Yasuki Arasaki

Femtosecond energy- and angle-resolved photoelectron spectroscopy

We present a formulation of energy- and angle-resolved photoelectron spectra for femtosecond pump–probe ionization of wave packets and results of its application to the ^1Σ^+_u double-minimum state of aligned Na_2. The formulation is well-suited for inclusion of the underlying dynamics of molecular photoionization and its dependence on molecular geometry. Results are presented for three typical pump laser energies selected so as to investigate qualitatively different patterns of the spatio-temporal propagation of wave packets on the double-minimum potential curve and of their associated photoelectron spectra. Photoelectron angular distributions are also reported for different orientations of linearly polarized pump and probe pulses. The resulting photoelectron spectra illustrate the importance of a proper description of the underlying photoionization amplitudes and their dependence on geometry for unraveling wave packet dynamics from pump–probe photoelectron signals in nonadiabatic regions where the electronic structure evolves rapidly with geometry. The dependence of these photoelectron angular distributions on relative orientation of the molecule and polarization of the probe pulse are also seen to be potentially useful for real-time monitoring of molecular rotation.

FEMTOSECOND ENERGY- AND ANGLE-RESOLVED PHOTOELECTRON SPECTRA

Abstract We present energy- and angle-resolved photoelectron spectra for femtosecond pump–probe ionization of wavepackets in the 1 Σ u + double-minimum state for aligned Na2. These results illustrate that a robust description of the underlying photoionization amplitudes can significantly enhance the utility of photoelectron spectroscopy as a probe of wavepacket motion and of the evolution of electronic structure, particularly in cases of avoided crossings and motion over large distances. The angular dependence of these photoelectron spectra provide insightful fingerprints of vibrational wavepacket dynamics and can be a useful real-time probe of molecular rotation.

Studies of electron transfer in NaI with pump–probe femtosecond photoelectron spectroscopy

We discuss an extension of our formulation of energy- and angle-resolved photoelectron spectra for femtosecond pump–probe ionization of wave packets to nonadiabatically coupled states and present results of its applications to wave packet motion on the ionic (Na^+I^−) and covalent (NaI) states of sodium iodide. The results of these studies suggest that the energy and angular distributions of these photoelectron spectra provide a useful mapping of the bifurcation of the wave packets through the crossing region and a valuable window on the intramolecular electron transfer occurring between the covalent and ionic states (NaI→Na^+I^−).

Chemical theory beyond the Born-Oppenheimer paradigm : nonadiabatic electronic and nuclear dynamics in chemical reactions

Basic Framework of Theoretical Chemistry Nuclear Dynamics on Adiabatic Electronic Potential Energy Surfaces Breakdown of the Born-Oppenheimer Approximation Classic Theories of Nonadiabatic Transitions and Ideas Behind Direct Observation of the Wavepacket Bifurcation due to Nonadiabatic Transitions Nonadiabatic Electron Wavepacket Dynamics in Path Branching Representation Dynamical Electron Theory for Chemical Reactions Molecular Electron Dynamics in Laser Fields

Controlled Dynamics at an Avoided Crossing Interpreted in Terms of Dynamically Fluctuating Potential Energy Curves

The nonadiabatic nuclear wavepacket dynamics on the coupled two lowest (1)Σ(+) states of the LiF molecule under the action of a control pulse is investigated. The control is achieved by a modulation of the characteristics of the potential energy curves using an infrared field with a cycle duration comparable to the time scale of nuclear dynamics. The transition of population between the states is interpreted on the basis of the coupled nuclear wavepacket dynamics on the effective potential curves, which are transformed from the adiabatic potential curves with use of a diabatic representation that diagonalizes the dipole-moment matrix of the relevant electronic states. The basic feature of the transition dynamics is characterized in terms of the notion of the collision between the dynamical crossing point and nuclear wavepackets running on such modulated potential curves, and the transition amplitude is mainly dominated by the off-diagonal matrix element of the time-independent electronic Hamiltonian in the present diabatic representation. The importance of the geometry dependence of the intrinsic dipole moments as well as of the diabatic coupling potential is illustrated both theoretically and numerically.

Energy- and angle-resolved pump–probe femtosecond photoelectron spectroscopy: Molecular rotation

We have incorporated a classical treatment of molecular rotation into our formulation of energy- and angle-resolved pump–probe photoelectron spectroscopy. This classical treatment provides a useful approach to extracting the photoelectron signal primarily associated with vibrational dynamics in cases where rotational motion is slow and the coupling between rotational and vibrational motion is weak. We illustrate its applicability with pump–probe photoelectron spectra for wave packets on the ^1Σ^+_u double-minimum state of Na_2.

