Femtosecond Time-resolved Photoelectron Spectroscopy Research Articles

Ultrafast vectorial and scalar dynamics of ionic clusters: Azobenzene solvated by oxygen

The ultrafast dynamics of clusters of trans-azobenzene anion (A-) solvated by oxygen molecules was investigated using femtosecond time-resolved photoelectron spectroscopy. The time scale for stripping off all oxygen molecules from A- was determined by monitoring in real time the transient of the A- rise, following an 800 nm excitation of A- (O2)n, where n = 1-4. A careful analysis of the time-dependent photoelectron spectra strongly suggests that for n > 1 a quasi-O4 core is formed and that the dissociation occurs by a bond cleavage between A- and conglomerated (O2)n rather than a stepwise evaporation of O2. With time and energy resolutions, we were able to capture the photoelectron signatures of transient species which instantaneously rise (<100 fs) then decay. The transient species are assigned as charge-transfer complexes: A.O2- for A- O2 and A.O4-(O2)n-2 for A-(O2)n, where n = 2-4. Subsequent to an ultrafast electron recombination, A- rises with two distinct time scales: a subpicosecond component reflecting a direct bond rupture of the A- -(O2)n nuclear coordinate and a slower component (1.6-36 ps, increasing with n) attributed to an indirect channel exhibiting a quasistatistical behavior. The photodetachment transients exhibit a change in the transition dipole direction as a function of time delay. Rotational dephasing occurs on a time scale of 2-3 ps, with a change in the sign of the transient anisotropy between A- O2 and the larger clusters. This behavior is a key indicator of an evolving cluster structure and is successfully modeled by calculations based on the structures and inertial motion of the parent clusters.

Primary processes underlying the photostability of isolated DNA bases: adenine.

The UV chromophores in DNA are the nucleic bases themselves, and it is their photophysics and photochemistry that govern the intrinsic photostability of DNA. Because stability is related to the conversion of dangerous electronic to less-dangerous vibrational energy, we study ultrafast electronic relaxation processes in the DNA base adenine. We excite adenine, isolated in a molecular beam, to its pipi* state and follow its relaxation dynamics using femtosecond time-resolved photoelectron spectroscopy. To discern which processes are important on which timescales, we compare adenine with 9-methyl adenine. Methylation blocks the site of the much-discussed pisigma* state that had been thought, until now, minor. Time-resolved photoelectron spectroscopy reveals that, although adenine and 9-methyl adenine show almost identical timescales for the processes involved, the decay pathways are quite different. Importantly, we confirm that in adenine at 267-nm excitation, the pisigma* state plays a major role. We discuss these results in the context of recent experimental and theoretical studies on adenine, proposing a model that accounts for all known results, and consider the relationship between these studies and electron-induced damage in DNA.

Ultrafast Electron Dynamics at Ice−Metal Interfaces: Competition between Heterogeneous Electron Transfer and Solvation

Microscopic insight into heterogeneous electron transfer requires an understanding of the participating donor and acceptor states and of their respective interaction. In the regime of strong electronic coupling, two limits have been discussed where either the states overlap directly or the states are separated by a potential barrier. In both situations, the transfer probability is determined by the magnitude of the wave function overlap, whereby in the case of the potential barrier, its width and height are rate limiting. In our study, we observe a dynamical crossover between these two regimes by investigating the electron-transfer dynamics of localized, solvated electrons at ice-metal interfaces. Employing femtosecond time-resolved two-photon photoelectron spectroscopy, we analyze the population dynamics of excess electrons in the ice layer, which experience the competing processes of transfer to the metal electrode and energetic stabilization in the ice by molecular reorientation. Comparing the dynamics of D(2)O on Cu(111) and Ru(001), we observe an early regime at t < 300 fs, where the transfer time is determined by wave-function overlap with the metal and a second regime (t > 300 fs), where the transfer proceeds nearly independent of the substrate. The assignment of these two regimes to the established mechanisms of electron transfer is backed by an empirical model calculation that reproduces the experimental data in an excellent manner.

