Energy transfer in collisions of excited Na atoms with NO moleculesJoel A. Silver, N. C. Blais, and G. H. Kwei Citation: The Journal of Chemical Physics 67, 839 (1977); doi: 10.1063/1.434848 View online: http://dx.doi.org/10.1063/1.434848 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/67/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Vibrational and rotational energy transfer in collisions of vibrationally excited HF molecules with Ar atoms J. Chem. Phys. 115, 257 (2001); 10.1063/1.1378815 Transfer of electronic excitation in collisions of metastable argon atoms with nitrogen molecules. II J. Chem. Phys. 77, 5855 (1982); 10.1063/1.443750 Transfer of electronic excitation in collisions of metastable argon atoms with nitrogen molecules J. Chem. Phys. 70, 3171 (1979); 10.1063/1.437904 Vibrational Energy Transfer in Gases. Atom—Diatomic Molecule Collisions J. Chem. Phys. 42, 1957 (1965); 10.1063/1.1696231 Energy Transfer between Molecules and Electronically Excited Atoms J. Chem. Phys. 41, 2021 (1964); 10.1063/1.1726199

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

129.49.170.188 On: Fri, 19 Dec 2014 08:01:26

Letters to the Editor 839

207 (1968) and B. H. Schechtman, thesis, Stanford University, 1968, Stanford Electronics Laboratories, Technical Report No. 5207-2.

6W• Pong and J. A. Smith, J. Appl. Phys. 44, 174 (1973). 7A. I. Belkind, S. B. Aleksandrov, V. V. Aleksandrov, and

V. V. Grechov, Proc. Karpaz Summer School 1974, p. 8. SA. J. Signorelli and R. G. Hayes, J. Chern. Phys. 64, 4517

(1976), and references therein. 9G. M. Bancroft, T. K. Sham, D. E. Eastman, and W. Gudat,

J. Am. Chern. Soc. (in press). lOS. A. Flodstrom, L. G. Peters son, and S. B. M. Hagstrom, J. Vac. Sci. Technol. 13, 280 (1975).

11L • Ley, R. Pollak, S. Kowalczyk, and D. A. Shirley, Phys.

Lett. A 41, 429 (1972); F. R. McFeely, L. Ley, S. P. Kowalczyk, and D. A. Shirley, Sol. State Commun. 17, 1415 (1975).

l2G• M. Bancroft,!. Adams, D. K. Creber, D. E. Eastman, and W. Gudat, Chern. Phys. Lett. 38, 83 (1976).

l3J • E. Demuth and D. E. Eastman, Phys. Rev. Lett. 32, 1123 (1974).

l4B• H. Schechtman and W. E. Spicer, J. Mol. Spectrosc. 33, 28 (1970).

l5B • F. Hoskins, S. A. Mason, and J. C. B. White, J. Chern. Soc. D 1969, 554.

l6A. Henriksson and M. Sandborn, Theoret. Chim. Acta 27, 213 (1972).

Energy transfer in collisions of excited Na atoms with NO molecules a)

Joel A. Silver, N. C. Blais, and G. H. Kwei

University of California. Los Alamos Scientific Laboratory. Los Alamos. New Mexico 87545 (Received 3 May 1977)

The collisional quenching of electronically excited alkali atoms has long served as a prototype for nonadia­batic processes. 1 The majority of these studies have been total quenching cross section measurements for collisions of excited Na(3 3p) atoms with rare gas atoms and diatomic molecules. 2- 4 Recently, relative rates of transfer into individual vibrational levels of CO mole­cules have been measured by infrared absorption, 5 and the recoil velocities of quenched Na atoms inelastically scattered from N2 molecules have been determined in a molecular beam scattering experiment. 6 However, neither of these studies has provided any information on the detailed mechanism for energy transfer or on the partitioning of internal energy between vibration and ro­tation of the diatomic molecule. 7 In this Communica­tion, we report initial results from a crossed molecular beam scattering study of the process

Na(3 2 P3/2) + NO(O, J) - Na(3 2S1/2 ) + NO(v', J ') ,

in which both scattering angle and product velocity dis­tributions are measured. These results provide the first direct evidence that energy transfer takes place via the formation and subsequent decomposition of a collision complex and that rotational excitation is neg­ligible, with most of the electronic energy partitioned between relative translation (- 25%) and vibration of the NO molecule (-75%).

