Berlin 2001 – wissenschaftliches Programm
Bereiche | Tage | Auswahl | Suche | Downloads | Hilfe
AMPD: EPS AMPD
AMPD 4: Sitzung 4
AMPD 4.1: Vortrag
Dienstag, 3. April 2001, 09:55–10:30, H104
From above threshold ionisation to statistical electron emission: The laser pulse-duration dependence of C60 photoelectron spectra — •E.E.B. Campbell1 and K. Hoffmann2 — 1Dept. of Experimental Physics, Göteborg University/Chalmers University of Technology, SE-41296 Göteborg, Sweden — 2Max-Born-Institut, Max-Born-Str. 2a, D-12489 Berlin, Germany
Fullerenes are very interesting and fruitful model systems for investigating the dynamics of complex molecular systems with a large number of degrees of freedom. One of the most interesting properties of C60 is its tendency to undergo delayed (µs) ionisation after excitation with ns laser pulses. This is generally described in terms of a statistical thermal electron emission, akin to thermionic emission from bulk metals. The delayed ionisation occurs due to the strong coupling of electronic to vibrational degrees of freedom during the timescale of the laser pulse leading to a strong vibrational heating to temperatures on the order of 3000-4000 K. Since the activation energy for the lowest energy fragmentation process (C2 emission) is higher than the ionisation potential in fullerenes, delayed ionisation is an observable reaction channel, in competition with neutral fragmentation. This work explores the ionisation behavior when the excitation timescale is reduced below the timescale for electron-phonon coupling. Three distinct ionisation mechanisms are observed for different excitation timescales. For the shortest laser pulses (25 fs), the ionisation is a direct multiphoton ionisation for intensities up to ca. 1014 W/cm2 where tunnel ionisation onsets. This is seen most clearly in the photoelectron spectra where clear above threshold ionisation (ATI) peaks are seen for intensities below this value. As the laser pulse duration is increased beyond 75 fs, keeping the fluence constant, the ATI becomes less important and is dominated by a thermal electron energy distribution corresponding to high electron temperatures (on the order of 30000 K). This is interpreted as being due to a statistical electron emission after the electronic excitation energy has been distributed among the electronic degrees of freedom in the molecule. The mass spectra are still characterised by strong multiple charging and sharp mass peaks (indicating prompt ionisation on the ns timescale). The third ionisation mechanism is observed for laser pulses longer than 500 fs. The thermal electron temperature drops dramatically for laser pulses of the same fluence. In addition, no multiply charged ions can be seen in the mass spectra and the C60+ parent ion peak develops a long "tail" extending to longer flight times. Such a tail is characteristic of delayed (µs) ionisation. This therefore marks the cross-over to a thermionic-like ionisation after the electronic excitation energy has coupled to the vibrational degrees of freedom.