Femtosecond Time-resolved Photoelectron Spectroscopy Research Articles

Pseudo-Bimolecular [2+2] Cycloaddition Studied by Time-Resolved Photoelectron Spectroscopy

The first study of pseudo-bimolecular cycloaddition reaction dynamics in the gas phase is presented. We used femtosecond time-resolved photoelectron spectroscopy (TRPES) to study the [2+2] photocycloaddition in the model system pseudo-gem-divinyl[2.2]paracyclophane. From X-ray crystal diffraction measurements we found that the ground-state molecule can exist in two conformers; a reactive one in which the vinyl groups are immediately situated for [2+2] cycloaddition and a nonreactive conformer in which they point in opposite directions. From the measured S(1) lifetimes we assigned a clear relation between the conformation and the excited-state reactivity; the reactive conformer has a lifetime of 13 ps, populating the ground state through a conical intersection leading to [2+2] cycloaddition, whereas the nonreactive conformer has a lifetime of 400 ps. Ab initio calculations were performed to locate the relevant conical intersection (CI) and calculate an excited-state [2+2] cycloaddition reaction path. The interpretation of the results is supported by experimental results on the similar but nonreactive pseudo-para-divinyl[2.2]paracyclophane, which has a lifetime of more than 500 ps in the S(1) state.

Ultrafast Dynamics at the Na/D2O/Cu(111) Interface: Electron Solvation in Ice Layers and Na+-Mediated Surface Solvation

We have studied the influence of sodium ions bound near the ice/vacuum interface on the electron solvation dynamics in amorphous D2O ice layers by means of femtosecond time-resolved two-photon photoelectron spectroscopy. Adsorption of submonolayer coverages of sodium on top of multilayers of amorphous ice leads to the formation of Na+ ions and to pronounced changes in the observed ultrafast dynamics compared to pure amorphous ice. We identify a Na+-induced species of excess electrons which exhibits a much longer lifetime compared to excess electrons in pure D2O ice and approximate the decay of the Na-induced contribution by two decay times τ2 = 880 fs and τ3 = 9.6 ps. In addition, a faster energetic stabilization of the excited electrons with a rate of Σ = 0.73 eV/ps is observed. The population of these electrons depends nonlinearly on the sodium coverage. We attribute the Na-induced contribution to a transient electron/ion/water complex which is located at the ice/vacuum interface. This interpretation is...

Wavelength Dependence of Electronic Relaxation in Isolated Adenine Using UV Femtosecond Time-Resolved Photoelectron Spectroscopy

Electronic relaxation pathways in photoexcited nucleobases have received much theoretical and experimental attention due to their underlying importance to the UV photostability of these biomolecules. Multiple mechanisms with different energetic onsets have been proposed by ab initio calculations yet the majority of experiments to date have only probed the photophysics at a few selected excitation energies. We present femtosecond time-resolved photoelectron spectra (TRPES) of the DNA base adenine in a molecular beam at multiple excitation energies between 4.7-6.2 eV. The two-dimensional TRPES data is fit globally to extract lifetimes and decay associated spectra for unambiguous identification of states participating in the relaxation. Furthermore, the corresponding amplitude ratios are indicative of the relative importance of competing pathways. We adopt the following mechanism for the electronic relaxation of isolated adenine; initially the S(2)(ππ*) state is populated by all excitation wavelengths and decays quickly within 100 fs. For excitation energies below ∼5.2 eV, the S(2)(ππ*)→S(1)(nπ*)→S(0) pathway dominates the deactivation process. The S(1)(nπ*)→S(0) lifetime (1032-700 fs) displays a trend toward shorter time constants with increasing excitation energy. On the basis of relative amplitude ratios, an additional relaxation channel is identified at excitation energies above 5.2 eV.

Ultrafast dynamics of o-fluorophenol studied with femtosecond time-resolved photoelectron and photoion spectroscopy

The ultrafast dynamics of o-fluorophenol via the excited states has been studied by femtosecond time-resolved photoelectron imaging. The photoion and photoelectron spectra taken with a time delay between 267 nm pump laser and 800 nm probe laser provide a longer-lived S1 electronic state of about ns timescale. In comparison, the spectra obtained by exciting the S2 state with femtosecond laser pulses at 400 nm and ionizing with pulses at 800 nm suggest that the S2 state has an ultrashort lifetime about 102 fs and reflects the internal conversion dynamics of the S2 state to the S1 state.

