Chemical
Physics
ELSEVIER
Chemical Physics 213 (1996) 439-448
High-resolution threshold photoelectron spectroscopy
of molecular fluorine
A.J. Cormack
a
A.J. Yencha b,* R.J. Donovan a, K.P. Lawley
A. Hopkirk c, G.C. King d
a
a Department of Chemistry, The University of Edinburgh, Edinburgh EH9 3JJ, UK
b Department of Physics and Department of Chemistry, State University of New York at Albany, Albany, NY 12222, USA
c CLRC Daresbury Laboratory, Daresbury, Warrington WA4 4AD, UK
a Department of Physics, Schuster Laboratory, Manchester University, Manchester M13 9PL, UK
Received 19 August 1996
Abstract
The threshold photoelectron spectrum of molecular fluorine has been recorded in the 15.6-21.9 eV photon energy range,
at resolutions ranging from 3 to 12 meV, using synchrotron radiation and a penetrating-field electron spectrometer. In
addition to observing the three known band systems of F2~ at higher resolution than previously achieved with conventional
photoelectron spectroscopy, extensive vibrational structure is found in the Franck-Condon gaps between the main electronic
systems of F~-. This extended vibrational structure is attributed to resonance autoionization of neutral Rydberg states.
I. Introduction
The electronic states of fluorine and its cation
have been investigated by absorption spectroscopy
[1,2], electron energy-loss spectroscopy [3-5], pbotoionization mass spectrometry [6-8], emission spectroscopy [9,10], photoelectron spectroscopy [ 11-16],
threshold photoelectron spectroscopy [17], resonance-enhanced multi-photon ionization spectroscopy [18], and by theory [4,14,15,19,20]. However, in spite of all these experimental and theoretical studies, the electronic states of fluorine and its
cation are not well-understood by comparison with
* Corresponding author. E-mail: [email protected]
the heavier diatomic halogens. This is particularly
true of the ionic states of fluorine owing to experimental difficulties resulting from their high formation energies. The most recent experimental report
on the electronic states of F f was a high-resolution
HeI photoelectron study by van Lonkhuyzen and de
Lange [16] in which they observed for the first time
vibrational structure in the A(2H/.u) band system,
including resolved spin-orbit structure. By observing directly the adiabatic ionization energies of both
the X(2Hi,g) a n d A(2Hi,u) states of F f , they were
able to confirm the vibrational numbering given by
Tuckett et al. [10] in the (A 2Hi,u-X 2Hi,g) emission
spectrum. Van Lonkhuyzen and de Lange [16] also
observed for the first time vibrational structure in the
B ( 2 ~ ; ) band system of F f from which they were
0301-0104/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved
PII S 0 3 0 1 - 0 1 0 4 ( 9 6 ) 0 0 2 8 8 - I
440
A.J. Cormack et a l . / Chemical Physics 213 (1996) 439-448
able to derive spectroscopic constants by performing
Franck-Condon (FC) calculations. Subsequently, a
detailed theoretical study of the potential energy
curves of F 2 and F f was reported providing further
information on the spectroscopic constants [20]. The
present study of F 2 follows our recent successes in
investigating the electronic states of the heavier diatomic halogen cations of C12 and Br 2 [21] and 12
[22] by threshold photoelectron spectroscopy (TPES).
The major advantages of TPES over conventional
photoelectron spectroscopy (PES) are: (1) the intensity of TPE spectra does not have to be corrected for
the transmission function of the analyzer; (2) in
TPES the energy resolution is essentially limited by
the resolution of the scanning photon source, whereas
in PES it is usually governed by the electron energy
analyzer; (3) in TPES Doppler broadening is reduced
to zero; and (4) TPE spectra yield extensive vibrational structure in the FC gap regions between electronic states that cannot generally be observed in PE
spectra. However, TPES and PES should be viewed
as complementary methods for studying the electronic state structure of molecular ions. The sole
TPES study of F 2 reported in the literature was by
Guyon et al. [17], covering only the lowest five
vibrational levels of the ground state of F~-.
