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materials which exhibit a metal - insulator transition concomitant with a structural phase transition naturally raise questions about the driving mechanism of the phase transition , i.e. if it is possible to single out what is the role played by the electronic or lattice degrees of freedom and how they affect the electronic structure of such materials . in this context , studies of the non - equilibrium dynamics have opened a new dimension in the investigation of correlated materials in recent years . time- and angle - resolved photoemission spectroscopy ( trarpes ) has proved to be a major technique , giving access to the momentum - resolved electronic band structure of solids out of equilibrium @xcite . in the case of charge density wave ( cdw ) materials , the most direct approach follows the photoinduced changes of the cdw spectral features , namely the closing of the cdw gap or the disappearance of the cdw backfolded bands directly in the time - domain . many studies have focussed on the first hundred femtoseconds following the photoexcitation , in order to figure out whether these changes occur faster or not than the lattice motion related to the structural distortion , the timing of which is given by the period of the relevant phonon mode . the recovery dynamics of cdw order have received less attention , however we will show below that they also provide fundamental information on the role of electronic and phonon degrees of freedom . cdw materials undergo a low temperature phase transition resulting in a new translational periodicity in the electronic density ( the cdw ) , as well as a periodic lattice distortion ( pld ) . one of these materials , 1_t_-tise@xmath0 , undergoes a peculiar second - order cdw transition@xcite below @xmath1 k. the mechanism of its cdw transition has been debated intensively over the last decades , in terms of either an electronic @xcite or a structural origin @xcite . time - resolved techniques seem to give a decisive answer by focussing on the ultrafast melting of the cdw and pld and disentangling the roles of electrons and lattice on the femtosecond timescale @xcite . these studies are consistent with a purely electronic mechanism of the cdw transition in 1_t_-tise@xmath0 , which might be an instability towards an excitonic insulator phase . in such a scenario , electrons and holes from distinct low - energy bands , are bound together in excitons by the interband coulomb interaction and their macroscopic spontaneous condensation at low temperature leads to a new ground state @xcite . a recent ultrafast terahertz spectroscopy study@xcite stressed the importance of the lattice degree of freedom , which is shown to stabilize the cdw phase together with the excitonic mechanism . in parallel , a few theoretical models@xcite have demonstrated that the cdw ( and its concomitant pld ) can be cooperatively stabilized by both the interband coulomb interaction and the electron - phonon coupling . in the light of these recent studies , what is then the role of the excitonic mechanism in 1_t_-tise@xmath0 ? does is alone provoke the cdw instability or does it merely assist the lattice in driving the cdw transition ?    here , in order to address these questions , we investigate the femtosecond dynamics of both the recovery of the cdw phase in 1_t_-tise@xmath0  at 30 k , as well as of high - temperature cdw fluctuations at 240 k , which precedes the cdw instability . we exploit the full capabilities of trarpes by looking at the energy- and momentum - resolved recovery dynamics of the cdw correlations . by doing so we unveil the dynamics of both the electron and lattice degrees of freedom . below @xmath2 , in the cdw phase , we observe the coherent oscillations of the cdw amplitude mode for the first time with trarpes . above @xmath2 , we single out a fast component in the recovery of the cdw correlations with a relaxation time @xmath3 100 fs . we interpret it as the consequence of the ultrafast recovery from the screening of the coulomb interaction responsible for the electron - hole correlations , which precedes the cdw phase transition . the observation of these two contributions - the coherent oscillations of the cdw amplitude mode at low temperature and the ultrafast electronic recovery at high temperature - supports the idea that both the interband coulomb interaction and the electron - phonon coupling cooperatively stabilize the cdw phase ( and the concomitant pld ) in 1_t_-tise@xmath0 . the paper is divided into two main parts : in part [ sec_lt ] , we study the low temperature ( 30 k ) cdw phase using a mid - infrared excitation . we show how the excitation generates a direct optical transition between the backfolded valence and conduction bands in section [ subsec_lt_res ] . the resulting coherent oscillation of the amplitude mode is then analyzed and explained as the result of a bonding - antibonding optical transition in section [ subsec_lt_phonon ] . part [ sec_rt ] shows the results obtained for the cdw fluctuation regime at 240 k using a near - infrared excitation . we demonstrate that the excitation generates a direct transition at the center of the brillouin zone in section [ subsec_rt_res ] and subsequently induces a change in the spectral function of the conduction band ( section [ subsec_rt_sw ] ) . the transient electronic temperature is extracted in section [ subsec_rt_te ] . in section [ sec_disc ] , we discuss all these results and their interpretation within the framework of the cdw phase and fluctations in 1_t_-tise@xmath0 . we conclude our study in section [ sec_conclusion ] . fig . [ fig_1 ] ( a ) presents the schematic electronic band structure of 1_t_-tise@xmath0  near the fermi level @xmath4 in the cdw phase . it consists of a valence band ( vb ) at @xmath5 , the center of the brillouin zone , and a conduction band ( cb ) at @xmath6 , the border of the brillouin zone ( see fig . [ fig_1](e ) for the corresponding hexagonal brillouin zone ) . these two bands are the main components of the low energy band structure at room temperature . below 200 k , in the cdw phase , a replica of the valence band , the cdw band , appears at @xmath6 as a direct consequence of the new periodicity of the electronic density in the cdw phase , see fig . [ fig_1 ] ( a ) ( dashed line ) . in addition , replicas ( at symmetry - equivalent @xmath6 point ) of the cbs appear above @xmath4@xcite . our pump - probe measurements were obtained with the artemis time - resolved arpes endstation at the central laser facility of the rutherford appleton laboratory @xcite . 1_t_-tise@xmath0  samples were cleaved in a base pressure of @xmath7 mbar and were excited by 100 fs @xmath8-polarized laser pulses both in the mid - infrared ( mir ) region , at 3100 nm ( 400 mev ) , and in the near - infrared ( nir ) region , at 1400 nm ( 890 mev ) . the sample was probed by 30 fs @xmath9-polarized laser pulses with 21 ev photons . the total energy resolution was about 300 mev .    in fig . [ fig_1 ] , we show trarpes data measured in the cdw phase at 30 k when exciting the sample with mir pump pulses at 3100 nm with a fluence of 0.4 mj/@xmath10 . at this wavelength , electrons are excited resonantly at @xmath6 between the backfolded valence band and the backfolded cb ( see the orange arrows on fig . [ fig_1 ] ( a ) ) . [ fig_1 ] ( b ) and ( c ) show trarpes intensity plots taken before the arrival of the pump pulse , @xmath11 fs ( i.e. at thermal equilibrium ) and after the arrival of the pump pulse , @xmath12 fs , at which time the pump - induced effects are the strongest . at 30 k , 1_t_-tise@xmath0  is in the cdw phase , meaning that a well - developed cdw band is measured at @xmath6 below the cb . however , due to the limited energy resolution of our experiment , these two bands can not be resolved fig . [ fig_1 ] ( b ) and ( c ) and the cb can hardly be distinguished . is very intense ( see supplementary materials@xcite ) and dominates over the cb , so that it dominates also the spectral weight distribution when convolved by the experimental energy resolution . ] nevertheless , comparing the spectra measured at @xmath12 fs with those at equilibrium , one distinguishes clear differences , which are more obvious in the corresponding difference image in red / blue false colour plot . such a difference intensity plot is shown in fig . [ fig_1 ] ( d ) , for @xmath12 fs , giving a clear view of the dynamics in our trarpes data . it emphasizes the main dynamical changes , namely the disappearance of the cdw band ( in blue ) and the accumulation of excited electrons in the cb ( in red ) . the resonant mir excitation ( 400 mev ) taking place at @xmath6 also transfers population from the top of the cdw band into the unoccupied states . this can be seen directly by looking at the time - dependent photoemission intensity integrated in the different energy versus momentum regions depicted by boxes in fig . [ fig_1 ] ( d ) . the energy position of such boxes is also indicated in fig . [ fig_1 ] ( f ) , where we plot energy distribution curves ( edcs ) taken before ( black ) and 50 fs after photoexcitation ( red ) . here the photoinduced changes below the fermi level @xmath4 are mostly due to the change of the spectral function of the cdw band . however , part of it is also due to the resonant optical transition taking place at @xmath6 with the mir pump pulse , as schematically displayed in fig . this can be observed in the transient photoemission intensity curves shown in fig . [ fig_1 ] ( g ) , integrated from the corresponding boxes in graph ( d ) . from box 1 ( blue curve ) , taken near @xmath4 , an oscillation is observed and will be discussed in the next section . at time 0 , an additional sharp peak can be distinguished , which is coming from the depopulation of the cdw band due to the pump pulse . there is correspondingly a peak at about 0.4 ev higher in energy , in the unoccupied states . this can be seen in the time - resolved intensity of box 2 ( red curve ) where we observe a transient population of a ( backfolded ) conduction band at @xmath6 . this indicates that at time 0 , electrons are transferred from occupied to unoccupied states at @xmath6 , i.e. that an optical transition takes place at @xmath6 . finally , the green curve shows the intensity integrated in box 3 , which is the region where the reduction of the photoemission intensity is the strongest . however , we avoid this region in our further analysis at low temperature , as the nature of the transient signal here is not clear . indeed , due to the significant energy resolution of our experiment ( about 300 mev ) , the signal coming from the cdw band can not be well separated from the one from the conduction band at @xmath6 exactly ( see also the supplementary materials for a high - energy resolution spectrum taken at @xmath6 exactly@xcite ) .     ( a ) photoemisson intensity map in false color showing the photoemission intensity difference map taken at 30 k for a mir pump pulses ( same as in fig . [ fig_1 ] ( d ) ) . ( b ) absolute value of the normalized photoemission intensity changes integrated in the black boxes of graph ( a ) , together with the fit of the coherent phonon oscillation ( total fit , green curve ) and the exponential part of the fit only ( dashed blue curve ) . ( c ) schematic electronic structure of 1_t_-tise@xmath0  in the cdw phase at @xmath6 . ( d ) schematic of the displacements involved by the @xmath13 coherent phonon excitation by the mir pump pulse for a tise@xmath14 octahedron . , width=321 ]      we now look in more detail at the recovery dynamics of the spectral features associated with the excited electrons and with the cdw . the ( normalized ) time - dependent photoemission intensity of these features is shown in fig . [ fig_2 ] ( b ) . the melting dynamics of the cdw band has been already extensively studied and shown to be faster in time for larger pump fluences , an effect assigned to the screening by the transient generation of free charge carriers and therefore consistent with a purely electronic mechanism . @xcite . here , these transient intensities have been integrated from the regions shown by boxes in fig . [ fig_2 ] ( a ) , corresponding to the excited electrons in the cb ( red curve with square markers ) and the bottom of the backfolded vb ( blue curve with discs ) . they both show an ultrafast response which recovers mostly within 2 ps followed by a slower recovery which is not over at 9 ps . most interesting are the dynamics within 2 ps . we emphasize that for states at binding energies larger than 400 mev ( the pump photon energy ) there are no unoccupied final states available for direct photoexcitation . the dynamics observed in the cdw band is therefore due to a photoinduced change of its spectral function . the photoemission intensity integrated at the bottom of the backfolded vb displays a clear oscillation on top of an exponential decay , up to about 1 ps . the oscillation can be well described by a cosine with a period of about 290 fs , which correspond to 3.4 thz , and a phase of about @xmath15 . these numbers are obtained by fitting the transient photoemission intensity of the coherent phonon oscillation using the following function @xmath16 this represents a sum of two incoherent exponential components , caracterized by two amplitudes , @xmath17 and @xmath18 , with two relaxation times , @xmath19 and @xmath20 , together with an oscillatory component , characterized by an amplitude , @xmath21 , a frequency , @xmath22 , a phase , @xmath23 , and damped by an exponential of damping time @xmath24 . the two incoherent exponential components are necessary to achieve the best fit . this function is then convolved with a gaussian having the experimental time resolution as a full width at half maximum ( 100 fs ) . the following values give the best fit . for the relative amplitudes , @xmath25 , @xmath26 and @xmath27 . for the relaxation times , @xmath28 fs , @xmath29 fs and @xmath30 fs . the phonon period is @xmath31 fs and the phase is 0 . this mode has already been observed in time - resolved reflectivity data from 1_t_-tise@xmath0  @xcite and was attributed to the @xmath13 amplitude phonon mode of the cdw @xcite , corresponding to a breathing mode of a tise@xmath14 octahedron , as depicted in fig . [ fig_2 ] ( d ) @xcite . this oscillation is highly localized in momentum space