- Email: [email protected]
Photon-induced fluorescence spectroscopy (PIFS)
Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1526–1528
Photon-induced fluorescence spectroscopy (PIFS) a . H. Schmoranzera,*, H. Liebela, F. Vollweilera, R. Muller-Albrecht , a b b A. Ehresmann , K.-H. Schartner , B. Zimmermann a
. Kaiserslautern, Erwin-Schro¨dinger-Strasse, D-67653 Kaiserslautern, Germany Fachbereich Physik, Universitat b . Giessen, D-35392 Giessen, Germany I. Physikalisches Institut, Justus-Liebig-Universitat
Abstract Photon-induced fluorescence spectroscopy (PIFS) using monochromatized synchrotron radiation for exciting the sample selectively and a fluorescence spectrometer with a position-sensitive photon detector for recording its fluorescence spectrum is a very powerful experimental technique especially for the investigation of atomic and molecular photoionization and photodissociation. To reduce the contributions of higher harmonics of the undulator radiation which are also transmitted through the primary monochromator a very compact He absorber was developed. With a He pressure of 30 mbar in the differentially pumped gas cell of the absorber the fraction of the third harmonic of the undulator transmitted through the monochromator was reduced by a factor of 175 for an exciting-photon energy of the first order of 24.18 eV. # 2001 Elsevier Science B.V. All rights reserved. PACS: 33.80.Gj; 33.80.b; 82.80.Ch; 85.60.Ha Keywords: Photon-induced fluorescence spectroscopy; PIFS; He absorber for higher-order radiation
1. Introduction The method of photon-induced fluorescence spectroscopy (PIFS), using monochromatized synchrotron radiation for exciting the sample selectively and a fluorescence spectrometer combined with a position-sensitive photon detector for recording its fluorescence spectrum for a number of selected exciting-photon energies has proved to be a powerful tool for investigating, e.g., atomic and molecular photoionization [1,2] and photodissociation of molecules [3]. By scanning the *Corresponding author. Tel.: +49-631-205-2329; fax: +49631-205-2394. E-mail address: [email protected] (H. Schmoranzer).
exciting-photon energy and recording a dispersed fluorescence spectrum for each excitation energy, one obtains fluorescence intensities convertible into emission cross sections as a function of excitation and fluorescence energies in a twodimensional plot. In contrast to conventional photoelectron spectroscopy (PES), PIFS offers several advantages: *
*
The bandwidth of the exciting-photon energy and the spectral resolution of the fluorescence radiation are decoupled, whereas in PES the excitation bandwidth is convoluted with the bandwidth of the electron spectrometer. Strongly varying photoelectron energies do not affect the spectral fluorescence resolution in
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 7 5 0 - 1
H. Schmoranzer et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1526–1528
*
*
*
1527
PIFS while they can cause transmission problems of the electron spectrometer in PES. Multiple photoionization can be studied by PIFS without coincidence techniques [4]. In PIFS the investigation of neutral dissociation with excitation [3] is not restricted to a few autoionizing atomic states like in PES. Alignment parameters of radiating ions can be determined directly [5,6].
2. Experimental setup In Fig. 1 the typical setup of a recent PIFS experiment is shown as performed at the beamline U2-FSGM at BESSY I in Berlin. Two crossed undulators generated the synchrotron radiation which was monochromatized by a focusing spherical-grating monochromator (FSGM) [7] equipped with a 500 lines/mm grating. The monochromatized synchrotron radiation was focused into a target cell filled with, e.g., molecular oxygen at a pressure of 26.6 mbar. The fluorescence radiation following this excitation was dispersed by a 1 m-NI-monochromator (McPherson 225) equipped with a 2400 lines/mm grating. Fluorescence photons were detected in the open VUV range by a position-sensitive open microchannelplate detector [8,9]. The flux of the exciting radiation was monitored using an aluminum photodiode. As an example of a recent measurement, Fig. 2 shows an excerpt of a two-dimensional fluorescence yield spectrum of the O2 molecule recorded in the exciting-photon energy range from 22.8 to 24.8 eV (in steps of 10 meV, at a primary band-width of 20 meV) and for the fluorescence wavelength range from 90.5 to 105.5 nm at a resolution of 0.09 nm. In Fig. 2 the fluorescence intensities are plotted on a linear 15step grey scale. The fluorescence lines observed were assigned to transitions of excited neutral O atoms produced by dissociation of superexcited molecular states. The spectrum consists mostly of ‘islands of intensity’ which reflect a state-to-state dissociation of molecular Rydberg states into atomic Rydberg states. It was observed that conservation of the effective quantum number n *
Fig. 1. Experimental setup (schematically).
