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The carbon X-ray spectrum of gaseous CO
Volume 4 1A, number 2
PHYSICS LETTERS
11 September 1972
THE CARBON X-RAY SPECTRUM OF GASEOUS CO KXEGBAHN, L.WERME, B.GRENNBERG, J.NORDGREN and C.NORDLING Uppsala University, Institute of Physics, Uppsala, Sweden
Received 27 June I912 A new grazing incidence grating spectrometer for ultra soft X-rays from gaseous samples is briefly described. Threefold differential pumping makes direct electron impact excitation feasible. The carbon X-ray emission spectrum of CO is shown as a preliminary result. Recent
years’ development
copy has provided
of electron
spectros-
a new tool for the study of molec-
ular structure. Ionization by means of X-rays and Wradiations liberates both inner (atomic core) and outer (valence) electrons, and through the analysis of the resulting electron spectra we can improve our understanding of molecular orbitals and chemical bonds. A natural complement to such studies would be to use X-ray emission spectroscopy to get information on outer molecular orbitals for free molecules. X-ray spectroscopy for free molecules, i.e. for gases, has been confined almost entirely to absorption spectra. The first few attempts with emission studies were made quite recently. Excitation was accomplished by means of X-rays [ 1,2] (fluorescence) and by electrons [3,4] . The cited investigations were performed either on crystal X-ray spectrometers, where only X-ray energies 2 1000 eV can be studied (e.g. chlorine Kp radiation) or with Langmuir-Blodgett type analyzers (e.g. lead stearate) when the purpose has been to study the ultrasoft X-rays from for example carbon. Both these analyzers have certain disadvantages. In the first case one has to resort to X-rays originating from a broad inner shell. The second type of analyzers are difficult to make thicker than -60 double layers, which limits the resolving power to a rather low value. It is also doubtful whether a higher number of layers would be of any use, since absorption would make deeper layers inefficient. These difficulties can be surmounted with the use of a grating spectrometer at grazing incidence for the ultrasoft X-ray region. With appropriate choice of the grating a resolving power of E/AE > 2000 can be obtained, although one must bear in mind that resolution is gamed at the expense of intensity. If one wants to resolve, for example, the vibrational structure in
ELECTRON
COLLISION”
FARADAV
CHAMBER
1
COOLING
GUN
CUP
WATER
Fig. 1. Principle of the high intensity electron gun with the electron optical arrangement for excitation of X-rays in gases.
carbon X-rays from CO, a resolving power of -2800 is required. A spectrometer for the study of ultrasoft X-rays from gases has recently been taken into use at our laboratory. A detailed discussion of the instrument will appear soon. A preliminary description is given in ref. [5] . The dispersive element is a 3m concave grating ruled with 540 lines/mm. The instrument is 111
Volume
4 1A, number
PHYSICS
2
ELECTRON SPECTRUM
co 1eV
X-RAY SPECTRUM
!I 36
ill
26
16
278.9 eV (44.5 &
281.8 eV (44.0A) c 1s
-
0 1s
Fig. 2. The carbon X-ray emission spectrum CO obtained with the grating spectrometer. is also shown the electron spectrum excited
from gaseous For comparison by hv = 2 I. 1 eV.
LETTERS
1972
hv = 21.21 eV. As can be seen in the figure, the X-ray transitions involving the 30 and In orbitals give rise to line profiles in agreement with what is expected from the electron spectrum The 30 orbital. which has the characteristics of a nonbonding orbital in the electron spectrum, appears as a narrow line in the X-lay spectrum. The In orbital exhibits the broad Franck-C’ondon envelope of a bonding orbital in both spectra. No line corresponding to transitions from the 20 orbital has been found in our spectrum. A calculation of intensities based on CNDO wave-functions predicts zero intensity for this transition. if we consider the relative emission probability to depend only on the amount of C2p character in the molecular orbital describing the vacancy in the final state. A previous analysis of the spectrum obtained by Mattson and Ehlert [3] based on extended Hiickel wavefunctions resulted in an intensity of -3% for this line [Xl. Ab-initio calculations also attribute very little C2p character to this orbital t -2%). No transitions from the la orbital were observed, although both CNDO and ab-initio calculations indicate -10% C2p character in this orbital. It must also be pointed out that the intense high energy satellites in Mattson and Ehlert’s spectrum arc of considerably lower intensity in our spectrum. These structures have been attributed to Wentzel-Druyvesteyn satellites 181. We have found that results obtained from CO strongly encourage further work on improvements of the high resolution ultrasoft X-ray spectroscopy as an approach to a better understanding of the electronic structure of the valence orbitals in free molecules. The unpublished electron spectrum of CO was kindly provided for us by Dr. T.Bergmark.
