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Israel Journal of Chemistry Vol. 45 2005 pp. 97–109

*Author to whom correspondence should be addressed. E-mail:[email protected]

Photochemistry of Molecules at Confined Environment:CD3Br/O/Ru(001) and CO2@Ice

RAPHAEL BERGER,a YIGAL LILACH,b YOUSIF AYOUB,a AND MICHA ASSCHERa,*aDepartment of Physical Chemistry and the Farkas Center for Light-Induced Processes, The Hebrew University of Jerusalem,

Jerusalem 91904, IsraelbDepartment of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, California 93106, USA

(Received 14 May 2004 and in revised form 17 June 2004)

Abstract. The photochemistry of molecules constrained within a confiningenvironment on surfaces has been studied. Orientation of methyl bromide could becontrolled methyl down or up by varying the pre-adsorbed oxygen coverage due toelectrostatic interactions on Ru(001) under UHV conditions. Irradiation of thecoadsorption system at 193 nm has shown that the resulting photochemical activityis sensitive to the molecular orientation. Photodesorption and dissociation crosssections were 1.0⋅10–19 cm–2 for methyl-down and 3.0⋅10–19 cm–2 for the methyl-upconfigurations. This observation represents the first report of the steric effect inelectron–molecule interaction due to the dissociative electron attachment mecha-nism of photochemical processes on surfaces.

A second system of CO2 molecules caged within ice has also been studied. Here

the trapped carbon dioxide molecules cannot leave the surface at their normaldesorption temperature near 100 K, but are explosively desorbing at the onset of iceevaporation near 165 K. Upon UV irradiation, enhanced dissociation to adsorbedCO and oxygen is recorded. In addition, a new reactivity channel is observed to formH

2CO, tentatively identified as formaldehyde. The relevance of photochemistry of

caged molecules within ice to interstellar hydrocarbon formation as a possible routefor the origin of life is discussed.

1. INTRODUCTIONChemical reactivity of molecules at constrained envi-ronment has been the focus of extensive research inrecent years. The role of solid and liquid surroundings inorienting molecules, restricting some reactivity chan-nels while enhancing others, is the basis for selectivitythat is particularly important in heterogeneous catalysisand biology. Proper mutual orientation of reactants hasbeen realized to be almost as important as energeticrequirements for a successful reactive encounter. Oneexperimental approach to the study of molecular orien-tation-dependent reactivity has been via molecularbeam studies in the gas phase, a focus of interest morethan a decade ago. In these studies, electrostatic hexa-pole technology was employed to orient gas phase mol-ecules in a beam and then let them collide with alkalimetals1–4 and other electron donor (reducing) atoms.5 In

cases such as those of methyl halides, the halogen sidewas found to be more reactive than the methyl tail.2,5

Electron–molecule interactions (electron transferprocesses) have been the focus of basic science interestin both chemistry and biology for decades. Studies inthis field have included inter- and intra-molecular elec-tron transfer,6 transmission7–9 conduction within andthrough molecular layers,10–11 and dissociative electronattachment (DEA) processes.12–14 Recent interest in mo-lecular electronics has brought the basic science in thisfield closer to modern technology than ever before.8–10

Based on the extensive research discussed above, it isinteresting and surprising to note that in spite of thetremendous importance of electron-induced processesin chemical and biological systems, previous studies

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have not attempted to directly investigate molecularorientation dependence or stereochemistry in electron–molecule interactions.

Here we introduce and demonstrate the effect ofmolecular orientation on electron–molecule interaction.Specifically, by tuning the oxygen coverage on ruthe-nium, the orientation of methyl bromide could be con-trolled bromine down or methyl down on O/Ru(001).Tuned electrostatic interactions by the surrounding oxy-gen-covered surface were used to orient adsorbed mol-ecules and consequently affect their (photo) reactivity.Similar behavior was reported in the co-adsorption sys-tem of potassium with water on Pt(111).15,16 The otherexample of the effect of surrounding molecules on(photo) reactivity will be that of caged CO2 withinamorphous solid water on clean Ru(001) – CO2@Ice.

