Surface ionization detector sensitive to degree of internal excitation of alkali halides in molecular beams

Volmiune 5, number 5

SURFACE

15 April 1970

CHEMICALPHYSICSLETTERS

IONIZATION

EXCITATION

DETECTOR OF

ALKALI

SENSITIVE HALIDES

TO

DEGREE

IN MOLECULAR

OF

INTERNAL

BEAMS*

K. T. GILLEN and R. B. BERNSTEIN Clmnistq~ Departnrenf and Tkoratical Ci~e?nistrJ Institute, Universitj~ of Wiscomin. hbdison, Wisconsin 53706, US4

Received 2 March 1970

The sensitivity, or ionization efficiency, f, of a low work-function (“desensitized”) Pt/W surface ionization detector for alkali halide (BLY)beams increases strongly with the internal

Eexc, of the RIX. For escited KI (formed with y %11/d.

in crossed

1. INTRODUCTION The suitability of a 92% Pt/8% W alloy as a differential surface ionization (SI) detector for alkali/alkali halide beams was established by Datz and Taylor [l] and employed in a multitude of crossed beam scattering experiments involving reactions of alkali atoms with halogen-containing molecules [2] **. The present note reports a new application of such an SI beam detector: as a discriminator for extent of internal excitation of alkali halide molecules. The possibility of a variation in the rate of an overall wrfam reastivn LiCl (v)

W(1400OK)

* Work supported by National Science Foundation Grant GB-i6665.

chemical.renction).

For reviews of later work see ref. (31.

fc

energy,

cxp [yEexJ&TI

study of the velocity analysis of the reactive scattering of K by 12. The present experiments involved the chemically excited M molecules, with bzmu?zinternal (excitation) energies in the range 1 - 2 eV. A Pt/W (92/8) SI filament was employed in the low work-function or “desensitized” mode [8], such that it had a very low sensitivity to a thermal KI beam. The ionization efficiency of the detector was found to depend exponentially upon Eat, the excitation energy of the M. This effect is not expected to be limited to this particular tion

alkali of the

ST detector

b Li+ + Cl + e’

with the alkali halide vibrational state ZJ(v = 0, 1,2,3) was suggested by Klemperer and Herschbach [4] on the basis of their analysis of some molecular beam resonance experiments of Marple and Trischka [5]. Unfortunately, subsequent careful measurements by Moran and Trischka [6] negated the earlier experimental results and no measurable (i. e., f = 1%) discrimination in SI efficiency with vibrational state could be found. It was recognized [7] however, that sufficiently highly excited states (e. g., o >> 3) of alkali halides might well be selectively ionized under favorable SI conditions. Such conditions were found, somewhat by chance, in this laboratory in the course of a detailed

**

beam

(E/S’%)

halide: indeed, qualitative erA~ancement in sensitivity

for

other

excited

;rkli

available from a number of sources,

coniirmaof such ZI halides

is

see ref.[9].

2. EXPERIMENTAL The present data were obtained in the course of a crossed beam study [lo] of the reaction of a velocity-selected K beam with a thermal I2 beam at 90° incidence. The product KI and K scattered at various Iaboratory angles passed through a velocity analyzer to the SI detector. This consisted of a Pt/W (92/8) ribbon 0.7 mm wide, 0.025 mm thick, ca. 1 cm in length, dc heated (by ca. 1 amp) to operate at ca. 1330°K. Under these conditions (the desensitized mode), the ionization efficiency for a thermaL KI beam was in the range O.l-0.4%, based on comparative measurements with the filament in the sensitized (oxidized) mode at ca. 1370°K, where it is essentially 1Opm efficient. 275

Volume 5. number

15 April 19’70

CHEMICAL PHYSICS LETTERS

5

1.5

E,, (eV) Fig. 1. Dependence of the logarithm off, the apparent fraction of Kl ionized, (in the unsensitized mode) on Ee,,, the (average) internal excitation (in eV) of the Kl. Each set of connected points is derived from data at specified relative kinetic energy nnd laboratory angle but different laboratory velocities (corresponding to different c. m. recoil energies and thus different Eexc for the Kl). With the knowledge

of the translational

veloci-

ty of the chemi-excited M and conventional conservation considerations commonly applied [ll] to crossed-beam velocity analysis experiments, the average internal excitation energy of the Kl molecules striking the detector could be calculated: E&xc (‘(I)

= Etr + Eint (12) - ADo

- gir

9

the inciwhere Etr and Eir are, respectively, dent and final (c.m.) relative translational (kinetic) energies. Etr is, typically, 0.12 eV; Eint('I.$ =

sents data at the same laboratory angle 0 but different laboratory velocities v ‘. Provided there is negligible translational energy effect, different points at the same calculated Eat (arising from different 0 and v’) should yield the same f: Within the estimated f 20% uncertainties in the points this appears to be the case. Also shown on the graph at low Eat are the re-

sults for a thermal Kt beam, which accord well with a linear extrapolation from the main body of data for the excited ICI.

= 0.05 eV; and A DQ = - 1.80 eV asstcnting the other reaction product, I, to be ;J? its ground (zPSn) state. The velocity analysis results [lOI combined with the results of a diffusion flame study by Roth and Scbay [12] of the K + 12 reac-

tion

indicate that there is little, if any, excited I(z P,b ) product [lo]. Fig. 1. shows the logarithm

efficiency

of the M.