2. Experimental
This study was performed at the synchrotron facility of the CLRC Daresbury Laboratory. The procedure for obtaining TPE spectra has been described in
detail elsewhere [23], so only a brief account will be
given here. Synchrotron radiation from the electron
storage ring that entered beamline 3.2 was dispersed
by a 5-m McPherson vacuum monochromator. Upon
exiting the monochromator, the light was focused
through a 1-mm bore, by 35-cm long, glass capillary.
The end of the capillary tube was positioned 15 mm
away from the center of the 5-mm diameter entrance
hole of the electron spectrometer, that was mounted
perpendicular to the direction of the photon beam
and 12 mm back from it. The sample gas entered the
vacuum system through a 0.8-mm bore platinum
tube mutually perpendicular to the photon beam and
the entrance to the electron spectrometer and was
positioned about 3 mm above the interaction region.
TPE spectra were recorded using a penetrating-field
electron spectrometer [24,25] tuned to accept threshold electrons ( < 20 meV). The argon doublet ion
lines at 15.759 eV (2P3/2) and 15.937 eV (2P~/2)
were used to tune the spectrometer, to establish the
resolution and to calibrate the photon energy scale.
The overall TPE spectrum of F 2 was generated by
taking several single-scan spectra of portions of the
full energy range studied. After normalization of
each single scan for the intensity of the light source,
the spectra were summed and joined to other parts of
the spectrum similarly obtained. The resolution of
the overall TPE spectrum was determined to be
AE/E=7.53 × 10 -4 (i.e. A E = 12.0 meV at
15.937 eV); this was determined to be largely controlled by the resolution of the photon source, e.g., in
this study the photon bandpass was fixed at 1.00 A,
corresponding to an energy resolution of I0.0 meV
at 15.937 eV. Higher-resolution TPE spectra were
recorded over selected regions of the full energy
range studied. The resolution of these spectra are
given in the text and in the respective figure captions.
Fluorine gas was generated in situ using an apparatus designed to produce essentially pure F 2 gas
( > 99% purity) safely (self-regulating) at a pressure
of = 8 Torr above atmospheric. F 2 was produced by
the electrolytic decomposition of a molten (80-90°C)
fused mixture of potassium fluoride and hydrogen
fluoride in the mole ratio of 1:2; HF was removed
from the F 2 stream by scrubbing the effluent. This
"fluorine on demand" generator (model no. Fluorodec 30) was supplied by BNFL Electrogas, British
Nuclear Fuels plc (Springfields, Preston, PR4 0XJ,
UK). It provided a continuously variable delivery
rate of F 2 of 0-210 cm3/min. The fluorine gas
exhausted from the experimental apparatus was removed completely using a scrubber made up of a
combination of activated alumina and activated charcoal filter elements. Prior to recording any data, the
fluorine generator was allowed to supply gas to the
experimental apparatus for several hours to ensure
the complete passivation of the connecting gas line.
The only contamination that was observed was CF4
which only appeared after about 24 h of continuous
operation of the generator. Purging the electrolytic
cell with argon gas and restarting the generator
removed all traces of CF4. The background pressure
A J . Cormack et a l . / Chemical Physics 213 (1996) 439-448
of fluorine gas in the apparatus was of the order of
8 × 10 -5 Torr.
tion. The high intensity of the lower vibrational
levels of the X(2Hi,g) state can be explained by
efficient vibrational autoionization of high Rydberg
levels (i.e. Rydberg levels converging on higher
vibrational levels of the same X-state ion core). This
mechanism was proposed by Berkowitz et al. [8] to
explain the pronounced autoionization structure observed in the photoion yield curve of F 2 in the region
immediately following the ionization onset. The extended vibrational structure in the FC gap regions
between ionic states is most likely a consequence of
resonance autoionization from Rydberg states residing in these regions into upper rovibrational levels of
the X(2H i,g) state and to the dissociative continua of
the X(21-li.g) and A(2[Ii,u ) states. Similar, extended
vibrational structure in the FC gap region has been
observed and analyzed in the case of all the other
diatomic halogens [21,22]. We now proceed to discuss the individual parts of the TPE spectrum of F 2
in detail.