2. EXPERIMENTAL

The photochemistry experiments described below were per-formed in an ultra high vacuum (UHV) apparatus, describedelsewhere in detail.19 It is equipped with LEED/Auger, a qua-drupole mass spectrometer (QMS) for temperature pro-grammed desorption (TPD) and for real-time detection ofdesorbing molecules during laser irradiation and electronbombardment. Adsorbates orientation could be determined bymeans of a Kelvin probe, operated in a differential ∆ϕ-TPDmode, as explained elsewhere in detail.19 A mini excimer laserwas operated at 193 nm, 1 mJ/pulse, 10-ns pulse duration at avariable repetition rate up to 100 Hz. Low energy electrons at10 eV kinetic energy bombarded the adsorbate system via aretarding grid that decreased the electrons’ energy from 100eV at a typical sample current of 0.1 µA. The Ru(001) sample,oriented to within 0.1 degree of the (001) plane, could becooled down to 82 K by pumping over a liquid nitrogenreservoir attached to the sample, heated to 1650 K for anneal-ing, temperature stabilized to 0.5 degrees, or ramp its tempera-ture via a resistive heating ac coupled routine. The temperaturewas monitored by means of a W5Re/W26Re thermocouple.

3. ADSORPTION OF CD3BR ON O/RU(001)

We found that by tuning the oxygen density above0.25 ML, the orientation of adsorbed methyl bromidecould be controlled bromine down or methyl down.Before discussing the interactions between CD3Br andadsorbed oxygen, the adsorption of oxygen on Ru(001)is briefly reviewed.

3.1 O/Ru(001)Oxygen dissociates on the ruthenium surface, and

forms a strong chemical bond with the metal, as indi-cated by its high temperature of desorption (Fig. 1b).Since adsorbed oxygen hinders further oxygen dissocia-tion, the sticking probability of oxygen decreases rap-idly above 0.5 ML coverage. The plot of coverage vs.

exposure in Fig. 1a indicates that at 5 Langmuirexposure the sticking probability (obtained by differen-tiating the curve in Fig. 1a) is 6 times lower than theinitial sticking probability. Exposure to thousands ofLangmuirs at 600 K is required in order to achieve amonolayer of oxygen.17,18

When an oxygen-covered surface with a coverage of0.1–0.25 ML is annealed above 300 K, it forms anordered (2 × 2)O-Ru(001) structure, which can be ob-served in LEED and was confirmed by STM measure-ments. Our LEED apparatus is rather insensitive to thecoverage in this case (we are not able to monitor the spotintensity), since already at 0.1 ML coverage, smallislands with the (2 × 2)O structure are formed. Above0.25 ML coverage, domains of (2 × 1)O-Ru(001) areformed,20, 21 but this structure results in a LEED patternsimilar to the (2 × 2)O because the domains do not have

Fig. 1. (a) Integrated area under O2 TPD peaks vs. exposure,indicating the diminishing sticking probability above 5 L. (b)∆P-TPD of O/Ru(001) following adsorption of 1.5–25 L oxy-gen on Ru(001) at 600 K. (c) ∆φ vs. oxygen exposure atsurface temperatures between 100 K and 600 K. At tempera-tures above 300 K, two distinct slopes are visible, before andafter the completion of the ordered (2 × 2)O layer structure.

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the same orientation. Therefore, another method wasneeded in order to calibrate the coverage.

Figure 1c presents the measured work functionchange during adsorption of oxygen at different tem-peratures. Above a sample temperature of 300 K at anexposure of about 2 L, a distinct change in slope isobserved. This change was associated with the satura-tion of the (2 × 2)O structure by Madey et al.,22 but thenit was believed to occur at 0.5 ML coverage. There is nosuch change at this exposure of the sticking probability(Fig. 1a), hence this change in slope is due to a 3 timeslarger dipole moment of the above-(2 × 2)O-adsorbedoxygen. This change in dipole is probably due to thesignificant structure changes in the ruthenium top layersdue to the adsorption of the first 0.25 ML of oxygen.20, 21

These changes in structure also change the surface’sreactivity, as will be discussed below.

We used the break in the ∆φ slope as our coveragecalibration point, with 2 ± 0.2 L exposure resulting in0.25 ML coverage, and the TPD peak area plot as cali-bration to other oxygen exposures at 600 K.

3.2 The Interaction between CD3Br and Oxygen on

Ru(001)Methyl bromide thermally dissociates on a clean

Ru(001),19 which results in adsorbed D atoms on thesurface; thus the desorption of D2 following CD3Bradsorption can be used as an indication of its dissocia-tion. D2 desorption in such an experiment, while gradu-ally approaching an oxygen precoverage of (2 × 2)O, isshown in Fig. 2. A coverage of 0.25 ML oxygen isenough to passivate the ruthenium surface and block thethermal dissociation channel of CD3Br.

