Electron stimulated desorption of H� from thin films of 5-halouracils

Marie-Anne Herve du Penhoat,y Michael A. Huels,* Pierre Cloutier, Jean-Paul Jay-Gerin and

Leon Sanchez

Groupe des Instituts de Recherche en Sante du Canada en Sciences des Radiations,Departement de Medecine Nucleaire et de Radiobiologie, Faculte de Medecine, Universite deSherbrooke, Sherbrooke, Quebec, Canada J1H 5N4. E-mail: michael.huels@USherbrooke.ca;Fax: +1 (819) 564 5442; Tel: +1 (819) 346 1110, ext. 14907

Received 23rd December 2002, Accepted 9th June 2003First published as an Advance Article on the web 26th June 2003

We present measurements of low-energy (1–20 eV) electron stimulated desorption (ESD) of H� from thinfilms of 5-halouracils (5-XU, X ¼ F, Cl, Br and I) condensed on polycrystalline Pt. The onset for H�

desorption is �5.0 eV. For all 5-XU a single dissociative electron attachment (DEA) H� peak is observed,which is attributed to at least two dissociation channels at approximately 7.1 and 8.6 eV, resulting from N–Hand C–H bond cleavage, respectively. Their relative contribution is parent-molecule dependent. Also, for agiven molecule, these dissociation channels are affected differently by the metal, and the DEA maximum shiftsby �0.5 eV as the film thickness increases. Above 10 eV, dipolar dissociation (DD) is the dominant H�

formation mechanism. The yields of H� vary in the following relative order (T ¼ thymine, U ¼ uracil):T > U > 5-FU > 5-ClU > 5-BrU > 5-IU. A model is developed to explain the film thickness dependence ofthe desorbed anion signal: above a coverage of �1 monolayer (ML), the film thickness dependence of the H�

ESD yield is interpreted in terms of incident electron and desorbing ion transmission within the films. The maindifference in the thickness dependence of H� produced via DEA and DD is attributed to the fact that electrontransfer to the metal from the transient molecular anion leads to dramatic decrease in the H� ESD signal below�1 ML, whereas it does not affect significantly DD, since its intermediate state is neutral.

Introduction

The interest in halouracil compounds, such as 5-bromouracil(5-BrU) and 5-iodouracil (5-IU), has been inspired by theirradiosensitization properties, which have great potential forthe therapy of cancer.1,2 The structural similarity of 5-halour-acils (5-XU, X ¼ F, Cl, Br and I) and thymine (T) allows forin vivo incorporation of the former (except for 5-FU) intoDNA instead of T.1 Such modified DNA molecules experienceenhanced radiation damage, which results in increased celldeath.Despite their promising potential for clinical applications,

the precise mechanism(s) of radiosensitization by 5-halouracilsare not yet fully understood, and thus their routine use as clin-ical sensitizing agents has proven to be elusive. Traditionally,it has been proposed that the hydrated electrons which areproduced by water radiolysis in irradiated biological media,may dissociate 5-BrU into Br�+U-yl, and that the very reac-tive uracil-5-yl (U-yl) radicals subsequently cause multiplydamaged sites in sensitized cellular DNA.3 Recently, X-rayphotoelectron spectroscopy (XPS) experiments in the con-densed phase4 have shown that near thermal electrons andthose produced by soft X-rays can both induce dissociationof 5-BrU and 5-IU into a halogen anion and a uracil-yl radical.These findings indirectly imply that not only hydratedelectrons contribute to 5-BrU dissociation but possibly alsothe majority of the abundant secondary electrons createdalong radiation tracks which have initial kinetic energies below20 eV.5

From a number of previous studies,6 it is well establishedthat the basic mechanisms responsible for the formation ofnegative ions following low-energy electron impact on thinmolecular films can be related to elementary processes knownfrom gas-phase studies, namely dissociative electron attach-ment (DEA) and dipolar dissociation (DD).7 In the first pro-cess, a transient molecular anion (TMA) M*� is formed byresonant electron capture, which may then dissociate into anegative ion and neutral fragments:

e� þM ! M�� ! RþX�; ð1aÞ

where the asterisk indicates that the TMA may also be electro-nically excited. Alternatively, the TMA may decay via electronautodetachment, i.e.,

M�� ! M� þ e�; ð1bÞ

leaving a vibrationally excited neutral molecule (which mayalso be electronically excited) and an electron with a loweredkinetic energy. The branching ratio between dissociation andelectron autodetachment depends in part on the intrinsic para-meters of the TMA, e.g., electron autodetachment lifetime,position and shape of the TMA in the autodetaching region,all of which are likely to be influenced by the local molecularenvironment in the solid or liquid phases. In any case, the elec-tron energy dependence for electron attachment and DEA isdefined by a peaked signature in the fragmentation or excita-tion yields.DD occurs when the impinging electron creates an electroni-

cally excited state of the neutral molecule, which then dissoci-ates into a positive and a negative ion fragment, i.e.,

e� þM ! M� þ e� ! Rþ þX� þ e�; ð2Þ

y Present address: Groupe de Physique des Solides, Tour 23,Universite Paris VII, 2 Place Jussieu, 75251 Paris Cedex 05, France.z Canada Research Chair in the Radiation Sciences.

