Resonant inelastic soft-x-ray scattering spectra at the N1s and C1s edges of poly(pyridine-2,5-diyl)

Abstract

Resonant inelastic scattering measurements of poly(pyridine-2,5-diyl) have
been performed at the N1s and C1s edges using synchrotron radiation. For
comparison, molecular orbital calculations of the spectra have been carried out
with the repeat unit as a model molecule of the polymer chain. The resonant
emission spectra show depletion of the p electron bands which is consistent
with symmetry selection and momentum conservation rules. The depletion is most
obvious in the resonant inelastic scattering spectra of carbon while the
nitrogen spectra are dominated by lone pair n orbital emission of s symmetry
and are less excitation energy dependent. By comparing the measurements to
calculations an isomeric dependence of the resonant spectra is found giving
preference to two of the four possible isomers in the polymer.

Full text

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Journal of Electron Spectroscopy and Related Phenomena 101–103 573 (1999)
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Resonant??  inelastic??  soft-­‐x-­‐ray??  scattering??  spectra??  at??  the??  N1s??  and??  C1s??  
edges??  of??  poly(pyridine-­‐2,5-­‐diyl)??  
??  
??  
M.??  Magnuson1,??  L.??  Yang2,??  J.-­‐??  H.??  Guo1,??  C.??  Såthe1,??  A.??  Agui1,??  J.??  Nordgren1,??  Y.??  Luo3,??  ??  
H.??  Ågren2,??  N.??  Johansson2,??  W.??  R.??  Salaneck2,??  L.??  E.??  Horsburgh4??  and??  A.??  P.??  Monkman4??  
??  
1Department??  of??  Physics,??  Uppsala??  University,??  Box??  530,??  S-­‐75121??  Uppsala,??  Sweden??  
2Institute??  of??  Physics??  and??  Measurement??  Technology,??  Linköping??  University,??  Box??  530,??  S-­‐58183??  Linköping,??  Sweden??  
3FYSIKUM,??  University??  of??  Stockholm,??  Box??  6730,??  S-­‐113??  85??  Stockholm,??  Sweden??  
4Department??  of??  Physics,??  University??  of??  Durham,??  South??  Road,??  Durham??  DH1??  2UE,??  England??  
??  
??  
??  
Abstract??  
??  Resonant??   inelastic??   scattering??   measurements??   of??   poly(pyridine-­‐2,5-­‐diyl)??   have??   been??  
performed??  at??  the??  N1s??  and??  C1s??  edges??  using??  synchrotron??  radiation.??  For??  comparison,??  molecular??  
orbital??  calculations??  of??  the??  spectra??  have??  been??  carried??  out??  with??  the??  repeat??  unit??  as??  a??  model??  
molecule??   of??   the??   polymer??   chain.??   The??   resonant??   emission??   spectra??   show??   depletion??   of??   the??   p??  
electron??  bands??  which??  is??  consistent??  with??  symmetry??  selection??  and??  momentum??  conservation??  
rules.??  The??  depletion??  is??  most??  obvious??  in??  the??  resonant??  inelastic??  scattering??  spectra??  of??  carbon??  
while??  the??  nitrogen??  spectra??  are??  dominated??  by??  lone??  pair??  n??  orbital??  emission??  of??  s??  symmetry??  and??  
are??  less??  excitation??  energy??  dependent.??  By??  comparing??  the??  measurements??  to??  calculations??  an??  
isomeric??  dependence??  of??  the??  resonant??  spectra??  is??  found??  giving??  preference??  to??  two??  of??  the??  four??  
possible??  isomers??  in??  the??  polymer.??  
??  
??  
??  
Introduction??  