A nearly effusive Na atom beam (at 720 K) is crossed with a H2-seeded nozzle beam of NO molecules. Under these conditions, the NO molecules are internally re­laxed and we estimate that the most probable initial ro­tational angular momentum is ::; 4 n. ApprOximately 1%-5% of the Na atoms in the collision region are excited to

a)Work done under the auspices of the United states Energy Re­search and Development Administration.

The Journal of Chemical Physics. Vol. 67. No.2. 15 July 1977

the 3 2 P3/2 state by an argon ion pumped cw dye laser. The dye laser is frequency stabilized by a feedback sys­temS which monitors the fluorescence at the scattering center. Processes involving electronically excited atoms are distinguished from ground state processes by modulation of the laser beam. A simple chopping wheel is used for total intensity measurement while a pseudo­random binary wheel is used for cross correlation time­of-flight measurements. 9 The detector, an electron bombardment ionizer followed by a quadrupole mass­filter, has been described elsewhere. 9

The angular distribution of NO molecules scattered from excited Na atoms is shown in Fig. l(a). The ve­locity distribution for NO at 0 = - 14 0 shown in Fig. 1(b), together with time-of-flight data taken at other labora­tory angles, confirm that NO molecules that are in­elastically scattered from excited Na atoms are re­sponsible for the signals that contribute to this angular distribution. The shape of the angular distribution, with the peak near - 10° and the rise at large angles, sug­gests that energy transfer takes place via a complex mechanism. Unfortunately, the backward peak (with respect to the initial NO direction) in the angular dis­tribution is expected to be at 0 - 105 0 and is not accessi­ble in these experiments. Further evidence for com­plex formation is provided by the velocity distribution of Na atoms scattered at 0=_18 0 as shown in Fig. 1(b). Scattering of Na at this angle corresponds to scattering of NO in the backward direction, and the measured Na velocity is commensurate with the velocity of the in­elastically forward scattered NO. An approximate transformation of the laboratory angular distribution to the center-of-mass (c. m.) system using only the most probable NO recoil velocity produces a c. m. angular distribution that is sharply peaked in the forward di­rection and displays apprOximate forward-backward symmetry over the limited range of c. m. scattering

Copyright © 1977 American I nstitute of Physics

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

129.49.170.188 On: Fri, 19 Dec 2014 08:01:26

840 Letters to the Editor

> 0...

T (0 )

3 No (32p3/2) + NO (v.J) --

No (3 2S,12 ) + NO (v'.J')

0 -30 0 0 0 300 600 90°

LABORATORY SCATTERING ANGLE @ 4

8 6 4 2 o V ,

I I

I I

I I

I I I (b)

.98 .75 .50 .25 o fint

'" 0

3

--6-- No

/~- -g, I II \ lJ. /0 \

/ \

NO

lJ,I \ 7 \ I \ I \ I \ 10 \ I \ I \

I ~ 1 I \

! \\ / ',I

0 0'-------10-'0-'-066- 2000 3000 ~l!-~-4-0-,-tO-0-J LABORATORY VELOCITY (m/s)

FIG. 1. (a) Laboratory angular distributions of NO molecules scattered from excited Na(32P3/2) atoms. Different symbols refer to data taken in separate runs, arrows denote the location of the primary beams, and error bars represent ± one standard deviation. At laboratory angles within ~ 5° of the NO beam, the data includes substantial contributions from NO molecules elas­tically scattered from excited Na atoms and from modulation of elastic scattering of ground state species resulting from laser depletion of the ground state Na concentration in the collision region. (b) Laboratory flux density velocity distributions for NO and Na at laboratory angles of -140 and _180