Non-adiabatic intramolecular and photodissociation dynamics studied by femtosecond time-resolved photoelectron and coincidence imaging spectroscopy

Time-resolved photoelectron spectroscopy (TRPES) is emerging as a useful tool for the study of non-adiabatic dynamics in isolated polyatomic molecules and clusters due to its sensitivity to both electronic and vibrational dynamics. A powerful extension of TRPES, coincidence imaging spectroscopy (CIS), based upon femtosecond time-resolved 3D momentum vector imaging of both photoions and photoelectrons in coincidence, is a new technique for the study of complex dissociative processes. Here we show how these spectroscopies can be used to study both non-adiabatic intramolecular and photodissociation dynamics in polyatomic molecules. Intramolecular dynamics in the alpha, beta-enones acrolein, crotonaldehyde and methyl vinyl ketone are studied using both TRPES and laser-induced fluorescence of HCO(X) product yields. The location of the methyl group is seen to have very dramatic effects on the relative electronic relaxation rates and the HCO(X) yield. Applying both TRPES and CIS to the 200 nm and 209 nm photodissociation of the nitric oxide dimer, (NO)2, we observe the fs time-scale evolution of the excited parent neutral via its photoelectron spectrum and the emergence of the NO(A) photofragment including its energy and angular distributions.

Mechanism and dynamics of azobenzene photoisomerization.

The excited-state dynamics of trans-azobenzene were investigated by femtosecond time-resolved photoelectron spectroscopy and ab initio molecular dynamics. Two near-degenerate pipi* excited states, S2 and S3,4, were identified in a region hitherto associated with only one excited state. These results help to explain contradictory reports about the photoisomerization mechanism and the wavelength dependence of the quantum yield. A new model for the isomerization mechanism is proposed.

Time-resolved photoelectron spectroscopy: Non-adiabatic dynamics in polyatomic molecules

Femtosecond time-resolved photoelectron spectroscopy is emerging as an important technique for investigating photodynamics in polyatomic molecules. Photoelectron spectroscopy is sensitive to both electronic configurations and vibrational dynamics and is therefore well suited to the study of ultrafast non-adiabatic processes. We emphasize the conceptual interpretation of wavepacket dynamics experiments, particularly stressing the critical role of the final state. We discuss the advantages of the molecular ionization continuum as the final state in polyatomic wavepacket experiments and show how the electronic structure of the continuum can be used to help to disentangle electronic from vibrational dynamics. We illustrate these methods with examples from internal conversion in polyenes and polyaromatic hydrocarbons, electronic relaxation in substituted benzenes, excited state intramolecular proton transfer, azobenzene photoiosomerization dynamics and non-adiabatic photodissociation dynamics.

Ultrafast electron solvation dynamics in D 2O/Cu(1 1 1): influence of coverage and structure

Using femtosecond time-resolved two-photon photoelectron spectroscopy we have studied the dynamics of photoexcited electrons injected from a Cu(1 1 1) substrate into the conduction band of ultrathin ice layers. Ultrafast localization of the injected electrons within <50 fs is followed by a stabilization on a time scale of 0.1–1 ps due to local rearrangements of water dipoles. The dynamics of this electron solvation process are very similar for amorphous and crystalline ice, but exhibit a pronounced coverage dependence with two different regimes, which are attributed to solvation sites in the interior and at the surface of the adlayer. Additional information on the adsorbate structure is obtained from low-temperature scanning tunneling microscopy.

Substituent Effects in Molecular Electronic Relaxation Dynamics via Time-Resolved Photoelectron Spectroscopy: ππ* States in Benzenes

We study the applicability of femtosecond time-resolved photoelectron spectroscopy to the study of substituent effects in molecular electronic relaxation dynamics using a series of monosubstituted benzenes as model compounds. Three basic types of electronic substituents were used: CC (styrene), CO (benzaldehyde), and C⋮C (phenylacetylene). In addition, the effects of the rigidity and vibrational density of states of the substituent were investigated via both methyl (α-methylstyrene, acetophenone) and alkyl ring (indene) substitution. Femtosecond excitation to the second ππ* state leads, upon time-delayed ionization, to two distinct photoelectron bands having different decay constants. Variation of the ionization laser frequency had no effect on the photoelectron band shapes or lifetimes, indicating that autoionization from super-excited states played no discernible role. From assignment of the energy-resolved photoelectron spectra, a fast decaying component was attributed to electronic relaxation of the ...

Femtosecond time-resolved photoelectron spectroscopy of polyatomic molecules.