Gradually increasing the annealed oxygen pre-cover-age above 0.25 ML and then covering it by 1.5 ML of

CD3Br results in the ∆P-TPD and differential (d(∆φ)/dT) ∆Φ-TPD spectra shown in Figs. 3 and 4. Threedistinct peaks appear, denoted β1, β2, and α. The rela-tively high temperature of the β1 peak and its width werefound by line shape analysis (not shown) to imply abinding energy of 17.8 kcal/mol of CD3Br molecules tothe partly oxygen-covered surface, and a significantrepulsive interaction among the adsorbates. The ∆Φ-TPD spectrum indicates that the β1 molecules areadsorbed with their bromine end down towards thesurface. The α peak at 120 K is due to molecules desorb-ing from the second layer, at a binding energy of7.7 kcal/mol. As previously discussed in detail,19 thesecond layer of CD3Br on the clean Ru(001) is adsorbedwith its methyl end down. Increasing the oxygen cover-age above 0.25 ML, towards local (2 × 1)O-Ru(001)

Fig. 2. ∆P-TPD of D2 recombinative desorption followingadsorption of 2 ML CD3Br on different pre-coverages ofannealed oxygen.

Fig. 3. (a) ∆P-TPD of 1.5 ML of CD3Br on different oxygenpre-coverages over Ru(001). The heating rate was 2 K/s. Asthe oxygen coverage increases, the CD3Br molecules at the β1

are shifted to the β2 site. For comparison, desorption of 1 MLof CD3Br from the clean surface is shown as the dashed line.(b) The amount of CD3Br at the β1 and β2 sites as a function ofoxygen coverage. The total coverage is constant.


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domains, creates a new adsorption site, β2, which des-orbs at 160 K (binding energy of 9.6 kcal/mol).

The total peak area of the methyl bromide ∆P-TPDspectra is invariant with oxygen coverage (indicatingthat the sticking probability of methyl bromide is in-sensitive to the oxygen coverage), as is the area ofβ1 + β2, to within 5% of our experimental uncertainty.The appearance of the β2 molecules suggests that as theoxygen precoverage increases, CD3Br molecules aredisplaced from the β1 adsorption sites into the β2 sites.The interesting observation in Fig. 4 is that the orienta-tion of the β2 molecules is sensitive to the local surfacedensity of adsorbed oxygen. This is inferred from theopposite sign of the relevant ∆Φ-TPD peak at 160 K.This is a justified interpretation of the work functionchange spectrum as long as no molecular dissociationtakes place at this temperature. In fact, work functionchange measurements in the differential mode are ex-pected to be more sensitive and relevant for the determi-nation of overall molecular orientation with respect tothe surface than, e.g., IR measurements. At oxygencoverages above 0.25 ML the β2 molecules are arrangedwith their methyl down, while at 0.6 ML oxygen theyare bromine down.

These orientation changes do not occur due to theheating ramp of the TPD, since the same orientationswere observed by monitoring ∆Φ changes upon adsorp-tion at 80 K (not shown here). As mentioned above, thefirst layer of CD3Br on the clean Ru(001) adsorbs in abromine-down orientation, which results in a decrease

of ∆φ of about –2eV. When adsorbed on 0.6 ML ofoxygen, two negative slopes are apparent, consistentwith population of the β1 (up to 0.6 L exposure) andsubsequently the β2 sites with bromine-down molecules.Adsorption of CD3Br on 0.35 ML of oxygen results in adecrease of ∆φ up to exposure of 1.4 L (bromine-downβ1), then an increase in ∆φ (methyl-down β2).

A qualitative explanation for the flip in β2 moleculesis based on two effects: Local electrostatic interactionsof the methyl bromide molecules with nearby adsorbedoxygen, combined with strong dipole–dipole interactionamong neighboring β1 molecules. At an oxygen pre-coverage of slightly more than 0.25 ML, when a β1

molecule is displaced to a β2 site, it becomes moreweakly bound to the metal (desorption at 160 K insteadof 240 K), and it is surrounded by neighboring β1 mol-ecules, aligned bromine-down on average. These β1

neighbors induce the β2 molecules to flip to the methyl-down configuration and lower the total energy of thesystem via attractive dipole–dipole attraction.19 A simi-lar hypothesis was proposed to explain the work func-tion change data obtained from the coadsorption systemof K + H2O.15,16 As the oxygen coverage increases, thereare on average fewer β1 neighbor molecules for each β2

site, hence the β2 molecules no longer flip. As a resultof the presence of methyl-down molecules in the β2

sites, bromine-down molecules are found in the secondlayer, as seen in the differential ∆Φ-TPD near 120 K(Fig. 4). The α peak in the differential work functionspectrum is split between methyl-down (negative peak)and bromine-down (positive peak) orientations. Athigher oxygen coverages, when no flipped β2 exist, thisα peak splitting is not detected anymore. These consid-erations are summarized in Fig. 5, as a schematic repre-sentation of the main interactions between the adsorbedmolecules.