Conjugated??   polymers??   have??   been??   the??   subject??   of??   much??   interest??   owing??   to??   their??   unique??  
electronic??  properties??  which??  can??  be??  technically??  exploited??  e.g.,??  as??  doping??  induced??  electrical??  
conductors??  and??  light??  emitting??  diodes??  [1].??  ??  Detailed??  experimental??  studies??  of??  the??  uppermost??  p-­‐
orbital??   levels??   at??   the??   valence??   band??   edges??   of??   these??   polymers??   are??   important??   to??   gain??   an??  
understanding??  of??  their??  properties.??  ??  Such??  studies??  have??  been??  carried??  out??  by??  many??  techniques??  
including??   photoelectron??   spectroscopy??   using??   photon??   excitation??   in??   both??   the??   x-­‐ray??   and??  
ultraviolet??  wavelength??  regimes.??  
??  
X-­‐ray??   emission??   (XE)??   spectroscopy??   provides??   a??   useful??   technique??   for??   studying??   conjugated??  
polymers??  but??  has??  yet??  not??  been??  exploited??  much.??  XE??  provides??  electronic??  structure??  information??  
in??  terms??  of??  local??  contributions??  to??  the??  Bloch??  or??  molecular??  orbitals??  (MO’s),??  since??  the??  x-­‐ray??  
processes??  can??  be??  described??  by??  local??  dipole??  selection??  rules.??  The??  method??  is??  atomic??  element??  
specific??  and??  also??  angular??  momentum??  and??  symmetry??  selective??  at??  high??  resolution.??  However,??  
the??  relatively??  low??  fluorescence??  yield??  and??  instrument??  efficiencies??  associated??  with??  XE??  in??  the??  

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Journal of Electron Spectroscopy and Related Phenomena 101–103 573 (1999)
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??  2??  
sub??  
considerable??   demands.??   ??   An??   intense??  
synchrotron??   radiation??   (SR)??   excitation??  
source??   is??   therefore??   required??   which??   has??  
earlier??  limited??  the??  experimental??  activity??  of??  
studying??   the??   behavior??   for??   radiative??  
emission??   spectroscopy??   of??   oligomers??   and??  
polymers.??  
??  
Non-­‐resonant??   XE??   spectra??   are??   obtained??  
when??  the??  energy??  of??  the??  incident??  photons??  
exceed??   far??   above??   the??   core??   ionization??  
threshold.??   In??   this??   case??   the??   XE??   spectral??  
profile??  is??  practically??  independent??  (besides??  
x-­‐ray??   satellites)??   of??   the??   excitation??   energy??  
and??  has??  been??  often??  described??  using??  a??  two-­‐
keV??  energy??  region??   lead??  to??  
step??  model??  with??  the??  emission??  step??  decoupled??  from??  the??  excitation??  step.??  On??  the??  other??  hand,??  
when??   the??   excitation??   energy??   is??   tuned??   at??   resonances??   below??   or??   close??   to??   a??   core??   ionization??  
threshold,??   the??   spectral??   distribution??   is??   strongly??   dependent??   on??   the??   excitation??   energy.??   The??  
description??  in??  the??  resonant??  case??  must??  therefore??  switch??  from??  a??  two-­‐step??  to??  a??  one-­‐step??  model??  
with??  the??  excitation??  and??  emission??  transitions??  treated??  as??  a??  single??  scattering??  event??  in??  resonant??  
inelastic??  x-­‐ray??  scattering??  (RIXS).??  
??  
In??  a??  recent??  work??  we??  used??  a??  set??  of??  poly(p-­‐phenylenevinylene)s;??  PPV,??  PMPV??  and??  PDPV??  to??  
demonstrate??  the??  feasability??  of??  studying??  the??  electronic??  structure??  of??  conjugated??  polymers??  by??  
means??  of??  resonant??  and??  non-­‐resonant??  XE??  measurements??  with??  monochromatic??  SR??  excitation??  