3. Results and discussion
3.1. The threshold photoelectron spectrum of molecular fluorine
The overall TPE spectrum of F 2 covering the
photon energy range of 15.6-21.9 eV is shown in
Fig. 1. This displays the three band systems of F~resulting from transitions to the X(2 H i,~), A(2 H i,u)
and B ( 2 £ g ) states known from conventional PES
[16]. One is immediately struck by two facts: (1)
nearly all of the intensity resides in the lower vibrational levels of the X(2Hi,g)-State band (see the
lowest curve in Fig. 1) and (2) there is extensive
vibrational structure between the X(2Hig) and
A(2[Ii,u ) band systems and between the A(2Hi.u )
and B( 2 ]~;) band systems, especially the former (see
the upper two curves in Fig. 1). These features
reflect the complex interplay of the dual production
mechanism for threshold electrons, i.e. direct photoionization and indirect photoexcitation/autoioniza-
3.1.1. The TPES o f F e in the 15.6-19.4 eV range
Fig. 2 show a part of the TPE spectrum covering
the X(2H/,g) and A(2Hi,u) band systems of F f at a
resolution of 12.0 meV as measured by the in situ
I
I
441
I
I
I
I
F2 +
X (2Fli,g)
,
15
16
I
]7
A
,
I
18
(2~Ii,u)
~
I
B
~
19
I
20
~
(2Zg+)
I
21
,
I
22
Photon Energy / eV
Fig. 1. Overall threshold photoelectron spectrum for fluorine (with a trace of argon gas present for calibration purposes) showing the bands
for the formation of the X(2H i,g), A(2II i,u) and B( 2-~g ) states of Fz*. The energy resolution of this spectrum is A E / E = 7.53 X 1 0 - 4 i.e.
AE = 12.0 meV at 15.937 eV (Ar+(2pI/2)).
442
AJ. Cormack et a l . / Chemical Physics 213 (1996) 439-448
tion potential of the A(2H3/2,u ) sub-state of F f of
18.300 eV (see below), we calculate a Rydberg
quantum defect of -- 1.2. This is very close to our
calculated quantum defect for an s-type Rydberg
state in atomic fluorine of = 1.3. We tentatively
assign these vibrational intensity distributions, I, II,
and III, to three Rydberg s t a t e s [(Org)2(Tru)3
(Trg*)4]inso-g(lI]u), where n = 4, 5, and 6, respectively. This assignment is supported by our analysis
of the TPES of CI 2 [21].
The assignment of the vibrational structure in the
x(ZI][i,g) band system is shown in Fig. 2. The halflength tick marks indicate no assigned peak. The
observed band head positions are listed in Table 1
together with calculated band head positions based
on our analysis of the experimental data using a
second-order vibrational Dunham expression [26].
Similarly, the A(2Hi,u ) band system has been assigned in Fig. 2 and the observed and calculated
band head positions tabulated in Table 2. The spectroscopic constants for the X(2Hcg) and A(21qi,u)
states of F f as derived from the fitted data in Tables
1 and 2 are given in Table 3 where they are com-
Ar+(2Pt/2) line at 15.937 eV (marked with asterisk
in Fig. 2). One immediately notes the extended
vibrational structure ( v + > 6 ) of both spin-orbit
components of the x(Z[][i,g) band system covering
the entire FC gap region, and in fact, extending
partially into the A(ZIIi,u ) band system. As mentioned above, we attribute this extended vibrational
structure to resonance autoionization of Rydberg
states of F 2 present in this energy region. It can be
seen from the upper curve in Fig. 2 that there are
three vibrational intensity distributions present, labeled I, II, and III, reflecting the transition probabilities to three Rydberg states converging on the
A(2H~,u) state ion. It can also be seen that the
extended vibrational structure terminates quite
abruptly with the onset of A(2IIi,o) state ion formation. In this energy range ( = 16.6-18.6 eV) extensive autoionization structure appears in the photoion
yield spectrum of F 2, and this has been similarly
attributed to a Rydberg series converging on the
A ( 2 H i , u ) state ion [8].
