Time-resolved Photoelectron Angular Distributions Research Articles

Tracing Photoinduced Hydrogen Migration in Alcohol Dications from Time-Resolved Molecular-Frame Photoelectron Angular Distributions.

The recent implementation of attosecond and few-femtosecond X-ray pump/X-ray probe schemes in large-scale free-electron laser facilities has opened the way to visualize fast nuclear dynamics in molecules with unprecedented temporal and spatial resolution. Here, we present the results of theoretical calculations showing how polarization-averaged molecular-frame photoelectron angular distributions (PA-MFPADs) can be used to visualize the dynamics of hydrogen migration in methanol, ethanol, propanol, and isopropyl alcohol dications generated by X-ray irradiation of the corresponding neutral species. We show that changes in the PA-MFPADs with the pump-probe delay as a result of intramolecular photoelectron diffraction carry information on the dynamics of hydrogen migration in real space. Although visualization of this dynamics is more straightforward in the smaller systems, methanol and ethanol, one can still recognize the signature of that motion in propanol and isopropyl alcohol and assign a tentative path to it. A possible pathway for a corresponding experiment requires an angularly resolved detection of photoelectrons in coincidence with molecular fragment ions used to define a molecular frame of reference. Such studies have become, in principle, possible since the first XFELs with sufficiently high repetition rates have emerged. To further support our findings, we provide experimental evidence of H migration in ethanol-OD from ion-ion coincidence measurements performed with synchrotron radiation.

Ultrafast decay dynamics of electronically excited 2-ethylpyrrole.

The excited-state decay dynamics of 2-ethylpyrrole following UV excitation in the wavelength range of 254.8-218.0 nm is investigated in detail using the femtosecond time-resolved photoelectron imaging method. The time-resolved photoelectron spectra and photoelectron angular distributions at all pump wavelengths are carefully analysed and the following picture is derived: at the longest pump wavelengths (254.8, 248.3 and 246.1 nm), 2-ethylpyrrole is excited to the S1(1πσ*) state having a lifetime of about 50 fs. At 248.3, 246.1 and 237.4 nm, another excited state of Rydberg character is excited. The lifetime of this state is ∼570 fs at 237.4 nm and becomes slightly longer at other two pump wavelengths. At the shortest pump wavelengths (230.8 and 218.0 nm), 2-ethylpyrrole is excited to a state which is tentatively assigned to the 11ππ* state, having a lifetime of 75 ± 15 and 48 ± 10 fs for the longer and shorter pump wavelengths, respectively. Internal conversion to the S1(1πσ*) state might be one of the decay mechanisms of the 11ππ* state.

Time-resolved photoelectron angular distributions from nonadiabatically aligned CO2 molecules with SX-FEL at SACLA

We performed time-resolved photoelectron spectroscopy of valence orbitals of aligned CO2 molecules using the femtosecond soft x-ray free-electron laser and the synchronized near-infrared laser. By properly ordering the individual single-shot ion images, we successfully obtained the photoelectron angular distributions (PADs) of the CO2 molecules aligned in the laboratory frame (LF). The simulations using the dipole matrix elements due to the time dependent density functional theory calculations well reproduce the experimental PADs by considering the axis distributions of the molecules. The simulations further suggest that, when the degrees of alignment can be increased up to , the molecular geometries during photochemical reactions can be extracted from the measured LFPADs once the accurate matrix elements are given by the calculations.

Time-Resolved Photoelectron Imaging of Molecular Coherent Excitation and Charge Migration by Ultrashort Laser Pulses

Charge migration is a fundamental and important process in the photochemistry of molecules and has been explored by time-resolved photoelectron angular distributions. A scheme based on UV pump and polarized soft X-ray probe techniques shows that photoelectron diffraction effects enable us to reconstruct electronic coherences encoding the information of the charge migration with extreme time resolutions. We discuss how probe pulse helicity influences the probing photoelectron spectra in the presence of molecular nonspherical Coulomb potentials. This phenomenon is analyzed theoretically and simulated via ab initio calculations for the molecular hydrogen ion, offering a reliable approach for measurements of charge migration and for the exploration of molecular structure in attosecond science.

