Surface Chemistry and Radiation Chemistry of Trifluoroiodomethane (CF 3I) on Mo(110)

Article abstract of DOI:10.1021/jp0310192

The surface-induced and electron-induced chemistry of trifluoroiodomethane (CF₃I), a potential replacement for chlorofluorocarbons (CFCs) and chlorofluorobromocarbons (halons), were investigated under ultrahigh vacuum conditions (p a?? 1 a?? 10^-10 Torr) on Mo(110). Results of temperature-programmed desorption (TPD) experiments indicate that dissociative adsorption of CF₃I leads only to nonselective decomposition on Mo(110), in contrast to reactions of CF₃I on other metal surfaces. Desorption of CF₃ radicals and atomic iodine was detected mass spectrometrically during low-energy (10-100 eV) electron irradiation of four monolayer thick films of CF₃I condensed at 100 K. Results of postirradiation temperature-programmed desorption experiments were used to identify CF₂I₂, C₂F₅I, C ₂F₆, C₂F₄I2, and CFI₃ as electron-induced reaction products of CF₃I. Except for CFI ₃, all of these electron-induced reaction products of CF₃I have been previously identified in ?3-radiolysis studies, supporting our earlier claim that temperature-programmed desorption experiments conducted following low-energy electron irradiation of multilayer thin films provide an effective method to investigate the effects of high-energy radiation, including radical-radical reactions.

Full text of DOI:10.1021/jp0310192

4080
J. Phys. Chem. B 2004, 108, 4080-4085
Surface Chemistry and Radiation Chemistry of Trifluoroiodomethane (CF3I) on Mo(110)
Nozomi Nakayama, Elizabeth E. Ferrenz, Denise R. Ostling, Andrea S. Nichols,
Janelle F. Faulk, and Christopher R. Arumainayagam*
Department of Chemistry, Wellesley College, Wellesley, Massachusetts 02481
ReceiVed: August 27, 2003; In Final Form: December 30, 2003
The surface-induced and electron-induced chemistry of trifluoroiodomethane (CF3I), a potential replacement
for chlorofluorocarbons (CFCs) and chlorofluorobromocarbons (halons), were investigated under ultrahigh
vacuum conditions (p ∼ 1 × 10-10 Torr) on Mo(110). Results of temperature-programmed desorption (TPD)
experiments indicate that dissociative adsorption of CF3I leads only to nonselective decomposition on Mo-
(110), in contrast to reactions of CF3I on other metal surfaces. Desorption of CF3 radicals and atomic iodine
was detected mass spectrometrically during low-energy (10-100 eV) electron irradiation of four monolayer
thick films of CF3I condensed at 100 K. Results of postirradiation temperature-programmed desorption
experiments were used to identify CF2I2, C2F5I, C2F6, C2F4I2, and CFI3 as electron-induced reaction products
of CF3I. Except for CFI3, all of these electron-induced reaction products of CF3I have been previously identified
in γ-radiolysis studies, supporting our earlier claim that temperature-programmed desorption experiments
conducted following low-energy electron irradiation of multilayer thin films provide an effective method to
investigate the effects of high-energy radiation, including radical-radical reactions.
