Excitation and cooling of large molecular and cluster ions

Transcription

1 Excitation and cooling of large molecular and cluster ions Andreas Wolf Max-Planck-Institut für Kernphysik, Heidelberg, Germany Laboratory Astrophysics Workshop, Tabarz, October 2014 Decay mechanisms in excited molecular ions Quantitative study of cooling rates for SF6 Laser probing of radiative cooling (Co6 ) Outlook

2 Photoexcitation and -emission of complex molecules S1 S0 Photoexcitation Internal conversion High vibrational excitation Intramolecular vibrational redistribution - Vibrational quasicontinuum - Emission on optical active modes Radiative cooling = Repeated emission of active mode frequency/ies within the vibrational quasicontinuum (stepwise allowed-harmonic emission)

3 Vibrational energy levels in large molecules Density of states SF6 (for SF6 vibrational modes) Vibrational quasicontinuum K. Hansen, Statistical Physics of Nanoparticles in the Gas Phase (Springer, 2013)

4 Decay of excited SF6 Broad thermal excitation 1000 K t=0 Vibrational autodetachment (SF6 )* SF60 + e VAD Inverse process of dissociation-less electron attachment

5 Decay of excited SF6 Broad thermal excitation 1000 K t = 1 ms Vibrational autodetachment (SF6 )* SF60 + e VAD Inverse process of dissociation-less electron attachment

6 Decay of excited SF6 Broad thermal excitation 1000 K t = 50 ms Vibrational autodetachment (SF6 )* SF60 + e VAD Inverse process of dissociation-less electron attachment

7 Decay of excited SF6 Broad thermal excitation 2000 K 1000 K t = 50 ms Vibrational autodetachment (SF6 )* SF60 + e VAD Inverse process of dissociation-less electron attachment

8 Time scales of excited SF6 decay processes ~10 μs 10 μs 50 ms ~30 ms

9 CTF: Cryogenic Trap for Fast Ion Beams ~ 8 kev Fast stored ions cm 0 5 ~ Ion beam Neutral fragment counting Aln, Cun, Con, SF6, N2+, etc. Trapping electrodes (pulsed) 10 K separate vacuum chamber ~103 cm3 rest gas density (<10 13 mbar equivalent)

10 CTF: Cryogenic Trap for Fast Ion Beams Injection beam line Ion beam Neutral fragment counting CTF cryostat Ion source (Cs sputter type) 10 K Injected ion beam

11 Vibrational autodetachment rates of SF6 (SF6 )* SF60 + e Delayed vibrational autodetachment Cryogenic ion beam trap CTF connected to the CSR cryogenics ~5 kev ions SF60 + e e Energy > 1 ev Multimode excitation from ion source 0.95 ev SF6 electron affinity SF6 Coordinate

12 Vibrational autodetachment rates of SF6 (SF6 )* SF60 + e Delayed vibrational autodetachment Cryogenic ion beam trap CTF connected to the CSR cryogenics ~5 kev ions SF6 + e 0 Energy SF60 rate 0.95 ev SF6 electron affinity SF6 Coordinate Storage time

13 Vibrational autodetachment rates of SF6 (SF6 )* SF60 + e Delayed vibrational autodetachment Cryogenic ion beam trap CTF connected to the CSR cryogenics ~5 kev ions SF60 + e Storage time dependence of the VAD rate Energy 0.95 ev SF6 electron affinity SF6 Coordinate Multi-exponential decay power law S. Menk et al., Phys. Rev. A 89, (2014) Previous experiments

14 Analysis of SF6 autodetachment rates VAD rate and energy distribution near EA Storage time dependence of VAD rate Effect of radiative cooling

15 Analysis of SF6 autodetachment rates Fit of measured VAD rate curves Effect of radiative cooling Storage time dependence of VAD rate Effect of radiative cooling

16 Analysis of SF6 autodetachment rates Fit of measured VAD rate curves Ion source conditions varied Storage time dependence of VAD rate Effect of radiative cooling Electron affinity (EA) and radiative cooling rate at ESF6 ~ EA Ion temperatures (short times)

17 Radiative cooling of Co4 Laser excitation + delayed detachment Cryogenic ion beam trap CTF connected to the CSR cryogenics ~5 kev ions Co40 delayed autodetachment rate Co40 + e Pulsed laser e Energy Co4 Coordinate Storage time

18 Radiative cooling of Co4 Laser excitation + delayed detachment Cryogenic ion beam trap CTF connected to the CSR cryogenics ~5 kev ions Co40 delayed autodetachment rate Co40 + e Pulsed laser Energy Co4 Storage time Coordinate Co4 delayed detachment Track radiative cooling of multimode vibrations Al : 4 M. Lange et al., New J. Phys. 14, (2012) Delayed Co40 rate (arb.un.) e 1.17 ev 1.46 ev 1.77 ev Storage time (s) In progress: C. Breitenfeldt, S. George et al. (collaboration with L. Schweikhard, Greifswald)

19 Summary and Outlook In stored ion beams, large numbers of cations or anions can be kept for long time in an interaction-free environment Stepwise allowed-harmonic emission during radiative cooling of excited ions Observed in cryogenic electrostatic ion beam trap with vibrational autodetachment as a probe Vibrational emission observable with sufficiently sensitive mid-ir detectors preferable: cryogenic environment and large number of ions Laser-induced vibrational emission: a tool of absorption spectroscopy for wide range of polyatomic ions (such as C60+) LIVE scheme, H. Kreckel et al., ISM SPP proposal using CTF

20 Acknowledgements CTF measurements, CSR laser experiments M. Lange S. George S. Menk C. Breitenfeldt (Univ. Greifswald) C. Meyer J. Göck D. Schwalm (MPIK/WIS Rehovot) CSR detectors, electron cooler C. Krantz O. Novotný (Columbia Univ.) C. Vogel A. Becker K. Spruck (Univ. Gießen) D. W. Savin (Columbia Univ.) S. Schippers (Univ. Gießen) Astrolab MPG WIS Funding H. Kreckel A. O'Connor F. Grussie ERC DFG Priority Program Physics of the Interstellar Medium NASA and NSF (Columbia Univ.) CSR team R. von Hahn M. Grieser F. Fellenberger S. George M. Lange R. Repnow P. Herwig C. Krantz K. Blaum S. Vogel K. Spruck (Univ. Gießen) A. Becker H. Kreckel (Astrolab) F. Grussie (Astrolab) A. O'Connor (Astrolab) and the MPIK workshops and labs Collaborations CSR and experiments Astrolab ERC/MPIK (H. Kreckel) WIS Rehovot (O. Heber, D. Zajfman) Univ. Greifswald (L. Schweikhard) Columbia Univ. (D. W. Savin) Univ. Gießen (S. Schippers) Univ. Louvain-la-Neuve (X. Urbain) Univ. Heidelberg, KIP (A. Fleischmann, C. Enss) Univ. Heidelberg, OCI (O. Trapp) Univ. Kaiserslautern (G. Niedner-Schatteburg) MPIK Quantum Dynamics and Control (T. Pfeiffer, C. D. Schröter, R. Moshammer)