The number of oxygen atoms needed to shift oneCD3Br molecule from a β1 to a β2 site can be estimatedby plotting the total area of the β1 peak vs. the totaloxygen coverage (Fig. 3b). This plot indicates that ad-sorption of 0.75 ± 0.05 ML of oxygen should shift all ofthe CD3Br molecules. It was estimated that a full mono-layer of CD3Br on the clean Ru(001) surface consists of3.6 ± 0.3 × 1014 molecules/cm2, which is about a quarterof the density of the ruthenium atoms. (1 ML of oxygenhas the same surface density as the Ru(001).21) There-fore, while keeping 1 ML defined by the Ru(001) den-sity, about a 0.5 ML of oxygen added to the 0.25 MLthat forms the (2 × 2)O structure shifts 0.25 ML ofCD3Br, suggesting that 2 oxygen atoms are necessary toshift 1 CD3Br to a β2 site. Due to the small number ofexperimental points, and the fact that the surface densityof CD3Br on the oxygen-covered surface may not be the

Fig. 4. Differential ∆Φ-TPD of 1.5 ML of CD3Br on differentoxygen precoverages over Ru(001). The heating rate was2 K/s. As the oxygen coverage increases, the CD3Br moleculesat the β1 are shifted to the β2 site, and the β2 site moleculeschange their orientation from methyl-down to bromine-down.


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same as on the clean Ru(001) surface, the possibilitythat 3 oxygen atoms are needed cannot be ruled out.

Once the ability to control molecular orientation hasbeen confirmed, it becomes an ideal model system toexamine the sensitivity of electron–molecule interactionto the molecular orientation by employing photo-medi-ated and direct electron-induced desorption and disso-ciation.

4. PHOTOCHEMISTRY OF CD3BR/O/RU(001)

Differential ∆Φ-TPD spectra following 6.4 eV photonirradiation of 0.8 ML CD3Br adsorbed on 0.6 ML O/Ruare shown in Fig. 6a, for different irradiation times. Theexcimer laser power density at the sample was about1 MW/cm2 or 2.4 × 1016 photons/s, a repetition rate of50 Hz. The same experiments were performed at oxygencoverage of 0.35 ML (Fig. 6b). Both the β1 and β2

molecules undergo photoreaction, desorption, and frag-mentation, which lead to an exponential decay of therelevant differential ∆Φ-TPD peak intensity. A plot ofthe logarithm of the integrated area under the d(∆φ)/dTspectra for both peaks vs. the number of photons shouldresult in a linear plot. The initial slope of such plotsdetermines the cross section for the photoreaction. InFig. 6c such plots are presented.

The cross sections obtained this way can clearly be

separated into two groups: The removal cross section ofthe β1 molecules in both oxygen coverages and that ofthe β2 molecules on top of 0.6 ML O/Ru coverage have across section (σ) of (1.0 ± 0.2)⋅10–19 cm2. In contrast, theβ2 molecules that reside on top of 0.35 ML O/Ru, andare oriented methyl down, are removed at σ = (3.0 ±0.4)⋅10–19 cm2.

We find that molecules adsorbed at the same orienta-tion (bromine down) but at different binding sites atdifferent adsorption energies (β1 and β2 over 0.6 MLO/Ru, an 8.2 kcal/mol energy difference) possess thesame overall removal (desorption + DEA) cross section.At the same time, the more weakly bound moleculesthat are flipped to the methyl-down configuration are

Fig. 5. An illustration of the proposed interactions betweenCD3Br and O/Ru(001), explaining the orientation changes ofthe β2 molecules.