Monitoring non-adiabatic dynamics in CS2 with time- and energy-resolved photoelectron spectra of wavepackets

We report results from a novel fully ab initio method for simulating the time-resolved photoelectron angular distributions around conical intersections in CS2. The technique employs wavepacket densities obtained with the multiple spawning method in conjunction with geometry- and energy-dependent photoionization matrix elements. The robust agreement of the calculated molecular-frame photoelectron angular distributions with measured values for CS2 demonstrates that this approach can successfully illuminate, and disentangle, the underlying coupled nuclear and electronic dynamics around conical intersections in polyatomic molecules.

Femtosecond x-ray photoelectron diffraction on gas-phase dibromobenzene molecules

We present time-resolved femtosecond photoelectron momentum images and angular distributions of dissociating, laser-aligned 1,4-dibromobenzene (C6H4Br2) molecules measured in a near-infrared pump, soft-x-ray probe experiment performed at an x-ray free-electron laser. The observed alignment dependence of the bromine 2p photoelectron angular distributions is compared to density functional theory calculations and interpreted in terms of photoelectron diffraction. While no clear time-dependent effects are observed in the angular distribution of the Br(2p) photoelectrons, other, low-energy electrons show a pronounced dependence on the time delay between the near-infrared laser and the x-ray pulse.

Time-Resolved Photoelectron Spectra ofCS2: Dynamics at Conical Intersections

We report results of the application of a fully ab initio approach for simulating time-resolved molecular-frame photoelectron angular distributions around conical intersections in ${\mathrm{CS}}_{2}$. The technique employs wave packet densities obtained with the multiple spawning method in conjunction with geometry- and energy-dependent photoionization matrix elements. The robust agreement of these results with measured molecular-frame photoelectron angular distributions for ${\mathrm{CS}}_{2}$ demonstrates that this technique can successfully elucidate, and disentangle, the underlying nuclear and photoionization dynamics around conical intersections in polyatomic molecules.

Probing ultrafast dynamics with time-resolved multi-dimensional coincidence imaging: butadiene

Time-resolved coincidence imaging of photoelectrons and photoions represents the most complete experimental measurement of ultrafast excited state dynamics, a multi-dimensional measurement for a multi-dimensional problem. Here we present the experimental data from recent coincidence imaging experiments, undertaken with the aim of gaining insight into the complex ultrafast excited-state dynamics of 1,3-butadiene initiated by absorption of 200 nm light. We discuss photoion and photoelectron mappings of increasing dimensionality, and focus particularly on the time-resolved photoelectron angular distributions (TRPADs), expected to be a sensitive probe of the electronic evolution of the excited state and to provide significant information beyond the time-resolved photoelectron spectrum (TRPES). Complex temporal behaviour is observed in the TRPADs, revealing their sensitivity to the dynamics while also emphasising the difficulty of interpretation of these complex observables. From the experimental data some details of the wavepacket dynamics are discerned relatively directly, and we make some tentative comparisons with existing ab initio calculations in order to gain deeper insight into the experimental measurements; finally, we sketch out some considerations for taking this comparison further in order to bridge the gap between experiment and theory.

Nuclear and electron dynamics from femto- and subfemto-second time-resolved photoelectron angular distributions

We explore the application of a quantum vibrational wavepacket dynamics formulation of femtosecond energy- and angle-resolved photoelectron spectroscopy to the study of nuclear and electron dynamics. The formulation incorporates geometry- and energy-dependent photoionization matrix elements and is well suited for studies of nonadiabatic dynamics where nuclear and electronic degrees of freedom are coupled. In this paper we explore two aspects of angle-resolved pump–probe photoelectron spectroscopy, using NO2 and NaI as examples. The first is a refinement of femtosecond photoelectron spectroscopy in which the time derivative of the velocity map image is seen to more readily characterize the nuclear wavepacket dynamics in non-adiabatic regions. The other is an analysis to extract the effects of electronic-state fluctuation from sub-femtosecond scale photoelectron dynamics. Within the framework of the Born–Huang (Oppenheimer) expansion where the total wavefunction is expressed in products of stationary-state electronic wavefunctions and time-dependent nuclear wavepackets, we seek to identify how the effect of electron dynamics might be manifested in principle.

Direct observation of field-free alignment of asymmetric molecules in excited states

The excited state dynamics of $o$-dichlorobenzene has been studied by femtosecond time-resolved photoelectron imaging. The lifetime of the first excited state ${S}_{1}$ of $o$-dichlorobenzene was determined to be 482 \ifmmode\pm\else\textpm\fi{} 10 ps. Field-free nonadiabatic alignment of the $o$-dichlorobenzene on the first excited state ${S}_{1}$ (with an asymmetry parameter k $=$ 0.153) by a femtosecond laser pulse was observed via time-resolved photoelectron angular distributions. Rotational wave packet revivals on the ${S}_{1}$ state of the typical asymmetric molecule $o$-dichlorobenzene at 296 ps has been measured.

