Photoelectron Imaging Technique Research Articles

Insights into ultrafast decay dynamics of electronically excited pyridine-N-oxide.

The ultrafast decay dynamics of pyridine-N-oxide upon excitation in the near-ultraviolet range of 340.2-217.6 nm is investigated using the femtosecond time-resolved photoelectron imaging technique. The time-resolved photoelectron spectra and photoelectron angular distributions at all pump wavelengths are carefully analyzed and the following view is derived: at the longest pump wavelengths (340.2 and 325.6 nm), pyridine-N-oxide is excited to the S1(1ππ*) state with different vibrational levels. The depopulation rate of the S1 state shows a marked dependence on vibrational energy and mode, and the lifetime is in the range of 1.4-160 ps. At 289.8 and 280.5 nm, both the second 1ππ* state and the S1 state are initially prepared. The former has an extremely short lifetime of ∼60 fs, which indicates that the ultrafast deactivation pathway such as a rapid internal conversion to one close-lying state is its dominant decay channel, while the latter is at high levels of vibrational excitation and decays within the range of 380-520 fs. At the shortest pump wavelengths (227.3 and 217.6 nm), another excited state of Rydberg character is mostly excited. We assign this state to the 3s Rydberg state which has a lifetime of 0.55-2.2 ps. This study provides a comprehensive picture of the ultrafast excited-state decay dynamics of the photoexcited pyridine-N-oxide molecule.

Wavelength dependence of the multiphoton ionization of CH3I by intense femtosecond laser pulses through Freeman resonances.

Multiphoton ionization (MPI) of methyl iodide, CH3I, has been investigated with the photoelectron imaging (PEI) technique, using high intensity femtosecond laser pulses at different central wavelengths. The use of high laser field strengths alters the way in which matter-radiation interaction takes place. This generates highly nonlinear phenomena, among which we can highlight the Stark shift effect. It can distort the potential energy surfaces of atoms and molecules, varying both the energy of electronic and rovibrational states of these systems and their ionization potentials. In this way, the Stark shift can generate resonances between intermediate states and an integer number of laser photons of a given wavelength, which would be absent in the low intensity regime. The main purpose of this work is the generation, detection and characterization of resonances produced by the Stark shift, commonly known as Freeman resonances, induced by multiphoton ionization of gas-phase CH3I at different laser wavelengths. The results obtained reveal that a multitude of resonances are induced in the ionization of CH3I in the range of intensities employed, involving several Rydberg states. Ionization pathways associated with different degrees of vibrational excitation in both the intermediate states and the molecular cation generated in each of the experiments are proposed.

Ultrafast decay dynamics of 2-hydroxypyridine excited to S1 electronic state

The S1 state decay dynamics of 2-hydroxypyridine following UV excitation at a wavelength range of 276.9−250.0 nm is investigated using femtosecond time-resolved photoelectron imaging technique. Based on pump wavelength dependence of the decay dynamics, a refined decay picture is proposed. At pump wavelength of 276.9 nm, the S1 state is depopulated through intersystem crossing to lower triplet state(s). At 264.0 nm, both intersystem crossing to lower triplet state(s) and internal conversion to the ground state are in operation. At 250.0 nm, internal conversion to the ground state becomes dominated.

Improved insights in time-resolved photoelectron imaging.

Time-resolved photoelectron imaging (TRPEI) is a highly differential technique for the detailed study of non-adiabatic energy redistribution dynamics operating in the electronically excited states of molecules following the absorption of ultraviolet light. This Perspective briefly reviews the main elements of the TRPEI method but also seeks to address some of its limitations. With the help of various examples drawn from our own recent work, we illustrate some of the challenges commonly encountered during the analysis and interpretation of experimental data and introduce some initial thoughts on approaches to help deal with them. We also discuss some novel methods that aim to expand the capabilities and utility of the TRPEI technique by extending the observation window along the photochemical reaction coordinate(s) and improving the temporal resolution. Given the widespread use of TRPEI and related ultrafast spectroscopies, we anticipate that this Perspective will be of broad interest to a sizeable research community. Furthermore, we hope it will also serve as a useful overview for those engaging with this topic for the first time.

Ultrafast Molecular Spectroscopy Using a Hollow-Core Photonic Crystal Fiber Light Source.