Fig. 6. ∆Φ-TPD of 0.8 ML of CD3Br adsorbed on 0.6 ML (a)and 0.35 ML (b) of oxygen, following different irradiationtimes by 193 nm UV laser irradiation. Photon flux was 2.4 ×1016 photons/s at the sample. (c) The cross sections for deple-tion of the β1 and β2 peaks for the two oxygen coveragesshown above. σBD = 1.0 ± 0.2 × 10–19 cm2 for all the CD3Brmolecules that are oriented bromine-down (both β1 and β2),and σMD = 3.0 ± 0.4 × 10–19 cm2 for the methyl-down β2

molecules.


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removed at a cross section that is three times larger. It isinteresting to note that in spite of the different electronicconfigurations of the surface dictated by different oxy-gen coverage, the molecular orientations methyl-downand bromine-down within the β2 sites are bound at ex-actly the same energy to the surface, as indicated by theTPD data in Fig. 3. This, we believe, emphasizes thedominance of the molecular orientation in determiningthe cross section for the (photo)-electron–molecule in-teraction. We cannot rule out, however, the possibilitythat the same factors that cause the molecular orienta-tion changes, namely the dipolar interactions around theβ2 site, are also contributing to the difference in the crosssection.

Orientation-dependent (photo)-electron–moleculeinteraction can be detected also by following the real-time desorption of CD3Br molecules during the UVirradiation. The QMS signal at mass 97 during continu-ous irradiation is shown in Fig. 7 (left), for the oxygen

and CD3Br coverages discussed above. The cross sec-tion is extracted from the expression I(t) = I(0)e–sFt,where I(t) is the QMS signal, F is the photon flux inphotons/cm2, and σ is the photo-desorption cross sectionin cm2. As before, only the initial 6 × 1018 photons areconsidered. This is important in order to minimize theinfluence of photo-fragments on the tail of the decaycurve.

The measured cross sections are σBr Down (σBD) = 2.0 ±0.1 × 10–19 cm2 and σMethyl Down (σMD) = 2.9 ± 0.1 × 10–19 cm2

for the 0.6 ML and 0.35 ML O/Ru, respectively.The QMS signal includes photo-desorption from

both β1 and β2 sites, but it is insensitive to photo-frag-mentation (DEA). The identical cross sections obtainedby the real-time QMS detection and the work functionchange measurements are explained assuming thatphoto-electron mediated desorption is the dominantprocess in the methyl-down configuration (0.35 MLO/Ru). Dominance of electron-mediated dissociative

Fig. 7. QMS signal at mass 97 (open circles) during irradiation of 0.8 ML CD3Br on 0.35ML O/Ru(001) (top) and 0.6 MLO/Ru(001) (bottom). Irradiation was photo-electron generated from 193 nm photons (left panels) and direct 10 eV electrons fromthe gas phase, discussed in section 5, (right panels). The initial decay was fitted to an exponent (solid line) to extract the crosssection for desorption.


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channel in the methyl-down configuration of the β2

molecules would have resulted in smaller integrateddifferential work function change near 160 K. This isdue to the negative work function change contributionof both dissociation fragments, methyl and bromine,19 ifthey adsorb on the surface. If fragments (e.g., methyl)are ejected to the gas phase, the overall work functionschange and thus the measured cross sections are againexpected to be similar to the electron-mediated parentmolecular desorption. The fact that about 15 ± 5% of theparent molecules dissociate and their methyl fragmentsstay on the surface is established by the post-irradiationTPD of D2 (and other products, see below), which is dueto several consecutive photo-generated dissociationevents of methyl and smaller fragments on the surface.

Post-irradiation stable molecular products that areobserved by TPD are CD4 (mass 20), C2D6 (mass 36), D2

(mass 4), and mass 32 (tentatively assigned as D2CObased on comparison to ref 24). The yield of these

photoproducts depends on the initial CD3Br surfacecoverage, as seen in Fig. 8.

The photoproducts that were produced after irradia-tion by 6 × 1018 193 nm photons (on a pre-coverage of0.4 ML oxygen) were determined by scanning throughmasses 2–100 au during the TPD ramp. When theCD3Br coverage is gradually increased, a signal at mass20 appears, and at higher coverage also at 32 and 36.This can be explained by considering the orientation andlocal environment of the molecules at each CD3Br cov-erage. Figure 9 shows a coverage-dependant TPD ofCD3Br on 0.4 ML oxygen, revealing the gradual popula-tion of the β1, β2, and α peaks. Up to a CD3Br coverageof 0.5 ML, only the β1 sites are populated. At coveragesbelow 1 ML both β1 and β2 sites are populated, andabove that coverage the α sites are also present. Theyield of the products after irradiation of 6 × 1018 photons(as determined by the integrated ∆P-TPD peak area inFig. 8) is presented in Fig. 9. The remaining oxygencoverage is shown as well.

