Time-resolved Photoelectron Imaging Research Articles

Studies of Ultrafast Dynamics of 3-Picoline with Femtosecond Time-Resolved Photoelectron Imaging

The ultrafast dynamics of excited states in 3-picoline was studied using femtosecond time-resolved photoelectron imaging, coupled with time-resolved mass spectroscopy. An ultrafast internal conversion from the S-2 state to the vibrationally excited S-1 state in about 910 fs was observed. This secondary-populated S-1 state further deactivates to the S-0 state in 2.77 ps. The photoelectron energy and angular distributions show ionizations from the singlet 3p Rydberg states. Two 400 nm photon excitations to the 3s Rydberg state suggest that this state probably decays to the ground state in 62 fs.

Time-resolved photoelectron imaging of the isolated deprotonated nucleotides

Time-resolved photoelectron spectroscopy of deprotonated nucleotides provides new insights into their relaxation dynamics.

Time-resolved photoelectron imaging spectra from non-adiabatic molecular dynamics simulations

We present an efficient method for the simulation of time-resolved photoelectron imaging (TRPEI) spectra in polyatomic molecules. Our approach combines trajectory-based molecular dynamics that account for non-adiabatic effects using surface hopping, with an approximate treatment of the photoionization process using Dyson orbitals as initial and Coulomb waves as final electron states. The method has been implemented in the frame of linear response time-dependent density functional theory. As an illustration, we simulate time- and energy-resolved anisotropy maps for the furan molecule and compare them with recent experimental data [T. Fuji, Y.-I. Suzuki, T. Horio, T. Suzuki, R. Mitrić, U. Werner, and V. Bonačić-Koutecký, J. Chem. Phys. 133, 234303 (2010)]. Our method can be generally used for the interpretation of TRPEI experiments allowing to shed light into the fundamental photochemical processes in complex molecules.

Simultaneous generation of sub-20 fs deep and vacuum ultraviolet pulses in a single filamentation cell and application to time-resolved photoelectron imaging

Sub-20 fs pulses of the third, fourth, and fifth harmonics of a Ti:sapphire laser are simultaneously generated using cascaded four-wave mixing in filamentation propagation of the fundamental frequency and the second harmonic pulses in Ne gas. Reflective optics under vacuum are employed after the four-wave mixing to minimize material dispersion of the optical pulses. The cross-correlation between 198 and 159 nm pulses of 18 fs is achieved without dispersion compensation. This new light source is applied to time-resolved photoelectron imaging of carbon disulfide (CS₂).

Following the relaxation dynamics of photoexcited aniline in the 273-266 nm region using time-resolved photoelectron imaging

Time-resolved photoelectron imaging was used to investigate the relaxation dynamics of electronically excited aniline in the gas-phase following ultraviolet irradiation in the 273-266 nm region. We find that at all wavelengths studied, excitation is predominantly to the long-lived (>1 ns) S1(ππ(*)) state, which exhibits ultrafast intramolecular vibrational redistribution on a <1 ps timescale. At excitation wavelengths centred on resonant transitions in the aniline absorption spectrum that have previously been assigned to the higher lying S2(3s∕πσ(*)) state, we also see clear evidence of this state playing a role in the dynamics. However, we see no indication of any non-adiabatic coupling between the S1(ππ(*)) and S2(3s∕πσ(*)) states over the range of excitation wavelengths studied.

Real-time observation of ultrafast internal conversion in ethylbenzene by femtosecond time-resolved photoelectron imaging.

The ultrafast dynamics of the second singlet electronically excited state (S2) in ethylbenzene has been studied by femtosecond time-resolved photoelectron imaging. The time evolution of the photoelectron signal can be well described by a biexponential decay: a rapid relaxation pathway with a time constant of 60 ( ± 9) fs and a longer-lived channel on a timescale of 2.58 ( ± 0.22) ps. The rapid relaxation is ascribed to the ultrafast internal conversion from the S2 state to the vibrationally hot S1 state. This internal conversion process has been observed in real time. The slow photoelectron signal reflects the depopulation of secondarily populated high vibronic S1 state.