and is absent from the dynamics of the excited electrons . such a coherent phonon oscillation is caused by the excitation of coherent ion motions by the pump laser pulse . the 0 phase of a cosine - like oscillation indicates that it is a displacive excitation , meaning that the potential energy surface of the corresponding ions , which is suddenly modified by the electronic excitation , triggers the coherent phonon oscillation . in fig . [ fig_2 ] ( b ) , the dynamics of the excited electrons and that of the exponential decay of the cdw band are very similar ( compare the exponential decay fit of the cdw band intensity ( dashed blue curve ) with the one of the hot electrons in fig . [ fig_2 ] ( b ) ) , supporting the idea that the transient change of the potential energy surface is modified by the excited electron distribution . ( a ) photoemisson intensity map in false color showing the vb along @xmath32 , near @xmath5 , measured at 30 k before time 0 with a 1300 nm pump photon wavelength . ( b ) corresponding false color plots showing the intensity difference induced by the pump pulse ( positive and negative intensity changes in red and in blue , respectively ) near @xmath5 . electrons and holes are excited resonantly at time 0 by the pump pulse , the energy of which is shown by the black arrow . ( c , d ) corresponding intensity maps along @xmath33 , near @xmath5 , where part of the conduction band and the cdw band can be seen on the right side of the graphs . ( e ) hexagonal brillouin zone of 1_t_-tise@xmath0 , with the two directions relevant to graphs ( a , b , c , d ) . ( f ) photoemission intensity traces as a function ot time delay integrated in the corresponding numbered black boxes on graphs ( b , d ) . the curves are normalized to their maximum . , width=321 ]        in the cdw phase , it has been shown that the electronic structure of 1_t_-tise@xmath0  can be well described by a mean - field hamiltonian describing one hole - like band at @xmath5 coupled to three symmetry equivalent electron - like bands near @xmath6 ( see the corresponding schematical fermi surface in fig . [ fig_1 ] ( e ) ) @xcite . at @xmath6 , the three conduction bands are degenerate ( without any coupling ) and the effective hamiltonian has the form @xmath34 where the order parameter @xmath35 describes the strength of the coupling between the single valence band @xmath36 and the three symmetry equivalent conduction bands @xmath37 ( @xmath38 ) , in the basis of the wave functions for the corresponding electronic states , namely @xmath39 . once the coupling between the valence and conduction bands takes a finite value , the valence band gets backfolded to @xmath6 and the conduction bands get backfolded to @xmath5 and other @xmath6 points . the new band dispersions along the high - symmetry direction @xmath33 are shown in fig . [ fig_2](c ) ( for realistic dispersions fitted from the experiment@xcite ) . for the extrema of the valence and conduction bands , we use the values @xmath40 mev , @xmath41 mev at @xmath5 and @xmath6 , respectively . for the order parameter , we use @xmath42 mev , which fits well with the experiment at low temperature . diagonalizing the hamiltonian @xmath43 leads to the following eigenvalues at @xmath6 @xmath44 the values @xmath45 and @xmath46 correspond to the backfolded valence and backfolded conduction bands ( at @xmath6 ) . the mid - infrared pump photon energy of 400 mev matches approximatively the resonance between these two bands , which can be defined as the cdw gap in 1_t_-tise@xmath0 . the eigenvectors corresponding to these eigenvalues are @xmath47 as a consequence , @xmath45 corresponds to the antibonding state and @xmath46 to the bonding state .    in 1_t_-tise@xmath0 , the vb at @xmath5 is mostly of se @xmath48 character and the cb at @xmath6 of ti @xmath49 character . as shown above , in the cdw phase , these bands will effectively hybridize due to their coupling and have a mixed character . we have just demonstrated that the states at the top of backfolded vb band have a bonding symmetry , while those at the bottom of the backfolded cb have an antibonding symmetry . this suggests how the resonant mir excitation triggers a displacive @xmath13 cdw amplitude mode by exciting electrons from bonding to antibonding states in 1_t_-tise@xmath0  ( see fig . [ fig_2](d ) ) . summarizing , in the low temperature phase of 1_t_-tise@xmath0 , the resonant mir excitation triggers a coherent lattice response of the cdw amplitude mode , observable in the recovery of the backfolded valence band of the material . this differs from the case of higher pump photon energies @xcite for which no coherent oscillations are observed . we explain it here by the fact that this resonant optical transition occurs between bonding and antibonding states ( at @xmath6 ) of the tise@xmath14 octahedron acting efficiently on the ionic position relevant to the cdw distortion and stimulating this way a displacive excitation . we now turn to the high temperature trarpes data . the phase transition in 1_t_-tise@xmath0  is continuous ( second - order ) @xcite and photoemission data@xcite have shown that short - range cdw fluctuations persist well above @xmath1 k. such fluctuations are known to lead to spectral features anticipating those of the low temperature cdw phase , e.g. with a cdw pseudo - gap@xcite . in the case of 1_t_-tise@xmath0 , no cdw band is observed , but a broad cb at @xmath6 ( see supplementary materials for a high - energy resolution arpes spectrum@xcite ) , the width of which results from the short - range cdw fluctuations . we have therefore also measured 1_t_-tise@xmath0  at 240 k , above @xmath2 , to investigate the dynamics of these spectral features . at 240 k , due to the disappearance of the sharp cdw bands at high binding energies , a resonant mir excitation can not be used ( accordingly , the cdw peak in the optical conductivity vanishes @xcite ) . we therefore used a shorter wavelength , 1300 nm , in the nir regime . the trarpes data of fig . [ fig_3 ] measured near @xmath5 , along @xmath32 ( see graph ( a ) ) , demonstrate that the nir pulses excite electrons by an optical transition taking place at around @xmath5 , as illustrated in fig . [ fig_4 ] ( a ) . this direction in the brillouin zone is depicted in fig . [ fig_3 ] ( e ) . shortly after time 0 ( at @xmath50 fs ) , one sees a clear depletion of intensity at the top of the valence band ( vb ) at @xmath5 and accordingly strong intensity above the vb , in a conduction band ( cb ) , which is slightly less than 1 ev above ( see graph ( b ) ) the top of the vb . the black arrow has a length of 890 mev , demonstrating this electronic resonance with the pump pulse . at later times , the excited hot electrons are scattered away from @xmath5 ( see graphs ( c ) and ( d ) ) and accumulate into the cb at @xmath6 . the time delay difference between these different events is shown in fig . [ fig_3 ] ( f ) . this behavior of the momentum path of the hot electrons in the unoccupied states is similar to what has been already observed by rohde _ @xcite for a much larger pump photon energy . photoemission intensity maps taken at 240 k with nir excitation are shown in fig . [ fig_4 ] ( b ) at negative ( @xmath51 fs ) and positive ( at @xmath52 fs ) time delays . at 240 k , i.e. above the critical temperature of the cdw phase , there is not clear cdw band at high binding energies anymore . our trarpes data have been acquired with a pump photon wavelength of 1300 nm resonant with an optical transition at @xmath5 ( see previous section ) . the difference image in red / blue false colour plot , fig . [ fig_4 ] ( c ) , shows the accumulation of excited electrons in the cb ( in red ) and a decrease of photoemission intensity at the bottom of the cb ( in blue ) the transient photoemission intensities of the excited electrons and of the bottom of the cb are displayed in fig . [ fig_4 ] ( d ) . the corresponding time - dependent photoemission intensities integrated in the corresponding boxes in this graph are shown in fig . [ fig_4 ] ( e ) . interestingly the decrease of intensity at the bottom of the cb at @xmath6 ( in blue ) displays a dynamics with 2 timescales . its recovery dynamics are therefore fitted with a bi - exponential function ( convoluted with a gaussian for experimental time resolution ) , involving a fast and a slow component , with decay constants @xmath53 fs and @xmath54 fs , respectively . we argue below that this decrease of intensity is due to a photoinduced change of the spectral function .    in fig . [ fig_6 ] , we demonstrate that there is no resonant excitation taking place at @xmath6 at 240 k with a pump photon wavelength of 1300 nm ( 0.95 ev ) , in the nir regime . graph ( b ) in fig . [ fig_6 ] displays edcs taken at @xmath6 , before time 0 ( black ) and at 130 fs ( red ) . graph ( c ) shows the transient photoemission intensity integrated in the boxes 1 and 2 in graph ( a ) . box 1 is situated in a region where the transient suppression of the photoemission signal is the largest and box 2 is situated 0.95 ev higher in energy , in the unoccupied states . the red curve from box 2 shows clearly that there is hardly any transient signal in this region , proving that there is no resonant excitation taking place at @xmath6 at 240 k for 1300 nm pump photon wavelength ( at variance with the case of mir pump photons at 30 k , see fig . [ fig_1 ] ) . we can therefore attribute the photemission intensity changes observed below @xmath4 to a photoinduced change of the spectral function of the conduction band , which we will explain further