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

Abstract

Strong-field excitation and energy redistribution dynamics of C 60 fullerenes are studied by means of time-resolved mass spectrometry in a two-color femtosecond pump-probe setup. Resonant pre-excitation of the electronic system via the first dipole-allowed HOMO→ LUMO+ 1͑t 1g ͒ ͑HOMO denotes highest occupied molecular orbital and LUMO denotes lowest unoccupied molecular orbital͒ transition with ultrashort 25 fs pulses at 399 nm of some 10 12 W cm −2 results in a highly nonequilibrium distribution of excited electrons and vibrational modes in the neutral species. The subsequent coupling among the electronic and nuclear degrees of freedom is monitored by probing the system with time-delayed 27 fs pulses at 797 nm of some 10 13 W cm −2 . Direct information on the characteristic relaxation time is derived from the analysis of transient singly and multiply charged parent and fragment ion signals as a function of pump-probe delay and laser pulse intensity. The observed relaxation times el Ӎ 60-400 fs are attributed to different microcanonical ensembles prepared in the pre-excitation process and correspond to different total energy contents and energy sharing between electronic and vibrational degrees. The characteristic differences and trends allow one to extract a consistent picture for the formation dynamics of ions in different charge states and their fullerenelike fragments and give evidence to collective effects in multiple ionization such as plasmon-enhanced energy deposition.

Ultrafast Multiphoton Thermionic Photoemission from Graphite

“…Thermionic emission corresponds to hot thermalized electrons excited by e-e scattering to energies above E v escaping into the vacuum. Thus, we expect TES to conform to a statistical Boltzmann energy distribution of a nondegenerate hot electron gas: IðEÞ ∝ exp ½−E=ðk B T e Þ , where k B is the Boltzmann constant [14,15,28]. The density of states of graphite for threshold photoemission is flat [53] and therefore not a factor in our analysis.…”

“…The density of states of graphite for threshold photoemission is flat [53] and therefore not a factor in our analysis. Similar assumptions of thermal equilibrium for the electronic system were made to analyze the ultrafast multiphoton excitation of blackbody luminescence from graphene, thermionic electron emission from carbon nanotube forests, and electron or ion emission from C 60 [14,15,28,86,87]. Indeed, fitting representative s-polarized mPP spectra for hν ¼ 1.…”

“…Hot nonthermal electrons and holes of this primary distribution interacting through ineffectively screened Coulomb interaction scatter throughout the available phase space, including the σ bands, to achieve a Boltzmann distribution with temperatures as high as 5500 K. Electrons excited by IR light must gain more than 3 eV by Auger scattering processes within 25 fs to be photoemitted. Such extreme electron correlation can be attributed to ineffective screening of the Coulomb interaction and resembles electron relaxation processes in molecular materials such as C 60 and carbon nanotubes rather than metals [28,87]. In metals, where the Coulomb interaction is more strongly screened and the photoexcited carrier density is only a small fraction of the Fermi-level density [38], reaching comparable T e requires about 100-times-larger fluences and electron thermalization takes about 10 times longer [67,73,94,95].…”

“…Moreover, strong modulation of the carrier density through ultrafast optical excitation and the ensuing hot carrier multiplication drives the electron and hole distributions to different chemical potentials, enabling applications in energy harvesting, ultrafast electronics, and coherent optics [1,3,[16][17][18][19][20]. These novel properties derive from graphene's Dirac fermion band structure, weak screening, and strong, moleculelike electron correlation [21][22][23][24][25][26][27][28][29][30][31], which distinguish it from conventional metals and semiconductors [22,32,33].…”

Coherent Electronic Wave Packet Motion inC60Controlled by the Waveform and Polarization of Few-Cycle Laser Fields

“…Angularly resolved photoionization by ultrashort laser pulses has been advocated for probing the electronic dynamics before the onset of significant nuclear motion (see, e.g., [27][28][29][30][31] [36,37], and efficient high-harmonic generation [38][39][40]. The ionization and fragmentation of C 60 have been investigated extensively in the past (see, e.g., [35,[41][42][43][44][45][46][47]). C 60 is very stable and is one of the few molecular systems for which the ionization energy is smaller than the lowest fragmentation threshold.…”