3270 Phys. Chem. Chem. Phys., 2003, 5, 3270–3277 DOI: 10.1039/b212552h

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This non-resonant mechanism, which involves a very specificelectronic excitation to a dissociative ‘‘ ionic ’’ state of the neu-tral molecule, usually occurs at electron energies slightly abovethe lowest ionization potential of the target molecule, i.e., nearand above 10–15 eV for most hydrocarbons in the gas orcondensed phases. Clearly, due to the unique dipolar (ionic)nature of the dissociative excited state M*, DD will likelyinvolve cleavage of only one particular bond, yielding an ionpair R++X� in a ‘‘diatomic-like ’’ dissociation process. Mostimportantly, when DD occurs via the direct excitation in eqn.(2), its electron energy dependence is characterized by a thresh-old above which the fragmentation yield is increasing monoto-nically. This is because unlike DEA, which requires a specificFranck–Condon overlap between the neutral and anion statefor the initial electron capture, and is allowed only for a rangeof specific electron energies for a given TMA, non-resonantdirect excitations of the neutral (or its cation) are allowed atany energy above threshold in the Franck–Condon region,and include the manifold of continuum states for each specificexcitation. However, if the dissociative state M* in eqn. (2) isnot formed by a direct excitation, but is rather the result of thedecay of a high lying autodetaching M*� state (see eqn. (1b)),the electron energy dependent ion pair yield signature willcontain some contributions from the initial resonance. Due tothe different fundamental nature of these two mechanisms, i.e.DEA and DD, it is important to study the various fragmen-tation pathways induced by low-energy electron impact onthymine, uracil and its 5-halogen-substituted derivatives, inorder to unravel the different basic processes involved in radio-sensitization of DNA by 5-halouracils. The need to understandthese basic mechanisms is furthermore underscored by theobservations that (a) single- and double-strand breaks inducedin unsensitized plasmid DNA by 3–15 eV electron impact8

were recently shown to be initiated by resonant electronattachment, and (b) substitution of thymidine by bromode-oxyuridine in short single-stranded oligonucleotides9 has beenshown to increase the neutral fragment yields by as much as afactor of 3 during 1–25 eV electron impact.Consequently, we have undertaken a series of gas- and

condensed-phase studies concerning electron-induced fragmen-tation of pyrimidine bases and their halo-derivatives. In gas-phase thymine,10 at least seven resonant dissociation pathwayshave been observed with onsets as low as 2.0 eV and reso-nances below 8.7 eV, yielding a wide variety of fragments:H�, O�, CH2

�, OCN�, CN�, OCNH� and OCNH2�. Except

for the last two fragments, those anions are also observed todesorb from electron-irradiated solid thymine films.11 How-ever, in the condensed phase, the onset for anionic fragmentdesorption is �5 eV, where numerous resonances take placein the gas phase. Gas-12,13 and condensed-phase14 fragmenta-tion of 5-BrU has also been reported, where it was found thatnew dissociative pathways, not observed in the gas phase,may occur in the condensed phase. In particular, H�, the mostintense anionic fragment measured in the condensed phase, isnot detected in the gas phase.In this paper, we present condensed-phase measurements of

electron-stimulated desorption (ESD) of H� fragments fromthin films of 5-XU (X ¼ F, Cl, Br and I) in the 0–20 eV inci-dent electron energy range. The results will be compared tothose from gas-phase 5-XU,12,13 in order to understand theeffect of the environment on DEA to 5-halouracils. Finally,the present results will be compared to those of thymine anduracil15 in the condensed phase, in an effort to establish howthis dissociation channel is affected by the different functionalgroups in the C5 position (see Fig. 1, insets).

Experimental method

The apparatus used for the condensed-phase studies of ESDyields of anionic fragments from 5-halouracils has been

described in detal elsewhere.15 Briefly, the 5-XU (X ¼ F, Cl,Br or I) powder, purchased from Sigma Chemicals, is loadedinto a miniature oven contained in a load-lock chamber, whichis then pumped to a base pressure of about 5� 10�9 Torr witha 250 l s�1 oil-free turbomolecular drag pump station. There,the compound is degassed by heating for several hours belowthe evaporation onset, as determined by recording the partialpressure of 5-XU by a residual gas analyzer as a function ofoven temperature. To reduce the possibility of thermal decom-position, film sublimation is carried out at the coolest possibletemperature compatible with evaporation times of a few min-utes for the thickest films of about 10 monolayers (ML). Oncethe oven has reached this temperature (in the present work,103, 105, 110 and 115 �C for 5-FU, 5-BrU, 5-ClU and 5-IU,respectively), it is introduced into the main chamber which isheld at a base pressure of 3� 10�10 Torr by a 400 liter/s ionpump. 5-halouracil films are then deposited by vacuum subli-mation onto a polycrystalline Pt substrate, held at room tem-perature, which is mounted on a rotatable sample holder. Thethickness of the film is then proportional to the time of deposi-tion (tD).

