The present paper deals with the benefits and difficulties of using ion scattering spectroscopy as a spectrometric technique

Weinberg, W. H.

Williams, E. D.

English

Caltech Authors - Main

Tsai, Friend, and Muetterties: Dehydrogenation of cyclohexane 
deuterium exchange between adsorbed C6llt, and C6D6 [in 
the case of Ni( l 00), annealing to tso·c of the surface with a 
chemisorbed mixture of C6H6 and C6D6 gave no evidence of 
hydrogen-deuterium exchange between chemisorbed ben-
zene molecules]. 
In dehydrogenation of c-C6H 10 and c-C6D 10 on the plati-
num surfaces, a mixture of all possible C6Hx 0 6 _ x mole-
cules were observed in both the thermal desorption and the 
displacement reactions, conducted at -125°C and -too•c, 
respectively, for the stepped and (Ill) surfaces. Since hydro-
gen-deuterium exchange was not observed2 between ad-
sorbed C6H6 and C60 6 on Pt( 111) up to - 130 ·c, the hydro-
gen-deuterium exchange process in the cyclohexene 
dehydrogenation on Pt( 111) must occur at some intermedi-
ate state before chemisorbed benzene is formed. A similar 
intermediate exchange probably occurs for cyclohexene on 
the stepped platinum surface. 
Cyclohexane did not react with any of the nickel surfaces 
at room temperature 'or above. The residence time of cycle-
hexane was probably too short for any significant dehydro-
genation to occur under these conditions. Only the Pt 
6( 111) X (Ill) surface displayed a high activity for the dehy-
drogenation of cyclohexane to benzene. Dehydrogenation of 
cyclohexane to benzene had a higher activation energy than 
the dehydrogenation of cyclohexene to benzene on this sur-
face. Evidence of qydrogen-deuterium exchange between 
adsorbed c-C6H 12 and c-C6D 12 on the Pt 6(1ll)X(lll) was 
obtained from a thermal desorption study subsequent to an 
attempted trimethylphosphine displacement whereby all 
possible C6 Hx 0 6 _ x molecules were desorbed at 170-
19o·c-a feature similar to that observed for benzene che-
misorbed on the stepped platinum surface. 
Dehydrogenation reactions were studied over carbon-
contaminated surfaces (submonolayer coverage). The 
Ni(llO)-C surface was far more reactive in dehydrogenating 
both cyclohexene and cyclohexadiene than the clean Ni( 11 0) 
surface. A substantial carbon effect on the chemistry of the 
other surfaces was not detected. 
Rate data for the dehydrogenation to benzene establish 
the cyclohexadiene transformation as the most facile pro-
cess, cyclohexene next and cyclohexane least. Thus, the data 
are not inconsistent with chemisorbed intermediates of cy-
clohexadiene in the cyclohexene transformation and of cy-
clohexene and cyclohexadiene in the cyclohexane transfor-
mation. In the cyclohexadiene and cyclohexene reactions, it 
is presumed that the initial chemisorption states primarily 
involved an interaction of appropriate metal surface orbitals 
with the rr and rr* orbitals of the olefinic or diolefinic sys-
tems. This necessarily places one C-H hydrogen atom of the 
saturated CH2 centers proximate to the metal surface, thus 
enabling C-H bond scission at these sites and the subsequent 
generation of chemisorbed benzene. Reversible C-H bond 
breaking was not detectable for any of the nickel surfaces 
under the ultrahigh uacuum conditions, but was for plati-
num-a result consistent with the relative electronic proper-
ties of nickel and platinum. 
There is some generality to the metal surface conversion of 
a cyclic hydrocarbon to a chemisorbed Cn Hn species. Ex-
ploratory experiments have demonstrated the conversion of 
1, 5-cyclo-octadiene to cyclo-octatetraene on Pt{lll). 
Acknowledgments: This work was supported by the Direc-
tor of the Office of Energy Research, Office of Basic Energy 
Sciences, Chemical Sciences Division of the U.S. Depart-
ment of Energy under Contract No. W-7405-ENG-48. 
ELM is indebted to the Miller Institute for Basic Research in 
Science for a grant in the form of a Miller Professorship. 
'C. M. Friend and E. L. Muetterties, J, Am. Chern, Soc. 103,773 (!98!), 
2M.-C. Tsai and E. L. Muetterties, J. Am. Chern. Soc. iin press). 
Summary Abstract: CO on Ru(001): Formation and dissolution of islands of 
CO at low coverages 
E. D. Williams and W. H. Weinberg 
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 
91125 
(Received 11 September 1981; accepted 1 October 1981) 
PACS numbers: 82.65.Nz, 6l.l4.Hg, 68.20. + t, 61.70.Wp 
Upon adsorption on the basal plane of ruthenium, CO forms 
a (v'3 X v'3)R 30" superstructure, H hereafter referred to as 
the y3 structure. Results of thermal desorption4 and in-
frared5 measurements on this system indicate that there is an 
attractive second-neighbor interaction between CO mole-
cules. This should cause the CO molecules to order into is-
lands ofthe 1/3 structure at low coverages if the temperature 
is sufficiently low. The size and mechanism of formation of 
islands has been studied for this system using low-energy 
electron diffraction (LEED). Measurements of the tempera-
