JOURNAL OF
ELECTRON SPSCTROSCOPY
andRelated
Phenomena
ELSEVIER
Journal of Electron Spectroscopy and Related Phenomena 70 (1994) 29-37
Threshold photoelectron
spectroscopy of CF4 up to 60.5 eV
A.J. YenchaaT*, A. Hopkirkb, A. Hiraya”, G. Dujardind, A. Kvarane,
L. Hellnerd, M.J. Besnard-Ramaged, R.J. Donovanf, J.G. Goodef, R.R.J. Maierf,
G.C. Kingg, S. Spyroud>’
aDepariment of Physics and Department of Chemistry, State University of New York at Albany, Albany, NY 12222, USA
bSERC Daresbury Laboratory, Daresbury, Warrington WA4 4AD, UK
‘Institute for Molecular Science, Myodaiji, Okazaki 444, Japan
‘Laboratorie de Photophysique Moleculaire du CNRS, Bdtiment 213. Universitk Paris-Sud, 91405 Orsay Cedex. France
eScience Institute, University of Iceland, Dunhaga 3. 107 Reykjavik, Iceland
‘Department of Chemistry, University of Edinburgh, West Mains Road. Edinburgh EH9 3JJ, UK
gDepartment of Physics, Schuster Laboratory, Manchester University, Manchester Ml3 9PL. UK
First received 7 January 1994; in final form 12 February 1994
Abstract
The threshold photoelectron spectrum of CF4 up to 60SeV has been recorded using synchrotron radiation and a
penetrating-field electron spectrometer of high resolution. Our results confirm the broad spectral features of the Z%*Ti, A
2T2 and B *E state bands of CFZ reported previously with He I PES. The influence of autoionizing super-excited states of
CF4 has been observed as an extension of the high-energy tail of the B *E state, as a structured band in the energy region
between the B *E and C *T2 states, and as additional vibrational structure extending the fi *A, state band. Improved
spectra of the C *T2 and fi *A, states have been obtained as a consequence of the high resolution of this study. The
overall electronic state band profiles observed here up to about 26eV agree with a recently published low-resolution
threshold photoelectron spectrum of CF* [Creasey et al., Chem. Phys., 174 (1993) 4411. Between 26 and 60.5 eV a
continuum-like background was observed with some broad-band features superimposed on it. One broad peak, centered on 40.3 eV, is tentatively assigned to the formation of the E *T2 and F *Ai states of CF:, but the formation of the
double-charged ion, CFY, may also be involved.
Keywords:
Carbon tetrafluoride; TPES
1. Introduction
Threshold
[l] is proving
photoelectron
spectroscopy
(TPES)
to be a very useful means of gaining
* Corresponding author.
’Permanent address: Theoretical and Physical Chemistry Institute, The National Hellenic Research Foundation, 48 Vassileos
Constantinou Avenue, Athens 116 35, Greece.
direct information
about the ionic states of molecules
and additionally,
indirect
information
about super excited states of neutral molecules.
As in the case of conventional
photoelectron
spectroscopy
(PES),
technological
advances
have
engendered
its development
and popularity.
For
PES [2], and its higher-energy
counterpart
X-ray
photoelectron
spectroscopy
(XPS) [3], it was the
0368-2048/94/$07.00
0 1994 Elsevier Science B.V. All rights reserved
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A.J. Yencha et al./J. Electron Spectrosc. Relat. Phenom. 70
the other part of the work reported here synchrotron radiation exiting the SRS of the Daresbury Laboratory on beam line 3.2 was dispersed
by a 5m normal incidence McPherson monochromator. The energy resolution of the TPES
spectra obtained was AE/E M 5.1 x 10e4 (AE z
10.2meV FWHM at 20.0eV). Both electron
spectra collected were corrected for the variation
in the photon flux as determined from the photoemission of a copper mesh located in line with the
photon beam.
The threshold electron spectrometer was based
on the one described by King et al. [5] and
improved by Hall et al. [6]. The spectrometer was
tuned to collect zero energy photoelectrons produced by the ionization of argon gas at 15.759 eV
(2P312)using the penetrating-field technique [ 161for
both calibration
and resolution
maximizing
purposes. By this method a potential well is formed
in the interaction region by field penetration of an
extracting electrode through a 5 mm aperture in a
screening electrode that surrounds the interaction
region. This penetrating field preferentially draws
out the zero energy electrons with a very large
collection angle and furthermore forms a crossover point in the electron trajectories which
ensures efficient transmission of the photoelectrons through the subsequent electron optics.