Introductory Lecture. Probing wavepacket dynamics with femtosecond energy- and angle-resolved photoelectron spectroscopy

Several recent studies have demonstrated how well-suited femtosecond time-resolved photoelectron spectra are for mapping wavepacket dynamics in molecular systems. Theoretical studies of femtosecond photoelectron spectra which incorporate a robust description of the underlying photoionization dynamics should enhance the utility of such spectra as a probe of wavepackets and of the evolution of electronic structure. This should be particularly true in regions of avoided crossings where the photoionization amplitudes and electronic structure may evolve rapidly with geometry. In this paper we present the results of studies of energy- and angle-resolved femtosecond photoelectron spectra for wavepackets in the diatomic systems, Na2 and NaI. Both cases involve motion through regions of avoided crossings. In Na2, however, wavepacket motion occurs on a single adiabatic potential with an inner and outer well and a barrier between them, while in NaI wavepackets move on the nonadiabatically coupled covalent (NaI) and ionic (Na+I−) potentials. Results of these studies will be used to illustrate the insight into wavepacket dynamics that time-resolved photoelectron spectra provide. For example, in the case of NaI these angle-resolved photoelectron spectra seem to offer some promise for probing real-time dynamics of intramolecular electron transfer occurring in the crossing region of the ionic and covalent states.

Time-resolved photoelectron spectroscopy of proton transfer in the ground state of chloromalonaldehyde: wave-packet dynamics on effective potential surfaces of reduced dimensionality.

We report on a simple but widely useful method for obtaining time-independent potential surfaces of reduced dimensionality wherein the coupling between reaction and substrate modes is embedded by averaging over an ensemble of classical trajectories. While these classically averaged potentials with their reduced dimensionality should be useful whenever a separation between reaction and substrate modes is meaningful, their use brings about significant simplification in studies of time-resolved photoelectron spectra in polyatomic systems where full-dimensional studies of skeletal and photoelectron dynamics can be prohibitive. Here we report on the use of these effective potentials in the studies of dump-probe photoelectron spectra of intramolecular proton transfer in chloromalonaldehyde. In these applications the effective potentials should provide a more realistic description of proton-substrate couplings than the sudden or adiabatic approximations commonly employed in studies of proton transfer. The resulting time-dependent photoelectron signals, obtained here assuming a constant value of the photoelectron matrix element for ionization of the wave packet, are seen to track the proton transfer.

Probing wavepacket dynamics with femtosecond energy- and angle-resolved photoelectron spectroscopy

Several recent studies have demonstrated how well-suited femtosecond time-resolved photoelectron spectra are for mapping wavepacket dynamics in molecular systems. Theoretical studies of femtosecond photoelectron spectra which incorporate a robust description of the underlying photoionization dynamics should enhance the utility of such spectra as a probe of wavepackets and of the evolution of electronic structure. This should be particularly true in regions of avoided crossings where the photoionization amplitudes and electronic structure may evolve rapidly with geometry. In this paper we present the results of studies of energy- and angle-resolved femtosecond photoelectron spectra for wavepackets in the diatomic systems, Na2 and NaI. Both cases involve motion through regions of avoided crossings. In Na2, however, wavepacket motion occurs on a single adiabatic potential with an inner and outer well and a barrier between them, while in NaI wavepackets move on the nonadiabatically coupled covalent (NaI) and ionic (Na+I-) potentials. Results of these studies will be used to illustrate the insight into wavepacket dynamics that time-resolved photoelectron spectra provide. For example, in the case of NaI these angle-resolved photoelectron spectra seem to offer some promise for probing real-time dynamics of intramolecular electron transfer occurring in the crossing region of the ionic and covalent states.

Communication: Induced photoemission from nonadiabatic dynamics assisted by dynamical Stark effect

Through nonadiabatic interaction due to electron transfer as that in alkali halides, vibrational dynamics on the ionic potential energy surface (large dipole moment) is coupled to that on the covalent surface (small dipole moment). Thus, population transfer between the states should cause long-range electron jump between two remote sites, which thereby leads to a sudden change of the large molecular dipole moment. Therefore, by making repeated use of the dynamical Stark effect, one may expect emission of photons from it. We show with coupled quantum wavepacket dynamics calculation that such photoemission can indeed occur and can be controlled by an external field. The present photoemission can offer an alternative scheme to study femtosecond and subfemtosecond vibrational and electronic dynamics and may serve as a unique optical source.