[2].??  It??  is??  of??  great??  interest??  to??  find??  out??  how??  the??  resonant??  and??  non-­‐resonant??  spectra??  show??  up??  in??  
more??  complicated??  hetero-­‐compounds,??  and??  separately??  measure??  the??  energy??  bands??  from??  the??  
different??   atomic??   elements??   involved.??   In??   the??   present??   work??   we??   analyze??   for??   this??   purpose,??  
resonant??   XE??   spectra??   of??   poly(pyridine-­‐2,5-­‐diyl)??   (PPy)??   which??   is??   an??   aza-­‐substituted??   poly(p-­‐
phenylene).??  The??  analysis??  based??  on??  ab??  initio??  canonical??  Hartree-­‐Fock??  theory??  indicate??  isomeric??  
dependence??  of??  the??  RIXS??  spectra??  [3].??  The??  resonant??  XE??  spectra??  show??  that??  the??  p??  electron??  bands??  
dissappear??  in??  the??  spectra??  due??  to??  symmetry??  selection??  and??  momentum??  conservation??  rules.??  ??  
??  
Experiment??  
The??  experiments??  were??  carried??  out??  at??  beamline??  7.0??  at??  the??  Advance??  Light??  Source??  (ALS)??  at??  the??  
Lawrence??   Berkeley??   National??   Laboratory.??   This??   undulator??   beamline??   includes??   a??   spherical-­‐
grating??   monochromator??   and??   provides??   linearly??   polarized??   SR??   of??   high??   resolution??   and??   high??  
brightness.??  
The??  XE??  spectra??  were??  recorded??  using??  a??  high-­‐resolution??  grazing-­‐incidence??  x-­‐ray??  fluorescence??  
spectrometer??   [4].??   During??   the??   XE??   measurements,??   the??   resolution??   of??   the??   beamline??  
monochromator??  was??  0.25??  eV??  and??  0.40??  eV??  for??  the??  carbon??  and??  nitrogen??  edges,??  respectively.??  
The??  x-­‐ray??  fluorescence??  spectrometer??  had??  a??  resolution??  of??  0.30??  eV??  and??  0.65??  eV,??  for??  the??  carbon??  
and??  nitrogen??  measurements.??  The??  sample??  was??  oriented??  so??  that??  the??  incidence??  angle??  of??  the??  
photons??  was??  20??  degrees??  with??  respect??  to??  the??  surface??  plane.??  During??  the??  data??  collection,??  the??  
N
N
N
N
N
N
N
N
(a)
(b)
(c)
(d)
head to head
HHC
HHT
HTC
HTT
head to tail
??  
??  
Figure??  1:??  (panels??  a,??  b,??  c??  and??  d)??  The??  isomeric??  head-­‐to-­‐
tail??  (H-­‐T)??  and??  head-­‐to-­‐head??  (H-­‐H)??  geometries??  of??  PPy.??  
The??  extra??  letter,??  T??  and??  C??  (trans??  and??  cis)??  denotes??  the??  
relative??  positions??  between??  adjacent??  nitrogen??  atoms??  
(180??  and??  0??  degrees??  torsion??  angle).??  ??  
??  

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Journal of Electron Spectroscopy and Related Phenomena 101–103 573 (1999)
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??  3??  
samples??   were??   scanned??   (moved??   every??   30??   seconds)??   in??   the??   photon??   beam??   to??   avoid??   photon-­‐
induced??  decomposition??  of??  the??  polymers.??  The??  base??  pressure??  in??  the??  experimental??  chamber??  
was??  4??  x??  10-­‐9??  Torr??  during??  the??  measurements.??  
??  
Calculations??  
Figure??   1??   illustrates??   head-­‐to-­‐head??   (HH)??   and??   head-­‐to-­‐tail??   (HT)??   repeat??   units??   of??   the??   four??  
different??  isomers??  possible??  in??  the??  PPy??  conjugated??  polymer.??  An??  extra??  letter,??  T??  or??  C??  (trans??  or??  
cis),??  denotes??  the??  relative??  positions??  between??  adjacent??  nitrogen??  atoms.??  The??  calculations??  were??  
carried??  out??  by??  taking??  a??  repeat??  unit??  (dimer)??  as??  a??  model??  molecule??  of??  the??  PPy??  polymer??  which??  
includes??  two??  polymer??  rings.??  The??  restriction??  to??  one??  single??  (dimer)??  unit??  is??  motivated??  by??  the??  