adjusted to an angle of incidence of 87”. This angle ensures a good reflectivity at wavelengths > 30 A (the K X-rays from N, at 3 1 .h have been recorded), and is therefore well suited for the study of e.g. carbon X-radiation (-45 A). The spectrometer is capable of a resolving power of -3000 in the first order diffraction. The instrument has been used with photographic detection and Kodak SWR plates have so far been used. A schematic view of the X-ray source is shown in fig. 1. An electron beam is produced in a high power electron gun (10 kV, 120 mA). This beam is then focussed by means of three magnetic lenses onto the collision chamber, where the sample gas is confined. In order to maintain the pressure ratio between the collision chamber (several torr) and the electron gun (1 0T6 torr) a threefold differential pumping system has been used as is indicated in the figure.
References
In fig.2 the carbon X-ray spectrum of CO is shown and compared to an electron spectrum excited by
[ 71
112
I1 September
[I] R.E. LaVilla and R.D. Deslattes, [2] ]3] [4] ]5]
[6]
[8]
J.Chem.Phys. 45 (1966) 3446. E. Gilberg, Z.Phys. 236 (1970) 21. R.A. Mattson and R.C. Ehlert, J.Chem.Phys.48 (1968) 5468 and 5471. R.E. LaVilla, J.Chem.Phys. 56 (1972) 2345. K. Siegbahn, in Electron spectroscopy, ed. D.A.Shirley Proc.Jntern.Conf.Asilomar, Pacific Grove, California, USA 1971 (North-Holland, Amsterdam 1972). K. Siegbahn et al., ESCA applied to free molecules (North-Holland, Amsterdam, 1969). T. Bergmark, unpublished results. R. Manne, J.Chem.Phys.52 (1970) 5733.
PHYSICS LETTERS
11 September 1972
THE CARBON X-RAY SPECTRUM OF GASEOUS CO KXEGBAHN, L.WERME, B.GRENNBERG, J.NORDGREN and C.NORDLING Uppsala University, Institute of Physics, Uppsala, Sweden
Received 27 June I912 A new grazing incidence grating spectrometer for ultra soft X-rays from gaseous samples is briefly described. Threefold differential pumping makes direct electron impact excitation feasible. The carbon X-ray emission spectrum of CO is shown as a preliminary result. Recent
years’ development
copy has provided
of electron
spectros-
a new tool for the study of molec-
ular structure. Ionization by means of X-rays and Wradiations liberates both inner (atomic core) and outer (valence) electrons, and through the analysis of the resulting electron spectra we can improve our understanding of molecular orbitals and chemical bonds. A natural complement to such studies would be to use X-ray emission spectroscopy to get information on outer molecular orbitals for free molecules. X-ray spectroscopy for free molecules, i.e. for gases, has been confined almost entirely to absorption spectra. The first few attempts with emission studies were made quite recently. Excitation was accomplished by means of X-rays [ 1,2] (fluorescence) and by electrons [3,4] . The cited investigations were performed either on crystal X-ray spectrometers, where only X-ray energies 2 1000 eV can be studied (e.g. chlorine Kp radiation) or with Langmuir-Blodgett type analyzers (e.g. lead stearate) when the purpose has been to study the ultrasoft X-rays from for example carbon. Both these analyzers have certain disadvantages. In the first case one has to resort to X-rays originating from a broad inner shell. The second type of analyzers are difficult to make thicker than -60 double layers, which limits the resolving power to a rather low value. It is also doubtful whether a higher number of layers would be of any use, since absorption would make deeper layers inefficient. These difficulties can be surmounted with the use of a grating spectrometer at grazing incidence for the ultrasoft X-ray region. With appropriate choice of the grating a resolving power of E/AE > 2000 can be obtained, although one must bear in mind that resolution is gamed at the expense of intensity. If one wants to resolve, for example, the vibrational structure in
ELECTRON
COLLISION”
FARADAV
CHAMBER
1
COOLING
GUN
CUP
WATER
Fig. 1. Principle of the high intensity electron gun with the electron optical arrangement for excitation of X-rays in gases.
