Soft X-ray irradiation effects in polymer films

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Soft X-ray irradiation effects in polymer films

M. Przybylski, M. Stamm, R. Zietz

To cite this version:

M. Przybylski, M. Stamm, R. Zietz. Soft X-ray irradiation effects in polymer films. Journal de

Physique, 1987, 48 (8), pp.1351-1355. �10.1051/jphys:019870048080135100�. �jpa-00210561�

Soft X-ray irradiation effects in polymer ^films

M. Przybylski, M. Stamm and R. Zietz

Max-Planck-Institut für Polymerforschung-6500 ^{Mainz, F.R.G.}

(Requ ^{le 24} février ^1987, accepté le 7 avril 1987)

Résumé.

Le rayonnement monochromatique (250 ^{eV hv 350} eV) ^du synchrotron ^{BESSY a} été utilisé pour étudier les excitations des niveaux fondamentaux dans ^une série de films de polymères. Le rendement

électronique ^{est lié au} spectre d’absorption des surfaces de polymères ^et révèle les caractéristiques ^des

transitions : niveau fondamental, niveau de valence ou niveau de Rydberg. ^Ce type d’excitation ne produit pas de rupture de liaison chimique ^{et de} désorption ^des ions, contrairement à ce qui ^se passe lorsque les excitations sont provoquées par des photons d’énergie supérieure à 290 eV. A ces énergies ^{on obtient un rendement en}

ions non négligeable. Des mécanismes d’excitation sont ensuite suggérés ^{et discutés.}

Abstract.

Monochromatic synchrotron radiation (250 ^{eV hv 350} eV) has been used to study ^{the core}

level electronic excitation processes in solid polymer ^{films of} PE, PB, PS and PMMA. The electron yield spectra ^are closely ^{related to the} absorption spectra ^{of the} polymer surface and reveal characteristic core to valence (Cls ~ 03C0*) ^{and core to} Rydberg (Cls ~ R ) transitions. These excitations, however, ^{do not} produce significant ^bond rupture ^{and ion} desorption contrary ^to excitations induced by photons of energy ~ 290 eV.

For those energies ^a significant ^ion yield is obtained. Possible excitation mechanisms are discussed in detail.

Classification

Physics ^Abstracts

61.40K

61.80

33.20

Introduction.

Exposure ^of polymers ^to high energy irradiation results in both temporary ^and permanent changes ⁱⁿ

their physical and chemical properties. ^{It is well}

known that fast charged particles like electrons lose their energy in ^two different ways : firstly by ^elec-

tronic excitation and secondly by collisions with nuclei [1, 2]. ^{There is no} evidence of permanent

damage ^{due to} low energy electronic excitation ^or ionisation in metals, semimetals or even some

semiconductors [2] because the conduction electrons

(usually ^of high mobility) ^restore quickly ^{the elec-}

tronic holes produced along ^the trajectories ^of charged particles. On the other hand the creation of permanent damage ⁱⁿ insulating ^molecular crystal,

ionic crystals ^and polymers ^{is due to} low energy electronic excitations and ionizations [1, 2]. ^For example, ^an 8 keV Cu Ka photon ^{is able to} produce approximately ^{8 keV} photoelectrons. ^{These elec-}

trons will, ^{in turn} lose their energy in the form of electronic excitations or ionizations. In the long-

chain polymers these processes lead ^to crosslinking, degradation (chain scission) changes in unsaturation.

However, little is known in organic ^materials (like polymers) ^about radiolysis ^{i.e. the main formation}

process of stable defects. One can consider the

radiolysis ^{as a local} chemistry ^{of excited states.} Only

in a few cases the experimental ^data give insight ^into primary processes leading ^{to bond} rupture, ^molecu- lar fragmentation following excitations of core elec- trons.

Recently ^the development ⁱⁿ synchrotron ^radi-

ation facilities, especially ^{for the vacuum} ultraviolet made possible ^to investigate excitations of deep

valence and core levels in simple molecules. The

binding energies ^of deep valence and core electrons of light elements like C, ^{N and} 0, ^{the basic}

« components ^{» of} organic ^{molecules in} particular polymers, ^are located in the range of about 600 eV.

At present ^the synchrotron ^{radiation can be used for}

selective excitation of the atoms of different elements

composing ^{the molecule. It is also} possible ^{even to}

excite the different atoms of the same element when

they ^{are in} different chemical surroundings (nearest- neighbours effect) [5].