15 The Pt substrate is cleaned prior to each depositionby resistive heating. XPS4 and thin film chromatography16 ofthe substances evaporated indicate that the films consist ofintact molecules.A modified ELG-2 electron gun (Kimball Physics Inc.)

allows irradiation of the sample films at incident electron ener-gies between 1 and 40 eV, with a 0.5 eV full width at half max-imum (FWHM) resolution at typical beam currents between 1nA and 0.5 mA. The beam diameter is about 3 mm and the elec-tron gun axis lies at an angle of �80� from the sample surfacenormal. The electron beam current at the platinum substrateis measured by a Keithley electrometer. A high resolution[(m/Dm) ¼ 400, 1–120 u and 1–500 u] ABB Extrel QuadrupoleMass Spectrometer (QMS), positioned with its axis normalto the film surface, simultaneously measures ESD yields of

Fig. 1 Transmission of the 19 eV incident electron beam through (a)uracil, (b) thymine, (c) 5-FU, (d) 5-ClU, (e) 5-BrU and (f) 5-IU con-densed films on a polycrystalline Pt substrate as a function of timeof deposition (tD). Each data point corresponds to a new film. Thesolid lines are fits to the experimental data according to eqn. (3).

Phys. Chem. Chem. Phys., 2003, 5, 3270–3277 3271

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anionic fragments from the condensed-phase target molecules.To give the electron beam access to the sample, smaller customelectrostatic lenses have been added to the QMS.In the present work, ion yields are given in counts s�1 nA�1

(cps nA�1) to enable comparison between experiments per-formed for different electron beam intensities. Thus, priorand after each measurement on a new film, the current incidenton the clean Pt substrate upon 19-eV electron bombardment(I0) is recorded. The signal from the QMS (in counts s�1) isthen divided by I0 . This calibration requires that the electronbeam current be constant at all energies; it is valid within a5% accuracy down to 4 eV. Below this energy the electronbeam current is no longer constant with energy and decreaseswith decreasing energy.Due to occasional clogging of the oven exit apperture during

evaporation of 5-IU, as well as temperature variations(usually, �1 �C), small errors in the ratio of the depositiontime, tD , to ‘‘ real ’’ film thickness may occur.15 For this reason,the transmission of the 19-eV electron beam (Tf ¼ If/I0 , whereIf is the electron current transmitted through the molecularfilm to the platinum substrate) is measured for each film.Fig. 1 shows the film transmission Tf plotted as a function oftD for uracil, thymine and 5-XU (X ¼ F, Cl, Br and I). Inthe case of 5-halouracils, the experimental data can be fittedto exponential decay curves (solid lines in Fig. 1):

T f ¼ 1þ T1½expð�tD=t1Þ � 1� þ T2½expð�tD=t2Þ � 1� ¼ f ðtDÞ;ð3Þ

where T1 , T2 , t1 and t2 are adjustable parameters (see Table 1).Except for 5-FU, all data were fitted with a single exponent.The deposition time for each film is then obtained viatD ¼ f�1(Tf). This method was not applied in previous experi-ments on 5-BrU,14 thymine or uracil15 films. It is useful todeduce intermediate film thicknesses, however, at large filmthicknesses, the transmission varies very slowly with film thick-ness, such that slight uncertainties in the transmission will leadto large variations of the estimated film thickness. As a result,the 0.6% uncertainty in film transmission measurements willlead to 5–10% uncertainties in the thickness determinationbetween 0.5 and 4 ML, and up to 20% at 5 ML in the caseof 5-ClU. Uncertainties are also more important for extremelythin films, i.e. up to 20% at 0.2 ML. In the case of 5-IU, uncer-tainties are always smaller than 15%. Finally, the values for tDthus calculated with eqn. (3) were compared to the experimen-tal times of deposition. In the case of 5-FU, calculated valuesfor tD are 4% above the measured values with a standarddeviation of 25%. In the case of 5-ClU and 5-IU, the calculatedover experimental ratio for tD decreases with thicknessbetween about 1.3 to 0.7 and 1.2 to 0.9, respectively, so thatsome distortion of the thickness scale might have been intro-duced. Also, this ratio is shifted by +0.4 for 5-IU in the wholethickness range, which is attributed to the gradual clogging ofthe oven aperture throughout the study on 5-IU. It should benoted that the transmission thickness dependence of 5-ClU canalso be fitted by two exponents. The ratios of calculated to

experimental values for tD are then randomly distributed asa function of thickness around an average of 1.0 with a stan-dard deviation of 9%. It thus appears that no distortion onthe thickness scale is introduced in that case.Determinations of the relative film thicknesses are accessible

through tD . However, in order to obtain absolute values (inunits of ML), a conversion factor must be determined for eachof the molecules studied. As will be shown for brevity else-where,17 for 5-FU the absolute calibration of film thicknessis based on the observation that the signal of F� ions producedby ESD under 15-eV electron impact is mainly the result ofnonresonant DD, and exhibits a maximum at low thickness(deposition time: 13 s), which corresponds to about 1 ML.17

