Photoelectron Kinetic Energy Distributions Research Articles

Wavepacket dynamics of the excited (S1) state of 2,5-difluoroaniline by accidental resonance with the Rydberg states.

Coherent nuclear wavepacket behavior obtained by time-resolved spectroscopy is a good choice to capture the real-time evolution of molecular configuration. Using femtosecond time-resolved photoelectron imaging, we investigate the real-time evolution of the vibrational wavepacket of 2,5-difluoroaniline following the coherent excitation of an out-of-plane vibrational mode in the S1 state at 289.8nm. Probed by an accidental resonance with the Rydberg states, the periodic oscillations with the frequency of 99cm-1 are observed from the photoelectron kinetic energy (PKE) distributions, corresponding to the energy difference between the out-of-plane mode X1 0 of C-F bond and the band origin. Moreover, phase reversal of π rad between 0.66-0.75 and 1.00-1.08eV is also observed in the PKE region. Combined with the scan of the potential energy surface in the ground cationic D0 state, the observed two ionization channels corresponding to different phases are attributed to the periodic geometry changes between the planar and the non-planar structures when the coherent wavepacket evolves from the initial vertical Franck-Condon region toward the global minimum of the S1 potential energy surface.

Identification of Above-Threshold Ionization by Imaging Photoelectrons from Ammonia Molecules in an Intense Femtosecond Laser Field

The above-threshold ionization process of ammonia molecules induced by a femtosecond laser field at 800 nm is studied in the intensity range from 1.6 × 1013 to 5.7 × 1013 W/cm2. Channel switching under different laser intensities is observed and identified in the photoelectron kinetic energy spectra of ammonia. Based on the photoelectron kinetic energy distributions and the photoelectron angular distributions, the characteristic peaks observed are exclusively assigned to the multiphoton resonance through certain intermediate states, followed by multiphoton above-threshold ionization.

Ultrafast dynamics of electron excited states of phenylacetylene

Interaction of light with matter has always been important in the field of natural science. Particularly, the ultrafast radiationless relaxation induced by UV light of molecular electronic excited states accompanied by ultrafast energy transfer plays an important role in the natural photophysical, photochemical and biological reactions. Generally, the molecular electronic excited state can be deactivated through a variety of decay channels, including dissociation, isomerization, internal conversion, intersysterm crossing, vibrational energy redistribution, and autoionization. This complexity of relaxation channels brings about a wide variety of deactivation mechanisms. The ultrafast nonadibatic relaxation dynamics of the excited state of phenylacetylene is studied by using femtosecond time-resolved photoelectron imaging and femtosecond time-resolved mass spectrometry. The first excited state S<sub>2</sub> of phenylacetylene is excited by 235 nm pump light, and the excited state deactivation process is detected by 400 nm probe light. The time-dependent curves of parent ions include two exponential curves. One is the fast component with a time constant of 116 fs, and the other is the slow component with a time constant of 106 ps. The time-resolved photoelectron kinetic energy distribution is obtained from the time-resolved photoelectron images. Combined with the time-resolved photoelectron spectroscopy data, the fast component with a time constant of 116 fs is found to reflect the internal conversion process from S<sub>2</sub> state to S<sub>1</sub> state. The experimental results also show that S<sub>1</sub> state is arranged by internal conversion, and the inter system jump process to T<sub>1</sub> state is an important attenuation channel. This work provides a clearer physical picture for S<sub>1</sub> state nonadibatic relaxation dynamics of phenylacetylene.

Intersystem crossing of 2-Methlypyrazine studied by femtosecond photoelectron imaging

