Internal energy exchange in molecular collisions with a crystalR. Grant Rowe and Gert Ehrlich Citation: The Journal of Chemical Physics 62, 735 (1975); doi: 10.1063/1.430480 View online: http://dx.doi.org/10.1063/1.430480 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/62/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Molecular dynamics simulation of energy exchanges during hydrogen collision with graphite sheets J. Appl. Phys. 107, 113533 (2010); 10.1063/1.3428447 Internal Energy Exchange and Dissociation Probability in DSMC Molecular Collision Models AIP Conf. Proc. 1084, 389 (2008); 10.1063/1.3076507 Semiclassical theory for lowenergy molecular collisions II. Atomic exchange reactions J. Chem. Phys. 60, 1386 (1974); 10.1063/1.1681208 Energy Exchange in Molecular Collisions J. Chem. Phys. 21, 1670 (1953); 10.1063/1.1698642 Transfer of Rotational Energy in Molecular Collisions II. Exchange of Energy in Collisions Between UnexcitedHgH and N2 Molecules J. Chem. Phys. 5, 831 (1937); 10.1063/1.1749949

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Letters to the Editor 735

producing an incipient paramagnetic hyperfine sp1itting.~ For slow relaxation one predicts from Griffith's results l3

for even-electron systems that the line arising from the I ± t) - I ± %) nuclear spin transitions will broaden first on lowering the temperature. The magnetic perturbation result [Fig. l(c)] confirms this, since on application of a large magnetic field the I ±t> - 1 ± t) line splits into an apparent triplet while the 1 ± t) - I ± %) line gives rise to a broadened doublet.

Attempts at a precise explanation of the causes of the slow relaxation observed here would be premature in the absence of further data. Clark's calculations2 suggest that for hexacoordinated Fe2+ ions under either Ok or D4k symmetry the spin-lattice relaxation is expected to be fast, as previously found. Fast relaxation down to 1. 8 K is also observed5 for Fe(H20)6SiF6, which is trigonally distorted with a singlet 10) ground state -1000 cm-! be­low the higher lying doublet.

Moreover, we find no relaxation broadening in the Mossbauer spectrum of Fe[(CaHshSO]a(CI04h at tempera­tures as low as 8.8 K. The latter complex is tetragonal­ly distorted by an axial field of - 500 cm-I, and has an 1 xy) ground state. a It is thus highly improbable that either the symmetry or magnitude of the axial field is of major importance here. Nor is the explanation likely to be that deduced2 for the line broadening in gillespite, where the absence of axial ligands leads to a loss of the Eg vibrational degree of freedom. It may be that the slow relaxation in Fe(C5H5NO)~+ results from the degen-

eracy of the orbital ground state, which presumably could have a significant effect on the orbit-lattice cou­pling coefficients.

We hope that studies on other FeL~· -type solvates now in progress will help to clarify these points. 14

1M. G. Clark, G. M. Bancroft, and A. J. stone, J. Chern. Phys. 47, 4250 (1967).

2M. G. Clark, J. Chern. Phys. 48, 3246 (1974). 3R. Zimmermann, H. Spiering, and G. Ritter, Chern. Phys.

4, 133 (1974), 4J. Reedijk and A. M. van der Kraan, Reel. Trav. Chim.

Pays-Bas 88, 828 (1969). 5C. E. Johnson, Proc. Phys. Soc. 92, 748 (1967), 6J. R. Sams and T. B. Tsin (unpublished work). lC. J. Bal1hausen, Introduction to Ligand Field Theory

(McGraw- Hill, New York, 1962), p. 68. 8Temperature measured with a calibrated Ge thermohm. OWing

to the design of our variable temperature cryostat, this is approximately the lowest temperature accessible in the sys­

tem. 9M. Blume, Phys. Rev. Lett. 14, 96 (1965). lOA. N. Buckley, G. V. H. Wilson, and K. S. Murray, Solid

state Commun. 7, 471 (1969), l1B. W. Fitzsimmons and C. E. Johnson, Chern. Phys. Lett.

6, 267 (1970). l2See also N. N. Greenwood and T. C. Gibb, Mossbauer Spec­

troscopy (Chapman and Hall, London, 1971). l3J. S. Griffith, Phys. Rev. 132, 316 (1963). l4This work was supported by the National Research Council of

Canada. TBT thanks the NRC for the award of a Postgraduate

Fellowship.

Internal energy exchange in molecular collisions with a crystal* R. Grant Rowet and Gert Ehrlich

Coordinated Science Laboratory t and Department of Meta/lu~v. University of Illinois at Urbana -Champaign, Urbana. Illinois 61801

(Received 23 October 1974)

One of the processes peculiar to the surface scattering of molecules, 1 as distinct from that of atoms, is the re­distribution of energy between translational and internal degrees of freedom during collision with a crystal. Theoretical estimates of this' effect have already been made. 2

-4 We wish to report here experimental observa­

tions which establish that redistribution occurs with a Significant probability in a coherent scattering event that does not involve transfer of energy to the lattice.