Substituent Effects on Dynamics at Conical Intersections: Cyclopentadienes

Substituent effects on dynamics at conical intersections are investigated by means of femtosecond time-resolved photoelectron spectroscopy for cyclopentadiene and its substituted analogues 1,2,3,4-tetramethylcyclopentadiene, 1,2,3,4,5-pentamethylcyclopentadiene, and 1,2,3,4-tetramethyl-5-propylcyclopentadiene. By UV excitation to the S(2) (1(1)B(2)) state, the influence of these substitutions on dynamics for the initially excited S(2) (1(1)B(2)) surface and the spectroscopically dark S(1) (2(1)A(1)) surface were investigated. We observed that the dynamics depend only on a small number of specific vibrations. Whereas dynamics at the S(2)/S(1)-conical intersection are independent of substitution at the 5-position, internal conversion dynamics on the S(1) (2(1)A(1)) surface slow down as the inertia of the 5-substituent increases. Contrary to the expectations of simple models of radiationless transitions, an increasing density of states does not lead to faster processes, suggesting that a true dynamical picture of vibrational motions at conical intersections will be required.

Photoemission with high-order harmonics: A tool for time-resolved core-level spectroscopy

A setup for femtosecond time-resolved photoelectron spectroscopy of solid surfaces is presented. The photon energies for core-level spectroscopy experiments are created by high-order harmonic generation from infrared 120-femtosecond laser pulses focused in a Ne gas jet. The present experimental realization allows the sample, located in an ultrahigh-vacuum chamber, to be illuminated by ∼ 10 6 65-eV photons per laser pulse at a 10 Hz repetition rate. The spectral width of a single harmonic is 0.77 eV (FWHM), and a few harmonics are selected by specially designed Mo/Si multi-layer mirrors. Photoelectrons from the sample are collected by a large-solid-angle time-of-flight electron spectrometer based on a parabolic-grid reflector. Results from experiments probing the Bi 5 d core-levels are presented, and the results of preliminary pump-probe experiments are described.

Ultrafast dynamics through conical intersections and intramolecular vibrational energy redistribution in styrene

We report a femtosecond time-resolved photoelectron spectroscopy (TRPES) investigation of internal conversion in the first two excited singlet electronic states of styrene. We find that radiationless decay through an S(1)/S(0) conical intersection occurs on a timescale of ∼4 ps following direct excitation to S(1) with 0.6 eV excess energy, but that the same process is significantly slower (∼20 ps) if it follows internal conversion from S(2) to S(1) after excitation to S(2) with 0.3 eV excess energy (0.9 eV excess energy in S(1)).

Competing ultrafast intersystem crossing and internal conversion in the “channel 3” region of benzene

We report new, detailed, femtosecond time-resolved photoelectron spectroscopy experiments and calculations investigating the competition between ultrafast internal conversion and ultrafast intersystem crossing in electronically and vibrationally excited benzene at the onset of "channel 3". Using different probe energies to record the total photoelectron yield as a function of pump-probe delay we are able to confirm that S(1), T(1) and T(2) electronic states are involved in the excited state dynamics. Time-resolved photoelectron spectroscopy measurements then allow us to unravel the evolution of the S(1), T(1) and T(2) components of the excited state population and, together with complementary quantum chemistry and quantum dynamics calculations, support our earlier proposal that ultrafast intersystem crossing competes with internal conversion (Chem. Phys. Lett., 2009, 469, 43).

Photodissociation dynamics of IrBr62− dianions by time-resolved photoelectron spectroscopy

We have used femtosecond time-resolved photoelectron spectroscopy to examine the photodissociation dynamics of doubly charged anions IrBr(6)(2-) after excitation at h nu(pump) = 1.6 eV and with a detachment photon energy of h nu(probe) = 4.8 eV. Excited state dynamics proceed by successive decay of the initially excited state, by way of an intermediate and back to the electronic ground state. This is associated with lifetimes of tau(1) = 2.1+/-0.3 ps and tau(2) = 21+/-2 ps, respectively. After nonadiabatic relaxation, the internal energy of the dianion is sufficiently large to induce fragmentation and delayed emission of Br(-) over the repulsive Coulomb barrier with a 79+/-21 ps time constant. As both fragments are negatively charged, Coulomb repulsion at early times (and correspondingly small separations) is reflected in the transient photoelectron spectra. Analysis of both shifts and intensities of the time-dependent bromide detachment features allows determination of the shape of the dissociation barrier. A lower limit of the outer height was retrieved from the kinetic energy release of KER > or = 1.6 eV. Modeling of the dissociation rate with statistical rate theory results in an inner barrier height of E(RCB) = 0.95 eV.

Excited‐State Dynamics of Isolated DNA Bases: A Case Study of Adenine

We present a summary of recent advances in the understanding of the UV photophysics of the isolated DNA base adenine, emphasizing a discussion of the mechanisms behind the ultrafast relaxation following excitation to the pipi* band. Drawing on our femtosecond time-resolved photoelectron spectroscopy experiments, we discuss differences in the ultrafast relaxation of adenine and 9-methyladenine and consider the relative merits of the various proposed mechanisms.