carbon X-rays from CO, a resolving power of -2800 is required. A spectrometer for the study of ultrasoft X-rays from gases has recently been taken into use at our laboratory. A detailed discussion of the instrument will appear soon. A preliminary description is given in ref. [5] . The dispersive element is a 3m concave grating ruled with 540 lines/mm. The instrument is 111
Volume
4 1A, number
PHYSICS
2
ELECTRON SPECTRUM
co 1eV
X-RAY SPECTRUM
!I 36
ill
26
16
278.9 eV (44.5 &
281.8 eV (44.0A) c 1s
-
0 1s
Fig. 2. The carbon X-ray emission spectrum CO obtained with the grating spectrometer. is also shown the electron spectrum excited
from gaseous For comparison by hv = 2 I. 1 eV.
LETTERS
1972
hv = 21.21 eV. As can be seen in the figure, the X-ray transitions involving the 30 and In orbitals give rise to line profiles in agreement with what is expected from the electron spectrum The 30 orbital. which has the characteristics of a nonbonding orbital in the electron spectrum, appears as a narrow line in the X-lay spectrum. The In orbital exhibits the broad Franck-C’ondon envelope of a bonding orbital in both spectra. No line corresponding to transitions from the 20 orbital has been found in our spectrum. A calculation of intensities based on CNDO wave-functions predicts zero intensity for this transition. if we consider the relative emission probability to depend only on the amount of C2p character in the molecular orbital describing the vacancy in the final state. A previous analysis of the spectrum obtained by Mattson and Ehlert [3] based on extended Hiickel wavefunctions resulted in an intensity of -3% for this line [Xl. Ab-initio calculations also attribute very little C2p character to this orbital t -2%). No transitions from the la orbital were observed, although both CNDO and ab-initio calculations indicate -10% C2p character in this orbital. It must also be pointed out that the intense high energy satellites in Mattson and Ehlert’s spectrum arc of considerably lower intensity in our spectrum. These structures have been attributed to Wentzel-Druyvesteyn satellites 181. We have found that results obtained from CO strongly encourage further work on improvements of the high resolution ultrasoft X-ray spectroscopy as an approach to a better understanding of the electronic structure of the valence orbitals in free molecules. The unpublished electron spectrum of CO was kindly provided for us by Dr. T.Bergmark.
adjusted to an angle of incidence of 87”. This angle ensures a good reflectivity at wavelengths > 30 A (the K X-rays from N, at 3 1 .h have been recorded), and is therefore well suited for the study of e.g. carbon X-radiation (-45 A). The spectrometer is capable of a resolving power of -3000 in the first order diffraction. The instrument has been used with photographic detection and Kodak SWR plates have so far been used. A schematic view of the X-ray source is shown in fig. 1. An electron beam is produced in a high power electron gun (10 kV, 120 mA). This beam is then focussed by means of three magnetic lenses onto the collision chamber, where the sample gas is confined. In order to maintain the pressure ratio between the collision chamber (several torr) and the electron gun (1 0T6 torr) a threefold differential pumping system has been used as is indicated in the figure.
References
In fig.2 the carbon X-ray spectrum of CO is shown and compared to an electron spectrum excited by
[ 71
112
I1 September
[I] R.E. LaVilla and R.D. Deslattes, [2] ]3] [4] ]5]
[6]
[8]
J.Chem.Phys. 45 (1966) 3446. E. Gilberg, Z.Phys. 236 (1970) 21. R.A. Mattson and R.C. Ehlert, J.Chem.Phys.48 (1968) 5468 and 5471. R.E. LaVilla, J.Chem.Phys. 56 (1972) 2345. K. Siegbahn, in Electron spectroscopy, ed. D.A.Shirley Proc.Jntern.Conf.Asilomar, Pacific Grove, California, USA 1971 (North-Holland, Amsterdam 1972). K. Siegbahn et al., ESCA applied to free molecules (North-Holland, Amsterdam, 1969). T. Bergmark, unpublished results. R. Manne, J.Chem.Phys.52 (1970) 5733.