Fig. 2. Two-dimensional fluorescence yield spectrum of O2 . Horizontal lines indicate the excitation energies of the assigned molecular Rydberg states.
of the Rydberg electron is the main rule for the investigated neutral dissociation processes [3].
3. Absorber for higher harmonics of the undulator During the investigation of the neutral photodissociation of O2 some contributions of higher harmonics of the undulator were found to be transmitted through the primary monochromator. In order to suppress the higher harmonics, a very compact absorber [10] of 48 cm total length has been built with a gas cell of 12 cm length and altogether five differential pumping stages (see Fig. 3). The absorber was inserted between the primary monochromator and our experimental
1528
H. Schmoranzer et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1526–1528
Fig. 3. Experimental setup of the absorber for higher harmonics of the undulator transmitted through the monochromator (schematically).
24.18 eV we measured the dependence of the XeIII 5s0 5p6 1 S0 ! 5s2 5p3 ð2 DÞ5d1 P1 (indicated as line no. 1 in Fig. 4) fluorescence production and of the XeII 5s2 5p4 ð3 PÞ6S4 P5=2 ! 5s2 5p52 P3=2 , (indicated as line no. 2 in Fig. 4) satellite fluorescence production on the He pressure. Note that the XeIII fluorescence can only be excited by the third harmonic of the undulator transmitted through the monochromator whereas the population of the XeII 5s2 5p4 ð3 PÞ6s4 P5=2 satellite state stems mainly from the first order. An exponential decrease of the third harmonic contribution as a function of the He pressure in the absorber was observed. At a He pressure of 30 mbar the third harmonic radiation of the undulator transmitted through the monochromator was reduced by a factor of 175. The second harmonic undulator radiation (off-axis contribution) transmitted through the monochromator was reduced much more. The suppression factor was calculated with the effective absorption length of 7.9 cm to be about 150 000.
Acknowledgements The technical assistance of H. Molter in setting up the experiment is greatly appreciated.
References
Fig. 4. Excitation of XeIII fluorescence (line no. 1) at E ¼ 24:18 eV due to the incompletely suppressed third harmonic of the undulator as a function of the helium pressure pHe in the gas cell of the absorber.
station. For the largest possible exciting-photon energy range free of higher-order radiation helium was chosen as absorber gas. The performance of the absorber was evaluated in a PIFS experiment, by observing double photoionization processes in xenon. At the exciting-photon energy of
[1] H. Schmoranzer et al., Phys. Rev. Lett. 79 (1997) 4546 and further references therein. [2] S. Lauer et al., Phys. Lett. A 247 (1998) 167 and further references therein. [3] H. Liebel et al., Phys. Lett. A 267 (2000) 357 and further references therein. [4] K.-H. Schartner et al., J. Phys. B 26 (1993) L445. [5] H. Schmoranzer et al., J. Phys. B 30 (1997) 4463. [6] B.M. Lagutin et al., J. Phys. B 33 (2000) 1337. [7] H.A. Padmore et al., in: J.A. Samson, D.L. Ederer (Eds.), Vacuum Ultraviolet Spectroscopy II, Academic Press, San Diego, Boston, New York, Sydney, Tokyo, Toronto, 1998. [8] H. Schmoranzer et al., Nucl. Instr. and Meth. A 246 (1986) 485. [9] B. Kraus et al., Proc. SPIE 1159 (1989) 217. [10] A.G. Suits et al., Rev. Sci. Instrum. 66 (1995) 4841.
