SUB-THRESHOLD EXCITED Cl Kβ (K-V) X-RAY
FLUORESCENCE FROM CF3Cl MOLECULE
R. Perera, P. Cowan, D. Lindle, R. Lavilla
To cite this version:
R. Perera, P. Cowan, D. Lindle, R. Lavilla. SUB-THRESHOLD EXCITED Cl Kβ (K-V) XRAY FLUORESCENCE FROM CF3Cl MOLECULE. Journal de Physique Colloques, 1987,
48 (C9), pp.C9-753-C9-756. <10.1051/jphyscol:19879130>. <jpa-00227240>
HAL Id: jpa-00227240
https://hal.archives-ouvertes.fr/jpa-00227240
Submitted on 1 Jan 1987
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JOURNAL DE PHYSIQUE
Colloque C9, suppl6ment au n012, Tome 48, decembre 1987
SUB-THRESHOLD EXCITED C1 Kf3 (K-V) X-RAY FLUORESCENCE FROM CF,C1
MOLECULE
R.C.C.
PERERA, P.L. COWAN*, D.W.
LINDLE* and R.E.
L~VILLA"
Center for X-Ray Optics, Lawrence Berkeley Laboratory,
Berkeley, CA 94720, U.S.A.
'Quantum Metrology Group, National Bureau of Standards,
Gaithersburg, MD 20899, U.S.A.
RFsm4z
Avec la disponibilite des sources accordables de la radiation du
synchrotron, on peut peupler selectivement les orbites moleculaires (au
dessous du niveau du vide) pour produire des ml~culesneutres fort excit6es.
On a obtenu les spectres des rayons X flourescents avec l'excitation moins de
la limite & l'ionisation. Les spectres obtenus de cette facon ont des
changements significants d1intensit6,des &placements absolus et relatives
des positions energetiques et changements de la largeur des lignes en
cornparaison des spectres qu'on a excites plus de la limite. On presente, par
example, le cas des spectres de C1 Ka de l'&ission de la vapeur du GF3C1. On
explique les &placements de l'energie et les variations d'intensite sous
forme &s effets de la perturbation A cause d'un electron qui occupe m e
orbite moleculaire libre. On explique les largeurs Btroites &s lignes sous
fome de la region "effective" de la production des trow clans m e codition
du noyeau limitee pour l'expansion du niveau libre. Come une nowelle
technique pour etudier les proprietes localisees et les effets reorganisants
d'un trou aux noyeau, on propose le changement de la largeur des lignes en
fonction de l'energie de l'excitation moins de la limite.
ABSTRACT
With the availability of tirneable synchrotron radiation sources,
unoccupied molecular orbits (below v a c m level) can be selectively populated
producing highly excited neutral molecules. X-ray fluorescence spectra from
molecules were obtained with excitation below the ionization threshold and
were observed to have significant intensity changes, absolute and relative
energy position shifts and line width changes as compared to fluorescence
spectra excited above the threshold. As an exauple, the C1 KB (K-V) emission
spectra from CF3C1 vapor are presented. The energy shifts and intensity
changes are explained in terms of perturbation effects due to the presence of
an electron in an unoccupied molecular orbital. The narrow line widths
obtained in the spectra excited below threshold are explained in terms of the
"effective" hole production region in a core state limited by the broadening
of the unoccupied level. The change in line widths as a function of belowthreshold excitation energy is proposed as a novel technique to study the
localizedproperties and reorganization effects of a hole in a core level.
In the present report, the C1 K absorption spectrum and C1 Kp(KV) fluorescent spectra of CF3C1 vapor were measured using the
monochromatic synchrotron radiation (band pass about 0.4 eV) from the
NBS x-24A beam line [l] at Brookhaven's NSLS. The unoccupied
molecular orbitals were selectively populated producing highly excited
neutral molecules (i.e., exciton states), resulting in x-ray emission
spectra from photon excitation below the ionization threshold [2]. A
detailed description of the beamline [I] and the monochromator [3] is
given elsewhere. The C1 Kp (K-V) fluorescent radiation was analyzed
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879130
JOURNAL DE PHYSIQUE
C9-754
by a variable radius curved crystal spectrometer [4] consisting of a
silicon (111) crystal of 2d = 6.271
and a linear position-sensitive
gas proportional counter [5] filled with a gaseous mixture of Xe (90%
vol) and CH4 (10% vol) at a pressure of 1 atm. The sample gas
contained in a cell by .001 in thick beryllium windows at a pressure
of 100 torr was located inside the Rowland circle of the spectrometer.