Polymers ^{like PE} (polyethylene), ^PB (poly- butadiene), ^PS (polystyrene) ^{and PMMA} (poly-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019870048080135100

1352

(methyl methacrylate)) ^are very simple ^{from the} viewpoint of their chemical structure. Namely ^the long ^{chain molecules} (typical ^{several thousand} A)

sometimes with side groups, consist mostly ^{of carbon}

atoms connected by single ^or double bonds. It is

obviously very tempting ^and interesting ^{to irradiate}

these materials by photons of energy in the range of the carbon Cls edge (h v

²⁸⁴ eV ). ^Eberhardt

et al. [6] ^have shown that in the region ^{of the Cls} absorption threshold, ^some organic molecules like CO and acetone (CH3)2CO decompose ^into frag-

ments not only after direct core ionization but also

following ^{core to 7T * or} Rydberg (bond state)

transitions. The core hole created in the initial photo absorption decays preferentially ^{via an} Auger ^KLL

transition rather than by X-ray emission, resulting ⁱⁿ highly charged molecules that is unstable and falls apart ^{in a Coulombic} explosion [7]. ^{The stimulated} desorption ^{in ionic solids} (like Ti02, V20,, W03) by low-energy ionizing ^radiation is accounted for by Auger decay ^{of a core hole state} resulting ⁱⁿ multiple

localized valence holes. The bonds around this excitation can vanish because the valence holes (due

to localization) ^are constrained to one pair ^{of atoms} (see e.g. Knotek [8]).

In this paper ^we present studies of the synchrotron

soft X-ray excitation _processes involving ^{carbon Cls}

electrons in some polymers. ^{We have measured}

electron and ion yields in the range of 250 eV , h v , 350 eV for solid polymer ^{films of} PS (H ) (polystyrene), PS(D3) (polystyrene deuterated in the alkyl position), PS (D8 ) (polystyrene totaly deuterated), ^PE (polyethylene), ^PB (polybutadiene)

and PMMA (poly(methyl methacrylate)). ^{The elec-}

tron kinetic energy distributions (photo ^electron spectra) ^{for some} photon energies ^{have been}

measured as well.

Experimental techniques.

The measurements were performed ^{at the} storage ring ^{BESSY in} Berlin, using the HE-PGM-2 plane grating monochromator providing ^an energy ^resol- ution of about 1 eV. The monochromator was cali- brated with LaAl2-plates revealing characteristic

resonances in the electron yield spectrum (due ^to

La 3d-4f transition). The electron yield ^of gold ^was

measured to characterize the monochromator trans- mission (the ^spectral dependence ^{of the} photon flux)

and used to normalize the electron yield spectra ^of studied samples. It turned out to be an important point ^{in our} studies since two intense minima in the transmission curve connected with the carbon K-

edge absorption ^{of PGM-2 at 284.7 eV and 291.0 eV}

were found. The total electron yield spectra ^were obtained from the total current of all electrons emitted from the sample. ^Ion yield spectra ^were obtained by ^{detection of ions at several kinetic}

energies in the range 0-300 eV (the upper value is limited by ^the photon energy). ^{The same} set-up ^was used to measure the electron kinetic energy distri- bution monitoring ^the photoelectron energy spectra

at constant incident photon energy h v (in ^the region

of carbon K-edge).

The solution casting ^{method on} chemically ^cleaned

aluminium plates ^{was used to obtain} polymer ^films

of different thicknesses 1-5 _pm. The polymer ^films

were kept ⁱⁿ ultrahigh-vacuum for about 7 days ^to degas ^the remaining ^{solvent and to eleminate con-}

taminations at the film surface. Vacuum better than 1 x 10- 9 torr necessary for ^our measurements, ^was reached after annealing ^{the vacuum chamber to}

200 °C for a few hours. The samples ^were kept always ^at temperatures ^{below 80 °C.} The thickness of polymer ^films appeared ^{to be an} important ^factor

in the photoemission measurements since the poly-

mers studied are electrical insulators. Namely, large charging ^{effects were} observed in the photoelectron spectra ^of polymers when the film thickness was

thicker than 4 _[Lm. The photoemitted ^{electrons are}

shifted to lower kinetic energies ^{as the surface} begins

to charge. This effect is already visible after 5-10 s of irradiation (especially ^at h v > 290 eV) and increases within 40-50 min reaching saturation were some

details of photoelectron spectra ^are shifted about 40 eV. This phenomenon ^causes difficulties in the determination of ion kinetic energies ^{but does not}

influence the total electron and ion yields.

Results and discussion.