below in our discussion of the results . ( a ) photoemisson intensity map in false color showing the photoemission intensity difference map taken at 240 k ( same as in fig . [ fig_4](c ) ) . the blue arrows indicate the size of the pump photon energy ( 0.95 ev , 1300 nm ) in comparison to the energy axis . ( b ) edcs taken at @xmath6 before time 0 ( black curve ) and at 130 fs ( red curve ) . ( c ) absolute value of the normalized photoemission intensity changes integrated in the black boxes of graph ( a ) . , width=321 ]      we now focus on the dynamics measured in the cb at @xmath6 near the fermi edge . we exploit here the fact that at this temperature , the cb in our sample is partially filled @xcite and analyse the slope of the corresponding fermi - dirac cut - off to monitor the transient electronic temperature , in a similar way to perfetti _ et al . indeed , at @xmath6 , at 240 k , there is a flat blue - red region in the difference plot of fig . [ fig_5 ] ( c ) ( see dashed box ) , typical from a heated metallic fermi - dirac edge . the ( normalized ) edcs in this box are averaged over momentum and plotted in fig . [ fig_5 ] ( a ) at different time delays , showing a clear increase of temperature . they are fitted by an exponential function@xcite above @xmath4 , in order to avoid the changes of the spectral lineshape at lower binding energies ( coming from the photoinduced changes in the spectral function ) . this is shown in fig . [ fig_5 ] ( b ) . the extracted electronic temperature ( black curve with discs ) is displayed in fig . [ fig_5 ] ( d ) ( after removing the contribution of the energy resolution@xcite about 300 mev ) . in the spirit of a two - temperature model , the electronic temperature decreases here in about 1 ps as the result of coupling to the lattice , the temperature of which should increase by about 40 k ( if heat diffusion is neglected ) for the pump fluence used here and the specific heat @xcite of 1_t_-tise@xmath0 . ( a ) edcs taken at @xmath6 at 240 k ( integrated inside the dashed box of graph ( c ) ) for different time delays . the dashed grey line is the reference edc at -600 fs . ( b ) the same edcs fitted by an exponential . ( c ) false colour plot of the transient photoemission intensity at @xmath6 ( same as in fig . [ fig_4](c ) ) . ( d ) comparison of the electronic temperature with the fitted transient photoemisson intensity at the bottom of the cb at @xmath6 ( black box in the blue region of fig . [ fig_4 ] ( c ) ) and the corresponding slow component fitted from the spectral feature ( see main text ) . , width=321 ] the reduction of photoemission intensity observed at the bottom of the cb in fig . [ fig_4 ] ( c ) comes from a photoinduced change of its spectral function . we interpret this effect as the suppression of the high binding energy tail of the cb , which is the spectral signature of the cdw fluctuations@xcite . this means that the intensity of the cdw fluctuations are reduced by the pump pulse . in fig . [ fig_4 ] ( e ) , we focus on the dynamics of these cdw fluctuations . the recovery dynamics are fitted with a bi - exponential function ( convoluted with a gaussian for experimental time resolution ) , involving a fast ( @xmath53 fs ) and a slow ( @xmath54 fs ) component .    to understand these two timescales , we plot in fig . [ fig_5 ] ( d ) the fitted time - dependent photoemission intensity at the bottom of the cb ( grey line ) together with the fitted slow component ( green ) and the extracted electronic temperature ( black ) . this comparison suggests that the two timescales of the dynamics of the cdw fluctuations do not match the decay behavior of the electronic temperature ( especially the sharp peak near time 0 ) . however , the slow component ( green ) timescale matches rather well the long timescale of the electronic temperature , indicating that part of the quenching of the cdw fluctuations is due to the transiently increased electronic temperature . we interpret the fast component of the suppression of the cdw fluctuations as a consequence of the ultrafast screening of the coulomb interaction , which is enhanced by the new charge carriers created by the optical excitation@xcite . the intensity of these fluctuations depends strongly on the strength of the interband coulomb interaction ( see supplementary materials for an analytical formula of the self - energy of the cb@xcite ) . in the framework of the excitonic insulator phase , this is the attractive interaction binding an electron and a hole in an exciton . in a previous study@xcite , the resulting self - energy leading to the broadened cb was derived and shown to have a particularly high value at the bottom of the cb . this reveals then that electrons at the bottom of the cb experience a high scattering rate@xcite , naturally explaining the ultrafast recovery of the cdw fluctuations .    