1. Introduction
bond that carbon forms. Desorption of CF4 has been reported
in the reactions of CF3I on Ni(111)12 and Ru(001).13,14 CF3
radical desorption has been observed during the reaction of CF3I
on Ag(111),15-17 Ni(100),18,19 Ni(111),12 Pt(111),20 and Ru-
(001).13 CF2 fragments desorb during temperature-programmed
desorption of CF3I on Pt(111),20 Ni(111),12 and Ru(001).14,13
Following adsorption of CF3I on Cu(111), cleavage of a single
C-F bond in adsorbed CF3 results in the selective formation
of adsorbed CF2 moieties whose subsequent coupling yields
gaseous C2F4.21
The surface chemistry and radiation chemistry of trifluor-
oiodomethane (CF3I) have recently come under increased
scrutiny because CF3I is being considered for many important
industrial applications. The attractiveness of trifluoroiodomethane
may be attributed to its excellent chemical and physical
properties as evidenced by near-zero ozone-depletion potential
(ODP), extremely low global warming potential (GWP),1 good
long-term stability at ambient conditions,2 and relatively low
toxicity.3 CF3I may eventually replace CF4, a gas with a high
global warming potential, in semiconductor device fabrication
because recent studies have demonstrated that CF3I can be used
for plasma treatment of silicon dioxide,4 an important fabrication
process which involves the etching of silicon surfaces with a
mixture of energetic neutrals, ions, and electrons. Because CF3I
has been extensively promoted as an alternate fire suppression
agent to Halon 1301 (CF3Br), a potent ozone-depleting halon,
the photon-induced degradation of CF3I has become an impor-
tant area of study.5 Radiation-induced dissociation of adsorbed
CF3I has recently been shown to be a facile method for the
fluorination of diamond surfaces,6-8 a potentially important
process in diamond thin film applications. Although CF3I was
recently rejected for use as a coolant for particle detectors in
the large hadron collider (LHC) at CERN due to its chemical
instability when exposed to ionizing radiation,9 CF3I, especially
as a component in mixtures, has been endorsed as a high-
performance, environmentally sound refrigerant for other in-
dustrial uses.10 On the basis of the results of pulse radiolysis
studies, it was concluded that CF3I is the most promising
candidate for an electron-beam initiated atomic iodine laser.11
In contrast to the reactions on Mo(110) we will describe here,
reactions of CF3I on other metal surfaces yield gaseous reaction
products containing at least one C-F bond, the strongest single
Electron-induced reactions of CF3I have been previously
investigated. The role of dissociative electron attachment in the
desorption of negative ions during the electron irradiation of
condensed and gaseous CF3I has been carefully delineated.22-24
Both CF2I2 and C2F3I have been identified in postirradiation
(with 100 eV electrons) temperature-programmed desorption
experiments of CF3I multilayers on Ag(111).25 The desorption
of CF3 and I- during photolysis of CF3I on Ag(111) has been
attributed to hot electrons produced at the Ag(111) surface by
the incident photons.26 Low energy electron diffraction (LEED)
and electron-stimulated desorption ion angular distribution
(ESDIAD) have been used to probe the electron-induced
dissociation of CF3I on Ru(001).27 Results of postirradiation
temperature-programmed desorption experiments demonstrate
carbon-carbon coupling in CF3I multilayer films on Ni(100).19
We report the use of temperature-programmed desorption
experiments to investigate the surface chemistry of CF3I on Mo-
(110) under ultrahigh vacuum conditions. In addition, we report
the use of isothermal electron-stimulated desorption experiments
to study the neutral species desorbed during the interactions of
low-energy (10-100 eV) electrons with multilayer adsorbed
films of CF3I on Mo(110). Such studies have previously been
shown to provide detailed information regarding the dynamics
of electron-induced condensed phase reactions relevant to
radiation chemistry and physics.28-30 We also report on the
results of postirradiation temperature-programmed desorption
* Corresponding author: Telephone 781-283-3326. Fax: 781-283-3642.
E-mail: carumain@wellesley.edu.
10.1021/jp0310192 CCC: $27.50 © 2004 American Chemical Society
Published on Web 03/06/2004

Chemistry of CF3I on Mo(110)
J. Phys. Chem. B, Vol. 108, No. 13, 2004 4081
studies of multilayer films of CF3I on Mo(110). We have
previously demonstrated that temperature-programmed desorp-
tion experiments conducted following low-energy electron
irradiation of multilayer thin films provide an effective method
to investigate the effects of high-energy radiation, including
ion-molecule and radical-radical reactions.31
2. Experimental Section
Experiments were performed in a custom-designed stainless
steel ultrahigh vacuum chamber with a base pressure of 1 ×
10-10 Torr described in detail previously.31 The Mo(110) single
crystal can be cooled to 100 K with liquid nitrogen and heated
to 800 K radiatively or to 2200 K by electron bombardment.