ture and coverage dependence of the v'3 beam profile have 
been performed using a standard Varian LEED system, 
modified by the installation of a channel electron multiplier 
into the Faraday cup.6 Electron multiplication in the detec-
534 J. Vac. Sci. Techno!., 20(3), March 1982 0022-5355/82/030534·02$01.00 @ 1982 American Vacuum Society 534 
535 E. D. Williams and W. H. Weinberg: Summary Abstract: Co on Ru(001): formation and dissolution 535 
tor allowed the use of low incident beam currents ( 1-10 nA) 
which prevented significant electron beam damage7 during 
the course of the measurements. 
At 100 K, the v3 structure is observed easily with LEED 
at fractional coverages as low as $· (The optimum coverage 
for the y3 structure is()= U This confirms the prediction 
of island formation. Ifthere were no attractive second-neigh-
bor interaction causing island formation, no ordered 113 
structure would form below a coverage of approximately 
() = 0.28 at any temperature. x.9 The coverage dependence of 
the island size was monitored by measuring the width of the 
!13 beam profile for five coverages between () = 0.10 and 
e =$at 100 K. The results are shown in the upper curve of 
Fig. l. The true widths, shown in the lower curve of Fig. l, 
were determined by a Fourier transform deconvolution of 
the measured profiles with the instrument response func-
tion. 10• 11 There is a strong increase in the full width at half 
maximum (FWHM) with decreasing coverage, indicating 
the formation of smaller islands at lower coverages. This 
rapid increase in the FWHM is inconsistent with a strict 
step-iimited model of island formation, for which there 
should be a linear change in island size with coverage. To 
generate the coverage dependence shown in Fig. 1, there 
must be increasing numbers ofislands at low coverages. This 
indicates that other limitations on surface mobility, in addi-
tion to step edges, constrain island formation, causing there 
to be more than one island per terrace at low coverages. 
The temperature dependence of the y'3 beam profiles at 
three coverages is shown in Fig. 2. As indicated by the ar-
rows, at 8 = 0.14 and 8 =ian abrupt onset of beam broad-
ening is observed after the intensity has decreased by ap-
proximately 20%. This delayed onset of beam broadening 
with respect to intensity decrease is a result of the distribu-
I 
~}' 
I I I 
'OCJK 
cJ.o:z - \ 
\ 
\ 
0.011) - o} 
"l hi 
~<1 O.iJOfJ I - ~~ 
::~ ~~ ' --------~ - .. --<\~ T 
~; 0.006 1\ r 
r -
'"-
0.004 1- t~f~ -- -- --- 1 -
--- ·r-0.00? 
l. 
I I I 
0.05 ~-).10 0.\5 0.20 0.2~~ 0.30 
e 
FIG. 1. Full width at half maximum ofthe V"3 LEED beam as a function of 
coverage of CO. Circles 0 are measured values. Error bars are the standard 
deviation from multiple measurements. Triangles /'::, are the true widths 
obtained from the measured values by deconvolution of the instrumental 
width. Dashed lines are empirical. 
J. Vac. Sci. Techno!., Vol. 20, No.3, March 1982 
"D 
~ 0.6 r--
CJ 
E 
2 0.5 
:;;; 
"if) 
2 0.4 
c: 
0.? -
0.1 -
0 
0 
0 
0 
0 
X 
0 
0 
0 
0 
0 
0 
0 
X 
B o().l4 · () 
T,K 
0 
0 
0 
X 
X 
FIG. 2. Variation of intensity with temperature at () = ~, triangles; () = ~' 
X 's; and 8 = 0. 14, circles. Each curve ha,; b<.-en normalized to unity at 100 
K. Arrows indicate the onset of beam broadening at () = f, and 0.!4. The 
solid curve is the intensity decrease calculated for the frustrated transla-
tional mode of CO parallel to the surface with a frequency of 45 em · 1• 
tion of islahd sizes. Although ma.ny different size distribu-
tions are consistent with the measured FWHM's, the ob-
served intensity-width behavior can occur with only one size 
distribution. 11 This size distribution was determined by 
modeling the equilibrium between the ordered phase (is-
lands) and the disordered phase. Once the size distribution 
was found, it was used in conjunction with the measured 
FWHM's to calculate the mean island size as a function of 
coverage. At (} = ;;, the mean number of CO molecules per 
island is 1 GOO; it is 500 at (} = 0.14, 300 at e = 0.12, and 160 
at (J = 0.10. 
It is not possible to make a determination of the island size 
from the width of a diffracted beam profile unless the size 
distribution is known by some other method. For CO on 
Ru(OO 1 ), the temperature dependence of the disordering has 
allowed a determination of the size distribution and thus the 
island sizes. In addition, the coverage dependence of the 
FWHM indicates that island formation is not strictly step 
limited, i.e., there is more than one island per terrace at low 
coverages for CO on Ru(OO l ). 
Acknowledgment: This research was supported by the 
Army Research Office Under Grant No. DAAG29-79-C-
0132. 


CO on Ru(001): Formation and dissolution of islands of CO at low coverages

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CO on Ru(001): Formation and dissolution of islands of CO at low coverages

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