We demonstrate, for the first time, the application of rare-gas-filled hollow-core photonic crystal fibers (HC-PCFs) as tunable ultraviolet light sources in femtosecond pump-probe spectroscopy. A critical requirement here is excellent output stability over extended periods of data acquisition, and we show this can be readily achieved. The time-resolved photoelectron imaging technique reveals nonadiabatic dynamical processes operating on three distinct time scales in the styrene molecule following excitation over the 242-258 nm region. These include ultrafast (<100 fs) internal conversion between the S2(ππ*) and S1(ππ*) electronic states and subsequent intramolecular vibrational energy redistribution within S1(ππ*). Compact, cost-effective, and highly efficient benchtop HC-PCF sources have huge potential to open up many exciting new avenues for ultrafast spectroscopy in the ultraviolet and vacuum ultraviolet spectral regions. We anticipate that our initial validation of this approach will generate important impetus in this area.

Femtosecond time-resolved observation of butterfly vibration in electronically excited o-fluorophenol

The butterfly vibration during the hydrogen tunneling process in electronically excited o-fluorophenol has been visualized in real time by femtosecond time-resolved ion yield spectroscopy coupled with time-resolved photoelectron imaging technique. A coherent superposition of out-of-plane C–F butterfly motions is prepared in the first excited electronic state (S1). As the C–F bond vibrates with respect to the aromatic ring, the nuclear geometry varies periodically, leading to the corresponding variation in the photoionization channel. By virtue of the more favorable ionization probability from the nonplanar minimum via resonance with the Rydberg states, the evolution of the vibrational wave packet is manifested as a superimposed beat in the parent-ion transient. Moreover, time-resolved photoelectron spectra offer a direct mapping of the oscillating butterfly vibration between the planar geometry and nonplanar minimum. The beats for the photoelectron peaks originating from the planar geometry are out of phase with those from the nonplanar minimum. Our results provide a physically intuitive and complete picture of the oscillatory flow of energy responsible for the coherent vibrational motion on the excited state surface.

Tracking ultrafast dynamics of n-propylbenzene cations by delayed photofragmentation and photoelectron spectroscopy

Ultrafast relaxation dynamics of n-propylbenzene (n-PB) cation were studied by delayed photofragmentation and photoelectron spectroscopy. Multiphoton ionization (MPI) by the fundamental pulse at ∼800nm was used to prepare the n-PB cations. The photoelectron image for MPI was captured and characterized by the photoelectron imaging technique, and the observed photoelectron bands were assigned to the corresponding cationic states. The ultrafast relaxation dynamics of these cation states were tracked by monitoring the time-dependent parent-ion (C6H5C3H7+) depletion and the fragment-ion (C6H5CH2+) appearance. An ultrafast relaxation time of 1.1 (±0.1) ps for the cations was obtained, which was ascribed to a dissipative intramolecular vibrational redistribution process within the ground D0 state.

Ultrafast excited-state dynamics in a prototype of the peptide bond: Internal conversion of the isolated N,N-dimethylformamide

A combination of femtosecond time-resolved photoelectron imaging technique and time-resolved mass spectroscopy technique are implemented to investigate the electronic excited-state dynamics in N,N-dimethylformamide (NNDMF). The ultrafast internal conversion (IC) of ${S}_{1}$ ($n$\ensuremath{\pi}${}^{*}$) and ${S}_{2}$ (\ensuremath{\pi}\ensuremath{\pi}${}^{*}$) excited states of NNDMF are observed in this experiment. The molecule is excited to the lowest-lying \ensuremath{\pi}\ensuremath{\pi}${}^{*}$ state (${S}_{2}$ state) following absorption of two 400-nm photons. It is found that the population of the ${S}_{2}$ (\ensuremath{\pi}\ensuremath{\pi}${}^{*}$) state undergoes ultrafast IC to the highly vibrationally excited ${S}_{1}$ ($n$\ensuremath{\pi}${}^{*}$) state within 99 fs by very fast C-N stretching, while the nonradiative deactivation of the ${S}_{1}$ ($n$\ensuremath{\pi}${}^{*}$) state occurs in 2.4 ps, and it is to a large extent due to the C-N bond cleavage from the ${S}_{1}$ potential energy surface, which would be able to efficiently compete with the IC of ${S}_{1}$ \ensuremath{\rightarrow} ${S}_{0}$ through ${S}_{1}$/${S}_{0}$ conical intersections.

Valence Electron Ionization Dynamics of Chromium by a Photoelectron Imaging Technique: J+-Dependent State and Angular Distribution

The photoionization of Cr at excited states is investigated using a velocity-map photoelectron imaging technique. Benzene chromium carbonyl or bis(η(6)-benzene) chromium was used as a precursor for the generation of excited Cr atoms. The a (5)S2 → x (5)P°3 and a (5)D3 → y (5)D°2 transitions are then employed for the preparation of resonant intermediate states in a two-color two-photon ionization process, in which an electronic configurational change from 3d(4)((5)D)4s4p((1)P°) to 3d(4)4s((6)D(J+)) occurs. The photoelectron kinetic energy distribution is found to be very sensitive to the ionization energy and the total angular momentum quantum number of the chromium ion (J(+)). Anisotropy parameters associated with departing electrons also show significant variation depending on the energy and total angular momentum quantum number, suggesting that direct and/or indirect ionization should be quantum-mechanically mixed, manifesting the complicated nature of angular momentum couplings in the ionization continuum.

