Time-resolved photoemission at the Si(100)-Ga surface using a femtosecond higher-harmonic laser source

Abstract

The Si͑100͒-͑2 ϫ 2͒-Ga surface was used to investigate time-dependent Ga͑3d͒ core-level shifts by pumping electrons from the valence to the conduction band. The pump-probe experiments were done by exciting carriers with 1.59 eV laser pulses and probing the Ga͑3d͒ core level with higher harmonics. The higher harmonics were generated by focusing laser pulses with 1.4 mJ energy, 30 fs pulse duration, and 779 nm wavelength from a multipass amplifier at a repetition rate of 1 kHz into argon. For the 23rd harmonic, the time resolution of the experiment was ϳ400 fs after a grating monochromator. The band bending of about 110 meV of the p-doped Si͑100͒-͑2 ϫ 2͒-Ga surface is completely lifted by illumination of the surface with the 1.59 eV laser pulses. The Ga͑3d͒ core level shows a slow time-dependent shift attributed to the rise ͑ϳ1 ns͒ and decay ͑ϳ100 ns͒ of the photovoltage. The Ga͑3d͒ core-level shift and broadening in the femtosecond range are determined to be Ͻ15 meV at the used pump-pulse intensity of 19 mJ/ cm 2 . Experiments with pump pulses of 3.18 eV photon energy showed similar results.

Time-resolved HAXPES at SACLA: probe and pump pulse-induced space-charge effects

“…When probe pulses in the UV to extreme ultraviolet (XUV) range are used and angular resolution is added, the technique is referred to as time-and angle-resolved photoemission spectroscopy (trARPES) and provides direct information on the momentum-resolved dynamics of valence electrons, including the temporal evolution of electronic populations, band structures, Fermi surfaces, and energy gaps [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. When the probe pulses have higher photon energies, in the extreme ultraviolet to soft x-ray range, the technique becomes time-resolved x-ray photoemission spectroscopy (trXPS), with element specificity, sensitivity to the chemical environment, and tunability of the probing depth, and provides a direct view on the local, atomic-site specific electron dynamics as reflected in the positions, widths, and shapes of coreelectron emissions [20][21][22][23][24][25][26].The recent experimental advances in time-resolved solid-state photoemission spectroscopy are intimately connected to the development of ultrashort pulsed light sources, with the evolution continuously progressing from the ultraviolet to ever higher photon energies in the xray regime. trARPES experiments are commonly based on Ti:sapphire laser systems delivering intense IR pump pulses, and UV and XUV probe pulses are produced by frequency quadrupling in beta barium borate crystals [27,28] and higher-harmonic generation (HHG) in rare gases [29][30][31][32], respectively.…”

“…trARPES experiments are commonly based on Ti:sapphire laser systems delivering intense IR pump pulses, and UV and XUV probe pulses are produced by frequency quadrupling in beta barium borate crystals [27,28] and higher-harmonic generation (HHG) in rare gases [29][30][31][32], respectively. For trXPS measurements, HHG-based setups [23,33] as well as soft and hard x-ray free-electron lasers [26,34,35] have been used. The highest probing photon energy used to date is 8 keV [35], opening the way to a novel technique: time-resolved hard x-ray photoemission spectroscopy (trHAXPES), i.e., trXPS with bulk (sub-surface) sensitivity.A fundamental factor limiting all variants of time-resolved pump-probe solid-state photoemission spectroscopy are vacuum space-charge effects, i.e., the spectral shifts and broadenings that develop after (and during) photoelectron emission from a surface whenever…”

Femtosecond time-resolved photoelectron spectroscopy with a vacuum-ultraviolet photon source based on laser high-order harmonic generation

“…It has been shown in recent studies that the photon energy range can be considerably extended into the water window of several 100 eV and beyond (34)(35)(36)(37)(38)(39)(40) but applications with these sources up to now are scarce. For ultrafast applications, time-resolved photoelectron spectroscopy (PES) (41) with pump-probe schemes where the sample is pumped with a laser pulse and probed with HHG pulses at a defined pump-probe delay has emerged as a powerful tool for the investigation of ultrafast dynamics in gas phase samples (42)(43)(44)(45)(46)(47) and on surfaces (48)(49)(50)(51)(52)(53)(54). A variety of set ups has been described to perform such experiments on gas phase samples (55)(56)(57)(58)(59)(60), on surfaces (61-64) or on both (65,66).…”

Tunable ultrafast extreme ultraviolet source for time- and angle-resolved photoemission spectroscopy

“…One possible method is high-harmonic generation ͑HHG͒, which provides coherent, ultrashort radiation covering the spectral region from vacuum UV to soft x-rays. 7,8 Several groups have successfully shown the implementation of HHG in timeresolved photoemission spectroscopy on solids: Haight et al 5,9 developed this technique to study carrier dynamics at semiconductor surfaces, Quere et al 10 observed energy relaxation in photoexcited quartz, Bauer et al 11 and Miaja-Avila et al 12 traced femtosecond surface chemistry reaction from molecular oxygen and xenon deposited on platinum, respectively, Read et al 13 studied the decay of excitations on a dye-doped organometallic material, Siffalovic et al 14 traced hot-electron dynamics from platinum surface, and Siffalovic et al 15 and Melzer et al 16 observed transient surface photovoltage in optically excited p-GaAs and Ga-covered Si surface, respectively.…”