The crystal temperature was measured by a tungsten-rhenium,
W-5% Re vs W-26% Re, thermocouple spot-welded to the edge
of the crystal. The Mo(110) crystal was cleaned in situ with
oxygen followed by heating briefly to 2200 K, exceeding the
desorption temperature of oxygen and sulfur. The carbon
coverage after repeated oxygen cleaning cycles was determined
to be less than the Auger detection limit. Low energy electron
diffraction was used to verify the surface structure of the Mo-
(110) single crystal.
Trifluoroiodomethane (Synquest Labs; lot assay > 99% pure)
was used without further purification. In situ mass spectrometry
was used to verify the absence of potential contaminants such
as hexafluoroethane. Direct dosers with precision leak valves
allowed for controlled deposition at normal incidence of the
CF3I onto the Mo(110) crystal surface at 100 K to obtain
multilayer coverages of approximately four layers prior to
electron-stimulated desorption and postirradiation temperature-
programmed desorption experiments. Film thickness was esti-
mated based on temperature-programmed desorption experi-
ments. One monolayer (1 ML) is defined as the maximum
exposure of CF3I that does not yield a multilayer peak.
The UTI model 100 C quadrupole mass spectrometer is
enclosed by a shield with a 1/8 in. diameter aperture to optimize
detection of the molecules desorbing directly from the center
of the crystal during temperature-programmed desorption and
isothermal electron-stimulated desorption experiments. Although
up to 100 masses could be monitored under a multiplexing
arrangement, typically only 10 masses were sampled during a
single experiment.
Figure 1. Low temperature thermal desorption data of CF3I on Mo-
(110) in the absence of electron irradiation demonstrating the multilayer
and monolayer desorption of CF3I at ∼130 and ∼160 K, respectively.
close to the mass spectrometer filament during temperature-
programmed desorption experiments, the crystal was negatively
biased (-100 V) prior to each thermal desorption experiment
to prevent further electron-induced reactions. As the surface was
heated from 100 to 700 K via radiative heating at ∼7 K/s,
temperature-programmed desorption data were collected with
the mass spectrometer and the thermocouple, both of which were
interfaced to a computer.
Charging of the thin film during irradiation may change the
incident electron energy. To reduce the effect of surface
charging, film thickness was minimized to decrease the distance
to the conducting metal surface. Surface charging may result
in an uncertainty of a few electronvolts in our energy scale.
3. Results and Discussion
3.1. Surface Chemistry of CF3I on Mo(110). Results of
temperature-programmed desorption experiments demonstrate
that multilayers of CF3I desorb at ∼130 K followed by the
monolayer at ∼160 K (Figure 1). The very small CF3I
desorption peak at ∼145 K is very similar to a desorption peak
attributed to the influence of atomic iodine on monolayer CF3I
desorption from Pt(111).20 During a comprehensive search of
fragments up to m/z ) 300, no gaseous reaction products were
detected in low temperature (up to 700 K) thermal desorption
experiments. Results of subsequent high temperature (up to 1400
K) thermal desorption experiments (data not shown), demon-
strate the desorption of atomic iodine (m/z ) 127 but not m/z
) 254) as a broad peak at ∼1100 K and atomic fluorine (m/z
) 19 but not m/z ) 38) as two very broad overlapping peaks
at ∼900 K and ∼1200 K. Oxygen titration experiments (data
not shown), following low temperature thermal desorption
experiments, demonstrate the presence of adsorbed atomic
carbon, providing additional evidence for the nonselective
decomposition of CF3I into its atomic constituents. In contrast
to reactions on other metal surfaces from which C-F-containing
species desorb, as described previously, dissociatiVe adsorption
of CF3I on Mo(110) results in the facile and total cleavage of
the very strong C-F bond. However, nondissociative adsorption
of CF3I on Mo(110) is a significant competing process as
evidenced by the monolayer peak at ∼160 K (Figure 1).