These optics consist of a combination of two
triple-aperture lenses which image the crossover
point onto the entrance slit of the 127” cylindrical
deflection analyzer (CDA). The lens combination
provides very good spatial definition of the electron
beam and gives the spectrometer very high sensitivity. The purpose of the CDA is to remove the highenergy tail in the spectrometer transmission due to
those energetic photoelectrons that are emitted in
the direction of the extraction electrode. Electrons
transmitted by the CDA are detected using a channel
electron multiplier (Mullard X919/BLOl). A personal
computer was used to control the experiment and to
store synchronously the incident photon energy and
flux and the detected electron counts.
The sample gas emanated as an effusive beam
from a platinum needle of internal bore 0.8mm,
the exit of which was located about 1 mm above
the photon beam and about 12mm from the
entrance of the electron spectrometer, all being
( 1994) 29-37
31
mutually perpendicular. The pressure of sample
gas in the doubly, mu-metal shielded main chamber was about 7 x 10M5Torr. It was possible to
adjust independently the potential applied to the
needle, which considerably aided the optimization
of the conditions necessary for threshold studies,
i.e. the elimination of local patch fields and fields
due to contact potential variations.
3. Results and discussion
The TPES of CF4 over the photon energy range
15.5-60.5eV is shown in Fig. 1. It consists of a
composite of two spectra using two different synchrotron light sources as described above. In the
low energy range (< 27 eV) the outer valence bands
for formation of the X 2T,, A 2T2, B 2E, C 2T2, and
D 2Ai states of CF: are clearly observed and identified, while at higher energy the formation of the
E 2T2 and F 2A, states of CF: can only be indicated, based on the XPS of CF4 [lO,ll]. The overall appearance of the TPE spectrum presented here
up to about 26 eV is similar to a recently published
low-resolution TPE spectrum of CF, [7b]. In Fig. 2
the three lowest energy bands are shown in
expanded form. These three bands are very similar
to those observed in the He I PES of CF4 [8] in that
the X 2T1 and A 2T2 state bands are broad and
structureless (except for the peaks due to a slight
nitrogen impurity, most of which are identified in
Fig. 2) confirming the strongly dissociative nature
of these states [17]. The vertical energies of the
X ‘Ti and A ‘T2 states of CF: measured here are
16.33 i 0.02 and 17.39 * 0.02 eV, respectively, in
good agreement with the previous He I PES results
of 16.20 and 17.40eV, respectively [8]. The rather
large uncertainties ascribed to our values are a
reflection of the broadness of the band maxima.
The B ‘E state of CF: (Fig. 2) shows some vibrational structure superimposed on a broad continuum background with a long structureless tail
extending out to about 20 eV. This vibrational structure, which has been identified as a doublet series in
vl, separated by one quantum of y [9], is essentially identical in position, separation and resolution to that found in the low temperature (168 K)
He I PES of CF4 [9]. The lack of improved
32
A.J. Yencha et al./J. Electron Spectrosc. Relat. Phenom. 70 (1994) 29-37
11.
15
Is
20
I
25
-
I
30
*
1
35
a
1
40
I
13
45
I.1
50
a
55
1.1
60
Photon Energy I eV
Fig. 1. Threshold photoelectron spectrum of CFd over the full photon energy range studied showing the bands representing the
formation of the X ‘T,, A *T,, R *E, C’T, and D*A, states of CF:, together with the band-maxima positions of the fi*Ts and
F *A, states of CF: as observed by X-ray photoelectron spectroscopy [lo,1 11.Also shown is the peak-maximum energy position for
the formation of the lowest energy doubly-charged cation, CFY (see text) and the Ar’ ion doublet peaks used for energy calibration.
The low-energy portion of the spectrum (15.5-350eV) was recorded at the SERC Daresbury Laboratory with a resolution of AE/E x
5.1 x 10m4, while the high-energy portion of the spectrum (27.0-60.5eV) was recorded at LURE with a resolution of
AE/E zz 1.2 x 10-3.
resolution in this higher-resolution, room temperature study implies that the natural vibrational line
width is observed in both the low temperature He I
PES [9] and the room temperature TPES of CF+
The adiabatic ionization potential for R ‘E has
been reported to be 18.37eV [9]. The energy of
the comparable first peak in our spectrum is
18.358 f 0.004eV; the positions of all the peaks
in band R are given in Table 1. The long-tail
feature seen between about 19 and 20eV in Fig. 2
appears to be an extension of the R 2E state band.