Figure ¹ shows the electron yield spectra (TEY) ^of polymer ^{films of} PE, PS(H), PS(D3), PS (D8 ),

PMMA and PB. The spectra obtained for the

photon energies ^{270-330 eV seem to} display ^some

similarities in spite ^of differences in their chemical structure. However, ^{there are also details in the} TEY-spectra allowing ^to distinguish ^between particu-

lar polymers ^or group of polymers (see ^discussion below).

Firstly ^{let us discuss some} general physical aspects of TEY in the soft X-ray range. The TEY from ^our

samples is dominated by inelastically scattered elec- trons [9, 10] ^{since most of elastic} Auger ^electrons

and photoelectrons ^{lose their} energy ^on their way out of the solids in the inelastic scattering ^events (electron-electron, electron-plasmon ^{or electron-} phonon scattering). ^{The total number of electrons y}

created in a depth ^{L of a} sample ^with absorption coefficient u ^is proportional ^to the number of absorbed photons ^{IL - 1-} exp (- ILL) [11]. ^For 1/ IL > L this relation reduces to y - ILL. Thus, ^the total electron yield ^is directly proportional ^{to the} absorption coefficient. The effective electron _escape

depth L in the soft X-ray region is about 50 A.

Electrons generated deeper inside the sample ^lose

Fig. ^1.

Total electron yield (absorption spectra) ^near

the carbon K edge ^{in some} polymer ^films (normalized ^to

incident photon flux).

their energy ^before reaching ^{the surface} (the photon

mean free path 1/u ^is always larger than L and is about 1 000 A in the discussed region). ^{For PMMA}

the calculated absorption coefficient is about 1 p,m-1 ¹ (for photon energy 200-300 eV) confirming ^{the valid-} ity of the relation _{y -,4} in the TEY-experiment.

Thus only ^electrons originating from the surface

layer (of depth L) contribute to the total electron

yield signal ^{of the} studied materials (the ^{TEY can}

also be used as a direct measure of extended X-ray absorption ^{fine structure EXAFS} [12], ^{above the} absorption edge). ^{There is an intense} sharp peak (A)

in the electron yield spectra ^of polymers (see Fig. 1) ; ^its position ^and shape ^is characteristic for excitation of the Cls electrons to an antibonding ^it

orbital in those hydrocarbon-molecules containing

1T-electrons [5]. ^For PS and its deuterated derivatives

(PS(D3), PS (D8 )) ^this peak is situated at about 285.2 eV which is identical to the experimental ^value

for the 1T ^* transition in gaseous benzene [5] (consist-

ent with spectra ^{of Hansen et al.} [13] ^as well).

The second group ^of polymers ^{i. e.} PB, PMMA, PE does not contain aromatic (benzene) groups in their structures. The peaks ^{shift to} somewhat lower energy, 284.8 eV. This value is in good agreement with the peak ^found in gaseous ethylene C2H4,

where the Cls 1T ^* transition was assigned ^{to a} peak ^{at 284.4 eV} (or ^284.8 sh) ^{in the electron} yield

spectra. ^In that group of polymers ^the intensity ^of peak ^{A is} appreciably ^lower (especially ^{for PE and} PB) ^{than for} PS-samples. Generally ^the binding

energy of Cls electrons for aromatic molecules (like benzene) ^{is smaller than that for} alkyl ^molecules (chain). ^{The strong 1T} *-resonance was also detected

by Crecelius et al. [14] ⁱⁿ energy-loss-experiments ^on

other polymers containing ^aromatic groups

(benzene). ^{In the} polymer films of PPP (poly(p- phenylene), ^PPS (poly(p-phenylene sulfide) ^and

PPO (poly(p-phenylene oxide) ^{the Cls 1T * transi-}

tion energies ^are consistent with the present ^results.

The second peak (B) is very apparent mainly ^for

PE but is not so intense and sharp ^{as A. It can be} assigned ^to Rydberg transitions (by comparison ^to hydrocarbon TEY-spectra) [5]. ^{It occurs between}

287.7 eV (for PS) and 288.2 eV (for PE) comparable

with an energy of 288.0 eV found for the excitation Cls 3p in methane CH4 and ethane CZH6 [5]. ^The

broad maximum in the electron yield spectra (C) ^at

about 291.0 eV is probably ^{due to} shape ^resonances (or ^* resonances) ^{which can} be observed just ^above

the ionization threshold [15, 16]. ^For simple hyd-

rocarbons the ionization potential of the carbon Is shell (I.P.) changes ^{from 290.3eV} (benzene) ^to

291.2 eV (acetylene) [5]. ^{The other features labelled}

as D and E at 297.8 eV and 301 eV, respectively, ^can probably ^be assigned ^to shake-up and shake-off effects (double (multiple) ionization (excitations))

or other a resonances occurring ^at energies higher

than 290 eV (like o- ^{resonance in} C2H4 ^and CZHb ^at

302 eV and 301 eV respectively [16].