Photon-induced fluorescence spectroscopy (PIFS) a . H. Schmoranzera,*, H. Liebela, F. Vollweilera, R. Muller-Albrecht , a b b A. Ehresmann , K.-H. Schartner , B. Zimmermann a
. Kaiserslautern, Erwin-Schro¨dinger-Strasse, D-67653 Kaiserslautern, Germany Fachbereich Physik, Universitat b . Giessen, D-35392 Giessen, Germany I. Physikalisches Institut, Justus-Liebig-Universitat
Abstract Photon-induced fluorescence spectroscopy (PIFS) using monochromatized synchrotron radiation for exciting the sample selectively and a fluorescence spectrometer with a position-sensitive photon detector for recording its fluorescence spectrum is a very powerful experimental technique especially for the investigation of atomic and molecular photoionization and photodissociation. To reduce the contributions of higher harmonics of the undulator radiation which are also transmitted through the primary monochromator a very compact He absorber was developed. With a He pressure of 30 mbar in the differentially pumped gas cell of the absorber the fraction of the third harmonic of the undulator transmitted through the monochromator was reduced by a factor of 175 for an exciting-photon energy of the first order of 24.18 eV. # 2001 Elsevier Science B.V. All rights reserved. PACS: 33.80.Gj; 33.80.b; 82.80.Ch; 85.60.Ha Keywords: Photon-induced fluorescence spectroscopy; PIFS; He absorber for higher-order radiation
1. Introduction The method of photon-induced fluorescence spectroscopy (PIFS), using monochromatized synchrotron radiation for exciting the sample selectively and a fluorescence spectrometer combined with a position-sensitive photon detector for recording its fluorescence spectrum for a number of selected exciting-photon energies has proved to be a powerful tool for investigating, e.g., atomic and molecular photoionization [1,2] and photodissociation of molecules [3]. By scanning the *Corresponding author. Tel.: +49-631-205-2329; fax: +49631-205-2394. E-mail address: [email protected] (H. Schmoranzer).
exciting-photon energy and recording a dispersed fluorescence spectrum for each excitation energy, one obtains fluorescence intensities convertible into emission cross sections as a function of excitation and fluorescence energies in a twodimensional plot. In contrast to conventional photoelectron spectroscopy (PES), PIFS offers several advantages: *
*
The bandwidth of the exciting-photon energy and the spectral resolution of the fluorescence radiation are decoupled, whereas in PES the excitation bandwidth is convoluted with the bandwidth of the electron spectrometer. Strongly varying photoelectron energies do not affect the spectral fluorescence resolution in
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 7 5 0 - 1
H. Schmoranzer et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1526–1528
*
*
*
1527
PIFS while they can cause transmission problems of the electron spectrometer in PES. Multiple photoionization can be studied by PIFS without coincidence techniques [4]. In PIFS the investigation of neutral dissociation with excitation [3] is not restricted to a few autoionizing atomic states like in PES. Alignment parameters of radiating ions can be determined directly [5,6].
2. Experimental setup In Fig. 1 the typical setup of a recent PIFS experiment is shown as performed at the beamline U2-FSGM at BESSY I in Berlin. Two crossed undulators generated the synchrotron radiation which was monochromatized by a focusing spherical-grating monochromator (FSGM) [7] equipped with a 500 lines/mm grating. The monochromatized synchrotron radiation was focused into a target cell filled with, e.g., molecular oxygen at a pressure of 26.6 mbar. The fluorescence radiation following this excitation was dispersed by a 1 m-NI-monochromator (McPherson 225) equipped with a 2400 lines/mm grating. Fluorescence photons were detected in the open VUV range by a position-sensitive open microchannelplate detector [8,9]. The flux of the exciting radiation was monitored using an aluminum photodiode. As an example of a recent measurement, Fig. 2 shows an excerpt of a two-dimensional fluorescence yield spectrum of the O2 molecule recorded in the exciting-photon energy range from 22.8 to 24.8 eV (in steps of 10 meV, at a primary band-width of 20 meV) and for the fluorescence wavelength range from 90.5 to 105.5 nm at a resolution of 0.09 nm. In Fig. 2 the fluorescence intensities are plotted on a linear 15step grey scale. The fluorescence lines observed were assigned to transitions of excited neutral O atoms produced by dissociation of superexcited molecular states. The spectrum consists mostly of ‘islands of intensity’ which reflect a state-to-state dissociation of molecular Rydberg states into atomic Rydberg states. It was observed that conservation of the effective quantum number n *
Fig. 1. Experimental setup (schematically).