, respectively. The upper scales show the fraction of the total aVailable energy converted to internal energy, hnt' and the final vibrational state of NO (if there were no rotational excitation) for given values of the NO laboratory veloCity. Arrows indicate the most probable recoil velocities for elastically scattered NO and Na (both 3 2SI/ 2 and 3 2

P 3/2)'

angle from 0 0 to 170 0• However, the lack of precise

knowledge of the c. m. distribution near 180 0 prevents us from estimating the mean lifetime of the complex. 10,11

The anisotropy in the c. m. angular distribution (ratio of cross sections at 0 0 and 90 0

) is unusually large with a value of - 8 and, since the initial rotational angular momentum is small, provides an estimate of -10 for the ratio of the maximum initial orbital angular momentum Lm to the most probable final rotational angular momen­tum J~~. 12 Since the quenching cross section is quite

large, 13 Lm is also expected to be large. Estimating Lm from the crossing between covalent and ionic surfaces in the "harpooning" mechanism14 gives a value of - 163 n; therefore J~p"" 16 no

The recoil velocity distributions in Fig. l(b) show that energy transfer is essentially nonresonant, with approxi­mately 75% of the available energy of - 200 kJ/mol ap­pearing as internal excitation. Since rotational excita­tion is negligible, these distributions provide a direct measure of the relative populations in the product vibra­tional states. As shown in Fig. l(b), the v' '" 7 and 8 levels of NO are preferentially populated with some shading to lower v'.

Energy transfer in collisions of excited alkali atoms with diatomic molecules has tradiationally beam treated as a process involving multiple potential surface cross­ings where a strongly attractive ionic state couples the upper and lower covalent states. Excited atoms and diatomic molecules which approach each other in states that cross adiabatically to the ionic state form complexes which later decompose to ground state products via a nonadiabatic transition. This picture has been used in theoretical treatments of the quenching of OeD) by N2 where the long complex lifetimes imposed by the spin forbidden transition result in statistical energy partition­ing. 15 In the Na(3 2P3 /2)+NO system, the nonadiabatic transitions have greater probability and we would not ex­pect statistical behavior. The final vibrational distri­bution we observe is in fact strongly reminiscent of the distributions predicted for the quenching of Na(3 2p) by N2 using a simple multiple-crossing electron-jump model. 16

We wish to thank R. A. Keller and R. K. Preston for several helpful discussions.

IA. C~ G. Mitchell and M. W. Zemansky, Resonance Radiation and Excited Atoms (Cambridge University, Cambridge, 1961).

2p. L. Lijnse, Report i398, Fysisch Laboratorium, Rijksuni­versiteit utrecht, The Netherlands (1972), provides an ex­tensive review of the literature through 1971.

3D. L. King and D. W. Setser, Ann. Rev. Phys. Chern. 27, 407 (1976).

4J • R. Barker and R. E. Weston, Jr., J. Chern. Phys. 65, 1427 (1976), and references cited therein.

5D. S. Y. Hsu and M. C. Lin, Chern. Phys. Lett. 42, 78 (1976).

61. V. Hertel, H. Hofmann, and K. A. Rost, Phys. Rev. Lett. 36, 861 (1976).

71. V. Hertel, H. Hofmann, and K. A. Rost, Chern. Phys. Lett. 47, 163 (1977).

8R: B. Green, R. A. Keller, G. G. Luther, P. K. Schenck, and J. C. Travis, IEEE J. Quantum Electron. QE-13, 63 (1977).

9N. C. Blais, J. B. Cross, and G. H. Kwei, J. Chern. Phys. 66, 2488 (1977).

lOWe have not yet measured the relative detection efficiency for Na and NO.

l1(a) G. A. Fisk, J. D. McDonald, and D. R. Herschbach, Discuss. Faraday Soc. 44, 228 (1967), and (b) M. K. Bullitt, C. H. Fisher, and J. L. Kinsey, J. Chern. Phys. 60, 478 (1974).

12W. B. Miller, S. A. Safron, and D. R. Herschbach, Discuss. Faraday Soc. 44, 112 (1967).