Time-Resolved Radiation Chemistry: Photoelectron Imaging of Transient Negative Ions of Nucleobases

Time-resolved photoelectron imaging has been utilized to probe the energetics and dynamics of the transient negative ion of the nucleobase uracil. This species was created through charge transfer from an iodide anion within a binary iodide-uracil complex using a UV pump pulse; the ensuing dynamics were followed by photodetachment with a near-IR probe pulse. The photoelectron spectra show two time-dependent features, one from probe-induced photodetachment of the transient anion state and another from very low energy electron signal attributed to autodetachment. The transient anion was observed to decay biexponentially with time constants of hundreds of femtoseconds and tens of picoseconds, depending on the excitation energy. These dynamics are interpreted in terms of autodetachment from the initially excited state and a second, longer-lived species relaxed by iodine loss. Hydrogen loss from the N1 position may also occur in parallel.

Time-resolved photoelectron imaging of the iodide–thymine and iodide–uracil binary cluster systems

The energetics and dynamics of thymine and uracil transient negative ions were examined using femtosecond time-resolved photoelectron imaging. The vertical detachment energies (VDEs) of these systems were found to be 4.05 eV and 4.11 eV for iodide-thymine (I(-) x T) and iodide-uracil (I(-) x U) clusters, respectively. An ultraviolet pump pulse was used to promote intracluster charge transfer from iodide to the nucleobase. Subsequent electron detachment using an infrared probe pulse monitored the dynamics of the resulting transient negative ion. Photoelectron spectra reveal two primary features: a near-zero electron kinetic energy signal attributed to autodetachment and a transient feature representing photodetachment from the excited anion state. The transient state exhibits biexponential decay in both thymine and uracil complexes with short and long decay time constants ranging from 150-600 fs and 1-50 ps, respectively, depending on the excitation energy. However, both time constants are systematically shorter for I(-) x T. Vibrational autodetachment and iodine loss are identified as the primary decay mechanisms of the transient negative ions of thymine and uracil.

Ultrafast imaging of electronic relaxation in o-xylene: a new competing intersystem crossing channel

The ultrafast dynamics of the second singlet electronically excited state (S2) in o-xylene was investigated by femtosecond time-resolved photoelectron imaging. A new competing relaxation channel of the S2 state was observed and assigned to the T3 ← S2 intersystem crossing. Interestingly, it is found that the relaxation via this channel occurs on a comparable femtosecond timescale as the S1 ← S2 internal conversion. A lifetime of ~60 fs for the initially excited S2 state, of 540 (±17) fs for the secondary populated S1 state, and of 7.23 (±0.21) ps for the T3 state could be inferred.

Time-resolved photoelectron imaging of excited state relaxation dynamics in phenol, catechol, resorcinol, and hydroquinone

Time-resolved photoelectron imaging was used to investigate the dynamical evolution of the initially prepared S(1) (ππ*) excited state of phenol (hydroxybenzene), catechol (1,2-dihydroxybenzene), resorcinol (1,3-dihydroxybenzene), and hydroquinone (1,4-dihydroxybenzene) following excitation at 267 nm. Our analysis was supported by ab initio calculations at the coupled-cluster and CASSCF levels of theory. In all cases, we observe rapid (<1 ps) intramolecular vibrational redistribution on the S(1) potential surface. In catechol, the overall S(1) state lifetime was observed to be 12.1 ps, which is 1-2 orders of magnitude shorter than in the other three molecules studied. This may be attributed to differences in the H atom tunnelling rate under the barrier formed by a conical intersection between the S(1) state and the close lying S(2) (πσ*) state, which is dissociative along the O-H stretching coordinate. Further evidence of this S(1)/S(2) interaction is also seen in the time-dependent anisotropy of the photoelectron angular distributions we have observed. Our data analysis was assisted by a matrix inversion method for processing photoelectron images that is significantly faster than most other previously reported approaches and is extremely quick and easy to implement.

Energy transfer of ethyl iodine studied by time-resolved photoelectron imaging

Ultrafast dynamics of electronically excited states in ethyl iodine is studied using femtosecond time-resolved photoelectron imaging coupled with mass spectroscopy. The dissociation constant of the A band was measured to be about 57fs. Upon two 400nm photon excitation to the B band, the time evolution of the parent ion with consists of two components. The fast component with a time constant of 50fs revealed the energy transfer from the higher Rydberg states to the B band. The slow one was determined to be 1.42ps, which was due to predissociation relaxation from the B band to the repulsive A band.