Measurements have been made on beams of molecular hydrogen and its isotopes colliding with a magnesium oxide surface. In a molecular beam experiment the in­terchange of energy between translational and internal degrees of freedom can be deduced from the angular distribution of the scattered intensity. Molecules that undergo such an energy interchange must obey two con­servation conditions. Conservation of energy relates the magnitude k of the wave vector for scattered mole-

cules to its initial magnitude, ko' by

k2 - k~= 2m" AE/1f 2

, ~1)

where mg is the mass of an incident particle and AE is the energy lost in the collision from the internal degrees of freedom and converted into translation of the mole­cule. The surface component K of the wave vector must also be conserved for coherent scattering from a planar periodic array, and therefore

K-Ko=21TG(m,n) ; (2)

G(m,n), as usual, is a reciprocal lattice vector of the surface. For elastic encounters t:.E vanishes; only the specular and diffracted peaks appear. For collisions in which the energy of internal degrees of freedom is exchanged entirely with translational motion, AE as­sumes values characteristic of transitions in the free molecule. If the probability of such an internal exchange

The Journal of Chemical Physics, Vol. 62, No.2. 15 January 1975 Copyright © 1975 American I nstitute of Physios

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736 Letters to the Editor

Ul

>­o >-

40

30

20-

}5 0-

<r o c CJ>

(f)

129 150 126

MgO HD

>- 1 __ ~o .2 <.) (])

Qj o

40 60 80 100 120 140 160 Angle from Incident Beam (Degrees)

180

FIG. 1. Scattering of HD molecules from the (001) plane of MgO at various angles of incidence, measured from the sur­face. The molecular beam strikes the surface along [010 l; beam and crystal are at 298°K. Detector signal for direct beam: 5025 units. Specular peak heights labeled at upper right.

is sufficiently great, new peaks should appear in the flux of scattered molecules5

; their angular position is fixed by the change in the internal state of the molecule.

For molecular hydrogen and its isotopes the vibra­tional spacing is so large that at room temperature we need consider only changes in the rotational energy; these will be denoted by tilllJ). 1=1 labels the transi­tion with the smallest energy, in which a molecule increases its translational energy by dropping into the rotational ground state from the next higher allowed level. For H2 and D2 this involves a change in the ro­tational quantum number6 l from 2 to O. 1=2 denotes the next larger tranSition, which amounts to a change of 1 from 3 to 1, and so forth. In HD, transitions between adjacent rotational states are allowed, and ~E(l) there­fore corresponds to a change of 1 from 1 to O. Negative values of I indicate loss tranSitions, in which a mole­cule loses translational energy in exciting higher rota­tional levels.

In our experiments, a molecular beam from a multi­capillary array at 298 OK strikes the (001) plane of MgO along the [OlD] direction. The surface is created just

prior to an experiment by cleavage in situ at pressures of '" 10-10 mm, and measurements are made under ultra­high vacuum conditions. Figure 1 shows the flux of HD molecules, scattered in the plane defined by the incident beam and the surface normal, for various angles of in­cidence. The locations of the elastic diffraction peaks are labeled by the indices (m, n); the angular profiles of these peaks reflect the velocity distribution expected for a thermal beam.

Most striking is the presence of other, stronger peaks in the intenSity data. These additio~ maxima have been identified as arising from the (11) diffraction peak, through conversion of translational into rotational mo­tion. The locations of (ii) loss peaks calculated from Eqs. (1) and (2) for thermal beams of HD are indicated in Fig. 1. Agreement between the peaks observed in the intensity distribution with the calculated pOSitions is generally very good. The - 1 rotational peak, however, is not resolved. This is expected, as the peak for this loss transition lies close to the (ii) elastic beam and is nearly as broad. Furthermore, in a beam at room temperature there are only half as many HD mole­cules in the proper rotational state (I = 0) for a - 1 as for a - 2 transition; the small height of this peak is there­fore reasonable. The first loss transition does, how­ever, overlap sufficiently with the elastic beam to af­fect the position of the (ii) peak to some extent. It should also be noted that for small angles of incidence (measured from the crystal surface), the rotational loss peaks become increaSingly prominent. At these angles the elastic peaks are broadened and their angular sepa­ration increases; this makes the rotational peaks stand out more clearly.

Quantitati ve estimates of the probability of loss tran­sitions have been made by fitting calculated intensity distributions to the experimental data. It appears from this fit that the number of HD molecules experiencing loss transitions is roughly equal to the number scat­tered into the (II) diffraction peak. Although only loss transitions from the (if) peaks are indexed in Fig. 1, gain peaks from the (11) are just as probable at higher angles of incidence. For none of the prominent peaks is it necessary to invoke phonon excitation; the ex­change of energy between internal degrees of freedom and translation gives a good account of the experiments.

Rotational transitions are not limited to HD scatter­ing. We have identified such transitions with D2 and H2 ,

as well as for crystals other than MgO. The conver­sion of rotational into translational motion in collisions with a crystal appears to be a general and important process affecting the scattering of diatomic molecules from a surface.