A similar feature observed in the He I PES, but not
observed in the He II PES of CF4, has been attributed to an autoionizing state lying at 21.2eV,
giving rise to the non-Franck-Condon
population
of the E)2E state in the former case [8]. This has
been confirmed by the observation of pronounced
autoionization structure at 584.5 A (21.2 12 eV) in
the absorption cross section of CF4 [19] at essentially the same energy as the He I resonance line
(21.217 eV). The similar observation of high vibra-
tional levels of CF: (R) in this TPES study implies
that the effect of an autoionizing state (or states) of
CF4 extends throughout the 19-20eV range. The
first member of a Rydberg series with an effective
quantum number of 1.61 converging on the fi 2A,
state of CF: has been identified in this energy
region through the observation of a broad, window-type minimum in the absorption cross section
of CF4 [19], and this is likely to be the source of the
R-state tail structure in the TPES of CF4. In a
related work, Carlson et al. [20] showed in a photoelectron study using a variable energy light source
that the R-state tail structure extends to higher
energies as the excitation energy varies from 20.5
to 22.5 eV in steps of 0.5 eV, disappearing completely at 23.0eV, corresponding to the high-energy
limit of the C 2T2 state vibrational progression.
They further showed, in a constant ionic state
study using an ionization potential of 19.75eV,
that in the photoexcitation region between 21.2
and 21.6eV the electron signal intensity follows
33
A.J. Yencha et al./J. Electron Spectrosc. Relat. Phenom. 70 (1994) 29-37
-I/
N2+ X2$
I
16
N2+ A=n Y
17
I
I
I
18
19
20
Photon Energy I eV
Fig. 2. Expanded view of a region of the high-resolution threshold photoelectron spectrum of CF4 showing the bands representing the
formation of the three lowest energy states of CF:. The Ar’ ion doublet peaks were used for calibration. The Nl ion peaks identified
were due to a slight impurity of Nz.
Table 1
Peak energy positions (eV) observed in the threshold photoelectron spectrum of CF., for formation of the l? *E, C *Tz and
fi ‘A, states of CF: together with the autoionizing structure
positions (eV) measured here in comparison with those observed in the total photoelectron yield (TPEY) spectrum measured previously [18]; uncertainty in the energy positions in the
present work is f0.004 eV
Autoionization band
CF$ bands
fi2E
C2T2
I?)‘A,
This work
TPEY [18]
18.358
18.418
18.463
18.534
18.564
18.624
18.734
18.759
18.835
21.671
21.769
21.861
21.952
22.044
22.130
22.219
22.307
22.391
22.479
22.556
22.618
22.712
22.782
25.099
25.198
25.286
25.380
25.471
25.565
25.653
25.741
25.827
20.342
20.436
20.533
20.621
20.711
20.800
20.876
20.968
21.056
21.149
21.239
21.340
21.432
21.530
20.44
20.53
20.62
20.71
20.79
20.87
20.97
21.07
21.16
21.29
21.38
21.49
21.58
the structured absorption cross section. This result
will be important in our interpretation of some
structured features in the TPES of CF4 in this
same photon energy range (see below).
Fig. 3 shows the TPES for the formation of
the C 2T2 state of CF: in the 21.7-23.1 eV region.
It consists of a long, well-resolved progression in
what appears to be the v1 vibrational frequency
(average vibrational separation being 88.5 meV
(= 714cm-‘)) which is better resolved than in any
previous PES study, reflecting the higher resolution
achieved in this work. Based on a comparative He I
and He II study of CF4, SiF4 and GeF4, Lloyd and
Roberts [9] concluded that the C *T2 state band of
CF: contains a doublet progression in vl (estimated separation of 770cm-‘) separated by one
quantum
of v4 (estimated
separation
of
71Ocm-‘). In fact, in our higher resolution TPES
spectrum shown in Fig. 3 there is clear evidence
that a second, minor progression is present that
begins to separate out in the valleys of the primary
progression for the higher vibrational levels. The
adiabatic ionization potential (first resolved vibrational band) for the C 2T2 state of CF,f determined
34
A.J. Yencha et al/J. Electron Spectrosc. Relat. Phenom. 70 (1994) 29-37
I
,
20.0
20.5
,
I
I
21.0
21.5
I
I
I
22.0
22.5
I
I
23.0
23.5
Photon Energy I eV
Fig. 3. Expanded view of a region of the high-resolution threshold photoelectron spectrum of CF4 showing the band representing the
formation of the c 2T2 state of CF: (21.7-23.1 eV) and some autoionizing structure preceding it (20.3-21.6 eV) (see text).
here is 21.677 f 0.004eV which is in good agreement with the best previous value of 21.70eV [8].