Based on the apparent onset of system I in Fig. 2
at = 16.6 eV and our determined adiabatic ioniza-
I
0
'
'
'
5
'
I
'
l0
'
'
15
'
I
20
'
'
'
'
I
25
I-q--I I I I ' I I J I I I I I I I II II IIIII111xdn3,_,g)
0
5
I0
15
20
I I I I I ' I I t I I I I [ [ I I I Ill
_172+
25
I1'111
0
x(2n,ag)
5
10
I' Ifllllllll A(2n3,2,~)
0
5
10
I-q-'FTq~[~ A (2pu2,u)
x 20
I
16
I
17
I
11
t III I
18
19
Photon Energy / eV
Fig. 2. Broad view of the X(2II,.g), and A(2Hi.u) threshold photoelectron band systems of F2~ with the assignment of the observed
vibrational progressions. The peaks marked with the asterisks ( * ) are the argon ion doublet peaks at 15.759 eV (2p3/2) and 15.937 eV
(2p~/2). The energy resolution of this spectrum is the same as in Fig. 1 (e.g., AE = 12.0 meV at 15.937 eV).
A.J. Cormack et a l . / Chemical Physics 213 (1996) 439-448
pared with similar data from the literature. The
agreement between the present results and those of
the literature is good.
Because considerable congestion occurs above
17.55 eV in Fig. 2, we have recorded another TPE
spectrum in the 17.5-19.2 eV range at a slightly
higher resolution (8.8 meV at 18.25 eV). This spectrum is shown in Fig. 3 with the assignment of
vibrational peaks indicated. We note that the last two
vibrational peaks observed in Fig. 3 occur at v + = 13
( 19.03 eV) in the A(2 IJ 3/2 u) subsystem and at v + =
12 (19.021 eV) in the A(2HI/2,u) subsystem, followed by a rise in intensity at 19.056 eV corresponding to the point of complete breakoff of vibrational
structure. The vertical arrows in Fig. 3 indicate the
Table 1
Observed and calculated vibrational band head positions (in eV)
for the transitions: e + F2~ (X 2H i , g , v') ,-- F,(X
1£~ , v" = 0)
.
u'
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
2H 3/2,g transition energy
2H /2,g transition energy
obs a
calc
obs a
calc
15.693 b
15.827 b
15.960
16.091
16.219
16.342
15.694
15.828
15.960
16.089
16.216
16.341
16.463
16.583
16.701
16.816
16.928
17.038
17.146
17.251
17.354
17.455
17.552
17.648
17.741
17.832
17.920
18.006
18.089
18.170
18.248
18.324
18.398
18.469
18.538
15.738 ~
15.870 b
16.004
16.131
16.261
16.382
15.738
15.871
16.003
16.132
16.259
16.383
16.505
16.624
16.741
16.856
16.968
17.078
17.186
17.291
17.394
17.494
17.592
17.688
17.781
17.872
17.960
18.046
18.130
18.211
18.290
18.366
18.440
18.512
16.585
16.699
16.810
16.924
17.033
17.149
17.256
17.356
17.452
17.551
17.651
17.738
17.840
17.918
18.005
18.093
18.170
18.242
18.326
18.403
18.467
18.535
16.631
16.738
16.848
16.966
17.079
17.187
17.296
17.392
17.493
17.589
17.684
17.788
17.882
17.957
18.045
18.133
18.204
18.361
18.441
18.515
a Uncertainty + 0.002 eV.
b Uncertainty +0.001 eV, from Fig. 5.