Femtosecond time-resolved photoelectron spectroscopy is emerging as a useful technique for investigating excited state dynamics in isolated polyatomic molecules. The sensitivity of photoelectron spectroscopy to both electronic configurations and vibrational dynamics makes it well suited to the study of ultrafast nonadiabatic processes. We review the conceptual interpretation of wavepacket dynamics experiments, emphasizing the role of the final state. We discuss the advantages of the molecular ionization continuum as the final state in polyatomic wavepacket experiments and show how the electronic structure of the continuum can be used to disentangle electronic from vibrational dynamics. We illustrate these methods with examples from diatomic wavepacket dynamics, internal conversion in polyenes and polyaromatic hydrocarbons, excited state intramolecular proton transfer, and azobenzene photoiosomerization dynamics.

Femtosecond time-resolved photoelectron spectroscopy with high frequency probe pulses

The interaction of an intense 2 eV femtosecond pulse with the Na 2 molecule prepares vibrational wavepackets in several electronic states. Subsequent ionization with a time-delayed probe pulse produces photoelectron kinetic energy distributions which depend on the pump–probe delay and reflect the vibrational dynamics in the various electronic states. We compare the resulting spectra with others obtained by direct one-photon ionization and show that in the latter case the contributions from different electronic states can be separated.

Ultrafast photochemistry in OClO molecules analyzed by femtosecond time-resolved photoelectron spectroscopy

Applying the coincident detection of photoions and photoelectrons with femtosecond time resolution we have analyzed the energetics and the dynamics of the ultrafast processes initiated by excitation of OClO molecules with 50 fs laser pulses at 398 nm. The major channel leading to the excitation of the A ̃ 2 A 2 electronic state is characterized by spin–orbit coupling to the 2 A 1 electronic state within 7 ps, the subsequent transition to the 2 B 2 state within 250 fs and, finally, the fragmentation to ClO+O and Cl+O 2 within about 750 fs. A minor channel is tentatively assigned to the direct excitation of the 2 A 1 state.

Femtosecond transition state dynamics of cis -stilbene

Femtosecond time-resolved photoion and photoelectron spectroscopy were employed to study the transition state dynamics of cis-stilbene in a molecular beam experiment. Two long-lived photoproducts were found with excitation of 267 nm radiation.

Methods and applications of femtosecond time-resolved photoelectron spectroscopy

Femtosecond time-resolved photoelectron spectroscopy is emerging as a new technique for investigating polyatomic excited state dynamics. Due to the sensitivity of photoelectron spectroscopy to both electronic configurations and vibrational dynamics, it is well suited to the study of ultrafast non-adiabatic processes such as internal conversion, often occurring on sub-picosecond time scales. We discuss technical requirements for such experiments, including laser systems, energy and angle resolved photoelectron spectrometers and new detectors for coincidence experiments. We illustrate these methods with examples from diatomic wavepacket dynamics and ultrafast non-adiabatic processes in polyatomic molecules. © 2000 Elsevier Science B.V. All rights reserved.

Analysis of the ultrafast photodissociation of electronically excited CF2I2 molecules by femtosecond time-resolved photoelectron spectroscopy

Applying the femtosecond pump–probe technique combined with the photoelectron–photoion coincidence detection we have studied the time-resolved photoelectron spectra of CF2I2 and its fragments after excitation with 4.65 eV photons. The time-dependent photoion signals reflect the complete dissociation of the CF2I2 molecules with a time constant of (100±30) fs which is preceded by an ultrafast relaxation process with (30±10) fs. The analysis of the electron spectra reveals that three electronic states with different vibrational energies are populated by one photon excitation during the pump pulse. Furthermore, the number of absorbed pump and probe photons for higher order excitation, the ionization potential of CF2I2 and its binding energies in the ionic state have been determined by the electron spectroscopy. Both the ion signals as well as the electron spectra demonstrate that the observed products CF2, I2, and I are formed by dissociation of the excited CF2I2 molecules, but no CF2I has been detected in all experiments with widely spread laser parameters. Thus, we conclude the concerted reaction mechanism to be the dominant dissociation channel while the sequential decay with the CF2I intermediate is negligible. The measured long-living signals for I2+ are suggested as due to molecular detachment after absorption of two pump photons. The detected electron spectra for I+ at longer delay times reflect the formation of highly excited neutral iodine atoms by absorption of at least three pump photons.