The positions of all the major vibrational bands for
the C 2T2 state are given in Table 1. The general
appearance of the TPES of the C 2T2 band of CF:
shown in Fig. 3 is quite similar to the He II results
[8,9], except for the higher resolution obtained
here, suggesting that there are no major effects
due to autoionizing Rydberg states in this energy
range.
Also shown in Fig. 3 is a structured band
between about 20.3 and 21.6eV, immediately preceding the onset of the C 2T2 state progression, that
has not been previously reported in any PES study
using resonance photon excitation. The vibrational
spacings appear fairly regular with an average
value of about 91 meV (about 734cm-‘), which is
similar to the average spacing found for the C *T2
state of CF,f . In fact, the number of resolved vibrational bands in both systems is the same (14),
although the vibrational distributions are somewhat different. It is entirely reasonable to associate
this structured band with the resonant ionization of
a Rydberg state or states, Ry(C), converging on the
C *T2 state of CF:. In fact, from a comparison of
the TPES of CF4 in this photon energy region with
the photoabsorption
cross section [19] and total
electron yield spectrum [18], a close correspondence in the autoionization structure is found by
the three methods, although the peak energy values
found in the absorption cross section study are
shifted by a progressive amount (increasing
towards higher energies) in comparison to those
of the other two studies, which are found to be
mutually consistent, as demonstrated in Table 1.
The autoionization structure in the photoabsorption cross section has been identified as resulting
from three Rydberg states with effective quantum
numbers n* = 2.96, 3.41 and 1.84. The n* = 2.96
progression was assigned to the 3t2 + 3d transition, while the n* = 3.41 progression was assigned
to the 3t2 + 4p transition [19]. The assignment of
the n* = 1.84 progression was uncertain, but it was
tentatively attributed to vibrational structure of a
Rydberg state converging on the fi 2A1 state of
CF,f. However, Carlson et al. [20] suggested that
this Rydberg series is likewise associated with the
C 2T2 state of CF: with which we concur. But how
can such autoionizing Rydberg states of CF4 yield
near-zero kinetic energy electrons while retaining
35
A.J. Yencha et al./J. Electron Spectrosc. Relat. Phenom. 70 (1994) 29-37
L
24.8
*
1.
25.0
I
25.2
-
I
I
25.4
I
25.6
I
I
25.8
*
12
26.0
I
26.2
I
I
26.4
Photon Energy I eV
Fig. 4. Expanded view of a region of the high-resolution threshold photoelectron spectrum of CF, showing the band representing the
formation of the fi ‘A, state of CF:.
the essential Franck-Condon
excitation distribution? The answer must lie in the coupling of the
Rydberg states to the ionization continuum, most
probably that of the fi 2E state, in which the final
ionic state is behaving as a quasi-continuum of
rovibrational states, thereby affording an effective
constant probability of threshold photoelectron
formation as a function of excitation energy. It
should be noted that the width of the autoionizing
Rydberg structure displayed in Fig. 3 is about double that of the vibrational structure in the C 2T2
state of CF: (see Fig. 3) probably reflecting the
shortening of the lifetime of the Rydberg states
due to autoionization.
In Fig. 4 is shown a region of the TPES of CF4
covering the formation of the fi 2A1 state of CFZ
in expanded form. As can be seen, there is a clear
vibrational progression extending out to 21’= 8 and
possibly 9, with an average separation of 9 1.OmeV
(5 734cm-‘), although there are substantial variations in the individual vibrational separations (see
Table 1) indicating that the vibrational levels are
significantly perturbed. Since only 21’= O-2 have
been observed in the He II PE spectrum [8,9], it is
likely that an autoionizing state lies in this energy
region that is isoenergetic with the fi state, thereby
populating vibrational levels of the fi state in a
non-Franck-Condon
fashion, yielding nearly
zero kinetic energy electrons. In addition to the
extended progression, it is of note that the w’= 2
band is more intense than the r~’= 1 band and
the w’= 1 band is relatively more intense than the
ZI’= 0 band compared with the He II PE spectrum
[8,9]. All these facts point towards the involvement
of two different ionization mechanisms, i.e. direct
photoionization
and autoionization,
following
photoexcitation to a Rydberg state of CF4. This
Rydberg state may be associated with innervalence-state excitation leading to the fi 2T2 and
F 2A1 states of CF:, whose vertical ionization
potentials are at 40.3 and 43.8 eV [lo, 111, respectively, or it may result from a two-electron excited
state. The adiabatic ionization potential for the
fi 2A1 state of CF:
is determined
to be
25.099 f O.O04eV, in good agreement with the
best previous value of 25.12 eV [8].