This is due to the fact that, at small film thicknesses, the effectsof a metal substrate on the neutral excited intermediate stateresulting in DD at higher energies are different than on theTMA resulting in DEA at lower energies. The following thick-ness calibration procedure was then adopted for the othermolecules: the H� ESD yield at the DEA peak is recorded asa function of tD . Assuming that the thickness dependence ofthese curves is the same for all 5-XU, the deposition times(tD) corresponding to the formation of 1 ML of 5-XU(X ¼ Cl, Br and I) are then equal to:

tDð5-XUÞ ¼ tDð5-FUÞ dð5-FUÞdð5-XUÞ

where d is a parameter (defined in the next section) which takesinto account both the electron and H� attenuation lengthswithin the film, which are assumed to be identical for all5-XU. Values of tD thus determined are 33� 12, 40� 10 and33� 8 s for 5-ClU, 5-BrU and 5-IU, respectively. It shouldbe mentioned that tD(5-ClU) is equal to 41� 13 s if the abovementioned two-exponent fit for the transmission thicknessdependence of 5-ClU is used to reassess deposition times.The uncertainties reported here (25–35%) are related to uncer-tainties in the determination of the parameter d. They onlyrepresent relative thickness uncertainties when comparingresults from two different molecules in the present article. Thisfactor should not be taken into account when comparingresults for the same molecule. However, when comparing pre-sent results to previous measurements on thymine or uracil,one should also take into account a factor 2, due to the thick-ness calibration of the present 5-FU condensed films.

Results and discussion

Fig. 2 shows the ESD yield of H� from 5-XU/Pt films (X ¼ F,Cl, Br and I) as a function of incident electron energy in the4–19 eV range for different film thicknesses. As discussed in theintroduction, only two processes yield negative ion desorptionfrom molecular films upon electron impact: DEA, which isknown to result in pronounced anion ESD peaks (see also refs.7 and 8, and work cited therein) usually at low electron ener-gies (here below 10 eV), and direct DD, which is always asso-ciated with a continuous increase of ESD anion signal abovethe energetic threshold of reaction (2) (here generally above10 eV). The onset for H� desorption occurs around 5 eV forall molecules, independent of film thickness. A single peakis observed at low film thickness around 7.3 eV (5-FU), 8.0eV (5-IU), 8.5 eV (5-ClU) and 8.6 eV (5-BrU) for the indicatedfilms respectively, indicating that DEA is responsible for theanion signal. The peak position shifts by about 0.5 eV tohigher incident energies as the thickness of the film increases.For all 5-halouracils, the signal starts to increase again above10 eV as DD becomes energetically allowed. In the case of theH� ESD yields for 5-ClU and 5-BrU, a weak structure appearsaround 20 eV (data not shown here) which is strongly depen-dent on film thickness and thus likely relates to multiple

Table 1 Fitting parameters (T1 , T2 , t1 and t2) used in eqn. (3) to

model the film transmission Tf as a function of deposition time tD(expressed in seconds or in equivalent monolayers). The solid curves

in Fig. 1c–f are the resulting fits of eqn. (3) to the experimental data

5-Halouracil T1 T2

t1 t2

/s /ML /s /ML

5-FU 0.20� 0.04 0.42� 0.03 2.3� 1.6 0.2 61� 13 4.7

5-ClU 0.57 — 39� 1 1.5 — —

5-BrU 0.55 — 77� 1 2.0 — —

5-IU 0.59 — 78� 3 2.4 — —

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scattering electron energy loss within the film, followed byDEA at the lower electron energy.In the gas phase,12,13 the formation of H� is not observed

for 5-halouracils. The sole structural difference betweenthymine, uracil and 5-XU is the functional group in the C5position, which is CH3 , H and X, respectively (see Fig. 1). Gas-phase experiments10 have shown that DEA to thymine leads toH� formation via four dissociation channels at 4.0, 5.2, 6.6 and8.0 eV incident electron energies. From those studies andothers on partially deuterated gas-phase thymine,18 it was con-cluded that the first three low energy peaks result from N–Hbond cleavage whereas the 8.0 eV peak can be ascribed toH� loss from the CH3 and CH sites. In thin films of thymineand uracil,15 the only H� peaks observed are near 8.6 to 8.7eV, and were also attributed to C–H bond cleavage. However,unlike for the present 5-XU films, for both condensed phasethymine and uracil, only a very weak H� desorption signalbetween 5 and 7 eV was observed, and tentatively attributedto N–H bond cleavage (i.e. the 6.6 eV dissociation channelin gas phase thymine10).15

Thus, the present observation of significant H� yieldsbetween 5–7 eV would suggest that in 5-XU films a portionof the H� yields arises from N–H bond cleavage in additionto C–H bond rupture (at the C6 site) at higher electron ener-gies, whereas in thymine and uracil films only C–H bond sitescontribute to the DEA H� yields observed by ESD. This isdemonstrated by the following example in Fig. 3: if the con-densed-phase DEA H� yield signature from uracil15 (after nor-malization to the present data at the uracil H� DEA peakenergy—dashed curve) is subtracted here from the 1.1 ML 5-FU curve, a peak remains in the present data that can be fittedto a Gaussian function centered around 7.1 eV (dotted curvein Fig. 3a—alternatively, a broadening of the DEA featuresbetween U and 5-FU would suggest contributions of twopeaks with similar intensities). It should be noted that sub-tracting instead the H�/thymine curve does not significantly