The C1 K absorption spectrum was obtained by measuring the incident
and transmitted flux with helium-filled ion chambers.
The C1 K absorption spectrum CF3C1 (gas) is shown in Fig. 1. The
energy calibration of the absorption spectrum and, in turn, the
monochromator calibration in this energy region were obtained using C1
K absorption of CF3C1 measured with a laboratory spectrometer. The
lowest unoccupied valence molecular orbitals (MOs) of CF3C1 are 6a1,
7al and 6e in the order of decreasing binding energy (BE). The first
discrete absorption maximum at 2823.1 eV results from transition of a
chlorine Is electron to the unoccupied 6al orbital. Using available
C1 Kp (K-V) emission data [6] and UPS [7] BEs, the C1 Is energy is
estimated to be 2829.6 eV.
2823.1 eV
-
CI K
Absorption Spectrum
5
4
CF3C1
CI
s- -
CF3C1
.
.
= -
1
3
2832.5 ev ~ r ~ l t e ~ b n
=+.
2823.8 eV
g-
-~j3
2832.5 .V
I
d
CI-1s
1aI.O
1a10.0
2126.0
2130.0
2820.0
Energy (aV)
Fig. 1. Experimental C1 K
absorption spectrum of CF3C1. The
position of C1 1s ionization
threshold energy is indicated.
?
2830.0
Energy (eV)
The C1 KB (K-V) spectrum of
CF-jCl excited with 2832.5 eV photon
energy along with the deconvolved
Voigt spectral components.
Fig. 2.
The C1 Kp (K-V) emission spectrum presented in Fig. 2 was excited
at 2832.5 eV energy, which is about 3 eV above the C1 1s ionization
threshold energy (K + 3 eV), but below the double vacancy threshold
energy. It was shown before [2,8] that such excitation produces no
multiple vacancy satellite contributions in this spectral region. The
energy calibration of the emission spectra was obtained using the
Also
position of the elastic peaks unaltered by self-absorption.
shown in Fig. 1 are the deconvolved Voigt spectral components along
with the sum of the components and experimental data. The Voigt
spectral components were derived using a 1.0 eV Gaussian total
instrumental broadening, estimated empirically by analyzing a series
of x-ray spectra in this energy region. This is consistent with
direct measurement of secondary spectrometer resolution. Presented in
Table 1 are energies of the (K + 3eV) spectral components, full width
at half maximum (FWHM) of the Voigt profile in eV, relative radiative
yields (the areas under the Voigt components) and Lorentzian FWHM
obtained by removing instrumental broadening from the Voigt width.
Peaks 1 and 2 in Fig. 2 are assigned as dipole-allowed transitions
from electrons in 5e and 5al valence MOs, whereas peak 3 is attributed
to the overlapping transitions from the 3e+2e+4al valence MOs.
In Fig. 3, the C1 Kp(K-V) spectrum excited with 2823.1 eV (K-6.5
eV) photon energy is presented, along with the deconvolved spectral
components. As seen in Fig. 1, the 2823.1 eV energy corresponds to
the first maximum in the absorption spectrum. Also presented in Table
1 are the energy positions, relative intensities and widths of the
spectral components of (K-6.5eV) excited spectrum. Comparing the C1
Kp (K-V) spectral components for (K - 6.5eV) excitation with the
spectral components for (K + 3 eV) excitation, large differences in
energy positions, relative intensities and line widths are observed.
The relative energy-position shifts and the significant intensity
changes can be understood in terms of perturbation effects due to the
presence of an electron in the first unoccupied MO, when excited with
(K - 6.5 eV) photon excitation [2].
Table 1.
T h e e n e r g i e s o f t h e s p e c t r a l c o m p o n e n t s , f u l l w i d t h a t half
m a x i m u m (FWHH) o f t h e V o i g h t profile in e V , t h e r e l a t i v e
i n t e n s i t i e s a n d l o r e n t z i a n F W H M in eV f o r 2832.5 e V , 2823.1
eV a n d 2823.8 eV photon excitation
2832.5 eV Excitation
Peak
Energy
A
2816.3
2814.2
2809.4
(.'-'I
B
C
Total
width
(eV)
1.5
1.5
3.0
2823.8 eV Excitation
2823.1 eV Excitation
Rel.
int.