Real-time visualization of the dynamic evolution of CS_2 4d and 6s Rydberg wave packet components

The dynamic evolution of CS2 4d and 6s Rydberg wave packet components has been experimentally visualized via femtosecond time-resolved photoelectron imaging coupled with time-resolved mass spectroscopy. The temporal evolution of the four components of the prepared Rydberg wave packet is directly observed as time-dependent changes of the intensities of different parts in the main photoelectron peak. Furthermore, time-resolved photoelectron angular distributions (PADs) clearly reflect the different component characters of 4d and 6s molecular orbitals. The lifetime of Rydberg wave packets is determined to be about 830fs and their decay is attributed to predissociation. Our results suggest the possibility of directly visualizing and determining the amplitudes and relative phases of different electronic and vibrational wave packet components in polyatomic molecules.

Ultrafast Dynamics of the First Excited State of Chlorobenzene

Ultrafast dynamics of the first excited state (S-1) of chlorobenzene was studied using a combination of femtosecond time-resolved photoelectron imaging and time-resolved mass spectroscopy. One-photon absorption at 266.7 nm was used to populate the S-1 state of chlorobenzene. The time evolution of the parent ion signals consists of different biexponential decays. One is a fast component on a timescale of (152 +/- 3) fs and the other is a slow component with a timescale of (749 +/- 21) ps. Time-resolved electron kinetic energies (eKE) and time-resolved photoelectron angular distributions (PADs) were extracted from time-resolved photoelectron imaging and are discussed in detail. The ultrafast process with a time constant of (152 +/- 3) fs is a population transfer within the S-1 state, and only a vibrational energy transfer process with strong coupling is a reasonable explanation. This is attributed to an ultrafast process of dissipative intramolecular vibrational energy redistribution (IVR). The lifetime of the S-1 state was determined to be (749 +/- 21) ps, and its deactivation was due to slow internal conversion to the ground state. Additionally, nonadiabatic alignment and rotational dephasing of the S-1 state of chlorobenzene, as a typical asymmetric top molecule, were observed. The first C-type and J-type recurrences are expected at delay time of 205.8 and 359.3 ps, respectively.

Monitoring the effect of a control pulse on a conical intersection by time-resolved photoelectron spectroscopy

We have previously shown how femtosecond angle- and energy-resolved photoelectron spectroscopy can be used to monitor quantum wavepacket bifurcation at an avoided crossing or conical intersection and also how a symmetry-allowed conical intersection can be effectively morphed into an avoided crossing by photo-induced symmetry breaking. The latter result suggests that varying the parameters of a laser to modify a conical intersection might control the rate of passage of wavepackets through such regions, providing a gating process for different chemical products. In this paper, we show with full quantum mechanical calculations that such optical control of conical intersections can actually be monitored in real time with femtosecond angle- and energy-resolved photoelectron spectroscopy. In turn, this suggests that one can optimally control the gating process at a conical intersection by monitoring the photoelectron velocity map images, which should provide far more efficient and rapid optimal control than measuring the ratio of products. To demonstrate the sensitivity of time-resolved photoelectron spectra for detecting the consequences of such optical control, as well as for monitoring how the wavepacket bifurcation is affected by the control, we report results for quantum wavepackets going through the region of the symmetry-allowed conical intersection between the first two (2)A' states of NO(2) that is transformed to an avoided crossing. Geometry- and energy-dependent photoionization matrix elements are explicitly incorporated in these studies. Time-resolved photoelectron angular distributions and photoelectron images are seen to systematically reflect the effects of the control pulse.

Time-resolved photoelectron angular distributions and cross-section ratios of two-colour two-photon above threshold ionization of helium

Time-resolved anisotropy parameters and cross-section ratios of the positive and negative sidebands from two-colour two-photon above threshold ionization of helium atoms are measured using photoelectron velocity map imaging with the selected 19th high-order harmonic at 29.1 eV in an 810 nm perturbative dressing field. The intensities of both the sidebands and the single-photon ionization depletion follow a Gaussian correlation function where the photoelectron angular distributions and cross-section ratios of the sidebands do not change as a function of the temporal delay between the extreme ultraviolet and infrared pulses. The experimental results are compared with theoretical predictions using the soft-photon approximation, showing poor agreement, and analytical expressions are derived using second-order perturbation theory to determine the relative magnitudes of the resulting S and D partial waves of the above threshold ionization features.