The ultrafast nonadibatic relaxation dynamics of the excited state of 2-methylpyrazine has been studied by using femtosecond time-resolved photoelectron imaging and femtosecond time-resolved mass spectrometry. The first excited state S<sub>1</sub> of 2-methylpyrazine was excited by 323 nm pump light, and the excited state deactivation process is detected by 400 nm probe light. The lifetime of S<sub>1</sub> state 98 ps is obtained by time-resolved mass spectroscopy. The intersystem crossing from the S<sub>1</sub> state to the T<sub>1</sub> state is observed on real time. The relaxation dynamics of S<sub>1</sub> state of 2-methlypyrazine is different from that of pyrazine, the results show that the intersystem crossing process between S<sub>1</sub> and T<sub>1</sub> is the main relaxation channel of S<sub>1</sub> state of 2-methlypyrazine, but the internal conversion process between S<sub>1</sub> and S<sub>0</sub> is also a main relaxation channel of S<sub>1</sub> state. By using the advantages of femtosecond time-resolved photoelectron imaging, the photoelectron angular distribution at different pump-probe time delay was obtained experimentally. From the photoelectron angle distribution combined with photoelectron kinetic energy distributions, we tried to observe the field-free nonadiabatic alignment. However, due to the fact that the molecular symmetry of 2-methylpyrazine is lower than that of pyrazine, it is more challenging to observe the phenomenon of molecular nonadiabatic alignment with lower symmetry. Therefore, it is fail to observe nonadiabatic alignment feature of 2-methylpyrazine in this experiment. This work provides a clearer physical picture for S<sub>1</sub> state nonadibatic relaxation dynamics of 2-methylpyrazine.

Time-Resolved Photoelectron Imaging of Acetone with 9.3 eV Photoexcitation

Ultrafast electronic relaxation following 9.3 eV photoexcitation of gaseous acetone was investigated with time-resolved photoelectron imaging spectroscopy. An intense photoionization signal due to a transition from the 41A1(π,π*) state to the D1(π-1) cationic state diminishes within 50 fs, owing to vibrational wave packet motion leaving our observation energy window. Additional photoionization signals were assigned to transitions from Rydberg states with principal quantum numbers of 3-8 to the D0(n-1) cationic state, created by strong vibronic couplings with the bright 41A1(π,π*) state. The deactivation processes of the 41A1(π,π*) and Rydberg states are discussed based on their decay profiles obtained from a time-energy map of photoelectron kinetic energy distributions.

Nonresonant multiphoton ionization of xenon atoms by femtosecond laser pulses

We experimentally study multiphoton ionization of xenon atoms by 266 nm, 400 nm and 800 nm femtosecond laser pulses. The measurements of photoelectron kinetic energy and photoelectron angular distribution indicate that nonresonant multiphoton ionization occurs in the present experimental condition. It obeys the dipolar selection rule and the angular momentum changes one quantum number for adjacent above-threshold ionization peaks.

Femtosecond photoelectron imaging of NO at 410 nm

The simultaneous measurement of the photoelectron kinetic energy and angular distributions of NO were performed as a function of 410 nm laser intensities by femtosecond photoelectron imaging. The photoelectron kinetic energies are nearly independent of the laser intensities. The insensitivity of the photoelectron kinetic energy to the laser peak intensity suggests that peaks in the photoelectron spectra result from Freemen resonance, characteristic of the occurrence of real population on the intermediate states. The relative amplitudes of photoelectron peaks indicate that lower laser intensities emphasize the resonant population while higher laser intensities favor nonresonant population. The A2 Σ+(υ = 2) state can be weakly coupled to the BΠ2(υ=4) state, results in a small part of population on BΠ2(υ=4) state transferred from A2 Σ+(υ = 2) state. The strong field effect leads to more anisotropy of ATI than that of non-ATI. The modulation of populations in electronic states can be achieved by adjusting laser peak intensities. The results can provide some important basis for realizing quantum manipulation of molecules experimentally.

Vibronic relaxation dynamics of o-dichlorobenzene in its lowest excited singlet state