change this result. Here, uracil was chosen for the two follow-ing reasons: (i) all H positions in 5-FU are also H sites in uraciland (ii) the DEA H� yield signature from uracil films is inde-pendent of thickness for uracil film thicknesses below �4 ML.Thus, the peak near 7.1 eV for H�/5-FU in the present experi-ment is attributed to the same dissociation channel near 6.6 eVobserved in gas-phase thymine as resulting from N–H bondcleavage. As a conclusion, the H� DEA yield from con-densed-phase 5-FU is believed to result from at least two dis-sociation channels at approximately 7.1 and 8.6 eV, most likelyrelating to N–H (at N1 and N3) and C–H (at C6) bond clea-vage, respectively. We note however that in the preceeding dis-cussion we assume that, due to the structural similarity of U(or T) and halouracils, the ESD peaks due not shift upon halo-gen substitution; furthermore the deconvolution of the data inFig. 3 into contributions at 7.1 and 8.6 eV is only valid withinthe energy resolution of the electron beam which is determinedmainly by the beam’s FWHM energy width of 0.5 eV. It is alsoworth noting here that even lower energy H� yield peaks, suchas those observed from N–H bond cleavage at 4.0 and 5.2 eVin gas-phase thymine,10 are not observed in the present experi-ment. Even if those low energy dissociation channels existed incondensed-phase 5-FU, the resulting H� ions would likely beformed with insufficient kinetic energy to overcome the chargeinduced polarization energy barrier induced in the organicfilms.15

A general feature is that the shape of all the H� yield curvesin Fig. 2 change significantly with increasing film thickness upto �3 ML. This is shown more clearly in Fig. 3, where some ofthe curves from Fig. 2 are superimposed after normalization at19 eV. Here a similar deconvolution of the H�/5-FU curve at2.5 ML shows that both the 7.1 and 8.6 eV DEA contributions

Fig. 2 H� desorption yield as a function of incident electron energy(E0) for (a) 5-FU, (b) 5-ClU, (c) 5-BrU and (d) 5-IU condensed on apolycrystalline Pt substrate for different film thicknesses. Beam cur-rents are, respectively, about 6.5, 12, 3.5 and 9 nA. Each curve is theaverage of uncharged scans (identified by overlapping of subsequentscans) from at least two films, except for 5-FU (only one film for thetwo lower thickness curves).

Fig. 3 H� desorption yield (gH�) in arbitrary units as a function ofincident electron energy (E0) for (a) 5-FU, (b) 5-ClU, (c) 5-BrU and(d) 5-IU condensed on a polycrystalline Pt substrate (symbols). Allthe curves are normalized at 19 eV. The experimental data were fittedin the 4–10 eV range (solid lines) to the following equation:gH�(E0) ¼ aG(E0)+ bU(E0), where a and b are fitting parameters,and G and U are, respectively, a Gaussian function centered around7.1 eV (dotted line) and a fit to the H� yield from condensed-phaseuracil/Pt films15 (dashed line).

Phys. Chem. Chem. Phys., 2003, 5, 3270–3277 3273

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increase relative to the DD signal at higher energies as thethickness of the film increases; however, the relative contribu-tion of the 7.1 eV peak (N–H bond cleavage) decreases relativeto the 8.6 eV peak (C–H bond cleavage) as the thicknessincreases (Fig. 3a). This may account for the shift of the globalH�/5-FU peak position towards higher electron energy as thefilm thickness increases. On the other hand, for the thickest5-FU film (5.8 ML in Fig. 2), such a deconvolution does notshow the same relative increase in DEA contribution relativeto that attributed to DD, nor decrease of the 7.1 eV featurerelative to the that at 8.6 eV. More generally, for thicknessesbelow �5 ML, the H� DEA peaks of all the 5-XU can beroughly deconvoluted into a peak centered around 7.1 eV(attributed here to N–H bond cleavage) and a ‘‘uracil-like ’’peak (see Fig. 3) near 8.6 eV assigned here to C–H bond clea-vage. However, in contrast to 5-FU, for the other 5-halouracilsthe deconvoluted low energy peak attributed to N–H bondrupture is always less intense than the high energy one asso-ciated with C–H bond rupture. We note that the signal inten-sity on the high energy side of the deconvolutions in Fig. 3 isbelieved to be due to the overlap of the signal attributed toDEA with the rising desorption signal associated with (mostlikely) non resonant mechanisms that become energeticallyallowed at such higher electron energies (i.e. DD). Finally,the presence of a weak shoulder in the measurements on thelow energy side of the fit (solid curves in Fig. 3) is observed,which could perhaps be attributed to a contribution of a disso-ciation channel similar to the 5.2 eV H� DEA peak from gas-phase thymine that is produced via N–H bond cleavage. Thisweak shoulder is larger for 5-ClU and 5-BrU, than for 5-IU.The dependence of ion yields on film thickness are pre-

sented in Fig. 4 for H� desorption from 5-XU/Pt films (X ¼F, Cl, Br and I), for two incident electron energies, namely

19 eV and that energy at which the DEA peak isobserved. It is generally found in the present experiments on5-ClU and 5-IU that the H� yield produced via DEA firstincreases slower for film thicknesses significantly below �1ML, then increases more steeply above 1 ML and saturateseventually above �6 ML. In the case of H� produced viaDD, the behavior is roughly the same, except that the curvespresent no initial slow rise. These shapes are very differentfrom that of H� desorption from condensed thymine and ura-cil thin films,15 for which the H� yield intensities (producedboth via DEA and DD) increase very linearly with film thick-ness and then saturate quickly, which was attributed to the factthat the thymine and uracil films grow in a cluster like fashion.For this reason, strong effects of the metal on the H� forma-tion were not observed in the low thickness range (0–2 ML).This is not the case in the present experiments, where strongmetal effects are observed to induce notable variations inthe H� energy-dependent yield curve shapes (see Fig. 3). Asalready mentioned, the overall peak position for H� de-sorption shifts towards higher energies as the thickness of thefilm increases. This behavior is shown in greater detail inFig. 5, where the peak energy of the DEA ion yield signal max-imum (Emax

