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Observation of an ionic excimer state in CsF+
Volume 56, number 4
OPTICS COMMUNICATIONS
15 December 1985
OBSERVATION OF AN IONIC EXCIMER S T A T E IN CsF
+
F. S T E I G E R W A L D , F. E M M E R T , H. L A N G H O F F Physikalisches Institut der Unioersiti~t, 14rftrzburg, Fed. Rep. Germany
W. H A M M E R and T. G R I E G E L lnstitut ff~r Strahlenphysik der Unioersiti~t, Stuttgart, Fed. Rep. Germany Received 27 August 1985
The radiative decay of the excimer state CsF + isoelectronic to XeF was observed. Continua appeared at 152 nm and 184.5 nm and were assigned to the D ~ X and B ~ X transitions, respectively. The failure to detect the corresponding transitions in CsCI + is discussed.
The rare gas halide excimers (RgX) are the most efficient media for lasers in the ultraviolet down to 193 nm (ArF). In order to extend the range for this type of lasers to even lower wavelengths, in a recent paper [1] it has been proposed to investigate excimer states in ionized molecules which are isoelectronic to the RgX molecules. Most of the possible isoelectronic ions tend to predissociate. The most stable ions to be expected are CsF + and CsC1+ which are isoelectronic to the well known XeF and XeC1 excimers. Model calculations [1 ] using a Rittner potential for the upper state and a Born-Mayer potential for the repulsive lower state predicted for the range of the emitted wavelengths 154-195 nm and 131-205 nm, respectively. The resulting potentials for the Cs/F system are shown in fig. 1. Analogous to the RgX extimers [2], the emitted continua should exhibit a fine structure, the main intensities are due to the D -> X, B ~ X and C ~ A transitions. The formation of CsF + and CsCI+ are energetically possible by charge transfer from He + or Ne + to the corresponding cesium halids. Another reaction could be the Penning ionization by He* and Ne*. For an experimental investigation, the arrangement shown in fig. 2 has been used. The 3.6 MeV argon beam of the Stuttgart dynamitron accelerator with a current of 100 #A entered the reaction cell without passing through foils. This was accomplished using a 240
f 25
/// CS2,.F -
20
Cs÷F"
15 I0 5
c~Ff~ / ~--.~
Cs**F ,,
Cs'*F-
~CsF
Fig. 1. Relevant potential curves of the Cs/F system labeled by their asymptotic atomic states. By excitation with He + and/or Ne +, He*, Ne*, molecules in the CsF ground state are transferred into the bound excimer state labeled Cs2++ F-. They decay radiatively to the unbound state Cs+ + F. The lowest excited atomic states Cs+ + F* and Cs + F + are located considerably higher than the minimum of the Cs2+F--cmve.
four stage differential pumping system which maintained the vacuum in the accelerator even when the cell was filled with 500 mbar buffer gas. In order to remove impurities, the buffer gas which was circulated by the first pump, was passed through a titanium 0 030-4018/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 56, number 4
OPTICS COMMUNICATIONS
MgF-Window Differential pumping system
/
i Ii T
"r
..._T_~ ....T_~. T
L__
Recording System
__/
Titonium Oven
Fig. 2. Experimental arrangement. The At + beam entered the reaction cell windowless. The cell was heated and contained cesium halide vapor and buffer gas.
oven (800°C) before re-entering the cell. Spectra o f the emitted radiation were observed by a VUV spectrograph placed in direction o f the beam. The cell contained a crucible filled with CsF or CsC1 and could be heated up to 800°C. Helium or neon served as the buffer gas. Fig. 3 shows two spectra obtained with CsF and 100 mb of helium. At a temperature o f 680°C corresponding to a CsF pressure of about 0.7 mbar (fig. 3a), two continua appear in the IV spectra. They are not present at lower temperatures. In addition, the spectrum contains only narrow lines due to excited states o f He, He + and same remaining impurities as carbon and nitrogen.
0
190
180
170
160
150 " ~ - ' 140 k/nm
Fig. 3. Spectra observed for CsF vapor and a helium pressure of 100 mbar. (a) cell temperature 680°C, 0.7 mb CsF pressure, (b) cell temperature 750°C, 3.5 mb CsF pressure. The continua at 184.5 n m and 1 5 2 n m are attributed to the B ~ X and D -* X excimer transitions in CsF+. The lines are caused by excitations of helium and impurities. The main lines are due to the 193.1 nm transition in CI and the 164.0 transition in He II.