3.2. Isothermal Electron Stimulated Desorption of CF3I.
Isothermal electron-stimulated desorption experiments were
The mass spectrometer filament or the LEED electron gun
was used to irradiate the CF3I thin films. All electron irradiation
experiments were conducted with the crystal grounded. At an
ionization energy setting of 70 eV, the mass spectrometer
filament provides an electron flux of ∼2 × 1013 cm-2 s-1 (lower
limit because scattered electrons are not counted) with an energy
of 55 eV at the grounded sample surface. Because the LEED
electron gun, with an electron flux of ∼3 × 1013 cm-2 s-1
,
provides a relatively monochromatic electron beam, the LEED
electron gun was used to perform electron-stimulated desorption
experiments as a function of electron energy. As described
below, the LEED electron gun was also used to verify that the
species desorbing during electron irradiation were neutral and
not positively charged ions. During electron-stimulated desorp-
tion experiments carried out with the LEED electron gun, the
shutter in front of the mass spectrometer ionizer was moved to
allow the mass spectrometer to sample the entire UHV chamber.
All electron-stimulated desorption experiments were conducted
at a surface temperature of ∼100 K.
Following electron irradiation (electron fluence of ∼2 × 1015
cm-2), temperature-programmed desorption experiments were
conducted to identify radiolysis products. Since the crystal is

4082 J. Phys. Chem. B, Vol. 108, No. 13, 2004
Nakayama et al.
Figure 3. Isothermal electron-stimulated desorption yield of CF3
radicals, corrected for small variations in incident electron flux, plotted
as a function of incident electron energy.
Figure 2. Isothermal experiment depicting the desorption of CF3I, CF3
radicals, and atomic iodine during 55 eV electron irradiation of CF3I
(four layers) initiated at t ∼ 13 s. The mass spectrometer filament was
used as the irradiation source to produce an incident electron flux of
1.6 × 1013 cm-2 s-1. The unusual time dependence of the I+ signal is
due to an upward sloping background signal.
conducted with CF4 to ascertain the CF4 fragment intensities in
our mass spectrometer. The relative CF3+:CF2+:CF+ intensity
ratios for CF4 and electron-stimulated desorption experiments
of CF3I (after subtracting the CF3I contribution) were found to
be 100:12:6 and 43:100:87, respectively. Since these two ratios
differ significantly, CF4 alone cannot account for the desorption
features observed during electron irradiation of CF3I thin films.
Our results are suggestive of CF3 radical desorption, although
we cannot rule out the concomitant desorption of CF, CF2, or
CF4.
Additional evidence for the desorption of CF3 radicals during
electron irradiation of CF3I is provided by a previously published
mass spectrum of CF3 radicals. The CF3+:CF2+:CF+ intensity
ratio of CF3 radicals produced in thermal desorption experiments
was found to be 42:100:42,19 in good agreement with the ratio
determined in the experiments described herein. This comparison
is reasonable given the work of White and co-workers who have
demonstrated that the fragmentation pattern of excited-state CF3
radicals desorbed during photon irradiation of CF3I is the same
as that of ground-state CF3 radicals produced in thermal
desorption experiments.26
conducted to investigate the desorption of radiolysis products
during electron irradiation. These experiments were conducted
as follows: while the surface temperature was kept constant at
∼100 K, the mass spectrometer signal was monitored as a
function of time as the CF3I-covered crystal was rotated to face
the mass spectrometer (or LEED gun). As described in detail
below, desorption of trifluoroiodomethane (CF3I), iodine (I),
and trifluoromethyl radicals (CF3) occurs when the crystal comes
into alignment with the mass spectrometer filament (or LEED
gun).
On the basis of the relative CF3I mass spectral intensities
found in temperature-programmed desorption data (Figure 1),
CF3I fragments account for only a fraction of the desorption
intensities for m/z ) 127, 69, 50, and 31 in the electron-
stimulated desorption experiments (Figure 2). Hence we con-
clude that one or more species, in addition to CF3I, must desorb
during electron irradiation of CF3I thin films.
A. Identification of Atomic Iodine. The presence of a
desorption feature for m/z ) 127 (and not for m/z ) 254) that
cannot be attributed to CF3I alone indicates that atomic iodine
(and not molecular iodine) desorbs during electron irradiation
of CF3I thin films (Figure 2).