in fig . [ fig_4 ] ( f ) , we depict how this electron - hole interaction between the valence and conduction bands occurs . here an electron jumps from the cb to the vb , scattering another electron from the vb to the cb . this can effectively been seen as the scattering of an electron in the cb with a hole in the vb . the strength of the cdw fluctuations depends strongly on the coulomb interaction and also on the electronic and lattice temperature@xcite . therefore they tend to be suppressed not only by enhanced screening , but also by the increased transient electronic temperature . the combination of these two observations explains the bi - exponential decay of the photoemission intensity suppression at the bottom of the cb ( see fig . [ fig_4 ] ( e ) ) , namely the recovery of the cdw fluctuations . we ascribe the fast component ( @xmath53 fs ) to the relaxation of the enhanced screening by the efficient electron - hole scattering and the slow component to a quasi - thermal recovery following the relaxation of the electronic temperature . our interpretation that the fast component is due to the electron - hole correlations as a strong scattering channel supports their occurence at temperature above but near @xmath2 . these correlations give rise to the cdw fluctuations in this temperature range@xcite and play an important role in stabilizing the low - temperature cdw phase .    at 30 k , the situation is different as a well - formed pld is present . the longer timescale dynamics of the cdw band display a clear response of the lattice degrees of freedom . the @xmath13 cdw amplitude mode is observed as a coherent phonon oscillation , provided that the cdw band is selectively excited by a resonant mir photon pulse generating excited electrons with a minimal excess kinetic energy with respect to the cdw gap . the fact that the intensity of the cdw band oscillates with the frequency of the @xmath13 cdw - related phonon mode implies that the order parameter of the cdw phase is strongly coupled to the corresponding atomic motion . furthermore , this coupling is very localized in momentum space , namely only for states in the cdw band . this means that both the excitonic interband coulomb interaction and the electron - phonon coupling contribute to the order parameter of the cdw phase @xcite . in the first 100 fs , the melting of the cdw order is dominated by the suppression of the excitonic part , as observed by rohwer _ @xcite with rise times as fast as 20 fs for large fluences ( up to 5 mj/@xmath10 ) .    in that sense , our experimental results support a model according to which strong electron - hole correlations , giving rise to short - range cdw fluctuations above @xmath2 , contribute to the stabilization of the cdw phase together with the electron - phonon coupling below @xmath2@xcite . in conclusion , we have probed both the femtosecond dynamics of the cdw phase in 1_t_-tise@xmath0  at 30 k , as well as the dynamics of the high - temperature fluctuation regime at 240 k using trarpes . below @xmath2 , in the cdw phase , we observe the coherent oscillation of the cdw amplitude mode for the first time with trarpes using a mid - infrared excitation resonant with the cdw gap . above @xmath2 , we single out a fast component with a relaxation time @xmath3 100 fs . we interpret it as the consequence of the ultrafast screening of the interband coulomb interaction responsible for the electron - hole correlations . we propose that these electron - hole correlations act as an efficient scattering channel in the relaxation of excited electrons . in this way , we have unveiled the dynamics of both the electron and lattice degrees of freedom in 1_t_-tise@xmath0 . the authors thank h. beck , p. aebi , k. rossnagel for useful discussions . c.m . acknowledges support by the swiss national science foundation under grant number @xmath55 , as well as by the alexander von humboldt foundation . access to artemis was provided by stfc .
using time- and angle - resolved photoemission spectroscopy with selective near- and mid - infrared photon excitations , we investigate the femtosecond dynamics of the charge density wave ( cdw ) phase in 1_t_-tise@xmath0 , as well as the dynamics of cdw fluctuations at 240 k. in the cdw phase , we observe the coherent oscillation of the cdw amplitude mode . at 240 k 
 , we single out an ultrafast component in the recovery of the cdw correlations , which we explain as the manifestation of electron - hole correlations . 
 our momentum - resolved study of femtosecond electron dynamics supports a mechanism for the cdw phase resulting from the cooperation between the interband coulomb interaction , the mechanism of excitonic insulator phase formation , and electron - phonon coupling .