15 December 1985
When the vapor pressure of CsF is further increased to 3.5 mb (750°C cell temperature), the intensity of the continua centered at 152 nm and 184.5 nm rises. With increased CsF density, the absorption of the UV radiation by the CsF vapor becomes larger as may be seen from the decreasing intensities of the impurity lines. After correction for the absorption, the intensities o f both continua increase with rising CsF pressure. The continua were also observed with neon as the buffer gas. The corresponding experiments using CsC1 as the donor yielded no continua. Instead, an intense band system around 258 nm appeared. This is ascribed to the formation of CI~ [3]. In the experiment with CsF, the corresponding lines of F~ at 157 nm, if any, were only weakly present. The two continua exhibit the same features as those known from the RgX spectra. The wavelengths coincide with the wavelengths evaluated for a decay of CsF + in the model calculation. In the RgX molecules, the energy difference between the D ~ X and the B ~ X transitions is slightly less than the energy difference o f the two asymptotic states. In the pres. ent experiment, the difference between Cs2+(P3/2_) and Cs2+(P1/2 ) _ _ is 1.72 eV. If the continua are assigned to the D -~ X and B ~ X transitions in Cs2+F - , their energy difference of 1.44 eV compares favorably with the tendency. The C ~ A transition was not observed. It might have been missed due to its large width or its intensity was low due to B/C mixing as observed in many RgX spectra [4]. The continuum at 184.5 nm shows an undulated structure which is to be expected due to a vibrational excitation of the upper state. Assuming that the most intense line at 184.5 nm is caused by a transition from the u = 0 state to the repulsive ground state, the line width is due to the gradient of the lower potential curve. The value o f about 1.0 × 108 eV/cm is similar to corresponding values for the RgX molecules. The Cs÷F potential curve resulting from the Born-Mayer potential had to be slightly corrected in order to reproduce the position and width o f the B -~ X spectrum. The corrected curve is shown in fig. 1. A vibrational quantum energy o f o~ = 540 cm -1 is estimated from the Rittner potential calculation [1]. The experimental spacing of the different peaks is about 1050 cm -1 . A semiclassical calculation of the spectral shape using the potential gradient reproduces the experimental curve in fig. 3b and predicts the observed vibrational spacing. 241
Volume 56, number 4
OPTICS COMMUNICATIONS
The continua were not observed in CsC1+. Presumably, the reason is the crossing of the Cs + C1+ and Cs+ + CI* potential curves with the CsC1+ excimer curve at a low Cs+-CI distance. While in the Cs+ + F system the asymptotic Cs+ + F* and Cs + F + states lie more than 4.8 eV above the minimum of the CsF + potential, in the Cs+ + C1 system the corresponding difference is only 1.6 eV. Therefore, instead of forming CsC1+, the reactions of CsC1 with He + and He* proceed to C1+ or CI*. In secondary reactions, C1+ and CI* react with CsC1 to form CIr. The observation of the strong CI~ fluorescence seems to support this hypothesis. The same behavior was observed in the RgX molecules with similar potential schemes like NeF or ArCI [5]. In conclusion, there is strong evidence that the CsF + exeimer was produced in a charge transfer or Penning ionization from CsF. The D state decays by
242
15 December 1985
emitting 152 nm, the B state by 184.5 nm to the unbound ground state Cs+ + F. The authors thank Dr. A. Ulrich and W. Kr6tz, Technische Universit/it MOnehen, for their support in data taking.
References [1] R. Sauerbrey and H. Langhoff, IEEE QE 21 (1985) 179. [2] E.g. Ch. A. Brau, in: Topics in Appi. Physics Vol. 30, ed. Ch.K. Rhodes (Springer, Berlin, 1979). [3] E. Scl~itzlein, W. Walter, R. Sauerbreyand H. Langhoff, Appl. Phys. B 27 (1982) 49. [4] E.g.J.J. Ewing, in: Laser handbook, Vol. 3, ed. M.L. Stitch (North-Holland, Amsterdam, 1979). [5 ] W. Walter and H. Langhoff,J. Chem. Phys., to be published.