443
Table 2
Observed and calculated vibrational band head postions (in eV)
for the transitions: e + Fz+ (A 211,.u, v') ~ F2(X 1£~, v" = 0)
0
l
2
3
4
5
6
7
8
9
10
11
12
13
2H3/2. u transition energy
2H~/2, u transition energy
obs a
calc
obs a
calc
18.300
18.300
18.375
18.448
18.517
18.583
18.646
18.706
18.762
18.815
18.865
18.911
18.954
18.994
19.031
18.410
18.481
18.549
18.617
18.679
18.737
18.792
18.850
18.896
18.942
18.984
19.021
18.333
18.409
18.481
18.551
18.617
18.679
18.738
18.794
18.847
18.896
18.941
18.984
19.023
18.581
18.647
18.706
18.762
18.816
18.864
18.912
18.956
18.993
19.030
a Uncertainty _+0.002.
thermodynamic dissociation limits for [F(2p3/2)+
F+(3P2)] formation of 19.025 eV and f o r [F(ePt/2 )
+ F+(3P2)] formation of 19.075 eV 1, which agrees
with the experimental vibrational breakoff points.
As can be seen in Fig. 3, there are a number of
peaks marked with asterisks that are not identified as
belonging to either t h e X(ZHi,g) or A(2H;,,,) band
systems. We have carefully considered all possible
impurities, but could not identify any plausible contaminants. In an attempt to account for these additional peaks, we have considered the potential energy
curves of F2+ given by Cartwright and Hay [20].
From their calculated results, we have identified
three possible doublet bound states of F~- whose
potential minima would place them in the right
energy range to correspond to the unidentified structure in Fig. 3. They are the 2 £- u+ , 2£~-, and 2A u
states of F2+. Although these three bound states
would be optically accessible with configuration
mixing from the ground state of F2, it is unlikely that
the bound portions of the states would lie in the FC
region due to their considerably larger re values. For
example, the re values for these potentials have been
computed to be 1.80 A [20], 1.80 ~, [19], and 1.775
~From D0(F2)= 1.602 eV [2], IP(F)= 17.423 eV (F+3p2)
[27], F(2P3/2) ~ F(2PI/2) = 0.050 eV [28].
444
A J . Cormack et al. / Chemical Physics 213 (1996) 439-448
Table 3
Summary of spectroscopic constants (in eV) derived from analyses of the F 2 TPES data presented here and from literature data
State
Ionization energy a
¢oc
103we x e
SOS b
Ref.
F~-X( 2113/2,g)
15.693(1)
15.694
15.697(3)
0.1369
0.1380
0.1354
0.1351
0.1363
-
0.045
0.044
0.043
[17]
0.1350
0.0793
0.0777
0.0798
0.0794
0.0780
- t.ll
- 1.64
- 1.59
- 1.67
- 1.69
- 1.71
F+ X(2H l/2.g)
15.738(1)
15.738
F2+ A(2 H 3/2,u)
18.300(2)
18.304(10)
F2+A(21]l/2.u )
18.333 c
1.22
1.21
1.07
1.11
1.20
this work
[16]
[10]
this work
[17]
[1o]
this work
[16]
0.033
0.035
[1o1
this work
[10]
a Experimental (0-0) transition value unless otherwise noted.
b Spin-orbit splitting between v = 0 bands in subcomponents.
c Determined from fitting procedure.
t h e h i g h e r - r e s o l u t i o n s p e c t r u m o f F i g . 3, it is n o t
p o s s i b l e to i d e n t i f y a n y r e g u l a r v i b r a t i o n a l f e a t u r e s ,
although some of the peak separations do approxim a t e l y c o r r e s p o n d to t h e v i b r a t i o n a l s e p a r a t i o n s t h a t
one might expect based on the computed vibrational
c o n s t a n t s [19,20]. T h e p e a k p o s i t i o n s o f t h e s e
[20], r e s p e c t i v e l y , w h i l e t h e r e o f t h e g r o u n d state
o f F 2 is 1 . 4 1 1 8 A [2]. H o w e v e r , it is p o s s i b l e t h a t
these states could be accessed via intermediate neutral R y d b e r g states, l y i n g in t h i s e n e r g y r a n g e ,
through resonant autoionization. Because of the cong e s t i o n t h a t still e x i s t s in this e n e r g y r e g i o n , e v e n in
'
'
16
I
'
'
i
.