Dynamic molecular interferometer: Probe of inversion symmetry in I2− photodissociation

Time-resolved photoelectron imaging of negative ions is employed to examine 780-nm dissociation dynamics of I2(-), emphasizing the effects of interference in time-resolved photoelectron angular distributions obtained with 390-nm probe. No energetic changes are observed after about 700 fs, but the evolution of the photoelectron anisotropy persists for up to 2.5 ps, indicating that the electronic wave function of the dissociating anion continues to evolve long after the asymptotic energetic limit of the reaction has been effectively reached. The time scale of the anisotropy variation corresponds to a fragment separation of the same order of magnitude as the de Broglie wavelength of the emitted electrons (lambda=35 A). These findings are interpreted by considering the effect of I2(-) inversion symmetry and viewing the dissociating anion as a dynamic molecular-scale "interferometer," with the electron waves emitted from two separating centers. The predictions of the model are in agreement with the present experiment and shed new light on previously published results [A. V. Davis, R. Wester, A. E. Bragg, and D. M. Neumark, J. Chem. Phys. 118, 999 (2003)].

Multidimensional calculation of time-resolved photoelectron angular distributions: The internal conversion dynamics of pyrazine

We present the first calculation of time-resolved photoelectron differential cross sections for a polyatomic molecule. The calculation is based on a nonperturbative quantum mechanical theory that accounts exactly for rotations and vibrations and describes the electronic dynamics within a density functional approach. Application is made to study the dynamics of a radiationless transition, as probed by time-resolved photoelectron imaging. Specifically, we consider the ultrafast S2→S1 internal conversion of pyrazine, induced by a short excitation pulse and probed by a time-delayed ionization pulse. Through calculation of total ionization signals, photoelectron energy distributions and energy-integrated and -resolved photoelectron angular distributions, we explore the potential of time-resolved photoelectron imaging. By comparing several models of the ionization dynamics, we examine the extent to which time-resolved photoelectron imaging can provide a general probe of ultrafast nonradiative transitions.

Theory of time-resolved photoelectron imaging: nonperturbative calculation for an internally converting polyatomic molecule.

We present the first calculation of time-resolved photoelectron angular distributions for a polyatomic system. Our method takes rotations into exact account, treats the laser field nonperturbatively, and computes the electronic dynamics from first principles. Our results point to the information content of time-resolved photoelectron imaging observables and illustrate the role played by the field intensity.

Time-resolved photoelectron angular distributions: concepts, applications, and directions.

The use of photoelectron angular distributions (PADs) as a probe in short-pulse, pump-probe scenarios is reviewed. We focus on concepts, on the insight that can be gained through theoretical analysis, on applications, and on future opportunities. Time-resolved PADs are sensitive to both the time-evolving rotational composition of wavepackets and their time-evolving electronic symmetry. The former feature renders this observable a potential probe of molecular structure, intensity effects, and rotational perturbations. The latter feature renders the PAD a potential probe of radiationless transitions.

Time-resolved photoelectron angular distributions as a probe of coupled polyatomic dynamics

A nonperturbative theory for calculating time-resolved photoelectron angular distributions in linear molecules [J. Chem. Phys. 107, 7859 (1997)] is extended to nonlinear systems and reformulated so as to expose and utilize the underlying electronic and rotational symmetries. A sequence of approximations is next introduced, systematically reducing the formally exact expression to cruder forms that are applicable to systems of increasing complexity. As an example of the potential applications of time-resolved photoelectron angular distributions in polyatomic dynamics we examine the information they convey about an ultrafast internal conversion.

Predictions of rotation–vibration effects in time-resolved photoelectron angular distributions

We investigate the physical origin of direct reflection of rotation–vibration coupling in time-resolved photoelectron angular distributions. The theory is developed for a general rotation–vibration coupling mechanism in a polyatomic system and applied to the simplest instance of such interaction, namely centrifugal coupling in a diatomic molecule. Our results suggest the possibility of determining coupling strengths from the observed time dependence of the ionization asymmetry parameters.