OPTICS COMMUNICATIONS
15 December 1985
OBSERVATION OF AN IONIC EXCIMER S T A T E IN CsF
+
F. S T E I G E R W A L D , F. E M M E R T , H. L A N G H O F F Physikalisches Institut der Unioersiti~t, 14rftrzburg, Fed. Rep. Germany
W. H A M M E R and T. G R I E G E L lnstitut ff~r Strahlenphysik der Unioersiti~t, Stuttgart, Fed. Rep. Germany Received 27 August 1985
The radiative decay of the excimer state CsF + isoelectronic to XeF was observed. Continua appeared at 152 nm and 184.5 nm and were assigned to the D ~ X and B ~ X transitions, respectively. The failure to detect the corresponding transitions in CsCI + is discussed.
The rare gas halide excimers (RgX) are the most efficient media for lasers in the ultraviolet down to 193 nm (ArF). In order to extend the range for this type of lasers to even lower wavelengths, in a recent paper [1] it has been proposed to investigate excimer states in ionized molecules which are isoelectronic to the RgX molecules. Most of the possible isoelectronic ions tend to predissociate. The most stable ions to be expected are CsF + and CsC1+ which are isoelectronic to the well known XeF and XeC1 excimers. Model calculations [1 ] using a Rittner potential for the upper state and a Born-Mayer potential for the repulsive lower state predicted for the range of the emitted wavelengths 154-195 nm and 131-205 nm, respectively. The resulting potentials for the Cs/F system are shown in fig. 1. Analogous to the RgX extimers [2], the emitted continua should exhibit a fine structure, the main intensities are due to the D -> X, B ~ X and C ~ A transitions. The formation of CsF + and CsCI+ are energetically possible by charge transfer from He + or Ne + to the corresponding cesium halids. Another reaction could be the Penning ionization by He* and Ne*. For an experimental investigation, the arrangement shown in fig. 2 has been used. The 3.6 MeV argon beam of the Stuttgart dynamitron accelerator with a current of 100 #A entered the reaction cell without passing through foils. This was accomplished using a 240
f 25
/// CS2,.F -
20
Cs÷F"
15 I0 5
c~Ff~ / ~--.~
Cs**F ,,
Cs'*F-
~CsF
Fig. 1. Relevant potential curves of the Cs/F system labeled by their asymptotic atomic states. By excitation with He + and/or Ne +, He*, Ne*, molecules in the CsF ground state are transferred into the bound excimer state labeled Cs2++ F-. They decay radiatively to the unbound state Cs+ + F. The lowest excited atomic states Cs+ + F* and Cs + F + are located considerably higher than the minimum of the Cs2+F--cmve.
four stage differential pumping system which maintained the vacuum in the accelerator even when the cell was filled with 500 mbar buffer gas. In order to remove impurities, the buffer gas which was circulated by the first pump, was passed through a titanium 0 030-4018/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 56, number 4
OPTICS COMMUNICATIONS
MgF-Window Differential pumping system
/
i Ii T
"r
..._T_~ ....T_~. T
L__
Recording System
__/
Titonium Oven
Fig. 2. Experimental arrangement. The At + beam entered the reaction cell windowless. The cell was heated and contained cesium halide vapor and buffer gas.
oven (800°C) before re-entering the cell. Spectra o f the emitted radiation were observed by a VUV spectrograph placed in direction o f the beam. The cell contained a crucible filled with CsF or CsC1 and could be heated up to 800°C. Helium or neon served as the buffer gas. Fig. 3 shows two spectra obtained with CsF and 100 mb of helium. At a temperature o f 680°C corresponding to a CsF pressure of about 0.7 mbar (fig. 3a), two continua appear in the IV spectra. They are not present at lower temperatures. In addition, the spectrum contains only narrow lines due to excited states o f He, He + and same remaining impurities as carbon and nitrogen.
0
190
180
170
160
150 " ~ - ' 140 k/nm
Fig. 3. Spectra observed for CsF vapor and a helium pressure of 100 mbar. (a) cell temperature 680°C, 0.7 mb CsF pressure, (b) cell temperature 750°C, 3.5 mb CsF pressure. The continua at 184.5 n m and 1 5 2 n m are attributed to the B ~ X and D -* X excimer transitions in CsF+. The lines are caused by excitations of helium and impurities. The main lines are due to the 193.1 nm transition in CI and the 164.0 transition in He II.