,
,
'
i
,
20
I I i
I
I I I I
I
'
[
I I
16
,
I[J
'
i
,
'
,
' 1 ' ' ' ' 1 ' '
F24-
25
I I I I [ x(Znm.~)
20
25
i,'z,,)
d
u2,o)-
.=
I
,
17.50
.
.
.
I
. . . .
17.75
I
. . . .
18.00
I
•
18.25
.
,
~
I
i
18.50
I
I
i
I
i
18.75
i
I
'
I
.
19.00
.
.
.
19.25
Photon Energy / eV
Fig. 3. An expanded view of a portion of the X(2[Ii,g), and A(2Hi,u) threshold photoelectron band systems of F2+ at a slightly higher
resolution ( A E / E = 4.82 X 10 -4, i.e. AE = 8.8 meV at 18,25 eV) than that shown in Fig. 2. The unassigned peaks marked with the
•
.
•
asterisks
( * ) may possthly
arise
from the 2 ]£,+ , 2 Eg- , a n d / o r 2 A states of F 2+ (see text). The vertical arrows indicate the thermodynamic
2
dissociation limits for [F(P3/2) 4- F + ( 3 P2)] and [ F (2P l / 2 ) + F + ( 3P2)] formation.
AJ. Cormack et al./ Chemical Physics 213 (1996) 439-448
I
I
'
I
I
I
I
445
I
I
5p' Z,
+
F2+
,
A
qa
6p Zu+
r~ ~
~
sys 1
li,u) k
~t
I
18.8
19.2
i
I
19,6
;
I
,
I
sys 2
[ - [ - T - ~ sys 3
[~]
t
I
20.0
20.4
20,8
Photon Energy / eV
,
I
21.2
l
I
21.6
I
I
22.0
Fig. 4. An expanded view of a portion of the threshold photoelectron spectrum of fluorine showing the high-energy tail of the A(2U~.u)
band system and the full B(ZZg) band system of F~". The energy resolution of this portion of the threshold photoelectron spectrum is the
same as in Fig. 1, i.e. AE = 15.0 meV at 20.0 eV.
unidentified features are given in Table 4 for completeness.
3.1.2. T h e T P E S o f F 2 in the 1 9 . 0 - 2 1 . 9 e V r a n g e
In Fig. 4 is shown a portion of the TPE spectrum
of F 2 covering the dissociative continuum end of the
A(21]i,u ) band system and the entire B ( : E ~ -) band
system of F f . What one notices immediately is that
there is a considerable amount of vibrational structure superimposed on the broad continuum tail of the
A(Zlqi,u) band system. We have identified two progressions as being due to the np Rydberg series for
n = 5, and 6, converging on the B(2E~ -) state of F~-,
with quantum defects of 1.62 and 1.69, respectively.
These Rydberg states undergo resonance autoionization populating the dissociative continuum of the
A(21-1i,u) state of F~. The autoionization part of the
threshold electron signal will clearly be proportional
to the product of the FC factors for populating the
Rydberg states and the FC factors for autoionization
into the continuum [29].
Upon further inspection of the TPE spectrum
shown in Fig. 4, one notices two regions with welldefined structure, labeled sys 1 and sys 2, with
widely different spacings in the 20.0-21.0 eV range.
The intensities of these systems appear anomalously
high if they are Rydberg in origin. We note that the
vibrational structure on the low energy side of the
B ( 2 ~ -) band system (sys 2) does" n o t precisely
correspond in energy with the vibrational structure
observed in the HeI PE spectrum of F 2 [ 16], although
the vibrational spacings are similar. We have carefully checked our calibration and conclude that our
measured positions are accurate. In an attempt to
identify the origin of the observed shifts, we note the
following facts. First, according to the ab initio
potentials for F f [20], the B ( 2 ~ -) state is formed
from the avoided crossing of two 2 ~ - potentials, a
repulsive one correlating diabatically with the [F(ep)
+ F+(3P)] asymptote and a bound one correlating
diabatically with the [ F ( 2 P ) + F + ( I D ) ] asymptote.