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.

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.

Superatom spectroscopy and the electronic state correlation between elements and isoelectronic molecular counterparts.

Detailed in the present investigation are results pertaining to the photoelectron spectroscopy of negatively charged atomic ions and their isoelectronic molecular counterparts. Experiments utilizing the photoelectron imaging technique are performed on the negative ions of the group 10 noble metal block (i.e. Ni-, Pd-, and Pt-) of the periodic table at a photon energy of 2.33 eV (532 nm). The accessible electronic transitions, term energies, and orbital angular momentum components of the bound electronic states in the atom are then compared with photoelectron images collected for isoelectronic early transition metal heterogeneous diatomic molecules, M-X- (M = Ti,Zr,W; X = O or C). A superposition principle connecting the spectroscopy between the atomic and molecular species is observed, wherein the electronic structure of the diatomic is observed to mimic that present in the isoelectronic atom. The molecular ions studied in this work, TiO-, ZrO-, and WC- can then be interpreted as possessing superatomic electronic structures reminiscent of the isoelectronic elements appearing on the periodic table, thereby quantifying the superatom concept.

Photoelectron imaging of 8p Rydberg states of atomic iodine following methyl iodide A-band decomposition

Photoelectron imaging technique has been applied to study (2 + 1) REMPI of atomic iodine through 8p Rydberg states around 253 nm. Full three-dimensional state-specific speed and angular distributions of the photoelectrons were recorded. The branching ratios among the different I + levels revealed that the perturbation on ( 3P 2)8p series is particularly large among the ( 3P 2)np series. The violation of core-conserving ionization is attributed to the interactions between the ( 3P 2)8p and ( 1D 2)6p series. The photoelectron angular distributions were found to be well characterized by P 2(cos θ) and P 4(cos θ). A relatively high positive β 2 and a relatively low β 4 observed in (2 + 1) REMPI process indicated that the ionization process can be approximately considered as single-photon ionization via the weakly aligned ( 3P 2)8p intermediate states.

Photoelectron imaging of atomic chlorine and bromine following photolysis of CH2BrCl

Photoionization of chlorine and bromine atoms following photodissociation of CH(2)BrCl was studied in the wavelength range of 231-238 nm by photoelectron imaging technique. Final state-specific speed and angular distributions of the photoelectron were recorded. Analysis of relative branching ratios to different levels of Cl(+) and Br(+) revealed that the final ion level distributions are generally dominated by the preservation of the ion-core configuration of the intermediate resonant state. Some J(c) numbers of the intermediate states were newly assigned according to this regulation. The configuration interaction between resonant states and the autoionization in the continuum were also believed to play an important role in the ionization process since some ions that deviate from the regulation mentioned ahead were observed. The angular distributions of the electrons were found to be well characterized by beta(2) and beta(4), although the ionization process of chlorine and bromine atoms involves three photons.

Anisotropies of photoelectron angular distribution in the vicinity of autoionization resonances

Using a photoelectron imaging technique, remarkable variations of photoelectron angular distributions are demonstrated in the region of autoionization resonance states of a two-photon excited I-atom. It is shown that the observed energy evolution of angular variation exhibits Fano-like behaviour and can be parametrized, for an isolated autoionization state, using the theoretically proposed scaling relationship for the width and the energy shift with respect to the absorption spectrum of the autoionizing state. In the case of overlapping resonances, however, it is found that the scaling relationship fails to account for the energy dependences of the β4 parameters, though it remains valid for the β2 spectra. Possible reasons are discussed.

A Valence Photoelectron Imaging Investigation of Chiral Asymmetry in the Photoionization of Fenchone and Camphor

Photoelectron circular dichroism (PECD) is investigated in the valence ionization of selected fenchone enantiomers using a photoelectron imaging technique and circularly polarized synchrotron radiation. Theoretical modelling of the results using electron scattering calculations demonstrates that the observed chiral asymmetry in the photoelectron angular distributions depends strongly upon the final state scattering, and upon the quality of the molecular potential used for these calculations. However, very pronounced dependence on the orbital from which ionization occurs is also observed. Comparison with analogous results previously obtained for camphor reveals striking differences in the PECD, even when the ionizing orbitals are themselves left substantially unaffected by the changes in methyl groups' substitution site. PECD measurements readily differentiate these molecules despite their very similar photoelectron spectra, demonstrating PECD to be a structurally sensitive probe.

