The rather broad shape of this excited l~g+ state characterizes it as one of the most unusual states in N2. In Fig. 1, we have indicated how this state may have an avoided crossing with the a" l~g+ Rydberg state.5

These are probably at most one or two vibrational levels corresponding to the a" l~g+ Rydberg state owing to this avoided crossing.

Neither of these states of N2 has been reported in absorption or emission spectra, although studies of the

1135. 6.3 0.90

684. 20.9 0.78

0.006

0.019

resonance structure of N2 through electron impact experiments suggest that several new bound states of N2 may exist between 11-12 eV.6-8 The 3~g state could be identified with state 17 in Mulliken's compilation9

after changes in the MO assignment and energy level. The more strongly bound l~g+ state reported here has apparently not been previously identified. A complete analysis of the lowest 102 non-Rydberg states of N2 is now in progress.lO

* Supported in part by AFWL, Kirtland Air Force Base under Contract F29601-69-C-0048, and the Air Force Office of Scientific Research under Contract A1'49(638)-1711. A part of this research work was done during the author's tenure as a Visiting Fellow of the Joint Institute for Laboratory Astrophysics at the Univer­sity of Colorado.

1 F. E. Harris and H. H. Michels, Intern. J. Quant. Chern. IS, 329 (1967).

2 H. F. Schaefer and F. E. Harris, J. Chern. Phys. 48, 4946 (1968) .

3 F. R. Gilmore, J. Quant. Spec try. Radiative Transfer 5, 369 (1965) .

4 A. Lofthus, "The Molecular Spectrum of Nitrogen," Physics Department, University of Oslo, Spectroscopy Rept. 2, 1960.

6 K. Dressler and E. L. Lutz, Phys. Rev. Letters 19, 1219 (1967).:-

6 V. Cermak, J. Chern. Phys. 44, 1318 (1966). 7 H. G. M. Heiderman, C. E. Kuyatt, and G. E. Chamberlain,

J. Chern. Phys. 44, 355 (1966). 8 E. N. Lassettre, A. Skerbele, and V. D. Meyer,]. Chern. Phys.

45, 3214 (1966). 9 R. S. Mulliken, The Threshold of Space (Pergamon, New York,

1957), p. 169. 10 H. H. Michels, F. E. Harris, and J. B. Addison, "The Nitrogen

Molecule. An ab initio Study of 102 Low-Lying States" (un­published) .

Study of the Rotational Kinetic Energy Dependence of the Reaction Cross

Section: Ar+ +H2-+ArH+ +Ht NEIL SBAR AKD J. DUllRIN

Department of Chemistry, M assaclzusetfs Institute of Technology, Cambridge, M assaclzusetfs 02139

(Received 5 May 1970)

The difference in the cross sections for Ar+ reaction wi th H2 in the J = 0 and J = 1 states has been measured at 0.13-eV relative kinetic energy, and an inverse dependence of the cross section on rotational energy was found. l

Measurements were made with a two chamber beam apparatus. The source chamber housed a standard electron impact source; the main chamber contained the decelerating-focusing stage of the ion gun, the scattering cell (liquid-N2 cooled), the retarding grid energy analyzer, and a quadrupole mass filter fixed along the primary beam axis. Para enriched H2 and

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J. CHEM. PHYS., VOL. 53, 1970 LEITERS TO THE EDITOR 843

n-H2 (75% ortho) were separately exposed to an Ar+ beam of 2.7 eV with a fwhm spread of 0.3 eV. At nOK, > 98 % of the para molecules are in the J = 0 level and > 99% of the ortho molecules are in the J = 1 level. In order to eliminate the need for scattering cell pressure measurements of high relative accuracy and to reduce the effect of changing primary beam conditions, deute­rium was employed as an in ternal standard. H2 (para)-D2 and H2(n)-D2 mixtures of the same isotopic composi­tion were prepared. For both mixtures ArH+ I ArD+ current ratios were measured. Data from a typical run are given in Fig. 1 (a). Eleven such determinations were made and the average