fact??  that??  PPy??  has??  relatively??  flat??  bands.??  A??  few??  test??  calculations??  on??  multiple-­‐unit??  spectra??  did??  not??  
give??  significant??  differences.??  The??  geometries??  of??  the??  model??  molecules??  were??  obtained??  by??  using??  
the??  AM1??  Hamiltonian??  [5]??  in??  the??  MOPAC??  program??  for??  a??  model??  molecule??  with??  four??  pyridine??  
rings.??  Calculations??  of??  the??  pyridine??  molecule??  were??  also??  carried??  out??  for??  comparison.??  The??  direct??  
self-­‐consistent??   field??   (SCF)??   program??   DISCO??   [6]??   has??   been??   employed??   for??   calculations??   of??   the??  
orbital??  energies??  and??  dipole??  transition??  moments.??  
??  
Results??  
Figure??  2??  shows??  a??  resonant??  XE??  spectrum??  of??  PPy??  excited??  at??  398.8??  eV??  photon??  energy??  at??  the??  N1s??  
threshold.??   The??   corresponding??   calculated??   spectra??   of??   the??   four??   different??   isomers??   and??   the??  
pyridine??   molecule??   are??   presented??   below.??   The??  
spectrum??  basically??  consists??  of??  two??  main??  parts.??  A??  
strong??  elastic??  (recombination)??  peak??  at??  398.8??  eV??  
has??   an??   energy??   position??   equivalent??   to??   the??  
excitation??  energy.??  The??  other??  part??  constitutes??  the??  
inelastic??  scattering.??  In??  PPy,??  five??  features??  (labeled??  
A-­‐E)??  can??  be??  identified??  in??  the??  inelastic??  scattering??  
part??  of??  the??  spectrum.??  Peak??  A??  corresponds??  to??  p-­‐
electron??  and??  n??  electron??  states??  (1a2??  and??  11a1(n)??  in??  
pyridine),??  peak??  B??  is??  due??  to??  both??  p??  and??  s,??  with??  s??  
dominating,??   while??   features??   C,??   D,??   and??   E,??   are??   all??  
related??   to??   s??   electronic??   states.??   The??   five??   bands??  
clearly??  show??  up??  in??  the??  calculation??  of??  the??  pyridine??  
molecule??  as??  well??  as??  for??  the??  four??  different??  isomers??  
of??  PPy??  which??  were??  obtained??  by??  taking??  the??  repeat-­‐
unit??  as??  a??  model??  molecule.??  In??  the??  polymer??  case,??  the??  
bands??   are??   broader??   due??   to??   a??   large??   number??   of??  
transitions??   involved??   in??   each??   band.??   It??   is??   also??  
interesting??  to??  note??  that??  the??  bands??  A-­‐E??  observed??  in??  
these??   RIXS??   spectra??   correspond??   well??   in??   energy??  
with??   the??   five??   bands??   observed??   in??   ultraviolet??  
photoelectron??   spectra??   (UPS)??   [7].??   However,??   the??  
intensities??  are??  different??  owing??  to??  the??  different??  nature??  of??  the??  transition??  moments??  between??  the??  
spectroscopies.??  
??  
??  
Figure??   2:??   Experimental??   and??   calculated??  
resonant??  N1s??  XE??  spectra??  of??  PPy.??  
??  

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Journal of Electron Spectroscopy and Related Phenomena 101–103 573 (1999)
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??  4??  
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In??  Fig.??  2??  the??  relative??  intensity??  of??  peak??  B??  is??  about??  73%??  as??  high??  as??  that??  of??  peak??  A,??  for??  the??  HHC??  
isomeric??  form,??  whereas??  for??  the??  other??  three??  isomers??  the??  ratio??  is??  about??  45%.??  Some??  differences??  
in??   the??   intensity??   distribution??   of??   the??   C-­‐bands??   are??   also??   predicted??   by??   the??   calculations.??   The??  
comparison??  with??  the??  experimental??  spectrum??  appear??  to??  favour??  the??  HTT??  and??  the??  HHT??  isomeric??  
forms??  and??  disfavour??  the??  HHC??  form.??  