276

of the ionization

f versus the internal excitation Eat Each connected set of points repre-

3. INTERPRETATION The process

of surface ionization of alkali

halides has received for example refs. [l,

considerable

study (see yet thor-

4,8,13]) bet is not

oughly understood. Adopting either of the thermodynamic OYthe kinetic models commonly invoked to eq.rlain the large body of SI experimental data for alkali halides, it can readily be

Volume 5. number

5

CHEXICAL

PHYSICS

shown that the depender.ce of the ionization efficiency upon the excess (excitation) energy of the MX should be of the form f =fo exp [+,,c/kT], where f,-Jis the efficiency of the detector for ground state MX VO
LETTERS

detector as an internal energy monitcr (i.e., for estimating average internal excitation of chemiexcited alkali halides, MX) one must evaluate (calibrate) 7 for the alkali ha!ide in question: then the set of reactions M + XY - MX + Y (for a11 Y) may be studied. The authors appreciate the valuable advice and assistance of Mr. A. M. Rulis in connection with these experiments.

REFERENCES 4. CONCLUDING REMARKS

[l] S. Datz and E. H. Taylor.

The slope of the line in fig.1 (i.e., the value of y obtained) implies that an increase of as little as 0.023 eV (the spacing between adjacent low-lying vibrational levels of Kt [14] would produce a 5% increase in the fraction f. The question arises as to the failure of the experiments of ref. [S] to observe vibrationally selective detection for the various LiX molecules. This may be due to either of two distinct differences in their work. They used a tungsten filament for ionization, and the mechanism of ionization may vary considerably with filament material. Also, their iow efficienty esperiments were done at higher temperatures than those corresponding to the maximum ionization efficiency; the high temperature

decrease

in sensitivity

is undoubtedly

caused by a different mechanism than that which lowers efficiency in the low temperature region. There exist a number of observations which suggest that the qualitative behavior observed for KI is somewhat general. The fvalues on desensitized Pt/W are usually low for all alkali halides, but there is a considerable span which correlates well with MX ionization potentials [15]. The CsX molecules are most efficiently ionized; f usually increases from Cl to Br to I.

Datz and Minturn [16] reported a value of f- 0.08 for a CsBr beam on desensitized Pt/W;

(this increased detection sensitivity over KI suggests that CsBr might be better from the viewpoint of detection of excitation). An analysis of literature data and experiments from this laboratory on the wide -angle (supposedly nonreactive) scattering of alkalis by several halogencontaining molecules (using desensitized Pt/W detectors) has provided evidence for interference due to selective (enhanced) detection of internally excited MX product (for X = Cl, Br, and I) *. in order to make use of this type of selective * The implications of the present findings alter the interpretation of the h1+ X2 scattering data obtained with Pt/W “dI.fferential” detectors (see ref. [S]).

J. Chem. Phys.

25 (1936)

339.395. [Z] E. Taylor and S. Dab. J. Chem. Phys. 23 (1955)

1711. [3] J. P. Toennies. in: Chemische Elementarprozesse. eds. H. Hartmannet al. (Springer. Berlin. L9GS)

p. 157; J. P. Toennies. Ber. Bunsengcs. Phpsik. Chem. 72 (1968) 92’7; H. Pauly and J. P. Toennies.

in: Methods of esperi-

mental physics. ed. L. 3Iarton. Vol. 7 of Atomic and electron physics; Atomic interactions. Part A. eds. B. Bederson and \V.L. Fite (Academic Press, New York. 1965) p_ 217: D. R. Herschbach. Advan. Chem. Phys. 10 (LSGG) 319.

[4] W. KIemperer and D. Herschbach. Proc. Sail. Acad. Sci. US 43 (1957) 429. [5] D. T. hlarple and‘J. \V[Triscbka. Phys. Rev. LO3 (1956) 597. [6] T. I. aforan and J. W. Trischka. J. Chem. Phys. 34 (19Gl) 923. [7] D. R. Herschbach. private communication. [S] T. R.Touw and J. W. Trischka, J. AppL. Phys. 3% (1963) 3635. [9] K. T. Gillen and R. B. Bernstein. Appendix of Univ. of Wisconsin Theoret. Chem. Inst. Rept. . [VIS-TCI377X (i970). [lo] K. T. Gillen, Ph. D. Thesis. Universi:>- of Wiscon-

sin (1970):

[ll]

K.T. Gillen, A. ikl.Rulis and R. B. Bernstein, to be published. A. E.Grosser. A. R. Blythe and R. B. Bernstein, J. Chem. Phys. 42 (19651 1268; K. T. Gillen, C. Riley and R. B. Bernstein. J. Chem.

Phys. 50 (1969) 4019. [121E. Roth and G. Schay, Z. Physik. Chem. B2Y (1935) 323.

[13] B. II. Zimm and J. E. &Iayer, J. Chem. Phys. 12 (1944) 362; G. E. Cog-in and G. E.Kimball, J. Chem.Phys. 16 (1948) 1035. K. R. Wilson and R. J. tvsnetich, Univ. of CaLifornia. Lawrence Radiation Lab. Rept. IL606 (1964); N.I.Ionov, Zh. Eksperixn. i Teor. Fiz. 18 (LQ-LS) 96,174. [14] S. A-Rice and W. Klemperer, J. Chem. Phys. 27 (l957) 573;

j.R.

[15] J. W.

kusk~andW. Cordy, Phys. Rev. L27 (1362) 817,

Hastie and J. L. Margrave, to be pubIished. [lG] S. Datz and R. E. hIinturn. J. Chem. Phys. 41 (1964) 1153.

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