Abstract
The electron stimulated desorption (ESD) of anions is used to explore the effects of electron irradiation on a thiophene film and we report measurements for electron impact on multilayer thiophene condensed on a polycrystalline platinum substrate. Below
22
eV
and at low electron dose, desorbed anions include
H
−
(the dominant signal) as well as
S
−
,
CH
2
−
,
SH
−
and
SCH
2
−
. Yield functions show that anions are desorbed both by dissociative electron attachment (DEA) with resonances observed at 9.5, 11, and
16
eV
, and for energies
>
13
eV
, by dipolar dissociation (DD). An increase in the
S
−
signal from electron irradiated (beam-damaged) thiophene films and the appearance of a new DEA resonance in the
S
−
yield function at
6
eV
are linked to rupture of the thiophene ring and the formation of sulfur-terminated products within the film. The threshold energy for ring rupture is
5
eV
. The desorption of new anions such as
C
4
H
3
S
−
(
Thiophene
-
H
)
−
is also observed from electron irradiated films and these likely arise from the decomposition of large radiation product molecules synthesized in the film. The yield functions of
H
−
,
S
−
,
SH
−
,
(
Thiophene
-
H
)
−
, and
(
Thiophene
+
H
)
−
anions from irradiated thiophene films that have been annealed to
300
K
, each exhibit a single resonant feature centered around
5.1
eV
, suggesting that all signals derive from DEA to the same molecular radiation product. In contrast, only
H
−
and
S
−
are observed to desorb from films of 2-2-bithiophene and no resonance is seen below
∼
10
eV
in the anion yield functions. These data suggest that electron irradiation causes formation of ring-opened oligomers, and that closed-ring or 'classical" oligomers, (similar to bithiophene) if formed, contribute little to the ESD of anions.