The desorption of CF3 radicals during electron irradiation of
CF3I thin films is also consistent with previous studies of
radiation-induced decomposition of CF3I. Excited-state CF3
radicals have been previously postulated to account for the
radiolysis products of CF3I.32 CF3 radicals are also produced
during the infrared-multiphoton decomposition of CF3I.33,34
C. Dependence of Desorption Yield on Electron Energy.
The electron energy dependence of the desorption yield (Figure
3) suggests electron impact excitation or ionization as the
mechanism for the formation of the CF3 radicals. As the incident
electron energy is increased from 10 to 100 eV, the electron-
stimulated desorption yield of CF3 radicals increases monotoni-
cally with electron energy. Because of the limitations of the
electron gun, isothermal electron-stimulated desorption experi-
ments were not conducted at incident electron energies below
10 eV, an energy regime where dissociative electron attachment
is strongest. Moreover, the signal at higher energies may be
due to dissociative electron attachment induced by very low
energy (<10 eV) electrons resulting from ionization in the CF3I
film. However, the use of a four-monolayer thick film minimizes
the possibility of energy loss due to multiple scattering followed
by dissociative electron attachment at lower electron energy.
On the basis of the results displayed in Figure 3, we suggest
that dissociative electron attachment is not dominating in the
B. Identification of CF3 Radicals. To verify that desorption
of positive ions such as CF3+ did not contribute to the observed
desorption features for m/z ) 69, 50, and 31, isothermal electron-
stimulated desorption experiments were conducted with the
LEED electron gun as the irradiation source. The desorption
features disappeared in these experiments when the mass
spectrometer ionization energy was reduced to ∼15 eV,
demonstrating that the observed ion signal is due to uncharged
species. Because 15 eV is sufficient to ionize most radicals and
neutral species, the inability to see a signal at 15 eV must be
due to the poor ionization efficiency at this low energy rather
than the ionization energy being below the ionization potential
of the desorbed species. These results demonstrate that the
electron-stimulated desorption signals at m/z ) 69, 50, and 31
are solely due to neutral species.
Additional experiments were conducted to verify the identity
of the desorbing neutral species. Because tetrafluoromethane
(CF4) does not have a parent peak at m/z ) 88 in its mass
spectrum, temperature-programmed desorption experiments were

Chemistry of CF3I on Mo(110)
J. Phys. Chem. B, Vol. 108, No. 13, 2004 4083
observed production of CF3 radicals and that the mechanism
involves electronic excitation or ionization of the condensed
CF3I.
Hence we propose three possible routes to the formation of
CF3 radicals.
1. Electron impact excitation (not followed by dipolar
dissociation):
CF3I f CF3I* f •CF3 + I•
2. Electron impact ionization followed by fragmentation:
(1)
CF3I f CF3I+ + e- f I++ •CF3 + e-
(2)
3. Electron impact ionization followed by ion-molecule
reaction
CF3I+ + CF3If CF3I2+ + •CF3
(4)
Figure 4. Postirradiation (40 eV electrons at a fluence of 2 × 1015
cm-2) temperature-programmed desorption data of CF3I multilayers
(four layers) showing the desorption of C2F6, C2F5I, CF2I2, C2F4I2, I2,
and CFI3.
D. Cross Section Measurement. The total effective degrada-
tion cross section (σeff) of condensed phase CF3I for electron-
induced processes including electron-induced desorption was
calculated from results of isothermal electron-stimulated de-
sorption experiments. The cross section analysis assumes that
CF3 derives exclusively from CF3I and not from any of the
products. The mass spectrometer signal as a function of time
for m/z ) 69 (CF3+) was fitted to the following equation to
obtain the total effective degradation cross-section:
tion of radiolysis products, the desorption temperature and
boiling point trends of the radiolysis products were also
compared.