The formation of C2D6 becomes possible due to thepresence of α molecules. Based on the scheme describ-ing the adsorption sites (Fig. 9), the C2D6 formation isfacilitated by the presence of neighbor methyl groups inthe first and second layers that are oriented toward eachother. The formation of CD4, produced via surfacerecombinative desorption process of CD3 + D, does notoccur before population of the β2 sites takes place.Photo-fragmentation of the β1 molecules should lead toejection to the gas phase of a methyl group, as indicatedin the scheme of Fig. 9. Similar arguments suggest thatdissociation of β2 molecules should lead to the ejectionof a bromine atom or negative ion, while the methylgroup remains on the surface. Further photo-fragmenta-tion of the methyl is the source of adsorbed D atoms,which thereafter lead to the formation of CD4 uponramping the surface temperature. Additional increase ofthe initial CD3Br coverage to populate the second layer(α) decreases CD4 production rate in spite of the factthat molecules are oriented methyl down. This is prob-ably because it is not possible for the methyl from thesecond layer to stick and adsorb on the fully covered(β1 + β2) surface.

Methyl groups that are oriented toward the surfaceand thus strike it as a result of the DEA event can alsoreact with the underlying oxygen atoms, as suggestedboth by the production of D2CO, and by the depletion ofthe surface oxygen. This is observed starting at methylbromide coverage of 0.7 ML. It is not fully understoodyet why the oxygen abstraction reactions do not startright at the population onset of the β2 site. Populating thesecond layer, however, does not seem to enhance orhinder the production of D2CO.

Fig. 8. ∆P-TPD of masses 20, 32, and 36, following adsorptionof different amounts of CD3Br on 0.4 ML oxygen pre-coveredRu(001) and irradiation by 6 × 1018 193-nm photons. TheCD3Br coverages are indicated on the graph in units of mono-layers (ML).


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4.1 Photo Hydro-Dehalogenation of CD3Br

The hydrodehalogenation25 of halogenated aliphaticcompounds is important as a way to clean hazardoussolvents in industry, such as CCl4. Both homogeneous25

and heterogeneous26 catalysis were employed forhydrodehalogenation, as was irradiation by UV light.The molecular details of these reactions are seldomunderstood. The photoreaction described above can bediscussed as a model system for a simple hydro-dehalogenation reaction28 :

CD3Br + D —hv→ CD

4 + Br

By utilizing the isothermal desorption analysismethod discussed in detail in ref 27, we were able toextract for the first time the activation energy for thefinal step in the reaction, where adsorbed methyl photo-fragments and deuterium atoms recombine and desorbas methane.

The isothermal desorption signal of CD4 followingadsorption of 0.7 ML CD3Br on 0.4 ML O/Ru andirradiation by 6 × 1018 193-nm photons is shown inFig. 10b for several temperatures. Briefly, plotting

1/ QMS vs. time for a second-order process results in astraight line (Fig. 10c) with a slope k′. This slope isproportional to the desorption rate, thus plotting ln k′ vs.1/T should result in a linear plot (Fig. 10d).

Activation energy for the recombinative desorptionreaction: CD3(ad) + D(ad) → CD4(g) (Edes ) was deter-mined from this plot to be 5.6 kcal/mol. This numbermost probably reflects the barrier for surface reaction. Incontrast, activation barriers for the diffusion of methylor deuterium fragments on a smooth surface such asRu(001) are often smaller. In other words, if this hadbeen a diffusion-limited surface reaction, it would havebeen detected at temperatures lower than 200 K.28

5. IRRADIATION BY 10 eV ELECTRONSReal time desorption was measured also for irradiationwith 10 eV electrons, as explained in the experimentalsection and demonstrated in Fig. 7 (right panels).

The cross sections for the electron-stimulated des-orption (ESD) are four orders of magnitude larger thanthose for the photon-stimulated desorption (PSD). This

Fig. 9. The yields of the photoproducts and remaining oxygen after the irradiation conditions shown in Fig. 8, as a function ofinitial CD3Br coverage. To the right: ∆P-TPD of CD3Br showing the gradual population of the β1, β2, and α peaks with a schemeof these adsorption sites.