The avoided crossing occurs near the bottom of the
bound potential, on the inner wall side, at very near
the r e of the ground state of F 2. Second, one might
expect one or more ion-pair states to be present in
this energy region, based on the asymptotic limits 2
of [F+(3P 2) + F-(1S0 )] at 15.624 eV, [ F + ( i D 2) +
2 From D0(F2)= 1.602 eV [2], EA(F)= 3.401 190 eV [30],
IP(F)= 17.423 eV (F+sp 2) [27], = 20.011 eV (F+~D2) [28],=
22.993 eV (F + ~S0) [28].
446
AJ. Cormack et a l . / Chemical Physics 213 (1996) 439-448
F - ( I S 0 )] at 18.212 eV, and [F+(IS0 ) + F - ( L S 0 )] at
21.194 eV. Third, based on the calculated potentials
involving the interaction of Rydberg and ion-pair
states further crossings are expected at higher energy
[20]. All that we can conclude at this point, based on
the present spectroscopic evidence, is the following:
(1) that resonance autoionization occurs from two
neutral states possessing different shapes, i.e. sys 1
must involve a rather narrow potential well because
of the large vibrational spacings observed, while sys
2 must be associated with a much shallower well,
similar to the unperturbed Rydberg states at slightly
lower energy, or similar to the ion-pair states expected in this energy region whose calculated o)e
values [20] are very close to those for the Rydberg
states; and (2) that the potentials in (1) have slightly
smaller r e values compared with the unperturbed
Rydberg states because of the increased intensity
resulting from the more favorable FC factors. A
more definitive analysis must await accurate potential energy curve calculations. Finally, we note that
the four features, labeled sys 3 in Fig. 4, at the top of
t h e B(2•;) band system of F~-, are probably true
vibrational structure associated with that system. The
second peak would correspond to the vertical ioniza-
I
I
tion potential of this state with a value of 21.112 +
0.002 eV in good agreement with the HeI PES [16].
The observed band head positions of all the features
identified in Fig. 4 are listed in Table 4.
3.1.3. The high-resolution TPES o f F 2 in the 15.6815.95 eV range
Fig. 5 presents the high-resolution (3.0 meV) TPE
spectrum of F 2 over the energy range covering the
first two vibrational bands (v ÷ = 0 and 1) of both
spin-orbit components ( 1 2 = 3 / 2 ,
1/2) of the
X(:FI i.g) band system of F~-. The argon ion lines are
also present and serve to establish the energy resolution of the spectrum (see below). In this spectrum we
see the unresolved rotational envelopes for the various vibrational bands with some distorting features
that will be discussed below. We also see in Fig. 5,
that the 12 = 3 / 2 component is more intense than
the 12 = 1 / 2 component, whereas in direct photoionization one finds a nearly statistical branching ratio
for the two spin-orbit components [16]. The enhancement in the 3 / 2 component in the TPE spectrum is attributed to spin-orbit autoionization. Another feature to be noted in the TPE spectrum is that
the v + = 0 sub-bands are more intense than the
I
I
I
v+=0
I
v += 1
[
1
3/2
1/2
X (2Fl~,g)
[
1
312
1/2
F2 +
At (2p3m)
Ar + [Ar*'(nl=l L~I')]
/
Ar +
,
15.68
I
15.72
,
I
15.76
a
I
t5.80
,
I
15.84
,
I
15.88
L
(2p,a
I
15.92
Photon Energy / eV
Fig. 5. High-resolution threshold photoelectron spectrum of the first two vibrational bands in the X(2H,.g) band system of F2+ showing the
effect of autoionizing transitions (vertical arrows) on the rotational contour of the bands. The energy resolution of this spectrum is the same
as the photon bandpass, A h v = 3.0 meV.