Vibronic dynamics of o-dichlorobenzene in its lowest excited singlet state, S1, is investigated in real time by using femtosecond pump-probe method, combined with time-of-flight mass spectroscopy and photoelectron velocity mapping technique. Relaxation processes for the excitation in the range of 276–252 nm can be fitted by single exponential decay model, while in the case of wavelength shorter than 252 nm two-exponential decay model must be adopted for simulating transient profiles. Lifetime constants of the vibrationally excited S1 states change from 651 ± 10 ps for 276 nm excitation to 61 ± 1 ps for 242 nm excitation. Both the internal conversion from the S1 to the highly vibrationally excited ground state S0 and the intersystem crossing from the S1 to the triplet state are supposed to play important roles in de-excitation processes. Exponential fitting of the de-excitation rates on the excitation energy implies such de-excitation process starts from the highly vibrationally excited S0 state, which is validated, by probing the relaxation following photoexcitation at 281 nm, below the S1 origin. Time-dependent photoelectron kinetic energy distributions have been obtained experimentally. As the excitation wavelength changes from 276 nm to 242 nm, different cationic vibronic vibrations can be populated, determined by the Franck-Condon factors between the large geometry distorted excited singlet states and final cationic states.

Visualization of the formation of cyclopentylcarbene using time-resolved photoelectron imaging spectroscopy

Ultrafast electronic relaxation dynamics in cyclohexene are studied using femtosecond photoelectron imaging spectroscopy. Transient photoelectron kinetic energy distributions and photoelectron angular distributions are obtained and analyzed. Photoelectron bands are discussed and assigned to the ionization of the related electronic states. The formation process of cyclopentylcarbene, together with the ultrafast relaxation of the involved electronic states, is evidenced by the transient photoelectron images. The lifetime for the observed carbene is very short and determined to be 395 (±67) fs. The ionization dynamics of the produced cyclopentylcarbene are also discussed via an analysis of the corresponding photoelectron band.

Real-time observation of cascaded electronic relaxation processes in p-Fluorotoluene

Ultrafast electronic relaxation processes following two photoexcitation of 400nm in p-Fluorotoluene (pFT) have been investigated utilizing time-resolved photoelectron imaging coupled with time-resolved mass spectroscopy. Cascaded electronic relaxation processes started from the electronically excited S2 state are directly imaged in real time and well characterized by two distinct time constants of ~85±10fs and 2.4±0.3ps. The rapid component corresponds to the lifetime of the initially excited S2 state, including the structure relaxation from the Franck-Condon region to the conical intersection of S2/S1 and the subsequent internal conversion to the highly excited S1 state. While, the slower relaxation constant is attributed to the further internal conversion to the high levels of S0 from the secondarily populated S1 locating in the channel three region. Moreover, dynamical differences with benzene and toluene of analogous structures, including, specifically, the slightly slower relaxation rate of S2 and the evidently faster decay of S1, are also presented and tentatively interpreted as the substituent effects. In addition, photoelectron kinetic energy and angular distributions reveal the feature of accidental resonances with low-lying Rydberg states (the 3p, 4s and 4p states) during the multi-photon ionization process, providing totally unexpected but very interesting information for pFT.

Kinetic energy distribution of photoelectrons in the tunnel ionization process in the case of ultrashort laser pulses

In this paper, we use semiclassical model to discusse the kinetic electron energy distribution in the single tunnel ionization process of an atom with respect on the pulse duration and the spatial–temporal laser beam distribution. Our theoretical study show that the pulse duration is an important parameter in the ionization process, as well as beam shape.

Resolving Nonadiabatic Dynamics of Hydrated Electrons Using Ultrafast Photoemission Anisotropy.

We have studied ultrafast nonadiabatic dynamics of excess electrons trapped in the band gap of liquid water using time- and angle-resolved photoemission spectroscopy. Anisotropic photoemission from the first excited state was discovered, which enabled unambiguous identification of nonadiabatic transition to the ground state in 60fs in H_{2}O and 100fs in D_{2}O. The photoelectron kinetic energy distribution exhibited a rapid spectral shift in ca. 20fs, which is ascribed to the librational response of a hydration shell to electronic excitation. Photoemission anisotropy indicates that the electron orbital in the excited state is depolarized in less than 40fs.