A Dynamic Landscape from Femtoseconds to Minutes for Excess Electrons at Ice−Metal Interfaces

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.

Excited‐State Dynamics of Cytosine Reveal Multiple Intrinsic Subpicosecond Pathways

Excited‐State Dynamics of Cytosine Reveal Multiple Intrinsic Subpicosecond Pathways

Towards time resolved core level photoelectron spectroscopy with femtosecond x-ray free-electron lasers

We have performed core level photoelectron spectroscopy on a W(110) single crystal with femtosecond XUV pulses from the free-electron laser at Hamburg (FLASH). We demonstrate experimentally and through theoretical modelling that for a suitable range of photon fluences per pulse, time-resolved photoemission experiments on solid surfaces are possible. Using FLASH pulses in combination with a synchronized optical laser, we have performed femtosecond time-resolved core-level photoelectron spectroscopy and observed sideband formation on the W 4f lines indicating a cross correlation between femtosecond optical and XUV pulses.

Excited state dynamics of metastable phthalocyanine-tetrasulfonate tetra-anions probed by pump/probe photoelectron spectroscopy

Femtosecond time-resolved pump-probe photoelectron spectroscopy was used to study elementary relaxation processes occurring in isolated phthalocyanine-tetrasulfonate tetra-anions ([MPc(SO3)4]4-, M=Cu,Ni, and "free-base" [H2Pc(SO3)4]4-) following Q band excitation by one-photon absorption at 775 nm. Whereas the Cu and Ni systems decay rapidly by means of internal conversion without electron loss, the free-base phthalocyanine primarily undergoes excited state tunneling electron emission. This reflects less efficient coupling to lower lying states within the corresponding spin manifold. Results are interpreted in terms of (time-dependent) density functional theory calculations of ground and electronically excited states and kinetically modeled to yield the associated rates.

Substituent Effects on Dynamics at Conical Intersections: α,β-Enones

Femtosecond time-resolved photoelectron spectroscopy and high-level theoretical calculations were used to study the effects of methyl substitution on the electronic dynamics of the alpha,beta-enones acrolein (2-propenal), crotonaldehyde (2-butenal), methylvinylketone (3-buten-2-one), and methacrolein (2-methyl-2-propenal) following excitation to the S2(pipi*) state at 209 and 200 nm. We determine that following excitation the molecules move rapidly away from the Franck-Condon region, reaching a conical intersection promoting relaxation to the S1(npi*) state. Once on the S1 surface, the trajectories access another conical intersection, leading them to the ground state. Only small variations between molecules are seen in their S2 decay times. However, the position of methyl group substitution greatly affects the relaxation rate from the S1 surface and the branching ratios to the products. Ab initio calculations used to compare the geometries, energies, and topographies of the S1/S0 conical intersections of the molecules are not able to satisfactorily explain the variations in relaxation behavior. We propose that the S1 lifetime differences are caused by specific dynamical factors that affect the efficiency of passage through the S1/S0 conical intersection.

Size-dependent dynamics in excited states of gold clusters: From oscillatory motion to photoinduced melting

Femtosecond time resolved photoelectron spectroscopy in combination with direct ab initio molecular dynamics "on the fly" based on density functional theory has been used to study the relaxation dynamics of optically excited states in small mass selected anionic gold clusters (Au(n) (-); n = 5-8). The nature of the dynamics strongly depends on the cluster size and structure. Oscillatory wavepacket motion (Au(5)(-)), a long lived excited state (Au(6)(-)), as well as photoinduced melting (Au(7)(-),Au(8)(-)) is observed in real time. This illustrates nonscalable properties of excited states in clusters in the size regime, in which each atom counts.

Stark-assisted population control of coherent CS2 4f and 5p Rydberg wave packets studied by femtosecond time-resolved photoelectron spectroscopy

A two-color (3+1(')) pump-probe scheme is employed to investigate Rydberg wave packet dynamics in carbon disulfide (CS(2) (*)). The state superpositions are created within the 4f and 5p Rydberg manifolds by three photons of the 400 nm pump pulse, and their temporal evolution is monitored with femtosecond time-resolved photoelectron spectroscopy using an 800 nm ionizing probe pulse. The coherent behavior of the non-stationary superpositions are observed through wavepacket revivals upon ionization to either the upper (12) or lower (32) spin-orbit components of CS(2) (+). The results show clearly that the composition of the wavepacket can be efficiently controlled by the power density of the excitation pulse over a range from 500 GWcm(2) to 10 TWcm(2). The results are consistent with the anticipated ac-Stark shift for 400 nm light and demonstrate an effective method for population control in molecular systems. Moreover, it is shown that Rydberg wavepackets can be formed in CS(2) with excitation power densities up to 10 TWcm(2) without significant fragmentation. The exponential 1e population decay (T(1)) of specific excited Rydberg states are recovered by analysis of the coherent part of the signal. The dissociation lifetimes of these states are typically 1.5 ps. However, a region exhibiting a more rapid decay ( approximately 800 fs) is observed for states residing in the energy range of 74 450-74 550 cm(-1), suggestive of an enhanced surface crossing in this region.

