98 Dependence of Molecular Shape Resonance Energy on Inter-nuclear Distance Probed by High-resolution Core-level Photoelectron Spectroscopy The advent of third generation synchrotron radiation facilities has provided a wealth of information about inner shell photoionization processes because they offer a combination of high-resolution and high- intensity beams. Shape resonances are one of the most interesting features observed in core-level molecular photoionization. By definition, shape resonances arise from the temporal trapping of a photoelectron in the intermediate states by a centrifugal barrier. The resonance can be viewed as a broad enhancement in the ionization cross section where the photoelectron is resonantly scattered. As illustrated in Fig. 1, one can infer that the shape resonance positions should be sensitive to the intramolecular distances [1]. Köppe et al . [2] showed that the C 1 s -1 σ * shape resonance in CO that appears at the photon energy h ν ≈ 305 eV in the photoabsorption spectrum is not only present in the C 1 s -1 single-hole ionization (SHI) partial cross sections for the individual vibrational components ν ’, but also that the energies of the shape resonances decrease with increasing vibrational quantum number, i.e., from ~307 eV for ν ’ = 0 to ~302 eV for ν ’ = 2. In this work carried out on the C branch of the soft X-ray photochemistry beamline BL27SU , the CO O 1 s photoelectron spectra were recorded at photon energies across the σ * shape resonance region [3]. The O 1 s -1 SHI cross sections for the vibrational components ν ’ = 0, 1 and 2 are shown together with the C 1 s -1 SHI cross sections from Köppe et al . [2] in Fig. 2. It is clear that the O 1 s -1 σ * shape resonance energies increase with increase in ν ’, i.e., opposite to the C 1 s -1 σ * shape resonance. The corresponding displacements of the nuclear separation from the equilibrium distance R e of the ionic states were determined to be + 0.025, + 0.068 and + 0.111 Å for C 1 s and - 0.020, - 0.074 and - 0.109 Å for O 1 s for C 1 O e O 1 C e ’=1 ’=1 ’=0 ’=0 ’=2 ’=2 R e R e R e O 1 -1 C 1 -1 1.0 1.1 1.2 1.3 R (Å) Fig. 1. Schematic diagram of nuclear motion upon photoionization- induced vibrational excitation showing potential curves for the ground and core-ionized C 1 s -1 and O 1 s -1 in CO, respectively. R e is the equilibrium internuclear distances: R e = 1.128 Å, 1.167 Å and 1.079 Å for the ground and core-ionized C 1 s -1 and O 1 s -1 states, respectively. ν ’ = 0, 1 and 2, respectively. This indicates that the photoelectron is trapped by a compressed ion and the compression increases with increasing ν ’ for O 1 s in contrast with an elongation that increases with n’ in the case of C 1 s [3] (see Fig. 1). This work on CO has been extended to other molecules (H 2 CO [4], CO 2 , N 2 O, C 2 H 2 , C 2 H 4 and C 2 H 6 ), which offers an interesting systematic study of these effects. The g-u symmetry-resolved C 1 s σ g and 1 σ u SHI partial cross sections of C 2 H 2 are shown in Fig. 3 for ν ’ = 0, 1 and 2 vibrational components. The shape resonance appears only in the C 1 s σ g photoionization and can be assigned to σ g *. The energy of the C 1 s σ g -1 σ u * shape resonance decreases with the increase in the vibrational quantum number ν ’, i.e., in a way similar to the result of CO C 1 s -1 σ * shape resonance discussed above. This is the first observation of the vibrational effects on shape resonance energy in the g-u symmetry-resolved SHI cross sections [5]. The analysis of the results for the other molecules is still in progress. Casten Makochekanwa a , Hiroshi Tanaka a, * and Kiyoshi Ueda b (a) Department of Physics, Sophia University (b) Institute of Multidisciplinary Research for Advanced Materials, Tohoku University *E-mail: h_tanaka@sophia.ac.jp References [1] J. L. Dehmer and D. Dill: Electron-Molecule and Photon-Molecule Collisions, edited by T. Rescigno, V. McKoy and B. Schneider (Plenum Publishing Corporation) (1979) 225. [2] H. M. Köppe et al .: J. Phys. B 30 (1997) L741. [3] D. A. Mistrov et al. : Phys. Rev. A 68 (2003) 022508. [4] A. De Fanis et al .: Phys Rev. A 71 (2005) 052510. [5] K. Nakagawa, T. Tanaka, C. Makochekanwa, N. Kuze, M. Kitajima, H. Tanaka, A. De Fanis, Y. Tamenori, M. Matsumoto, K. Fujiwara and K. Ueda: - to be published. 99 ’ = 0 ’ = 1 ’ = 2 ’ = 0 ’ = 1 ’ = 2 CO C 1 s 300 305 310 5 10 15 Photon Energy (eV) 10 5 0 5 0 10 Cross Section (arb. units) Kinetic Energy (eV) CO O 1 s (a) (b) Fig. 2. SHI cross sections for vibrational components ν ’ = 0, 1 and 2 for (a) C 1 s -1 σ * from Köppe et al. [2] and, (b) current results for O 1 s -1 1 σ *. The photon energy scale above the top panel applies only to the CO C 1 s data, whereas the kinetic energy scale applies to both panels. 300 320 3 40 Photon Energy (eV) 0 .0 0.5 C ross Section (arb.units) Fig. 3. g-u symmetry-resolved C 1 s σ g and 1 σ u SHI partial cross sections of C 2 H 2 for ν ’ = 0, 1 and 2 vibrational components.
home
- JP
- EN