Ultrafast Deactivation of the ππ*(V) State of Ethylene Studied Using Sub-20 fs Time-Resolved Photoelectron Imaging

The ultrafast deactivation process of ethylene in the ππ*(V) state was studied using time-resolved photoelectron imaging with sub-20 fs pulses at 159 and 198 nm. The photoelectron kinetic energy distribution observed upon 159 nm photoexcitation exhibited a continuous downward shift within 20 fs, attributed to both C-C twist and pyramidalization motions. A partial revival of the vibrational wave packet was observed with the period of about 18 fs, which is attributed to the C-C twist from 0 to 180° on the ππ*(V) potential energy surface. Signature for internal conversion from the ππ*(V) state to a lower-lying π3s Rydberg (R) state, which has been previously suggested, was not detected in the time-dependent photoelectron kinetic energy and angular distributions.

Ultrafast imaging of electronic relaxation in n-propylbenzene: Direct observation of intermediate state

The ultrafast dynamics of the second singlet electronically excited state (S2) in n-propylbenzene has been investigated by femtosecond time-resolved photoelectron imaging coupled with photofragmentation spectroscopy. The intermediate state for the deactivation of the S2 state is observed by transient photoelectron kinetic energy distributions and photoelectron angular distributions. An ultrafast electronic relaxation process on timescale of the fitted ∼50fs was observed in the S2 state by time-resolved photoelectron imaging and it is attributed to the S1←S2 internal conversion (IC). The time constant of 1.23 (±0.2)ps is determined for the further deactivation of the intermediate S1 state.

Excited-state dynamics of guanosine in aqueous solution revealed by time-resolved photoelectron spectroscopy: experiment and theory.

Time-resolved photoelectron spectroscopy is performed on aqueous guanosine solution to study its excited-state relaxation dynamics. Experimental results are complemented by surface hopping dynamic simulations and evaluation of the excited-state ionization energy by Koopmans' theorem. Two alternative models for the relaxation dynamics are discussed. The experimentally observed excited-state lifetime is about 2.5 ps if the molecule is excited at 266 nm and about 1.1 ps if the molecule is excited at 238 nm. The experimental probe photon energy dependence of the photoelectron kinetic energy distribution suggests that the probe step is not vertical and involves a doubly-excited autoionizing state.

Ultrafast Breakdown of dielectrics: Energy absorption mechanisms investigated by double pulse experiments

We investigate the mechanisms involved in the modification of dielectric materials by ultrashort laser pulses. We show that the use of a double pulse (fundamental and second harmonic of a Ti–Sa laser) excitation allows getting new insight in the fundamental processes that occur during the interaction. We first measure the optical breakdown (OB) threshold map (intensity of first pulse versus intensity of second pulse) in various materials (Al2O3, MgO, α-SiO2). Using a simple model that includes multiphoton excitation followed by carrier heating in the conduction band, and assuming that OB occurs when a critical amount of energy is deposited in the material, we can satisfactorily reproduce this evolution of optical breakdown thresholds. The results demonstrate the dominant role of carrier heating in the energy transfer from the laser pulse to the solid. This important phenomenon is also highlighted by the kinetic energy distribution of photoelectrons observed in a photoemission experiment performed under similar conditions of double pulse excitation. Finally we show, in the case of α-SiO2, that the initial electronic excitation plays a key role in the formation of surface ripples and that their characteristics are determined by the first pulse, even at intensities well below OB threshold.

Femtosecond time-resolved XUV + UV photoelectron imaging of pure helium nanodroplets.

Liquid helium nanodroplets, consisting of on average 2 × 10(6) atoms, are examined using femtosecond time-resolved photoelectron imaging. The droplets are excited by an extreme ultraviolet light pulse centered at 23.7 eV photon energy, leading to states within a band that is associated with the 1s3p and 1s4p Rydberg levels of free helium atoms. The initially excited states and subsequent relaxation dynamics are probed by photoionizing transient species with a 3.2 eV pulse and using velocity map imaging to measure time-dependent photoelectron kinetic energy distributions. Significant differences are seen compared to previous studies with a lower energy (1.6 eV) probe pulse. Three distinct time-dependent signals are analyzed by global fitting. A broad intense signal, centered at an electron kinetic energy (eKE) of 2.3 eV, grows in faster than the experimental time resolution and decays in ~100 fs. This feature is attributed to the initially excited droplet state. A second broad transient feature, with eKE ranging from 0.5 to 4 eV, appears at a rate similar to the decay of the initially excited state and is attributed to rapid atomic reconfiguration resulting in Franck-Condon overlap with a broader range of cation geometries, possibly involving formation of a Rydberg-excited (He(n))* core within the droplet. An additional relaxation pathway leads to another short-lived feature with vertical binding energies ≳2.4 eV, which is identified as a transient population within the lower-lying 1s2p Rydberg band. Ionization at 3.2 eV shows an enhanced contribution from electronically excited droplet states compared to ejected Rydberg atoms, which dominate at 1.6 eV. This is possibly the result of increased photoelectron generation from the bulk of the droplet by the more energetic probe photons.