Results of postirradiation temperature-programmed desorption
experiments of CF3I are shown in Figure 4. Desorption peaks
for m/z ) 100 (C2F4+) and 119 (C2F5+) at ∼129 K were
assigned to C2F6. The absence of the parent peak m/z ) 138
(C2F6+) at ∼129 K is consistent with a previously published
effFt
Y ) Y0e-σ
(5)
mass spectrum of C2F6.37 Desorption peaks for mass fragments
m/z ) 246 (C2F5I+), 227 (C2F4I+), 119 (C2F5+), and 100 (C2F4
)
+
where Y and Y0 are the mass spectrometer signal at time t and
t ) 0 respectively, and F is the electron flux. The electron flux
is given by F ) I/Ae where I is the incident electron current, A
is the area of the electron beam incident on the crystal, and e is
the charge of an electron. The measured electron flux is a lower
limit because scattered electrons are not counted. The effective
total degradation cross section for condensed phase CF3I at
incident electron energy of 70 eV was calculated to be σeff ) 3
× 10-16 cm2. This value is of the same order of magnitude as
the gas-phase CF3I electron impact ionization cross section of
9.0 ( 0.9 × 10-16 cm2 at an electron energy of 70 eV.35
3.3. Postirradiation Temperature-Programmed Desorp-
tion Experiments. Results of postirradiation temperature-
programmed desorption experiments were used to identify CF2I2,
CFI3, C2F5I, C2F4I2, and C2F6 as radiolysis products of CF3I as
described in detail below.
A. Identification of Radiolysis Products. The electron-
induced reaction products were identified by comparing frag-
ments found at a given temperature in the thermal desorption
data to those found in known mass spectra.36,37 When monitoring
mass fragments such as CF+, CI+, CFI+, and CF2I+ that were
also fragments of CF3I, the crystal was heated to ∼160 K, above
the desorption temperature of CF3I, following irradiation, to
remove the multilayers and monolayer of CF3I before conduct-
ing temperature-programmed desorption experiments. This
procedure allowed for the detection of radiolysis product
desorption peaks at temperatures above 160 K that would have
been too small to detect in the presence of the large CF3I
multilayer and monolayer peaks. In identifying the radiolysis
products, the results of the postirradiation temperature-
programmed desorption experiments were also compared to
previously identified γ-radiolysis products38-44 of CF3I: I2, CF4,
C2F6, CF2I2, C2F5I, and C2F4I2. To further verify the identifica-
observed at ∼165 K were assigned to C2F5I. Desorption peaks
for mass fragments m/z ) 285 (CFI2+), 266 (CI2+), 254 (I2+),
177 (CF2I+), 158 (CFI+), 139 (CI+), 31 (CF+) detected at ∼193
K were assigned to CF2I2. The presence of the fragment m/z )
254 (I2+) is consistent with published36 and unpublished,45 mass
spectra of CF2I2. Although the mass fragments m/z ) 285 and
266 are absent in published mass spectra,37,36 these fragments
are present in an unpublished mass spectrum45 of CF2I2.
Desorption peaks for mass fragments m/z ) 254 (I2+), 227
(C2F4I+), 100 (C2F4+), and 81 (C2F3+) at ∼213 K were assigned
to C2F4I2. Desorption features for mass fragments m/z ) 285
(CFI2+), 158 (CFI+), and 139 (CI+) at ∼233 K were identified
as CFI3 and not CF2I2 because a peak was absent for the mass
fragment m/z ) 177 (CF2I+) at ∼233 K. A mass spectrum of
CFI3 was not available for direct comparison. The parent peaks
of CFI3 (m/z ) 412), CF2I2 (m/z ) 304), and C2F4I2 (m/z )
354) could not be monitored because the UTI model 100C mass
spectrometer is only capable of monitoring mass fragments up
to 300. Previous γ-radiolysis studies of CF3I have identified
CF2I2, C2F5I, C2F4I2, and C2F6 as radiolysis products of
CF3I.38-44 Our identification of CFI3, however, represents a new
finding.
The results of postirradiation temperature-programmed de-
sorption experiments discussed above do not provide evidence
for CF4, a previously known γ-radiolysis product of CF3I.38-40,43
The failure to identify CF4 as a radiolysis product of CF3I may
be attributed to a combination of three factors: (1) CF4 does
not have a parent peak (m/z ) 88) in its mass spectrum, (2) the
other mass spectral fragments (CF3+, CF2+, CF+, and F+) of
CF4 are also fragments of CF3I,37 and (3) CF4 has a lower
desorption temperature than CF3I.
The boiling point trend parallels the desorption temperature
trend for the radiolysis products, providing additional evidence

4084 J. Phys. Chem. B, Vol. 108, No. 13, 2004
Nakayama et al.
for the identification of CF2I2, C2F5I, C2F4I2, CFI3, and C2F6.