0 ) is plotted as a function of film thickness. As aconsequence, we believe that 5-halouracil films seem to growmore in a layer-by-layer fashion.In order to illustrate these effects on the thickness depen-

dence of the present H� yields in Fig. 4, we use the followingsimple model: an electron beam of incident energy E0 ,impinges on a film of molecules condensed on a metallic sub-strate. As the thickness (L) of the film increases, the numberof molecules that can undergo dissociation increases. How-ever, anion formation at a distance x from the metal surfacedepends on the attenuation of the electron beam intensitythrough the overlayer Tel(L� x) and on the cross sectionsDP(x) for the dissociation process (DP) considered (i.e.,DEA or DD). As a result, the number of anions created perunit time in the layer between x and x+dx is:

dN ¼ I0T elðL� xÞsDPðxÞN0S dx; ð4Þ

Fig. 4 H� desorption yields upon electron impact of incident energyE0 as functions of film thickness for 5-XU condensed films, whereX ¼ F, Cl, Br and I. Beam currents are, respectively, 6, 12, 10 and11 nA. Solid symbols correspond to the value of E0 (i.e. E

max0 ) at which

the DEA H� ion signal peaks, and open symbols to E0 ¼ 19 eV. Eachdata point corresponds to a new film. The DEA peak ion yield maxi-mum and the corresponding value for Emax

0 are estimated as follows.Several successive scans are recorded across the DEA peak. Unchargedscans (identified by overlapping of subsequent scans) are fitted by aGaussian function. The solid and dashed lines are fits to the experi-mental data according to eqn. (8).

Fig. 5 Values for Emax0 , estimated as described in the legend of Fig. 4,

as a function of film thickness for 5-XU condensed films, whereX ¼ F, Cl, Br and I.

3274 Phys. Chem. Chem. Phys., 2003, 5, 3270–3277

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where I0 and S are the electron beam intensity and area at thefilm surface, respectively, and N0 is the molecular density ofthe film. Among the anion fragments produced by electronimpact, only a fraction will scatter through the overlying mole-cules into the vacuum:

dNESD ¼ T ionðL� xÞdN; ð5Þ

where Tion is the anion transmission through the condensedfilm. As a consequence, using eqns. (4) and (5), the total ionyield is a complex function of film thickness:

NESDðLÞ ¼ I0N0S

ZL

0

TionðL� xÞTelðL� xÞsDPðxÞdx: ð6Þ

In order to model the thickness dependence of the ion yieldshere, the influence of x on Tion(L� x), Tel(L� x) and sDP(x)must be estimated by means of the following considerations.(a) As the thickness of the spacer between a molecule M that

undergoes DEA and the metal increases, the interactionbetween the metal and the TMA (M*�) changes, which in turninfluences the magnitude of sDEA . At submonolayer coverage,electron transfer from the transient molecular anion to themetal is dominant, causing an appreciable decrease in the mag-nitude of DEA.19 Another effect of the metal substrate is thatof the image charge induced within it by the anions in the film.The magnitude of the polarization energy, Epol , is sensitive tothe distance (x) of the anion from the metal and the molecularpolarizability of the solid film. In the simplest models, thepolarization energy increases from its thick limit Epol(x!1)with an approximate 1/x dependence.20 The image force hasseveral effects. First, Epol shifts the energy of the TMA statedownward in the Franck–Condon (FC) region with respectto the neutral (M) ground state. As a consequence, an increasein Epol (that is, a decrease in x) will result in an enhancement ofthe DEA cross section as less time is required to travel fromthe FC region to the stabilization region. In the case of DD, theinitial state being neutral, the two points mentioned aboveshould not change significantly the magnitude of sDD . A sec-ond effect of the image charge induced in the metal substrateon the DEA and DD processes involves the number of vibra-tional and/or electronic intermediate states (i.e., amount ofconfiguration space) which have sufficient energy to yield H�

ions.19,21 For the O2/Ar/Pt system,19 it has been shown thatthis configuration-space effect leads to a large decrease (smallincrease) of the O� ESD signal produced via DD (DEA) asthe distance of the transient O2*(O2*