Femtosecond Dynamics of the tert-Butyl Radical, t-C4H9

The excited-state dynamics of the tert-butyl radical, t-C4H9, was investigated by femtosecond time-resolved photoionization and photoelectron spectroscopy. The experiments were supported by ab initio calculations. tert-Butyl radicals, generated by flash pyrolysis of azo-tert-butane, were excited into the A 2A1 (3s) state between 347 and 307 nm and the 3p band at 274 and 268 nm and ionized by 810-nm radiation, in a [1 + 2'] or [1 + 3'] process. Electronic structure calculations confirm that the two states are of s and p Rydberg characters, respectively. The carbon framework becomes planar and thus ion-like in both states. The photoelectron spectra are broad and seem to be mediated by accidental intermediate resonances in the probe step. All time-resolved photoelectron spectra can be described by a single decay time. For the A 2A1 state, lifetimes between 180 and 69 fs were measured. Surprisingly, a much longer lifetime of around 2 ps was found for the 3p state. To understand the decay dynamics, the potential energy was computed as a function of several important nuclear coordinates. A [1,2] H-atom shift to the isobutyl radical seems not to be important for the excited-state dynamics. Qualitative considerations indicate curve crossings between the ground state, the 3s state, and a valence state along the asymmetric C-C stretch coordinate that correlates to the dimethylcarbene + methyl product channel. The implications of the present study for earlier work on the nanosecond time scale are discussed.

Energetics and dynamics of unoccupied electronic states at theh−BN∕Ni(111)interface

Adsorbed on the nickel 111 surface, hexagonal boron nitride h-BN forms a commensurate monolayer, which retains much of the band structure of layered bulk h-BN, specifically the wide band gap. This article explores its unoccupied band structure with femtosecond time-resolved two-photon photoelectron spectroscopy in a two-color setup, and presents spectra, dispersion, and cross-correlation measurements. Two distinct, dispersive intermediate states are observed at the origin of the surface Brillouin zone : a -related interface state at 1.51 eV above the Fermi level, which forms the conduction band, and an image-potential state at 0.65 eV below the vacuum level. With 260 fs for the image state and 110 fs for the interface state the lifetimes of the two unoccupied states are remarkably high due to the very small overlap of the corresponding wave functions with the nickel bulk states.

Coherent polyatomic dynamics studied by femtosecond time-resolved photoelectron spectroscopy: Dissociation of vibrationally excited CS2 in the 6s and 4d Rydberg states

The dissociation dynamics of the 6s and 4d Rydberg states of carbon disulfide (CS(2)*) are studied by time-resolved photoelectron spectroscopy. The CS(2) is excited by two photons of 267 nm (pump) to the 6s and 4d Rydberg states and probed by ionization with either 800 or 400 nm. The experiments can distinguish and successfully track the time dynamics of both spin [1/2] (upper) and [3/2] (lower) cores of the excited Rydberg states, which are split by 60 meV, by measuring the outgoing electron kinetic energies. Multiple mode vibrational wave packets are created within the Rydberg states and observed through recurrence interferences in the final ion state. Fourier transformation of the temporal response directly reveals the coherent population of several electronic states and vibrational modes. The composition of the wave packet is varied experimentally by tuning the excitation frequency to particular resonances between 264 and 270 nm. The work presented here shows that the decay time of the spin components exhibits sensitivity to the electronic and vibrational states accessed in the pump step. Population of the bending mode results in an excited state lifetime of as little as 530 fs, as compared to a several picosecond lifetime observed for the electronic origin bands. Experiments that probe the neutral state dynamics with 400 nm reveal a possible vibrationally mediated evolution of the wave packet to a different Franck-Condon window as a consequence of Renner-Teller splitting. Upon bending, symmetry lowering from D(infinityh) to C(2v) enables ionization to the CS(2) (+) (B (2)Pi(u)) final state. The dissociation dynamics observed are highly mode specific, as revealed by the frequency and temporal domain analysis of the photoelectron spectra.