Direct imaging of the electronic dephasing in benzene: Experimental evidence for ultrafast intersystem crossing ofT3←S2states

The electronic dephasing of benzene on the second excited state has been observed using a femtosecond time-resolved photoelectron imaging technique coupled with time-resolved mass spectroscopy. Upon absorption of two 400-nm photons, benzene is excited to the S-2 state, and the time evolution of the parent ion can be well described by a biexponential decay: a rapid dephasing pathway with 43 fs and a longer-lived channel with 1.06 ps. The fast dephasing is ascribed to the internal conversion through the S-2-S-1 conical intersections, and the slower one is attributed to the intersystem crossing from the S-2 state. It is possible that the vibrational excitation modes (9(0)(1)16(1)(1), 5(0)(1), and 9(0)(1)) from the S-2 state enhance the spin-orbit coupling greatly, which leads to the S-2-T-3 intersystem crossing. The photoelectron kinetic energy distributions between 1.1 and 1.7 eV exhibit a rapid energy transfer, revealing nuclear motion on the S-2 adiabatic surface.

Femtosecond Photoelectron Imaging of Aligned Polyanions: Probing Molecular Dynamics through the Electron-Anion Coulomb Repulsion.

The first time-resolved photoelectron imaging study of a polyanion is presented. Using the alignment induced through resonance excitation, the photoelectron angular distributions can be qualitatively understood in terms of the position of localized excess charges on the molecular skeleton, which influence the photoemission dynamics. Pump-probe experiments are used to demonstrate that the photoelectron angular distribution is also sensitive to molecular dynamics. This is shown here for the rotational dynamics of a polyanion, in which the photoelectron anisotropy tracks the rotational coherence as it dephases. The methodology can in principle be applied to general molecular dynamics in large polyanions, providing a new route to studying ultrafast structural dynamics in complex gas-phase systems.

Thermal effects on energetics and dynamics in water cluster anions (H2O)n−

The electron binding energies and relaxation dynamics of water cluster anions (H(2)O)(n)(-) (11 ≤ n ≤ 80) formed in co-expansions with neon were investigated using one-photon and time-resolved photoelectron imaging. Unlike previous experiments with argon, water cluster anions exhibit only one isomer class, the tightly bound isomer I with approximately the same binding energy as clusters formed in argon. This result, along with a decrease in the internal conversion lifetime of excited (H(2)O)(n)(-) (25 ≤ n ≤ 40), indicates that clusters are vibrationally warmer when formed in neon. Over the ranges studied, the vertical detachment energies and lifetimes appear to converge to previously reported values.

Ultrafast Internal Conversion Dynamics of Benzyl Chloride by Femtosecond Time-Resolved Photoelectron Imaging

The ultrafast internal conversion of benzyl chloride (BzCl) was studied with femtosecond time-resolved photoelectron imaging (TRPEI) coupled with time-resolved mass spectroscopy. Time-energy maps of the photoelectron intensity and the angular anisotropy were generated from a series of photoelectron images. Upon absorption of two 400 nm photons, benzyl chloride was excited to the S-4 and S-2 states at the same time. The time evolution of the parent ion with different pump-probe delays can be well described by biexponential decay. The fit yielded T-1=50 fs and T-2=910 fs. By analysis of time-resolved photoelectron kinetic energy distributions, it is concluded that the excited S-4 state has coupled with and decayed to the S-2 state in a short time scale and then converted to the S-1 state through ultrafast internal conversion (IC). Within 50 fs, the molecule electronically relaxes into S-1 through IC and from there, decays to the S-0 ground state with the relatively slow time constant of 910 fs. The anisotropy parameters of photoelectron angular distributions changed from 0.87 at the delay time of 0 fs to 0.94 at 25 fs and then to 0.59 at 190 fs, which also reflects the coupling from the S-4 state to the S-2 state and the following IC to the S-1 state.