Nonadiabatic Electronic Dynamics in Isolated Molecules and in Solution Studied by Ultrafast Time–Energy Mapping of Photoelectron Distributions

Abstract Nonadiabatic electronic dynamics contribute to the diversity of chemical reactions. We investigate nonadiabatic dynamics in isolated molecules and aqueous solutions by time-resolved photoelectron spectroscopy. For isolated molecules in the gas phase, we combine two-dimensional imaging detection of electrons and a sub-20 fs deep UV and vacuum UV light source to measure time-evolution of the photoelectron kinetic energy and angular distributions. Time–energy mapping of photoelectron angular anisotropy reveals the S2 → S1 internal conversion dynamics through conical intersection in pyrazine, benzene, and toluene. The time–energy mapping is also employed to extract the photoelectron angular distribution in the molecular frame for nitric oxide. For electronic dynamics in aqueous solution, we employ time-resolved photoelectron spectroscopy using a liquid beam and an electrostatic or a time-of-flight electron energy analyzer. Successful observation of ultrafast electron-transfer reactions in aqueous solutions and measurement of electron binding energies of solvated species suggest promising future developments of this very young field.

Ultrafast Internal Conversion of Aromatic Molecules Studied by Photoelectron Spectroscopy using Sub-20 fs Laser Pulses

This article describes our recent experimental studies on internal conversion via a conical intersection using photoelectron spectroscopy. Ultrafast S2(ππ*)–S1(nπ*) internal conversion in pyrazine is observed in real time using sub-20 fs deep ultraviolet pulses (264 and 198 nm). While the photoelectron kinetic energy distribution does not exhibit a clear signature of internal conversion, the photoelectron angular anisotropy unambiguously reveals the sudden change of electron configuration upon internal conversion. An explanation is presented as to why these two observables have different sensitivities to internal conversion. The 198 nm probe photon energy is insufficient for covering the entire Franck-Condon envelopes upon photoionization from S2/S1 to D1/D0. A vacuum ultraviolet free electron laser (SCSS) producing 161 nm radiation is employed to solve this problem, while its pulse-to-pulse timing jitter limits the time resolution to about 1 ps. The S2–S1 internal conversion is revisited using the sub-20 fs 159 nm pulse created by filamentation four-wave mixing. Conical intersections between D1(π−1) and D0(n−1) and also between the Rydberg state with a D1 ion core and that with a D0 ion core of pyrazine are studied by He(I) photoelectron spectroscopy, pulsed field ionization photoelectron spectroscopy and one-color resonance-enhanced multiphoton ionization spectroscopy. Finally, ultrafast S2(ππ*)–S1(ππ*) internal conversion in benzene and toluene are compared with pyrazine.

Optical control of the vibrational excitation of the polyatomic ions via strong field multi-photon ionization

The dynamics of multi-photon ionization of CH3I under strong field has been studied experimentally by femtosecond photoelectron imaging. Ultrafast optical control of the vibrational excitation in a polyatomic ion by strong field multi-photon ionization is experimentally realized. The present work enhances the intensity of the ionization beam from 1.6×1013 to 2.5×1013 W/cm2. In the order of this higher field, a new energy component is observed and attributed. From the photoelectron imaging, photoelectron kinetic energy distributions and the photoelectron angular distributions are obtained. The discussions of the previous letter are mostly based on the photoelectron kinetic energy information, and the present study emphasizes on the trend of the photoelectron angular anisotropy. More detailed dynamics on vibrational optical control is further explored.