??  
Figure??   3??   shows??   a??   resonant??   XE??   spectrum??   of??   PPy??  
excited??   at??   284.4??   eV??   photon??   energy??   at??   the??   C1s??  
threshold.??  In??  this??  case??  the??  elastic??  (recombination)??  
peak??   is??   stronger??   due??   to??   a??   different??   localization??  
character??   of??   the??   emitting??   levels.??   The??   carbon??  
spectrum??  obviously??  map??  the??  same??  final??  states??  of??  
the??   inelastic??   scattering??   part??   as??   the??   nitrogen??  
spectrum??   but??   with??   different??   transition??   moments??  
owing??   to??   the??   different??   intermediate??   states.??   Thus,??  
the??  carbon??  spectrum??  show??  a??  similar??  relative??  peak??  
position??   of??   the??   bands??   while??   the??   intensity??  
distribution??  is??  different.??  In??  both??  cases??  the??  spectra??  
show??   strong??   elastic??   (recombination)??   peaks.??   The??  
calculated??  C1s??  spectrum??  of??  the??  pyridine??  molecule??  
shows??   a??   similar??   band??   structure??   as??   the??   N1s??  
spectrum,??  
distribution??   among??   the??   bands,??   reflecting??   the??  
localization??   of??   the??   core-­‐hole??   orbitals.??   The??   most??  
conspicuous??  difference??  between??  the??  nitrogen??  and??  
carbon??   spectra??   is??   the??   intensity??   of??   peak??   A.??   It??  
becomes??  less??  intense??  in??  the??  resonant??  C1s??  spectrum,??  
since??  the??  n-­‐lone??  pair??  MO’s??  have??  only??  little??  overlap??  
with??  the??  localized??  C1s??  core??  orbital.??  On??  the??  other??  hand,??  the??  MO’s??  of??  band??  B??  have??  larger??  2p??  
contributions??  from??  the??  carbon??  atoms??  and??  so??  a??  larger??  dipole??  overlap??  with??  the??  C1s??  core??  orbitals??  
than??  with??  the??  N1s??  core??  orbitals??  is??  expected.??  For??  the??  inner??  MO’s,??  such??  as??  band??  E,??  the??  intensities??  
are??  weak??  in??  both??  the??  C1s??  and??  N1s??  spectra??  due??  to??  the??  larger??  2s??  character??  of??  the??  MO’s??  and??  also??  
due??  to??  a??  breakdown??  of??  the??  MO??  approximation??  accompanying??  correlation??  state??  splittings??  [8].??  
The??  calculated??  resonant??  spectra??  for??  the??  different??  isomers??  show??  an??  isomeric??  dependence.??  As??  
in??  the??  nitrogen??  case??  the??  best??  agreement??  between??  experiment??  and??  theory??  is??  obtained??  for??  the??  
HTT??  and??  the??  HHT??  isomeric??  forms.??  It??  has??  also??  been??  found??  that??  the??  calculated??  density-­‐of-­‐states??  
distribution??  in??  comparison??  to??  the??  measured??  UPS??  spectrum??  of??  PPy??  by??  Miyamae??  et??  al.??  [7],??  gives??  
some??  preference??  to??  the??  head-­‐to-­‐tail??  isomeric??  form.??  
??  
Discussion??  
For??  p-­‐electron??  systems,??  the??  momentum??  conservation??  leads??  to??  depletion??  of??  emission??  from??  the??  
p-­‐levels??  which??  has??  previously??  been??  observed??  in??  RIXS??  spectra??  of??  Benzene??  [9]??  and??  aniline??  [10].??  
A??  similar??  effect??  has??  also??  been??  found??  in??  conjugated??  polymer??  compounds??  such??  as??  PPV,??  PMPV??  
and??   PDPV??   [2].??   In??   both??   cases??   a??   similar??   depletion??   of??   the??   p-­‐levels??   could??   be??   obtained??  
but??   with??  a??  different??   intensity??  
??  
??  