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large difference in magnitude was reported before forother systems, e.g., NF3/Pt(111).23 As in the PSD case,different cross sections are obtained for the differentmolecular orientations. The difference, however, hasthe opposite trend, namely the removal cross section ofthe methyl-down molecules is smaller than that of thebromine-down molecules. As in the PSD case discussedabove, also here the difference in the cross section isattributed to electron–molecule steric effect, where thereaction CD3Br + e– has a larger cross section when theelectron approaches the molecule from the methyl end.

One should consider alternative mechanisms to ex-plain the enhanced probability for both photo-inducedand electron-induced desorption of methyl bromidefrom the O/Ru(001) surface when the methyl groupfaces the direction of approach of the electrons. Onesuch explanation is the enhanced quenching rate at thebromine-down configuration. Such a mechanism maybe justified if we consider that most of the negativecharge resides on the bromine side of the adsorbedmolecule following electron attachment. In this case,the larger charge density on the bromine side should

overlap higher electron density of the metal when thebromine faces the surface, since it is closer to the surfacethan when it is in the methyl-down configuration.Larger charge density overlap often results in shorterexcited-state lifetime or faster quenching back to theground state. Shorter lifetime results, therefore, in asmaller probability for electron-induced desorption, aspreliminary wave packet dynamical simulations indi-cate.29 While this argument qualitatively explains thesteric effect found in the photo-induced substrate-medi-ated desorption of methyl bromide at a photon energy of6.4 eV, it cannot simultaneously rationalize the oppositesteric effect found in the enhanced electron-stimulateddesorption data. While it will be inaccurate to com-pletely ignore the role of quenching, we tend to believethat kinematic (mass) effects lead to the larger photo-electron-induced desorption of the parent methylbromide molecule when the methyl faces the substrate’ssurface.29 The initial electron attachment step is prob-ably also sensitive to the molecular orientation dis-cussed in this work; however, its exact contribution isdifficult to estimate.

Fig. 10. Second-order isothermal desorption analysis of formation of CD4. 0.7 ML CD3Br was adsorbed on 0.4 ML O/Ru andirradiated by 6 × 1018 193-nm photons. Temperatures of 224 K, 233 K, 241 K, and 248 K are shown. An activation energy of5.6 kcal/mol was obtained by an Arrhenius plot.


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6. PHOTOCHEMISTRY OF CAGED MOLECULES:CO

2@ICE

In the previous parts of this work the effect of localelectrostatics among neighbor molecules on reorienta-tion of adsorbates has been discussed. The photochemi-cal consequences that result in the observation of stericeffect in electron–molecule interaction were presented.41

In this section we shall present another type of neighboror environment effect on the organization and chemistryof molecules on surfaces. Here, the confinement of CO2

molecules within a matrix of ice will be described,together with the resulting photo-excitation of suchsystem.

As was demonstrated in several other molecularsystems, N2@Ice,30 CCl4@Ice,31 CH3Cl@Ice,32 CO2 isalso compressed by post-adsorbed water molecules andeventually, as the water coverage increases beyond25 bilayers (BL), this molecule becomes caged withinthe ice matrix—CO2@Ice. This is demonstrated inFig. 11, where the desorption temperature of CO2 gradu-ally shifts from 100 K when adsorbed in the absence ofwater to the typical explosive desorption that peaks at165 K, 2 K in width, which coincides with the onset ofH2O desorption from the “ice” layer.

A fraction of 0.18 of the full first layer of CO2

thermally dissociates on clean Ru(001). This is deter-

mined from the ratio of the integrated TPD signal of13CO at mass 29 divided by the sum of integrated areasunder the TPD signals at masses 29 and 45 (13CO2).Cage formation decreases the dissociation probabilitydown to about half its value on the clean rutheniumsurface. This is demonstrated in Fig. 12 by the decreas-ing TPD signal at mass 29. This observation may berationalized by a shortage in vacant adsorption sitesnecessary to accommodate the dissociation productswithin the cage.

Exposing a monolayer of CO2 adsorbed on cleanRu(001) to excimer laser irradiation at 193 nm results inphoto-desorption and in enhanced dissociation. The dis-sociation channel is indicated by the gradual increase ofmass 29 TPD signal at desorption temperature near460 K, as a function of irradiation time. This is ex-plained by an indirect substrate-mediated dissociativeelectron attachment (DEA) mechanism of CO2. An ex-cited (CO2)– is momentarily formed following the laserirradiation, which subsequently dissociates to COad +Oad or it may undergo molecular desorption. The photo-desorption cross section for the parent carbon dioxide(σ = 8⋅10–21 cm2) was measured by monitoring the mass45 signal during irradiation, as shown in Fig. 13. Thecross section for the competing photo-dissociation wasdetermined to be five times smaller. Long irradiationtimes, however, result in the accumulation of more 13COproducts. 0.56 and 0.64 of the original 1 ML 13CO2 hasdissociated within the cage following 10 and 20 minutesirradiation times, respectively.