J. Chem. Phys., Vol. 67, No.2, 15 July 1977

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

129.49.170.188 On: Fri, 19 Dec 2014 08:01:26

Letters to the Editor 841

13Total quenching cross sections of 32 and 46 ,\2 at relative velocities of 2.4 and 2.8 km/s, respectively, have been re­ported by V. Kondratiev and M. Siskind [Physik. Z. Sowjetun­ion 8, 644 (1935»). However, these results display a velocity dependence that is reverse to those found more recently for other systems (see Ref. 4).

14For a "harpoon" model, a value of rc =4.75 A. can be obtained from the electron affinity of NO [0.024 eV from M. W. Siegel, R. J. Celotta, J. L. Hall, J. Levine, and R. A. Bennett,

NOTES

Phys. Rev. A 6, 607 (1972») and the ionization potential of Na(3 2p) (3,033 eV). The quenching cross section is then Q =f7rr~ =27 A.2, wheref=3/8 is the fraction of excited potential surfaces that correlate with the ionic surface in Cs symmetry.

15(a) J. C. Tully, J. Chem. Phys. 61, 61 (1974), and (b) G. E. Zahr, R. K. Preston, and W. H. Miller, ibid. 62, 1127 (1974).

16(a) E. Bauer, E. R. Fisher, and F. R. Gilmore, J. Chem. Phys. 51, 4173 (1969), and (b) J. R. Barker, Chem. Phys. 18, 175 (1976) and references cited therein.

Translational energies of the products of ion-molecule reactions

Duane E. Carte~)

Chemistry Department, Rice University. Houston, Texas 77001 (Received 25 January 1977)

A recent article by Klots 1 has stimulated renewed hope that statistical kinetics theories can successfully predict the translational energies of products of ion­molecule reactions that pass through a "long-lived" col­lision complex. In that paper the product energy distri­butions of the reaction

(1)

were accurately predicted using phase-space theory combined with the Langevin cross section for the re­verse ion-molecule reaction. This theory avoids many of the important approximations made in an earlier ap­proach to the problem by Safron, Weinstein, Hersch­bach, and Tully. 2

I have applied the same theory to the following reac­tions:

CH+ +H2 - [cH;]*- CH;+H ,

CH;+D2 - [CH2D;]*- CHD;+H ,

NH; + H2 - [NH;] * - NH; + H .

(2)

(3)

(4)

These reactions have been studied experimentally using a crossed beam technique3

-S and theoretically by Mayer6

using the approximations of Safron et al. 2 I have used a Monte Carlo technique of phase-space integration de­scribed in a previous paper7 and used for Reaction (1). Briefly, the technique considers that the complex is characterized only by its energy and total angular mo­mentum. All product states (specified by vibrational states, rotational angular momentum of each of the frag­ments and their projection on a particular axis) are con­sidered as equally probable if they are allowed by energy and angular momentum conservation. An additional con­straint is placed on the orbital angular momentum of the

The Journal of Chemical Physics, Vol. 67, No, 2, 15 July 1977

pair of fragments by the long-range forces. These long­range forces are assumed to be those of an ion-induced dipole as in the Langevin treatment of ion-molecule re­actions. All possible products (including a return to re­actants) were allowed for in the calculations. The fig­ures in this note compare the results of the present cal-

1.0 ,.......,.....,....",......"....."T""-----r------,-------,

: I

05 , , , , ...

CH+ + H2 -+ CH; +H

Ecal = 0.66 eV

...... ~ ..•... ......" Etot .~ ol~~~r---~----------L'~ .. ~'~"~ ... ~~~--------~ Q)

E '0 Q)

,!::! "0 E ~ .5 Z

"

" .....

C H++H2 -+ CH; + H

Etal= 0.39 eV

....... O~------~------~~--L_ __ ~~------~

0.5 1.0 1.5 2.0

c.m. Product Translational Energy (eV)

FIG. 1. Distribution of center-of-mass translational energies for Reaction (2) at collision energies of 0.39 and 0.66 e V. -.--- experimental; ----- present work; ••••• theory from Ref. 6.

Copyright © 1977 American Institute of Physics

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

129.49.170.188 On: Fri, 19 Dec 2014 08:01:26