Using femtosecond time-resolved photoelectron spectroscopy we demonstrate that photoexcitation transforms monoclinic VO 2 quasi-instantaneously into a metal. Thereby, we exclude an 80 fs structural bottleneck for the photoinduced electronic phase transition of VO 2 . First-principles many-body perturbation theory calculations reveal a high sensitivity of the VO 2 band gap to variations of the dynamically screened Coulomb interaction, supporting a fully electronically driven isostructural insulatorto-metal transition. We thus conclude that the ultrafast band structure renormalization is caused by photoexcitation of carriers from localized V 3d valence states, strongly changing the screening before significant hot-carrier relaxation or ionic motion has occurred. DOI: 10.1103/PhysRevLett.113.216401 PACS numbers: 71.27.+a, 71.20.Be, 71.30.+h, 79.60.-i Since its discovery in 1959 [1], studies of the VO 2 phase transition (PT) from a monoclinic (M 1 ) insulator (Fig. 1, top left) to a rutile (R) metal at T C ¼ 340 K (Fig. 1, top right) have revolved around the central question [2][3][4][5] of whether the crystallographic PT is the major cause for the electronic PT or if strong electron correlations are needed to explain the insulating low-T phase. While the M 1 structure is a necessary condition for the insulating state below T C , the existence of a monoclinic metal (mM) and its relevance to the thermally driven PT is under current investigation [6][7][8][9][10][11][12]. In particular, the role of carrier doping at temperatures close to T C by charge injection from the substrate or photoexcitation has been increasingly addressed [6,8,[13][14][15][16].One promising approach to disentangling the electronic and lattice contributions is to drive the PT nonthermally using ultrashort laser pulses in a pump-probe scheme. Time-resolved x-ray [17,18] and electron diffraction [16,19] showed that the lattice structure reaches the R phase quasithermally after picoseconds to nanoseconds. Transient optical spectroscopies have probed photoinduced changes of the dielectric function in the terahertz [20][21][22], near-IR [9,10,17,23], and visible range [23]. The nonequilibrium state reached by photoexcitation (hereinafter transient phase) differs from the two equilibrium phases, but eventually evolves to the R phase [17][18][19][20][21][22][23][24][25][26][27][28]. The observation of a minimum rise time of 80 fs in the optical response after strong excitation (50 mJ=cm 2 ), described as a structural bottleneck in VO 2 [24], challenged theory to describe the photoinduced crystallographic and electronic PT simultaneously [15,25].Time-resolved photoelectron spectroscopy (TR-PES) directly probes changes of the electronic structure. Previous photoelectron spectroscopy (PES) studies of VO 2 used high photon energies generating photoelectrons with large kinetic energies to study the dynamics of the electronic structure; however, with a low repetition rate (50 Hz [27]) and inadequate time resolution (> 150 fs) the ultrafast dynamics of t...

The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a photoinduced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, to identify when and how fast these phase transitions occur, methods that can probe the symmetry change in the time domain are required. Here we show that a time-dependent change in the coherent phonon spectrum can probe a change in symmetry of the lattice potential, thus providing an all-optical probe of structural transitions. We examine the photoinduced structural phase transition in Vo 2 and show that, above the phase transition threshold, photoexcitation completely changes the lattice potential on an ultrafast timescale. The loss of the equilibrium-phase phonon modes occurs promptly, indicating a non-thermal pathway for the photoinduced phase transition, where a strong perturbation to the lattice potential changes its symmetry before ionic rearrangement has occurred.

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We report on the nonequilibrium dynamics of the electronic structure of the layered semiconductor Ta_{2}NiSe_{5} investigated by time- and angle-resolved photoelectron spectroscopy. We show that below the critical excitation density of F_{C}=0.2  mJ cm^{-2}, the band gap narrows transiently, while it is enhanced above F_{C}. Hartree-Fock calculations reveal that this effect can be explained by the presence of the low-temperature excitonic insulator phase of Ta_{2}NiSe_{5}, whose order parameter is connected to the gap size. This work demonstrates the ability to manipulate the band gap of Ta_{2}NiSe_{5} with light on the femtosecond time scale.

Stabilization of excess electrons is studied at crystalline ice-metal interfaces by femtosecond time-resolved two-photon photoelectron spectroscopy and ab initio calculations. Following optical excitation into delocalized image potential states, electrons localize at pre-existing defects which are located at the ice-vacuum interface. Remarkably, the stabilization of these trapped electrons is monitored continuously from femtoseconds up to minutes. By first principle calculations we identify defect structures, that support excess electrons in front of crystalline ice surfaces, and suggest that the stabilization proceeds through subsequent conformational substates. The excess electron wave functions are efficiently screened from the underlying bulk and explain the long residence times observed in the experiment. Thereby, we shed light on the collective character of the nuclear rearrangement that determines the energetics over all timescales.