The main question which arises from the electron

yield studies is why ^{s 1T} *-resonance is observed for

polyethylene ^which only contains saturated hyd-

rocarbon-bonds. For saturated hydrocarbons only

excitations to Rydberg ^{orbitals can be} expected ⁱⁿ

this photon energy range (i.e. ^Cls absorption).

Consequently ^{one can} ask further whether the presence of only ^one double bond in the monomer

unit of PB and PMMA (i.e. ^C

0) ^{can be respon-}

sible for the relative intense peak (A) ^{in TEY}

spectra. ^{In order to answer this} question ^{we have to}

refer to the consideration at the beginning ^{of the} chapter. ^{The electon} yield spectra ^{in the} photon

energy range 200-300 eV confine us to surface

1354

properties ^{i.e. to a surface} layer ^not deeper ^than

approx. 50 A. The chemical structure in that layer might ^{be different to} that in the bulk polymer. ^One might expect that in the surface layer end-groups ^of polymer ^{chains are enriched which can} have different bonds compared ^{to the} repeating unit in the polymer

chains. This the observed 7r ^* resonance in the TEY- spectra ^{of PE can be accounted for} by existence of unsaturated hydrocarbon groups (double bonds) ⁱⁿ

the surface layer ^{of the} polymer.

Figure 2 shows the total ion yield of studied

polymers ^{as a} function of photon energy (260-

380 eV). ^{There is a} photon energy threshold in the ion desorption ^{as the} photon energy is scanned above the excitation of Cls-electron. The position ^of

Fig. ^2.

Total ion yield spectra ^of PE, PS, PS (D3 ), PS (Dg ), PMMA and PB around the Cls absorption

threshold.

this threshold does not correspond ^to the energy of

peak ^{A in the} TEY-spectra. ^For PB, PMMA, PS (H ), PS (D3 ) ^and P (Dg ) the rise of the ion yield signal ^{versus the} photon energy ^{commences at} about 290 eV and for PE even about 295 eV. It means that the excitation of carbon atoms in the benzene ring ^or

in the main chain, ^{via 7T * resonance} (that ^{is a core}

hole and electron in the ^{7r *} orbital) ^{does not lead to} significant ^bond rupture ^ion desorption. The electron kinetic _energy distribution changes drastically ^{as the} photon energy exceeds 285 eV by the appearance of

Auger ^electrons characterized by hv-independent

kinetic energy (due ^to Auger ^KLL transition of about 270 eV). ^{Thus, the core hole} produced by photon of energy > 285 eV in the polymer ^molecule decays predominantly (similary ^{to small}

molecules [7]) ^via Auger processes which ^can result in final states with two or multiple ^{holes in} bonding

orbitals and electrons in antibonding ^orbitals [8].

The typical single-hole lifetimes in covalent system

are of order of the inverse bandwidth (W), ^{which is} typically ^{about 10-15 s} what is small compared ^with

time needed for desorption (10-14 s ). ^{In cases where}

the hole-hole repulsion energy U is greater ^{than the} value bandwidth W, ^{holes cannot move} by ^resonant

processes ^and they ^{will be localized on the same site}

for time much longer ^than 11W [17], facilitating ^the

ion desorption. ^The Auger decay of the Cls 7r ^*

bond state excitation (in ^aromatic rings ^{or main}

chains of polymer) ^result probably ⁱⁿ delocalized

multiple-hole ^{final states.} The substantial delocaliza- tion of the 7T-system ^and strong ^nearest neighbour

interaction between atoms in covalent polymer ^chain (or ^side groups) ^{cause holes not be} bound with one

pair ^{of atoms} (not favourable for bond damage).

Eberhardt et al. [6] have shown that in the region ^of

the Cls absorption ^threshold organic ^{molecules de-}

compose following ^{core to 7T * or} Rydberg ^trans-

itions. The Cls to 7T* excitation observed at

288.5 eV (in ^the CO-group) ^{in acetone} results selec-

tively ^{in C+ and 0+} ions whereas the excitation of C atoms in CH3 ^groups (alkyl groups) ^at higher ^ener- gies (Rydberg transition) produces ^{other ionic} frag-

ments. However, in the studied polymers ^{the excita-}

tion of core electron to Rydberg orbitals does not result in a significant ^ion desorption ^{as well.} Namely

the peak ^{B at} about 288 eV in TEY spectra (PE, ^for example) ^{does not have its analog} in the total ion

yield-spectra ^of polymers.