�) from the metal (x)increases.(b) However, if the distance between the metal surface and

the molecule M is large enough (x�Lc), the DEA and DDcross sections can be considered as constant. The long-rangeprocess is the polarization energy shift. This range dependson the dielectric constant of the medium. Michaud andSanche20 showed that as the thick-film limit for N2 depositedon the surface of an Ar multilayer film of variable thicknessgrown on Pt is reached at about 32 ML (�100 A), Epol

increases by about 0.72 eV between 1 and 32 ML. However,�70% (50%) of the shift in the polarization energy is com-pleted after 4 ML (2 ML). Furthermore, in an ESD experi-ment,19,21 monitoring O� produced via DEA from O2(0.1 ML)/Ar(variable)/Pt films as a function of Ar thickness between0.37 and 4.3 ML, it appears that the O� yield intensity variesvery slowly below �1 ML, then increases somewhat faster atlarger Ar thicknesses. However, the ESD O� signal remainsfairly constant beyond �2 ML (within 5%). As a result,the shift in Epol seems to affect the DEA cross section onlywithin about 2 ML. In the case of 5-halouracil condensedfilms, the dielectric constant is expected to be higher than thatof rare-gas solids, so that the expected maximum shift in Epol

should be smaller.15

(c) The desorption probability of anions created below or atthe surface, depends strongly on their interactions or reactionswith molecules in the surface layers on top. In a number ofexperiments,22 the transmission of low-energy anions (feweV) through overlayers was shown to generally follow an expo-nential law:

TðxÞ ¼ exp½�N0sionðL� xÞ�; ð7Þ

where sion is the attenuation cross section of the anionsthrough the film. In these experiments, the anions are gener-ated via ESD from an adsorbed molecular monolayer. Theirkinetic energy distribution remains unchanged as the overlayerthickness increases and peaks at �1 eV. In films condensed ona metallic surface, the effect of the image charge is to reducethe in vacuo kinetic energy distribution of the anionic frag-ments produced by DEA at a given incident electron energyby bEpol , where b is the ratio of the anion mass to the massof the neutral parent molecule.23 However, for desorbed H�

from condensed 5-XU, b is smaller than 0.01, so that the ion’sin vacuo kinetic energy can be considered to be independent ofEpol (and thus x).24

Therefore, for a fixed incident electron energy, we mayassume that sDP(x) as well as the kinetic energy imparted tothe anions remain constant for x�Lc , and that both theelectron and anion transmissions Tel(L� x) and Tion(L� x)through the overlayer (L� x) essentially follow eqn. (7).Hence, the ion yield thickness dependence for L�Lc , cannow be obtained by integrating eqn. (6) between Lc and Land can be written as:

gðLÞ ¼ gmax 1� exp �ðL� LcÞd

� �� �; ð8Þ

where gmax is the maximum ion yield and 1/d ¼ N0(sion+sel)is the inverse sum of the ion and electron attenuation lengthsthrough the film [lion ¼ 1/(N0sion) and lel ¼ 1/(N0sel), respec-tively]. This model is in good agreement with the film-thicknessdependence of ESD yields of O� from O2 condensed on Pt,reported previously.25 We find that this latter data correspond-ing to the O� DEA yield produced by 7 eV electron impact canbe fitted quite well to eqn. (8) here, for thickness greater than1–2 ML. Below 1 ML, the O� signal is very low, which hasbeen attributed to metal neutralization of the transient anion.The polarization energy of solid O2 is about 0.7 eV,

26 while it isabout 0.62 eV for solid Ar.20 As a result, the Epol shift is some-what smaller in the O2/Pt

25 than in the (0.1 ML O2)/Ar/Pt19

experiment (by <0.1 eV). In the latter experiment, the shift inEpol was shown to affect the O� ESD signal formed by DEA inthe first two layers only, and it should thus have a slightlysmaller range in the O2/Pt experiment. This is consistent withthe fact that the O� ESD yield produced by DEA in O2/Ptcondensed films can be fitted to our model (eqn. (8)) onlyabove 1–2 ML.Thus, eqn. (8) has been fitted to the data in Fig. 4 for H�

desorption from condensed 5-XU (X ¼ F, Cl, Br and I) forthe two incident electron energies considered. For the lowenergies, the anions are formed via DEA, while at 19 eV,DD is the predominant process. Generally, above a certainfilm thickness, the data can be fitted fairly well to eqn. (8),and the fitting parameters (gmax , Lc and d) are presented inTable 2.Due to large uncertainties on the absolute film thickness

determination, H� ESD yields from different parent moleculescan be compared in the saturation region only. It is interestingto note that the H� yield at saturation (gmax) is higher in con-densed 5-FU than in uracil15 under the same experimental con-ditions. This is probably due to a higher capture probabilityof the electron by halouracils owing to the presence of theelectron affinic halogen atom. The yield of H� at saturationdecreases as the mass of the halogen increases, in the following

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relative order: 5-FU > 5-ClU > 5-BrU > 5-IU. This relativeorder also follows that of the C–X bond energy of the 5-XUmolecules considered.Another general feature of the curves in Fig. 4 relates to the

