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Vacuum-UV negative photoion spectroscopy of CF 3 Cl , CF 3 Br , and CF 3 I
Ist Teil von
The Journal of chemical physics, 2009-05, Vol.130 (19), p.194302-194302-11
Ort / Verlag
American Institute of Physics
Erscheinungsjahr
2009
Quelle
Alma/SFX Local Collection
Beschreibungen/Notizen
Using synchrotron radiation, negative ions are detected by mass spectrometry following vacuum-UV photoexcitation of trifluorochloromethane
(
CF
3
Cl
)
, trifluorobromomethane
(
CF
3
Br
)
, and trifluoroiodomethane
(
CF
3
I
)
. The anions
F
−
,
X
−
,
F
2
−
,
F
X
−
,
CF
−
,
CF
2
−
, and
CF
3
−
are observed from all three molecules, where
X
=
Cl
, Br, or I, and their ion yields recorded in the range of 8-35 eV. With the exception of
Br
−
and
I
−
, the anions observed show a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation. Dissociative electron attachment, following photoionization of
CF
3
Br
and
CF
3
I
as the source of low-energy electrons, is shown to dominate the observed
Br
−
and
I
−
signals, respectively. Cross sections for ion-pair formation are put onto an absolute scale by calibrating the signal strengths with those of
F
−
from both
SF
6
and
CF
4
. These anion cross sections are normalized to vacuum-UV absorption cross sections, where available, and the resulting quantum yields are reported. Anion appearance energies are used to calculate upper limits to 298 K bond dissociation energies for
D
o
(
CF
3
−
X
)
, which are consistent with literature values. We report new data for
D
o
(
CF
2
I
+
-
F
)
≤
2.7
±
0.2
eV
and
Δ
f
H
298
o
(
CF
2
I
+
)
≤
(
598
±
22
)
kJ
mol
−
1
. No ion-pair formation is observed below the ionization energy of the parent molecule for
CF
3
Cl
and
CF
3
Br
, and only weak signals (in both
I
−
and
F
−
) are detected for
CF
3
I
. These observations suggest that neutral photodissociation is the dominant exit channel to Rydberg state photoexcitation at these lower energies.