Figure??  3:??  Experimental??  and??  calculated??  
resonant??  C1s??  XE??  spectra??  of??  PPy.??  
??  

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Journal of Electron Spectroscopy and Related Phenomena 101–103 573 (1999)
??  
??  5??  
theoretically??  by??  a??  full??  symmetry??  and??  interference??  analysis??  of??  the??  resonant??  process??  using??  the??  
repeat??  unit??  as??  a??  model.??  The??  depletion??  has??  about??  the??  same??  strength??  in??  the??  PPy??  polymer??  as??  for??  
the??  PPV??  compounds??  although??  a??  smaller??  effect??  would??  be??  expected??  for??  PPy??  due??  to??  the??  stronger??  
chemical??   shifts??   of??   the??   core-­‐excited??   states.??   However,??   a??   corresponding??   depletion??   is??   not??  
observed??   in??   the??   resonant??   nitrogen??   spectrum.??   This??   can??   be??   explained??   by??   the??   fact??   that??   the??  
strong??  high-­‐energy??  band??  in??  the??  nitrogen??  spectrum??  is??  dominated??  by??  the??  lone-­‐pair??  n??  orbitals??  
localized??  on??  the??  nitrogen??  sites,??  which??  have??  s??  symmetry.??  It??  is??  also??  relevant??  to??  consider??  the??  
different??  localization??  characters??  of??  the??  emitting??  levels??  in??  the??  interpretation??  of??  the??  resonant??  
spectra.??  
??  
Summary??  
Resonant??   inelastic??   soft-­‐x-­‐ray??   scattering??   spectra??   of??   poly(pyridine-­‐2,5-­‐diyl)??   have??   been??  
measured??  with??  monochromatic??  synchrotron??  radiation.??  The??  spectra??  exhibit??  peak??  structures??  
far??  into??  the??  valence??  energy??  region.??  The??  relative??  energy??  positions??  of??  the??  emission??  bands??  are??  
similar??  in??  the??  carbon??  and??  nitrogen??  spectra??  i.e.,??  the??  same??  types??  of??  final??  states??  are??  involved??  but??  
different??  relative??  intensities??  are??  observed??  since??  the??  localization??  properties??  of??  the??  energy??  
bands??  are??  different.??  In??  particular,??  the??  lone-­‐pair??  n??  levels??  emphasize??  the??  high??  energy??  part??  of??  
the??  nitrogen??  spectrum??  compared??  to??  the??  carbon??  spectrum.??  The??  most??  conspicuous??  difference??  
is??  the??  partial??  depletion??  of??  the??  p??  band??  in??  the??  carbon??  case??  which??  is??  also??  expected??  from??  the??  
theory??  of??  resonant??  inelastic??  x-­‐ray??  scattering??  spectra??  of??  p-­‐electron??  systems.??  The??  comparison??  
to??  canonical??  Hartree-­‐Fock??  calculations??  indicate??  isomeric??  dependence??  of??  the??  resonant??  x-­‐ray??  
emission??   spectra,??   and??   gives??   preference??   for??   two??   of??   the??   four??   isomers??   contained??   in??   the??  
conjugated??  polymer.??  
??  ??  
Acknowledgements??  
This??   work??   was??   supported??   by??   the??   Swedish??   Natural??   Science??   Research??   Council??   (NFR),??   the??  
Swedish??  Research??  Council??  for??  Engineering??  Sciences??  (TFR),??  the??  Göran??  Gustavsson??  Foundation??  
for??   Research??   in??   Natural??   Sciences??   and??   Medicine??   and??   the??   Swedish??   Institute??   (SI).??   The??  
experimental??  work??  at??  the??  ALS,??  Lawrence??  Berkeley??  National??  Laboratory??  was??  supported??  by??  
the??  Office??  of??  Energy??  Research,??  Office??  of??  Basic??  Energy??  Science,??  Material??  Science??  Division??  of??  the??  
U.??  S.??  Department??  of??  Energy,??  under??  contract??  No.??  DE-­‐AC03-­‐76SF00098.??  
??  
??  
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