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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) X-RAY
FLUORESCENCE FROM CF3Cl MOLECULE. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-
753-C9-756. �10.1051/jphyscol:19879130�. �jpa-00227240�
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 ~ V I L L A "
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~cules neutres 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 below- threshold 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(K- V) 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
C9-75 4 JOURNAL DE PHYSIQUE
Fig. 1. Experimental C1 K Fig. 2. T h e C1 K B ( K - V ) spectrum of absorption spectrum of CF3C1. The CF-jCl excited with 2832.5 eV photon position of C1 1 s ionization energy along with the deconvolved threshold energy is indicated. Voigt spectral components.
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 position of the elastic peaks unaltered by self-absorption. Also 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 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
4
5
CI - ~ j 3 2832.5 ev ~ r ~ l t e ~ b n
g -
3
s -
-
dCF3C1
CI
-
K Absorption Spectrum.
= -
1
2832.5 .V.
CI-1s
I
?
CF3C1
2823.8 eV = +.
1 a I . O 1a10.0 2126.0 2130.0 2820.0 2830.0
Energy (aV) Energy (eV)
Kp (K-V) spectral components for (K - 6.5eV) excitation with the spectral components for (K +
3eV) 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 h a l f m a x i m u m ( F W H H ) o f t h e V o i g h t p r o f i l e i n 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 p h o t o n e x c i t a t i o n
2832.5 eV Excitation Peak Energy Total Rel. Lorntz.
width int. width (.'-'I (eV) (eV)
A 2816.3 1.5 100 0.7
B 2814.2 1.5 28 0.7
C 2809.4 3.0 6 2.6
2823.1 eV Excitation Energy Total Rel. Lorntz.
width int. width
(eV) (eV) (eV)
2814.9 1.2 100 0.3
2811.7 1.5 7 0.7
2806.8 2.6 3 2.2
2823.8 eV Excitation Energy Total Rel. Lorntz.
width int. width
(eV) (eV) (eV)
2815.7 1.5 100 0.7
2812.2 1.6 8 0.9
2807.7 3.1 3 2.7
A A
.,
2.-
= - e-
- 3
--
dd
(tbttic paat
. . . ..
. . ,2100.0 P110.0 2810.0 2ItO.O 2810.0
Energy (eV) Energy (eV)
Fig. 3. T h e C 1 K B ( K - V ) s p e c t r u m of Fig. 4. T h e C 1 Kg ( K - V ) s p e c t r u m C F 3 C 1 e x c i t e d w i t h 2 8 2 3 . 1 eV p h o t o n o f C F 3 C 1 e x c i t e d w i t h 2823.8 eV
e n e r g y . p h o t o n e n e r g y .
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
MOinvolved in the x-ray transition.
But, when a core electron is excited to an unoccupied orbital below the vacuum level, the 11effective91 hole production region in the core 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 "effective1I core hole region is a fraction of the total
1slifetime broadening, the line width of x-ray emission components from the exciton state will
besmaller 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.
d l
CI - )(P 2823.1 eV Excltstlon
U - KB 2823.8 aV Excll~tlllon
CF3C1
2 -
5 E
- 4
--
.,GI
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
*leffectiveW core 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 above- threshold 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 excitation limits the *8effective1* hole production region in the Is 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 measurements
,
and Dana ~erkeland- for he; invaluable assistance in analyzing the data. One of the authors (R.C.C.P.) acknowledges the support by the U.S. Department of Energy under contract No. DE-AC03- 76SF00098. This work was performed at NSLS, which is supported by the U.S. Department of Energy under contract No. DE-AC020-76CH00016.References
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