The TPES of CF4 in the higher energy region
studied (27.0-60SeV) is shown in Fig. 1. It con-
36
A.J. Yencha et al./J. Electron Spectrosc. Relat. Phenom. 70 (1994) 29-37
sists of a continuum-like signal with an onset at
about 27.0eV that persists out to 60.5 eV with
several broad features superimposed on it. This
spectral behavior is in stark contrast to that
observed at lower photon energies where welldefined peaks are found, and at first glance, it is a
surprising result owing to the type of measurement
being made, namely the detection of near-zero
kinetic energy electrons. Beside the underlying continuum there is a shoulder peak with a minimum
change-in-slope at about 34eV and an onset at
about 31.2 eV, followed by a much more prominent peak with a maximum at about 40.3eV and
an onset at about 37.2eV. At still higher photon
energies there is a broad peak at approximately
55eV. Analysis of these features and the underlying continuum is difficult because of the number
of ionization/dissociation
channels available, but
some tentative conclusions can be drawn here. It
is very probable that the prominent peak with an
onset at 37.2 f 0.2 eV is at least partially associated
with the opening of the lowest double ionization
channel: CF4 + hv -+ CF,f + F+ + 2e-, where it
is understood that both electrons have zero kinetic
energy for a “true” threshold process and are
thereby detectable in this study, although this is
known to be a relatively weak process. The experimentally determined threshold for this process has
been found to be 37.6 f 0.6eV [21] by photoelectron-photoion-photoion
coincidence measurements associated with the (It;’ 4t;‘)3T state of
CFi+. This threshold value is in agreement, within
experimental uncertainty, with the thermodynamic
threshold for the formation of CFZ + F+ of
32.2eV [22] plus the experimentally determined
ion kinetic energy release of 5.0 f 0.2 eV [21,23]
yielding a total energy of 37.2eV, a value in exact
agreement with the results presented here. Both of
these threshold findings are supported by recent
double charge transfer studies for the formation
of the lowest singlet and triplet states of CF:’
giving a peak-maximum value of 38.0 f 0.4eV
[24] as indicated in Fig. 1. Clearly, from the
width (about 6eV FWHM) and integrated intensity of the peak at about 40.3 eV additional contributions can be expected, the most obvious ones
being the formation of the E 2T2 and F *A, states
of CF:.
4. Conclusions
This high-resolution threshold photoelectron
study confirms the main (structureless) features of
the 2 2T, and A 2T2 states of CF,f , while the limited,
partially resolved vibrational structure observed for
the formation of the B 2E state appears to be similar
to that found in a lower-resolution, low-temperature
He I PES study, indicating that natural line-width
profiles are observed in both cases. In contrast, the
structure observed for both the C 2T2 and D 2A1
states of CFZ shows a considerable improvement in
the resolution of the vibrational structure in this
higher-resolution study. The influence of autoionizing super-excited states of CF4 is observed through,
(1) a structureless high-energy tail to the B 2E state,
(2) vibrational structure in the energy region just
preceding the C 2T2 state, and (3) additional vibrational structure in the D2Ai state, of CF:. At photon
energies above 27 eV an extensive, quasi-continuous
signal is observed that persists out to 60.5eV with
several broad features superimposed on it. One
broad peak, centered on 40.3 eV, can be associated
with the formation of the E 2T2 and F 2A, states of
CF,f , and several doubly ionized states are also
expected in this energy region. However, it is very
likely that the broad underlying continuum is a
result of dissociative single and double ionization
processes in which the dissociating heavy particles
possess substantial kinetic energy.
Acknowledgments
We sincerely appreciated the use of the synchrotron facilities at LURE and the SERC Daresbury
Laboratory. We thank R.P. Tuckett, P.A. Hatherly
and colleagues for sharing with us their results on
radiative decay and fragmentation of CF: prior to
publication. We also thank the SERC for support
for JGG and partial support for AJY. Partial support
of this research by a NATO travel grant (No. 9 10777)
and the British Council is gratefully acknowledged.
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