C 60 in intense short pulse laser fields down to 9fs: Excitation on time scales below e-e and e-phonon coupling

Abstract

The interaction of C60 fullerenes with 765-797 nm laser pulses as short as 9 fs at intensities of up to 3.7 x 10(14) W cm(-2) is investigated with photoion spectroscopy. The excitation time thus addressed lies well below the characteristic time scales for electron-electron and electron-phonon couplings. Thus, energy deposition into the system is separated from energy redistribution among the various electronic and nuclear degrees of freedom. Insight into fundamental photoinduced processes such as ionization and fragmentation is obtained from the analysis of the resulting mass spectra as a function of pulse duration, laser intensity, and time delay between pump and probe pulses, the latter revealing a memory effect for storing electronic energy in the system with a relaxation time of about 50 fs. Saturation intensities and relative abundances of (multiply charged) parent and fragment ions (C60(q+), q=1-6) are fingerprints for the ionization and fragmentation mechanisms. The observations indicate that for final charge states q>1 the well known C60 giant plasmon resonance is involved in creating ions and a significant amount of large fragments even with 9 fs pulses through a nonadiabatic multielectron dynamics. In contrast, for energetic reasons singly charged ions are generated by an essentially adiabatic single active electron mechanism and negligible fragmentation is found when 9 fs pulses are used. These findings promise to unravel a long standing puzzle in understanding C60 mass spectra generated by intense femtosecond laser pulses.

Fullerene Dynamics with X-Ray Free-Electron Lasers

“…The single-activeelectron (SAE) method was used to calculate the ionization of C 60 in intense, 4 × 10 13 W/cm 2 laser pulses with durations between 27 and 70 fs, and for a wide range of wavelengths ranging from 395 to 1800 nm [19]. This calculation agreed with measurements by Shchatsinin et al [12]. For a long IR wavelength of 1800 nm and 70 fs pulse duration, the SAE picture predicts "over the barrier" ionization for a peak intensity of 10 15 W/cm 2 , leading to non-fragmented but highly charged C 60 q+ (q = 1-12) [7].…”

“…The behavior of molecules in short, intense laser fields [6] was extended to large molecules, such as C 60 , which is intriguing due to the numerous nuclei-electron responses exhibited, because it is a cage of 60 atoms with 240 valence-electrons [7][8][9][10][11][12][13][14][15][16]. The interaction of such a large system is key to investigating many-body problems induced on the system's electrons by the photon electric field.…”

“…The photon interaction with the electronic fullerene's degrees of freedom results in electronic dynamics that lead to nuclei dynamics, because they are both interconnected. Fullerenes, including endohedral fullerenes, are ideal candidates to explore their many-body responses to electromagnetic fields because they respond in different ways-depending upon the field parameters [7][8][9][10][11][12][13][14][15][16][17]. Ionization, which is one of the possible reactions, has shown to occur on different time scales.…”

Ultrafast energy redistribution in C60 fullerenes: A real time study by two-color femtosecond spectroscopy

“…[5][6][7][8][9][10] SAE is assumed to be predominant for very short pulses as it reflects the simple fact that the fundamental interaction of photons with a many-electron system is given by an independent sum of one-electron dipole interactions, while NMED is mediated by electron correlations which require, in addition to a sufficient number of electrons, also an electron-photon interaction time at least comparable to the electron-electron interaction time which is considered to be approximately 50-100 fs. Of similar interest is the role of intermediate electronically excited states in the absorption process [11][12][13] and efficient coupling of electronic to nuclear motion.…”

“…Essentially three different ionization regimes can be distinguished, which depend sensitively on the excitation time scale. 9,21 For very short pulses ͑FWHMϽ 70 fs͒ ͑FWHM denotes full width at half maximum͒ the excitation energy tends to remain mainly in the electronic systems and multiply charged C 60 q+ ions are observed, formed by direct multiphoton ionization ͑MPI͒, possibly enhanced by transient, quasiresonant multiphoton excitation 9 of the plasmon resonance 22 between 10 and 28 eV. Relatively few fullerenelike fragments C 60−2m q+ are generated and detected in typical mass spectra under these conditions.…”