Lorntz.
width
(eV)
Energy
100
28
6
0.7
0.7
2.6
(eV)
Total
width
(eV)
2814.9
2811.7
2806.8
1.2
1.5
2.6
Rel.
int.
100
7
3
Lorntz.
width
(eV)
Energy
0.3
0.7
2.2
Total
width
(eV)
(eV)
2815.7
2812.2
2807.7
1.5
1.6
3.1
Rel.
int.
100
8
3
Lorntz.
width
(eV)
0.7
0.9
2.7
A
A
l
d
-
CI )(P
2823.1 eV Excltstlon
U - KB
CF3C1
-.,
= e
-3- -
2823.8 aV Excll~tlllon
.,GI
2.
2
-
-5
E
4-
d
-
d
.......
(tbttic paat
,
2100.0
P110.0
2ItO.O
2810.0
Fig. 3.
T h e C 1 K B ( K - V ) s p e c t r u m of
C F 3 C 1 e x c i t e d w i t h 2823.1 eV p h o t o n
energy.
2810.0
Energy (eV)
Energy (eV)
Fig. 4.
T h e C 1 Kg ( K - V ) s p e c t r u m
o f C F 3 C 1 e x c i t e d w i t h 2823.8 eV
photon energy.
When a Is hole is produced using photon excitation above the
ionization threshold, the electron is excited beyond the bound
orbitals of the closed electron core to a continuum state. Therefore,
the virtual core hole can be located anywhere in the core level width
[9], such that the Lorentzian line width of the C1 Kp (K-V) emission
component is the sum of the natural line width of the core level
(Lorentzian) and the valence MO involved in the x-ray transition.
But, when a core electron is excited to an unoccupied orbital below
hole production region in the core
the vacuum level, the 11effective91
state [9] is limited by convolution of the the broadening of the
unoccupied level and the band pass of the monochromator (neglecting
the core hole reorganization effects). Since this "effective1Icore
hole region is a fraction of the total 1s lifetime broadening, the
line width of x-ray emission components from the exciton state will be
smaller than the line widths of the above-threshold excited spectrum.
This agrees with the observed narrow line widths in (K-6.5 eV)
spectral components shown in Fig. 3 compared to (K+3 eV) spectral
components in Fig. 2. These narrow line widths also indicate that the
core hole relaxation process [9] is slower than the fluorescent decay
process.
C9-756
JOURNAL DE PHYSIQUE
The 2823.8 eV (K-5.8 eV) excited C1 KJ3 (K-V) spectrum of CF3C1 is
presented in Fig. 4, along with the deconvolved spectral components.
Also presented in Table 1 are the energy positions, relative
intensities and widths for the (K-5.8 eV) excited spectrum. As shown
in Fig. 1, the 2823.8 eV excitation corresponds to half-height of the
first discrete maximum in the C1 K absorption spectrum. Comparing
energy positions in Table 1, the energy of spectral components in the
(K-5.8) eV spectrum is about 0.8 eV higher than the corresponding
component energy in the (K-6.5 eV) spectrum. This indicates that the
*leffectiveWcore hole production region for (K-5.8 eV) is in a lower
energy region of the core level, approximately the difference in
excitation energy [lo]. This phenomenon is possible since the core
level (1s) line width is mainly due to the lifetime broadening. The
line widths in the (K-5.8 eV) spectrum is much larger than
corresponding line widths in the (K-6.5) spectrum, indicating the
localized properties of relaxation and reorganization effects of the
virtual core.
In summary, the C1 KI3 (K-V) spectra from a CF3C1 molecule are
obtained by selectively populating the unoccupied MOs below the vacuum
level and comparing them to the C1 KB (K-V) spectrum from abovethreshold excitation. The energy shifts and relative intensity
changes observed are due to perturbation effects resulting from the
electron in an unoccupied molecular orbital. The below-threshold
hole production region in the Is
excitation limits the *8effective1*
level, resulting in narrow line widths in the x-ray emission spectrum.
ACKNOWLEDGEMENT
We would like to thank R.D. Deslattes for his encouragement and
continuins interest in this study. Barry Karlin for assistins in the
for he; invaluable assistance in
measurements , and Dana ~erkelandanalyzing the data. One of the authors (R.C.C.P.) acknowledges the
support by the U.S. Department of Energy under contract No. DE-AC0376SF00098. This work was performed at NSLS, which is supported by the
U.S. Department of Energy under contract No. DE-AC020-76CH00016.
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