15 December 1985
When the vapor pressure of CsF is further increased to 3.5 mb (750°C cell temperature), the intensity of the continua centered at 152 nm and 184.5 nm rises. With increased CsF density, the absorption of the UV radiation by the CsF vapor becomes larger as may be seen from the decreasing intensities of the impurity lines. After correction for the absorption, the intensities o f both continua increase with rising CsF pressure. The continua were also observed with neon as the buffer gas. The corresponding experiments using CsC1 as the donor yielded no continua. Instead, an intense band system around 258 nm appeared. This is ascribed to the formation of CI~ [3]. In the experiment with CsF, the corresponding lines of F~ at 157 nm, if any, were only weakly present. The two continua exhibit the same features as those known from the RgX spectra. The wavelengths coincide with the wavelengths evaluated for a decay of CsF + in the model calculation. In the RgX molecules, the energy difference between the D ~ X and the B ~ X transitions is slightly less than the energy difference o f the two asymptotic states. In the pres. ent experiment, the difference between Cs2+(P3/2_) and Cs2+(P1/2 ) _ _ is 1.72 eV. If the continua are assigned to the D -~ X and B ~ X transitions in Cs2+F - , their energy difference of 1.44 eV compares favorably with the tendency. The C ~ A transition was not observed. It might have been missed due to its large width or its intensity was low due to B/C mixing as observed in many RgX spectra [4]. The continuum at 184.5 nm shows an undulated structure which is to be expected due to a vibrational excitation of the upper state. Assuming that the most intense line at 184.5 nm is caused by a transition from the u = 0 state to the repulsive ground state, the line width is due to the gradient of the lower potential curve. The value o f about 1.0 × 108 eV/cm is similar to corresponding values for the RgX molecules. The Cs÷F potential curve resulting from the Born-Mayer potential had to be slightly corrected in order to reproduce the position and width o f the B -~ X spectrum. The corrected curve is shown in fig. 1. A vibrational quantum energy o f o~ = 540 cm -1 is estimated from the Rittner potential calculation [1]. The experimental spacing of the different peaks is about 1050 cm -1 . A semiclassical calculation of the spectral shape using the potential gradient reproduces the experimental curve in fig. 3b and predicts the observed vibrational spacing. 241
Volume 56, number 4
OPTICS COMMUNICATIONS
The continua were not observed in CsC1+. Presumably, the reason is the crossing of the Cs + C1+ and Cs+ + CI* potential curves with the CsC1+ excimer curve at a low Cs+-CI distance. While in the Cs+ + F system the asymptotic Cs+ + F* and Cs + F + states lie more than 4.8 eV above the minimum of the CsF + potential, in the Cs+ + C1 system the corresponding difference is only 1.6 eV. Therefore, instead of forming CsC1+, the reactions of CsC1 with He + and He* proceed to C1+ or CI*. In secondary reactions, C1+ and CI* react with CsC1 to form CIr. The observation of the strong CI~ fluorescence seems to support this hypothesis. The same behavior was observed in the RgX molecules with similar potential schemes like NeF or ArCI [5]. In conclusion, there is strong evidence that the CsF + exeimer was produced in a charge transfer or Penning ionization from CsF. The D state decays by
242
15 December 1985
emitting 152 nm, the B state by 184.5 nm to the unbound ground state Cs+ + F. The authors thank Dr. A. Ulrich and W. Kr6tz, Technische Universit/it MOnehen, for their support in data taking.
References [1] R. Sauerbrey and H. Langhoff, IEEE QE 21 (1985) 179. [2] E.g. Ch. A. Brau, in: Topics in Appi. Physics Vol. 30, ed. Ch.K. Rhodes (Springer, Berlin, 1979). [3] E. Scl~itzlein, W. Walter, R. Sauerbreyand H. Langhoff, Appl. Phys. B 27 (1982) 49. [4] E.g.J.J. Ewing, in: Laser handbook, Vol. 3, ed. M.L. Stitch (North-Holland, Amsterdam, 1979). [5 ] W. Walter and H. Langhoff,J. Chem. Phys., to be published.