Upon irradiation of the caged CO2 molecules by the193-nm photons, however, a new reactivity channelopens up. In addition to photo-dissociation and photo-desorption of carbon dioxide, also water molecules par-tially dissociate. As a result, a new peak appears in themulti-mass TPD scan shown in Fig. 14. At mass 31, thedesorption of formaldehyde is detected as a result of hotchemistry between CO2 and water photo-fragments. It isimpossible to determine at this point whether H2

13COforms during a single excimer laser pulse or if this is theresult of accumulated photo-products. Only 0.1 of thephoto-dissociated carbon dioxide molecules undergothe reactivity channel of formaldehyde formation,which is about 0.01 of the initial parent molecule popu-lation. This product, however, is a unique consequenceof the caged CO2 in ice and cannot be formed otherwise.

7. CONCLUSIONSIn conclusion, we have discussed two cases of photo-chemistry of molecules constrained by their neighboradsorbates. Steric effect has been identified and mea-sured, for the first time, in the interaction of collimated(photo)-electrons with pre-oriented molecules. Control

Fig. 11. TPD at mass 45 demonstrating the gradual caging of13CO2 under the indicated number of water bilayers (BL).


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Fig. 13. Real-time detection and cross section extraction ofphoto-desorbing mass 45 during irradiation of 13CO2/Ru(001)system by an excimer laser at 193 nm.

Fig. 14. A TPD taken simultaneously at all masses between 0–80 amu in a 3-D representation. The blow-up reveals the signalat mass 31 (H2

13CO), which is the result of photochemistry ofCO2@Ice.

Fig. 12. TPD of 13CO as a result of thermal dissociation of 13CO2 on Ru(001). The decreasing signal is demonstrated as a functionof the water layer thickness at thick (A) and thin (B) layers.


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over the molecular orientation of adsorbed CD3Br wasachieved by varying oxygen pre-coverage on Ru(001).When a flux of photo-electrons generated at the bulkmetal strikes adsorbed methyl bromide from the methylend, the overall electron-mediated reactivity has a crosssection of σ = 3.0⋅10–19 cm2, which is three times largerthan the cross section determined for the BD-downconfiguration. Qualitatively similar results were ob-tained by direct bombardment with 10 eV electronsfrom the gas phase, but at cross sections that are fourorders of magnitude larger.

The steric effect reported here is central to our basicunderstanding of electron transfer processes, which areamong the most fundamental concepts in chemical reac-tivity.33 Many biological systems function via a cascadeof electron transfer processes, e.g., photosynthesis.Only in recent years has the detailed mutual structure ofthe complex molecular elements of photosystem-I beenscrutinized by means of X-ray diffraction analysis of therelevant proteins.34 Steric effects in such complex sys-tems, if studied, may reveal some of the evolutionarysteps that have led to a given structure over the others inplants and living organisms.

In the second example, CO2 molecules were found tobe trapped or caged within ice layers on Ru(001),CO2@Ice. Caging has decreased thermal dissociation ofcarbon dioxide on the one hand, but also has producednew photochemical products upon irradiation at 193 nm.Formaldehyde at mass 31 (H2

13CO) has been found as anew product, reported also to be formed in interstellarspace, possibly under similar caging conditions of CO2

within ice on solid grains.35–40 The study of caged mol-ecules under or within layers of ice under controlledexperiments at UHV conditions is considered as a con-venient and proper model for the formation of hydrocar-bons in interstellar space. In space, layers of ice areknown to accumulate on top of solid particles or grainstogether with other small molecules abundant in thislow-temperature environment, such as carbon dioxide,ammonia, and CO, and then are exposed to UV andVUV light.35–40 Integrated over a long time, new photo-chemical products (e.g., formaldehyde and other hydro-carbons) can be released if the grains are warmed up tothe ice desorption temperature.

Acknowledgments. This work was partially supported by agrant from the United States-Israel Binational Science Foun-dation and by the Israel Science Foundation.

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