Direct observation of the dynamics of electronic excitations in molecules and small clusters

Femtosecond time-resolved photoelectron spectroscopy is applied to study relaxation paths of excited states of mass-selected negatively charged clusters. As a first example, the lifetime of an excited state of the carbon trimer anion is measured directly. In addition, the mechanism of the decay, i.e., the configurations of the participating electronic states, is determined from the photoelectron spectra. In general, this method can be used to study all kinds of electronic excitation and relaxation processes in mass-selected nanoparticles.

Electron configuration changes in excited pyrazine molecules analyzed by femtosecond time-resolved photoelectron spectroscopy

Using the pump–probe technique with 130 fs laser pulses near 200 nm and near 266 nm the internal conversion of the pyrazine molecule excited to the S2 state has been studied. The lifetime of the S2 state due to internal conversion to the lower electronic states is τIC(2)=(20±10) fs while the lifetime of the secondarily populated S1 state is τIC(1)=(22±1) ps. The results of femtosecond time-resolved electron spectroscopy directly demonstrate the variation of the electron configuration during the internal conversion: The electron spectrum changes significantly on the fs time scale for pyrazine ions produced by ionization via the S2 state with ππ* character and by ionization of S1 state molecules with nπ* configuration after the internal conversion, respectively. The results obtained confirm theoretical estimations of Domcke and co-workers [J. Chem. Phys. 95, 7806 (1991); J. Phys. Chem. 97, 12466 (1993)] who describe the internal conversion in the pyrazine molecule on the basis of a conical intersection of the corresponding potential energy surfaces.

Towards disentangling coupled electronic–vibrational dynamics in ultrafast non-adiabatic processes

Femtosecond time-resolved photoelectron spectroscopy is emerging as a new technique for investigating polyatomic excited state dynamics. Due to the sensitivity of photoelectron spectroscopy to both electronic configurations and vibrational dynamics, it is well suited to the study of non-adiabatic processes such as internal conversion, which often occur on sub-picosecond time scales. We discuss the technical requirements for such experiments, including lasers systems, energy- and angle-resolved photoelectron spectrometers and new detectors for coincidence experiments. We present a few examples of these methods applied to problems in diatomic wavepacket dynamics and ultrafast non-adiabatic processes in polyatomic molecules.

Potential-energy function for intramolecular proton transfer in the malonaldehyde cation

The reaction path and the corresponding energy profile for intramolecular hydrogen transfer in the ground state of the malonaldehyde cation have been characterized employing the UHF/MP2, CASSCF and CASPT2 methods. It is found that the malonaldehyde cation represents the interesting case of a very low-barrier intramolecular hydrogen bond. The relevance of these findings for the infrared spectroscopy of cations of intramolecularly hydrogen-bonded π-systems and for femtosecond time-resolved photoelectron spectroscopy of the exited-state dynamics of these systems are briefly discussed.

Real time observation of hydrogen transfer: Femtosecond time-resolved photoelectron spectroscopy in the excited ammonia dimer

The energy flow in ammonia dimers excited to the electronic à state is analyzed by combining the femtosecond pump–probe technique and the photoelectron–photoion coincidence detection. We use ∼140 fs laser pulses (200 nm for excitation and 267 nm for ionization). For the dimer ion the photoelectron spectra change drastically from a rather broad shape (≳1 eV) at small delay times between pump and probe pulse to a rather narrow peak (0.25 eV) at some picoseconds. This is explained by the dynamics of an internal H-atom transfer in the electronic à state to an NH4…NH2 configuration. The measured photoelectron energies are consistent with ab initio potential energy surface calculations. The observed picosecond lifetime of the hydrogen-transfer state NH4…NH2 can be understood by a conical intersection with the charge-transfer state NH4+…NH2−.

Nonadiabatic dynamics in polyatomic systems studied by femtosecond time-resolved photoelectron spectroscopy

We investigate the use of time-resolved photoelectron spectroscopy for studying nonadiabatic polyatomic dissociation dynamics. In particular, we emphasize the importance of the electronic structure of the ionization continuum in interpreting the results and provide an experimental example of these effects in the dissociation dynamics of the NO dimer.