more rapid initial increase as a function of film thickness of the19-eV H� yield signal (due essentially to DD) relative to thatdue to DEA. This result is also expressed differently in the nor-malized data of Fig. 3. This general behavior may be linked tothe difference between the dissociating states. In DD, the disso-ciating state is neutral and therefore much less affected by themetal than a dissociating transient anion. Close to the metal,electron transfer can occur within the resonance lifetime, sothat quenching of the transient anion is highly effective. Hence,only when the transient anion is sufficiently far from the metalsubstrate does it possess a lifetime of the order of, or longerthan the dissociation time, necessary to produce a significantanion desorption signal. As a result, in the case of DEA, it isseen from Table 2 that Lc varies in the range of �0.54–0.80ML for the 5-XU films considered, so that experimental datamatch the solid curves for film thicknesses larger than 1 ML.In the case of DD, the parameter Lc is set to zero, and thedotted curves thus obtained fit the experimental data. If thetwo-exponent fit of eqn. (3) for the transmission thicknessdependence of 5-ClU is used to reassess deposition times, thenthe only parameter that changes in the fit to the solid symbolsin Fig. 4b is Lc , which would then be somewhat smaller(0.25� 0.08 ML instead of 0.54� 0.10 ML). Under such con-ditions, experimental data would thus match the fit well below1 ML, which is consistent with the other 5-XU molecules. As aresult, we used the one-exponent fit to the transmission thick-ness dependence of 5-ClU in the present work. For most com-pounds, the parameter d at 19 eV is roughly equal (withinexperimental uncertainties) to that at the incident electronenergy at which the DEA peak is observed, except for 5-IUwhere it is slightly smaller.

Conclusion

We have shown that resonant interaction of low-energy elec-trons with solid 5-halouracil films leads to the production ofH� (and associated neutral radical) fragments. The onset forH� formation is �5.0 eV. A single dissociative electron attach-ment (DEA) peak is observed, which are attributed here to aconvolution of at least two dissociation channels at approxi-mately 7.1 and 8.6 eV resulting from N–H and C–H bondcleavage, respectively, with relative contributions to the frag-mentation yield that are parent-molecule dependent. Of coursethis general interpretation assumes that, due to the overallstructural similarity of T, U, and 5-XU, the intrinsic reson-ances associated with CH cleavage do not shift appreciably

in energy upon substitution of a functional group (e.g.between U (or T) and 5-XU); this is in part supported byH� ESD experiments on condensed films of deuterated andundeuterated toluene, pyrimidine, and phenol,27 benzene,28

and aniline.29 In the present experiments, for a given molecule,the dissociation channels are affected differently by proximityto the metal, and the DEA maximum shifts by �0.5 eV tohigher energies as the film thickness (L) increases. The low-energy dissociation channel related to N–H bond cleavagehas been observed in gas-phase thymine,10 and the high energyone, associated with C–H bond cleavage, in gas and con-densed15 phase thymine and condensed phase uracil. Thepresence of a weak shoulder in the H� yield from 5-ClU and5-BrU suggests a contribution of the 5.2-eV peak from gas-phase thymine that is also related to N–H bond cleavage.Above 10 eV, dipolar dissociation (DD) is the dominant disso-ciation mechanism. The yield of H� varies in the followingrelative order: T > U > 5-FU > 5-ClU > 5-BrU > 5-IU. Thefilm thickness dependence of the H� yield produced by DEAis can be modeled as {1� exp[(L�Lc)/d]} above �1 ML,where d is a parameter taking into account the attenuationlengths of the ions and electrons in the film, and Lc is the thick-ness range where the DEA cross section and the probability foranion desorption are affected by the metal. In the case of DD,the thickness dependence of the H� ESD yield is proportionalto [1� exp(L/d)]. The main difference in the thickness depen-dence of both processes (DEA and DD) is thus attributed tothe fact that electron transfer to the metal from the transientmolecular anion leads to dramatic decrease in the H� ESD sig-nal below 1 ML, whereas it does not affect significantly DD,since its intermediate state is neutral. The radiosensitizingeffect of 5-halouracils is likely related to both: (i) the differentreactivity of the different radical fragments produced fromthem by DEA compared to thymine and uracil, (ii) unlike inthymine, DEA to 5-halouracils can occur at near thermal elec-tron energies,4,12,13 where it leads to formation of either uracil-5-yl or free halogen radicals.

Acknowledgements

This study was supported by the Canadian Institutes of HealthResearch and the National Cancer Institute of Canada.

References

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Table 2 Fitting parameters (gmax , d and Lc) used in eqn. (8) to model

the ion yields as a function of film thickness (in units of monolayer).

The curves in Fig. 4 are the resulting fits of eqn. (8) to the experimental

data for L�Lc . E0 is the incident electron energy (in eV)

5-Halouracil E0/eV gmax/cps nA�1 d/ML Lc/ML

5-FU 7.35–7.7a 734� 22 1.8� 0.2 0.72� 0.08

19 508� 20 1.6� 0.3 0

5-ClU 8.5–8.9a 300� 20 1.8� 0.3 0.54� 0.10

19 220� 10 1.7� 0.2 0

5-BrU 8.95� 0.06a 96� 1 1.8� 0.1 0.74� 0.07

19 119� 2 1.6� 0.1 0

5-IU 8–8.6a 59� 1 1.8� 0.1 0.80� 0.05

19 57� 1 1.3� 0.1 0

a Here E0 corresponds to the energy for which the experimental H�

yield curves have their maximum peak intensity (Emax0 ).

3276 Phys. Chem. Chem. Phys., 2003, 5, 3270–3277

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Sanche, Degradation of Substituted Benzene Thin-Films by LowEnergy Electron Impact (0–20 eV): The Effects of the FunctionalGroups and the Chemical Environment on Dissociative Attachment,‘‘Surfaces and Interfaces of Advanced Materials ’’ (SIAM), EcolePolytechnique, Montreal, PQ, Canada, 1996, p. 11.

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