A beginning ^of strong ^ion desorption ^{has been}

detected at photon energies of about 290 eV (see Fig. 2), very close ^to ionization potentials ^values

(Is

a ^* transition, just ^above I.P.) ^for simple

hydrocarbon molecules. In this _case, the Auger decay ^{of core holes at C atoms} probably ^{leads to}

two- or multiple-hole ^{final states.} Anyway, ^the

energy of 290 eV deposited ⁱⁿ polymer molecules is

sufficient to produce ^{the bond} rupture ^between

carbon atoms and consequently ^the strong ^{ion de-} sorption. On the other hand the strong excitation of

core electrons to 7T ^* or Rydberg ^orbitals by photons

of energies ^{2-5 eV} only ^lower decay ^{to states which}

do not enable to brake effectively the bonds between C-atoms (or involving C-atoms).

Conclusions.

We have demonstrated that electron yield spectra

near the carbon K edge ^{of some} polymer ^films display ^a characteristic absorption ^{structure similar}

to that observed in saturated or unsaturated hyd-

rocarbons like methane, ^{ethane or benzene} [5]. ^We

have interpreted the observed structure in terms of

core to antibonding 7T *, ^{core to a a*} (shape

resonances above the ionization threshold) ^{and core}

to Rydberg transitions. The presence of the Cls 7T* transition in TEY of polyethylene (saturated

bons in monomer unit) ^suggests that the surface

layer ^of polymers may have ^a different chemical structure than bulk material. It can be accounted

for, e.g. by end-groups ^of polymer ^{chains or small}

amounts of added stabilizer, ^{which are} favourably

concentrated in the surface area. These groups ^seem to have unsaturated bonds giving ^the strong ^{7T *}

resonance signal ⁱⁿ absorption spectra. ^{It turns out} from our studies that absorption spectroscopy ^includ- ing ^{more advanced} techniques like SEXFAS can be

a very useful method for the investigation ^of polymer

surfaces. It was found also that replacing hydrogen (H) ^atoms by ^deuterium (D) (isotopic labelling) only

in the benzene ring ^{or even} in all molecules of PS does not affect essentially ^the absorption spectrum

(differences smaller than 0.5 eV). ^{The ion} yield spectra ^have proved that excitations from core to

7r ^* and Rydberg orbitals do not lead to a significant

creation of damaged bonds and ion desorption. ^On

the other hand the excitation of polymers by photons

of energy higher than 290 eV results in a significant

rise of the ion yield ^of polymers. The threshold energy for the strong ^ion desorption ^{can be related}

to the ionization potential of the carbon Is shell (or

Is - a * transition, ^above I.P.). ^{The Cls core holes}

created by ^this excitation predominantly decay ^via Auger processes probably leading ^{to two or} multiple

hole final states which cause favourably ^bond rupture and ion desorption. ^{In the} present state, however, ^it

is difficult ^to indicate unequivocally the site of the excitation where the bond can break. The under-

standing ^of these processes is believed ^{to answer} the main questions connected with the permanent ^radi- ation damage ⁱⁿ polymers. ^{The bond} rapture ^{and ion} desorption ^are primary steps ⁱⁿ radiolysis ^{of stable}

defects in polymers ^{like :} crosslinking ^and degra-

dation. This can also be important ^for X-ray lithogra- phy (based ^on synchrotron radiation) ^since polymers proved ^{to be} good candidates for resist materials.

Another application in technical processes ^{are cross-}

linking reactions, ^{which are used to} improve e.g.

mechanical properties ^of polymeric ^materials.

We believe that mass spectroscopy detection of ions would give ^more insight ^to stimulated desorp-

tion of ions and photochemistry of processes leading

to permanent damage ⁱⁿ polymer molecules. Investi-

gations ^{in this direction are} planned for future

experiments.

Acknowledgments.

We would like to thank Prof. Dr. K. Baberschke and D. Arvanitis for experimental help and valuable discussions. We acknowledge ^the support ^{of Prof.}

Dr. E. W. Fischer and the help during ^the operation

of the HE-PGM-2 monochromator by ^Dr.

H. Petersen.

This work was supported by ^{BMFT under} grant- No. 03-FIMPG 7.

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