Fig. 2. Two-dimensional fluorescence yield spectrum of O2 . Horizontal lines indicate the excitation energies of the assigned molecular Rydberg states.
of the Rydberg electron is the main rule for the investigated neutral dissociation processes [3].
3. Absorber for higher harmonics of the undulator During the investigation of the neutral photodissociation of O2 some contributions of higher harmonics of the undulator were found to be transmitted through the primary monochromator. In order to suppress the higher harmonics, a very compact absorber [10] of 48 cm total length has been built with a gas cell of 12 cm length and altogether five differential pumping stages (see Fig. 3). The absorber was inserted between the primary monochromator and our experimental
1528
H. Schmoranzer et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1526–1528
Fig. 3. Experimental setup of the absorber for higher harmonics of the undulator transmitted through the monochromator (schematically).
24.18 eV we measured the dependence of the XeIII 5s0 5p6 1 S0 ! 5s2 5p3 ð2 DÞ5d1 P1 (indicated as line no. 1 in Fig. 4) fluorescence production and of the XeII 5s2 5p4 ð3 PÞ6S4 P5=2 ! 5s2 5p52 P3=2 , (indicated as line no. 2 in Fig. 4) satellite fluorescence production on the He pressure. Note that the XeIII fluorescence can only be excited by the third harmonic of the undulator transmitted through the monochromator whereas the population of the XeII 5s2 5p4 ð3 PÞ6s4 P5=2 satellite state stems mainly from the first order. An exponential decrease of the third harmonic contribution as a function of the He pressure in the absorber was observed. At a He pressure of 30 mbar the third harmonic radiation of the undulator transmitted through the monochromator was reduced by a factor of 175. The second harmonic undulator radiation (off-axis contribution) transmitted through the monochromator was reduced much more. The suppression factor was calculated with the effective absorption length of 7.9 cm to be about 150 000.
Acknowledgements The technical assistance of H. Molter in setting up the experiment is greatly appreciated.
References
Fig. 4. Excitation of XeIII fluorescence (line no. 1) at E ¼ 24:18 eV due to the incompletely suppressed third harmonic of the undulator as a function of the helium pressure pHe in the gas cell of the absorber.
station. For the largest possible exciting-photon energy range free of higher-order radiation helium was chosen as absorber gas. The performance of the absorber was evaluated in a PIFS experiment, by observing double photoionization processes in xenon. At the exciting-photon energy of
[1] H. Schmoranzer et al., Phys. Rev. Lett. 79 (1997) 4546 and further references therein. [2] S. Lauer et al., Phys. Lett. A 247 (1998) 167 and further references therein. [3] H. Liebel et al., Phys. Lett. A 267 (2000) 357 and further references therein. [4] K.-H. Schartner et al., J. Phys. B 26 (1993) L445. [5] H. Schmoranzer et al., J. Phys. B 30 (1997) 4463. [6] B.M. Lagutin et al., J. Phys. B 33 (2000) 1337. [7] H.A. Padmore et al., in: J.A. Samson, D.L. Ederer (Eds.), Vacuum Ultraviolet Spectroscopy II, Academic Press, San Diego, Boston, New York, Sydney, Tokyo, Toronto, 1998. [8] H. Schmoranzer et al., Nucl. Instr. and Meth. A 246 (1986) 485. [9] B. Kraus et al., Proc. SPIE 1159 (1989) 217. [10] A.G. Suits et al., Rev. Sci. Instrum. 66 (1995) 4841.