*Supported by the U. S. Air Force Office of Scientific Research (AFSC), USAF, under Grant AFOSR 72-2210.

t American Vacuum SoCiety Scholarship 1970-1972. fOperated under JSEP Contract DAAB-07-72-C-0259. l For modern studies of molecular scattering from surfaces see

D. R. O'Keefe, R. L. Palmer, H. Saltsburg, and J. N. Smith, Jr., J. Chem. Phys. 49, 5194 (1968); D. H. O'Keefe, J. l'\. Smith, Jr., H. L. Palmer, and H. Saltsburg, J. Chern.

J. Chern. Phys .• Vol. 62, No.2, 15 January 1975

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Letters to the Editor 737

Phys. 52, 4447 (1970). 2J. M. Jackson and A. Howarth, Proc. R. Soc. Lond. A 152,

515 (1935). 3R• M. Logan, Mol. Phys. 17, 147 (1969). 4G• Wolken, Jr. J. Chern. Phys. 59, 1159 (1973); Chern.

Phys. Lett. 21, 373 (1973). 5This was first pointed out by Logan, in Ref. 3. 6G. Herzberg, Molecular Spectra and Molecular Structure 1.

SPectra oj Diatomic Molecules ·(Van Nostrand, Princeton, NJ 1950), p. 139.

X-ray photoelectron spectroscopy of reduced porphins Yoshio Niwa

National Chemical Laboratory for Industry, Honmachi, Shibuya-ku, Tokyo, Japan (Received 21 October 1974)

The oxidation-reduction properties of porphyrins play an important role in their biological function. 1 It is well established that reduction of porphyrins and phthalocy­anines takes place by successive addition of electrons.2

,3

A number of ESR studies have been carried out to eluci­date the nature of acceptor orbitals. 4-9 All the reduction stages, however, do not yield paramagnetic species_ In the case of Co II(d 7) and CuII(d 9) complexes, for example, the dinegative ion is the first to give an ESR signal with a g value close to free spin, indicating that an unpaired electron is delocalized over a porphin 7T orbital. 6,9 In such a case, an electron uptake by a metal-centered or­bital is evidenced by ESR spectra rather indirectly.

X -ray photoelectron spectroscopy (XPS) offers further informations regarding charge distribution in a molecule irrespective of its magnetic property. XPS studies on porphyrins and azaporphyrins have presented valuable informations about the bonding scheme of inner protons in the metal-free bases10

,l1 and electronic structures in the metal complexes. 12 In this paper, we present the di­rect evidence obtained from XPS study that the central metal is in a lower oxidation state on the first reduction stage of CulI and COlI Ci, {3, 'Y, 0 -tetraphenylporphins and report other interesting results.

The reduction was run under high vacuum in a chamber adjacent to a photoelectron spectrometer since reduced species are rapidly oxidized by trace amounts of oxy­gen. Tetraphenylporphins (TPP) were exposed to reduc­ing vapor .such as sodium or magnesium metals.

Figure 1 shows the behavior of Cu 2p spectra obtained for Na/CuTPP. When the sodium was evaporated onto CuTPP, extra Cu 2p peaks appeared at a lower binding energy (1. 8 eV) than the 2p peaks due to CuIITPP [Fig. 1(b)]. The intensities of the peaks of lower binding en­ergy increased with increasing amounts of the deposited sodium. These peaks, therefore, resulted from the re­duction of CuTPP. The shift of the 2p peaks to lower binding energy reveals that the first reduction stage in­volves an electron capture by a metal-centered orbital, resulting in the formation of CUI species. Associated changes of N 1s and C Is peaks were quite small in their half-widths and binding energies. Thus the added elec­tron is largely localized on the metal atom and gives rise to only a little effect on the charge distribution on

the porphyrin ring. CuTPP was reduced also by the magnesium and the same results were obtained. Con­versely, no change in photoelectron spectra was ob­served for CoTPP and ZnTPP on exposure to the mag­nesium vapor, which indicates that CuTPP is reduced more easily than CoTPP and ZnTPP.

CoTPP, however, was reduced by the sodium. The results obtained were very similar to those for CuTPP and revealed the formation of COl species. When ZnTPP is exposed to the sodium vapor, Zn 2p spectra remain unchanged, which is consistent with a 3d10 closed-shell of ZnIl ion. However, the substantial shift of 0.7 eV to higher binding energy was found for C 1s spectra.

After the Cu 2p peaks due to the CuI! species disappear owing to further exposure of CuTPP to the sodium vapor [Fig. 1(c)], the shift of the C 1s peak to higher binding

C) CU2P3l2

960 950 940

Binding Energy, eV

FIG. 1. Copper 2p photoelectron spectra of Na/CuTPP: (a) CuTPP; (b) Partial reduction of CuTPP by the sodium; (c) Pro­longed exposure of CuTPP to the sodium vapor. Binding energy scale is not corrected. SAT denotes a satellite peak with which each copper 2p peak is accompanied. Figure l(b) shows the presence of both cuI and CuI! species. In Fig. l(c), copper 2p peaks arise only from the CUI species and the satellite peaks are completely missing.

The Journal of Chemical Physics, Vol. 62, No.2, 15 January 1975 Copyright © 1975 American I nstitute of Physics

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