(ArH+I ArD+) para

(ArH+/ArD+)n

was 1.03. The over-all procedure was checked for any unknown, large systematic errors by performing 13 blank runs [H2(n)-D2 mixtures in both cases] and the corresponding value was 1.00. Adjusting the former ratio for the para concentrations of the mixtures, one obtains the cross section ratio: UR(J=O)luR(J=I) = 1.05±0.02s. The identification of this value with the total reaction cross section is quite reasonable since, as seen from Fig. 1 (b), the product collection efficiency

:-,

10

8

6

4

g 2 Q)

6- 0 Q) ....

lJ... 8

6

4

2 O~--~~~~~~~~~~~----~~

(b)

Detector Acceptance

Angle' ± 4.5 0

FIG. 1. (a) Distributions (relative) of 25 measured product ratios. (b) Velocity diagram illustrating effect of x and Q (transla­tional exoergicity) sizes on collection efficiency: acceptance angle measured from entrance aperture of the scattering cell; U(ArH+) and (J are product CM velocity and laboratory scatter­ing angle, respectively; Q= 1.2-eV contour is estimated maximum exoergicity. The effects of the primary angular spread and target thermal motion have not been included.

is both large and relatively insensitive to the exact eM product, angular-velocity distribution.2

At 0.13-eV relative energy, the reaction cross section is ,.....,10-14 cm2, and it is valid to consider the above result in terms of the familiar ion-induced dipole model4

which predicts a cross section variation as (alE) 1/2. In the past, a has been taken as the weighted average of the parallel and perpendicular polarizabilities, al('-'1.3a.L. Our finding is consistent with a phenome­nological orientation effect in which the Ar+ tends to align the H2 in the direction of maximum polarizability, all. However, the presence of intrinsic angular momen­tum (J = 1) causes a reduction in the aligning ability of the long range Ar+-H2 interaction potential. This follows from an argument analogous to that used in the classical treatment of the dipolar rotator in a static field.s Furthermore, cross sections have been theoreti­cally obtained for low-energy ions colliding with polar molecules, and it has been found that the addition of rotational energy decreases the effect of the dominant long range, ion-dipole potential.6

Presently we are improving the experimental accu­racy, and in addition, determinations are anticipated at higher relative energies where the reaction proceeds largely by a spectator stripping mechanism. Together with other previously obtained microscopic information,! these results will aid in the development of models for this very basic chemical reaction. Of related interest would be a formal quantum mechanical treatment of this internal energy dependence.

Discussions with Professor R. Silbey are greatly appreciated. We also wish to thank Dr. Adir Jacob for his aid in testing certain components of the apparatus.

t This work was supported in part by the United States Atomic Energy Commission through funds provided under Contract AT(30-1)-905.

1 This system has been studied in great detail; see, e.g., (a) Z. Herman, J. Kerstetter, T. Rose, and R. Wolfgang, Discussions Faraday Soc. 44, 123 (1967); (b) A. Henglein, Advan. Chem. Ser. 58, 63 (1966).

2 The ArH+ from Ar+ reaction with H2 ( "-'90% in J>O) is strongly forward peaked with a QMP~O.l eV.a An important difference in the collection efficiencies could result from a mecha­nism giving "sideways" peaking and a very large positive Q for H2 ( J = 0). However, this seems quite unlikely.

3 R. Wolfgang, Accounts Chem. Res. 2,248 (1969). 4 G. Gioumousis and D. P. Stevenson, J. Chem. Phys. 29, 294

(1958) . 5 C. H. Townes, Microwave Spectroscopy (McGraw-Hill, New

York,1955). 6 J. V. Dugan, Jr. andJJohn L. Magee,'J. Chem. Phys. 47,3103

(1967).

Radical Fonnation in the Photolysis of Od3-Methylpentane Glasses

at X>190 nm

J. K. Roy AND P. K. LUDWIG

The Radiation Laboratory,* University of Notre Dame, Notre Dame, Indiana 46556

(Received 1 May 1970)

The long-wavelength absorption edge of aliphatic hydrocarbons is usually located at X< 190 nm. Photo-

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