The boiling points of CF3I and its radiolysis products are as
follows: C2F6 (195 K)37 < CF3I (250.6 K)37 < C2F5I (286 K)37
< CF2I2 (374 K)45< C2F4I2 (386 K)37 < CFI3 (477.5 K).46 The
increase in boiling points is consistent with the increase in
desorption temperatures: C2F6 (∼129 K) < CF3I (∼132 K) <
C2F5I (∼165 K) < CF2I2 (∼193 K) < C2F4I2 (∼215 K) < CFI3
(∼233 K).
necessary to obtain a better understanding of the electron-
induced reaction mechanisms of CF3I.
4. Conclusion
The surface chemistry and radiation chemistry of trifluor-
oiodomethane (CF3I), a potential replacement for chlorofluo-
rocarbons (CFCs) and chlorofluorobromocarbons (halons), were
investigated under ultrahigh vacuum conditions. In contrast to
reactions on other metal surfaces, dissociative adsorption of CF3I
leads only to nonselective decomposition on Mo(110). Low-
energy electron-induced reactions of CF3I produce gas-phase
CF3 radicals during electron-stimulated desorption experiments.
Postirradiation thermal desorption studies demonstrate that CFI3,
CF2I2, C2F6, C2F5I, and C2F4I2 are electron-induced radiolysis
products of CF3I. Our results demonstrate that temperature-
programmed desorption experiments conducted following low-
energy electron irradiation of multilayer thin films provide an
effective method to investigate the effects of high-energy
radiation, including radical-radical reactions.
Additional postirradiation temperature-programmed desorp-
tion experiments to probe the dependence of product yield on
electron energy and electron fluence are currently underway.
B. Postulated Mechanisms for CF3I Radiolysis. Radical-
radical reactions play a central role in the radiation chemistry
of CF3I. As described previously, we have identified both CF3
and I radicals in electron-stimulated desorption experiments.
•
Formation of CF2I radicals has been hypothesized previously
in γ-radiolysis studies of liquid and gas-phase CF3I39,40 and
electron-induced chemistry of adsorbed CF3I.19,25 Formation of
difluorocarbene (:CF2) has been previously postulated in the
radiolysis of CF3I.38 Below we postulate several reaction
mechanisms, involving the aforementioned intermediates, to
explain the radiolysis of CF3I.
Acknowledgment. C.R.. gratefully acknowledges partial
support from the donors of the Petroleum Research Fund,
administered by the American Chemical Society, a Cottrell
College Science Award of the Research Corp., a Henry-Dreyfus
Teacher-Scholar award from the Camille and Henry Dreyfus
Foundation, and a Brachman Hoffman fellowship from Welle-
sley College. Support for student researchers came from the
Beckman Foundation, the National Science Foundation, and the
Howard Hughes Medical Institute. We gratefully acknowledge
several useful discussions with Professor Evan Williams (Uni-
versity of California, Berkeley) and Dr. Andrew Bass (Univer-
sity of Sherbrooke, Sherbrooke, Canada). We thank Amanda
Mohling and Julia Forman for help with the initial experiments.
Three different reaction channels can be proposed for the
formation of CF2I2. The •CF2I radicals could abstract an iodine
atom from molecular iodine (eq 6)47 or CF3I (eq 7),39 or react
with atomic iodine (eq 8).39
•CF2I + I2 f CF2I2 + •I
•CF2I + CF3I f CF2I2 + •CF3
•CF2I + •I f CF2I2
(6)
(7)
(8)
C2F5I could be formed by a carbene insertion reaction (eq 9) or
a radical-radical reaction (eq 10):
References and Notes
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(4) Sanabia, J. E.; Moore, J. H.; Tossell, J. A. J. Chem. Phys. 2002,
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:CF2 + CF3I f C2F5I
•CF2I + •CF3 f C2F5I
(9)
(10)
•
C2F4I2 may be formed by the dimerization of two CF2I
radicals (eq 11) or by a carbene insertion reaction (eq 12).
(5) Nyden, M. R. NIST Spec. Publ. 1995, 890, 77-95.
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