Photoelectron Imaging of Atomic Iodine Following A-Band Photolysis of CH3I

Photoionization of the iodine atom following methyl iodide A-band photodissociation was studied over the wavelength range of 245.5-261.6 nm by photoelectron imaging technique. Final state-specific speed and angular distributions of the photoelectron were recorded. Two types of the photoelectron resulted from ionizing the I atom from the photodissociation of CH3I were identified: (a) (2+1) REMPI of the ground state I atom, and (b) two-photon excitation of spin-orbit excited I(2P1/2) to autoionizing resonances converging to the 3P1 state of I+. In addition, some weaker signals were attributed to one-photon ionization of I atoms produced in some higher excited states from multiphoton ionization of CH3I followed by dissociation. Analysis of relative branching ratios to different levels of I+ (in case a) revealed that the final ion level distributions are generally dominated by the preservation of the ion-core configuration of the intermediate resonant state. A qualitative interpretation of the electron angular distribution from an autoionization process is also given.

VUV photoelectron imaging of biological nanoparticles: Ionization energy determination of nanophase glycine and phenylalanine-glycine-glycine

The ionization energies of biological nanoparticles are determined using the velocity map photoelectron imaging technique. A beam of nanoparticles produced by aerosol methods is photoionized with tunable vacuum ultraviolet (VUV) synchrotron radiation. The resulting photoelectrons are detected and their angular and energy distributions are measured, yielding an angle-resolved photoelectron spectrum. The ionization energies of the nanoparticles are derived from plots of the photoelectron spectrum versus incident photon energy. The ionization energies of nanophase glycine and phenylalanine-glycine-glycine are 7.6 +/- 0.2 eV, and 7.5 +/- 0.2 eV, respectively. X-Ray powder diffraction studies on the glycine nanoparticles indicate that they are crystalline in nature. The reduced ionization energy when compared to gas phase results suggests that the polarization energy in the solid is significant. The difference in the ionization energy between the nano and gas phase reflects this polarization energy and is derived to be 1.7 +/- 0.2 eV and 1.6 +/- 0.2 eV for glycine and phenylalanine-glycine-glycine, respectively. Using these results the molecular polarizability of glycine is estimated to be 4.7 +/- 0.3 A3 (31.9 +/- 1.9 au).

Photoelectron imaging of negative ions: atomic anions to molecular clusters.

The negative ion photoelectron imaging technique is illustrated using two relatively simple atomic and molecular anion systems, and then applied to the study of a cluster system. Photoelectron images of I- and CS2- at 267 nm and 800 nm respectively are presented. Photoelectron spectra and angular distributions are obtained from the images and the concepts underlying these and their interpretation are outlined. The imaging technique is then applied to (CS2)n - (n = 2-4) cluster anions, for which 400 nm images are presented. Features of these images are highlighted and discussed with reference to solvation effects and structural properties of the cluster anionic moiety. Photoelectron signatures of different forms of the cluster core are discussed. These core structures are anionic monomer units solvated by the remaining n - 1 CS2 molecules or covalent dimer units solvated by the remaining n - 2 molecules. Images of the n = 2 anion at 400, 530 and 800 nm reveal information about the electron detachment processes within the different cluster types and both direct detachment and autodetachment are seen. The direct transitions are seen from clusters with either core type, while autodetachment is only seen from clusters with the covalent dimer core. The imaging work also reveals evidence of a previously unreported electronic transition within the direct detachment band due to the covalently bound core type.

High-resolution angle-resolved measurements in atoms and molecules using advanced photoelectron spectroscopy at the ALS

A two-dimensional photoelectron imaging technique using time-of-flight spectrometry and a high-resolution SES-200 electron energy analyzer have been used with photons from the atomic and molecular undulator beamline of the Advanced Light Source to study the structure and dynamics of atoms and molecules. The analyzers are rotatable around the direction of the photon beam, allowing angular-dependent studies to be performed. Results on the photoexcitation and decay mechanisms of doubly excited resonances in neon illustrate the importance of spin-orbit effects for such a light atom, since significant breakdown of LS-coupling was measured for such a light atom . Also, fully vibrationally resolved Auger electron spectra measured under Resonant Raman conditions following the C 1s→π* excitation in a diatomic molecule, CO, is presented which allows us to probe the core excited states, their excitation and de-excitation dynamics with unprecedented resolution.