A.l. Cormack et a l . / Chemical Physics 213 (1996) 439-448
Table 4
Band head positions (in eV) for the unidentified vibrational
features in Fig. 3 (marked with asterisks) and the identified
vibrational features in Fig, 4
System
Energy
System
Energy
position a
position a
unidentified
5p ~';+
~u
6p '~+
17.619
17.701
17,850
18.075
18.186
18.262
18,568
19.541
19,603
19.649
19.698
19.744
19.796
19.835
19.885
19.931
sys 1
20.171
20.311
20.446
20.577
sys 2
20.658
20.710
20.774
20.827
20.879
20.932
20.980
21.024
sys 3
20.073
21.112
21.159
21,199
20.002
20.063
20.121
20.175
20.227
20.269
20.310
Uncertainty ±0.002 eV.
v += 1 sub-bands, and we attribute this to vibrational
autoionization. The FC factors for transitions from
the ground state of F 2 predict the opposite intensity
ratio, as observed in the HeI PES [16]. The vertical
arrows in Fig. 5 indicate the positions of autoionizing Rydberg states (15.702, 15.744, 15.748, 15.753
and 15.831 eV) that effectively and selectively populate various parts of the rotational envelope, thereby
drastically distorting the appearance of the rotational
profile.
The argon (2P1/2) ionization peak in Fig. 5 helps
to establish the effective resolution in the spectrum
at 3.0 meV, which is exactly the photon bandpass of
the monochromator. This confirms that the penetrating-field electron spectrometer used has a negligible
effect on the resolution of the spectrum. The observation of a high-energy shoulder on the main
Ar+(2P3/2) line, due to the autoionization from the
447
( l l s ' ) Rydberg state (converging on Ar+(2PI/2)),
establishes that the spectrometer is detecting electrons within ~ 3 meV of threshold. The 0 - 0 transition for the X(2H3/2) ionic state of fluorine is
obtained from this spectrum as 15.693 _+ 0.001 eV,
in excellent agreement with the previous, lower-resolution TPE result of 15.694 eV [17] obtained in a
rather novel way and only slightly below the PES
finding of 15.697 eV [16]. All of the data derived
from this spectrum is given in Table 1.
4. Summary and conclusions
We have measured for the first time the threshold
photoelectron spectrum of molecular fluorine over
the full valence ionization region (15.6-21.9 eV) at a
resolution of 12.0 meV. Higher-resolution spectra
(7.7 and 3.0 meV) were also obtained over limited
regions, and in particular for the lower vibrational
levels of the X( 2 Hi, ~) system. These spectra clearly
show the vibrationally resolved X(2 II i.g), A(2 11 s,u)
and 8( 2 .3£,;) band systems of F f familiar in conventional PES [16], and in addition, extensive structure
in the FC gap region between the ionic states due to
resonance autoionization involving Rydberg states
lying in these energy regions. Several of these Rydberg systems have been tentatively identified from
an analysis of the spectral data. Analysis of the
vibrational structure in the X(2IIa,g) and A(2Hi,u)
band systems of F~- yielded spectroscopic constants
for these states in good agreement with previous
data. There is some evidence for the formation of
additional bound states of F2+ not previously observed and for the presence of perturbed neutral
potentials in the energy region of the B ( 2 £ g ) state
ion. Analyses of threshold photoelectron spectra in
conjunction with conventional photoelectron spectra
has been shown to provide valuable new information
on both the neutral and ionic states of molecular
systems.
Acknowledgement
The authors acknowledge support of this project
by the CLRC in the form of a grant and for providing beamtime at the Daresbury Synchrotron Light
448
AJ. Cormack et al. / Chemical Physics 213 (1996) 439-448
Source facility. One of us (AJC) gratefully acknowledges funding from the CLRC and EPSRC for a
CASE studentship. We would like to thank Mr.
Thomas Reynolds and Mr. Martin Hearne of BNFL
for their assistance in providing the fluorine generator used in this study.
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