Ultrafast Internal Conversion Dynamics of 2-Chloropyridine by Femtosecond Time-Resolved Photoelectron Imaging

Ultrafast internal conversion dynamics of 2-chloropyridine were studied by femtosecond time-resolved photoelectron imaging spectroscopy coupled with time-resolved mass spectroscopy. The ultrafast internal conversion from the second excited state (S-2) to the first excited state (S-1) via an adjacent conical intersection within (162 +/- 5) fs was clearly observed from the time-dependence of the photoelectron spectra. The subsequent deactivations involved the coupling of S-2/S-0 (the ground state) and S-1/S-0 conical intersections, which occurred on a timescale of about (5.5 +/- 0.3) ps, and led to the internal conversion to the ground state from the S-2 and S-1 states.

On the intrinsic photophysics of indigo: a time-resolved photoelectron spectroscopy study of the indigo carmine dianion.

The intrinsic photophysics of indigo has been studied using gas-phase time-resolved photoelectron imaging of the indigo carmine dianion (InC(2-)). The action spectrum reveals that the gas-phase absorption spectrum arising from the S(1) <-- S(0) transition in InC(2-) has a similar solvent shift to that of neutral indigo. Femtosecond spectroscopy shows that the S(1) state decays on a 1.4 ps timescale. Through isotopic substitution, the primary mechanism on the S(1) excited state can be assigned to an intra-molecular proton transfer, which is the same as that which has been observed in solution. However, the excited state lifetime is significantly shorter in vacuum. These similarities and differences are discussed in terms of recent theoretical investigations of the S(1) excited state of indigo.

Ultrafast dynamics of aniline following 269–238 nm excitation and the role of the S2(π3s/πσ*) state

Femtosecond time-resolved photoelectron imaging is employed to investigate ultrafast electronic relaxation in aniline, a prototypical aromatic amine. The molecule is excited at wavelengths between 269 and 238 nm. We observe that the S2(pi3s/pi sigma*) state is populated directly during the excitation process at all wavelengths and that the population bifurcates to two decay pathways. One of these involves ultrafast relaxation from the Rydberg component of S2(pi3s/pi sigma*) to the S1(Pi Pi)* state, from which it relaxes back to the electronic ground state on a much longer timescale. The other appears to involve motion along the pi sigma* dissociative potential energy surface. At higher excitation energies, the dominant excitation is to the S3(pi pi*) state, which undergoes extremely efficient electronic relaxation back to the ground state. Our study supports some conclusions reached from H-atom photofragment translational spectroscopy measurements and pump-probe photoionization measurements and contradicts some others.

Shedding new light on the role of the Rydberg state in the photochemistry of aniline

Efficient electronic relaxation following the absorption of ultraviolet light is crucial for the photostability of biological chromophores, so understanding the microscopic details of the decay pathways is of considerable interest. Here, we employ femtosecond time-resolved photoelectron imaging to investigate the ultrafast intramolecular dynamics of aniline, a prototypical aromatic amine, following excitation just below the second absorption maximum. We find that both the second ππ* state and the Rydberg state are populated during the excitation process. Surprisingly, the dominant non-radiative decay pathway is an ultrafast relaxation mechanism that transfers population straight back to the electronic ground-state. The vibrational energy resolution and photoelectron angular distributions obtained in our experiments reveal an interesting bifurcation of the Rydberg population to two non-radiative decay channels. The existence of these competing non-radiative relaxation channels in aniline illustrates how its photostability arises from a subtle balance between dynamics on different electronically excited states and importantly between Rydberg and valence states.

Direct imaging of the Fermi resonance interaction inpara-difluorobenzene: A special insight into energy redistributions in theS1low-energy regime

We have experimentally observed the dynamics of the Fermi resonance interaction in the ${S}_{1}$ low-energy regime of para-difluorobenzene at $+$819 cm${}^{\ensuremath{-}1}$ by femtosecond time-resolved photoelectron imaging technique. The Fermi resonance interaction is clearly demonstrated by the time dependence of the photoelectron spectra; furthermore, the corresponding photoelectron angular distributions (PADs) clearly reflect each Fermi component character of the resonance ${5}^{1}\ensuremath{\cdots}{6}^{2}$. The oscillation period for the Fermi resonance is estimated to be less than 20 ps from the time behavior of the photoelectron spectra, which is very close to our calculated oscillation period value of 13.3 ps. Our results demonstrate the vibrational energy transfer between the two Fermi resonance components of 5${}^{1}$ and 6${}^{2}$, suggestive of a special insight into the vibrational energy redistribution in the ${S}_{1}$ low-energy regime of para-difluorobenzene.