Femtosecond time-resolved photoelectron spectroscopy

It is noted that the approximations given above virtually reduce the costs of explicit pump-probe simulations to the costs of a standard time-dependent wave-packet propagation. Apart from interpretative purposes, the Franck-Condon formulation therefore cuts the computational effort considerably. Since the theory requires no explicit discretization of a continuum of electronic states, this is particularly true for the description of femtosecond time-resolved photoelectron spectroscopy. 

Although in the case of pyrazine the interpretation is complicated by cross-term signals and auto-ionizing Rydberg states, the experimental and theoretical investigations nevertheless demonstrate the considerable potential of femtosecond time-resolved photoelectron spectroscopy. As a beautiful example of a more favorable case, Stolow and coworkers have recently presented time-resolved ionization studies on decatetraene. In direct analogy to the discussion presented above, they managed to monitor the S2 Si internal conversion associated with a conical intersection in real time. 

In this section we present the formulation and computation of femtosecond time-resolved photoelectron spectroscopy employing geometry-and energy-dependent photoionization amplitudes [15]. 

Nuclear wavepacket description of femtosecond time-resolved photoelectron spectroscopy... 

In conclusion, the realm for femtosecond time-resolved photoelectron spectroscopy to cover has been extended such that it is applicable to simultaneous electronic and vibrational dynamics in molecules. 

The [2.2]paracyclophane derivative (20) has been employed as model system in which formyl and vinyl groups are arranged in a pre-reactive conformation for the Paterno-Buchi reaction. Using femtosecond time-resolved photoelectron spectroscopy and ab initio calculations, it has been found that intersystem crossing occurs within two picoseconds, and... 

Stolow, A. (2003a). Femtosecond time-resolved photoelectron spectroscopy of polyatomic molecules. Ann. Rev. Phys. Chem. 54, 89. 

In addition to the natural improvements expected in the accuracy of the measurements, and the increased scope in the types of systems examined, new techniques go beyond the issue of thermochemistry to allow for very detailed studies of reaction dynamics. The investigation by Zewail and co-workers of the reactivity of planar COT" on the femtosecond time scale is likely only the beginning. Time-resolved photoelectron spectroscopy, for example, has recently been used to map the potential energy surfaces for the dissociation of simple ions IBr and l2. " Although applications in the field of organic reactive molecules are likely far off, they are now possible. 

D. M. Neumark We are currently carrying out somewhat different femtosecond experiments in which time-resolved photoelectron spectroscopy is used to probe the photodissociation dynamics of negative ions. In these experiments, an anion is photodissociated with a femtosecond laser pulse. After a time delay, the dissociating anion is pho-todetached with a second femtosecond pulse and the resulting photoelectron spectrum is measured. The photoelectron spectrum as a function of delay time provides a detailed probe of the anion photodissociation dynamics. First results have recently been obtained for the photodissociation of I2. 

Another approach of great importance for studies of excited state dynamics is sub-picosecond time resolved spectroscopy. A number of authors have reported femtosecond pump-probe measurements of excited state lifetimes in A, C, T, and G [13-16] and base pair mimics [17]. Schultz et al. have reported time resolved photoelectron spectroscopy and electron-ion coincidence of base pair mimics [18]. these studies can also be compared with similar measurements in solution [19-24], While time resolved measurements provide direct lifetime data, they do have the limitation that the inherent bandwidth reduces the spectral resolution, required for selecting specific electronic states and for selecting single isomers, such as cluster structure and tautomeric form. 

In Chapter 5, we have studied some of the effects of laser fields on chemical dynamics. In particular, we have investigated how time-resolved photoelectron spectroscopy can be used as a very good means to monitor the femtosecond-scale nuclear dynamics such as the passage across nonadia-batic regions. The modulation of nonadiabatic interactions (both avoided crossing and conical intersection) is also among the main subjects from the view point of control of chemical reaction. Chapter 7, on the other hand, has treated nonadiabatic electron wavepacket dynamics relevant to chemical reactions. Here in this chapter, we therefore rise to the theory of electron dynamics in laser fields mainly associated with chemical dynamics. 

A qualitatively different approach to probing multiple pathways is to interrogate the reaction intermediates directly, while they are following different pathways on the PES, using femtosecond time-resolved pump-probe spectroscopy [19]. In this case, the pump laser initiates the reaction, while the probe laser measures absorption, excites fluorescence, induces ionization, or creates some other observable that selectively probes each reaction pathway. For example, the ion states produced upon photoionization of a neutral species depend on the Franck-Condon overlap between the nuclear configuration of the neutral and the various ion states available. Photoelectron spectroscopy is a sensitive probe of the structural differences between neutrals and cations. If the structure and energetics of the ion states are well determined and sufficiently diverse in... 

Structure, then the time-resolved photoelectron spectra [20, 21] could reveal signatures of two different intermediate structures, representing two different pathways on the PES. Transient absorption spectroscopy and other femtosecond time-resolved techniques may also be applicable to this problem. 

D. R. Cyr and C. C. Hayden, /. Chem. Phys., 104,771 (1996). Femtosecond Time-Resolved Photoionization and Photoelectron-Spectroscopy Studies of Ultrafast Internal Conversion in 1,3,5-Hexatriene. 

When considering the femtosecond photoionization dynamics of complex systems, a completely exact evaluation of the time and energy resolved photoelectron spectrum is often not really necessary. Approximative schemes which require significantly lower computational effort are valuable in such cases. Within the nonperturbative formalism, Meier et al. have proposed an efficient computational scheme which incorporates the multi-configuration time-dependent Hartree method.An approximate method which is based on a classical-trajectory description of the nuclear dynamics has been elaborated by Hartmann, Heidenreich, Bonacic-Koutecky and coworkers and applied, among other systems,to the time-resolved photoionization spectroscopy of conical intersections in sodium fluoride clusters. 

To illustrate the practical application of the theoretical formalism, we have discussed several representative examples that demonstrate the potential of femtosecond time-resolved spectroscopy. Since typically only a few degrees of freedom (say, 2-4) are involved in the ultrafast photodynamics via a conical intersection, explicit simulations of spectroscopic experiments for polyatomic molecules are nowadays possible on a routine basis. As an example of a state-of-the-art simulation of a femtosecond experiment on a polyatomic system, we have presented first-principles calculations of the time-resolved photoelectron spectra of pyrazine. Although in this case the interpretation was found to be quite complicated, the investigations nevertheless demonstrated the considerable potential of a joint experimental/theoretical study of the dynamics at conical intersections. Employing... 

I. Fischer, M.J.J. Vrakking, D.M. Villeneuve, and A. Stolow, Femtosecond Time-Resolved Zero Kinetic Energy Photoelectron and Photoionization Spectroscopy Studies of I2 Wave Packet Dynamics , Chem. Phys. 207, 331 (1996). 

In this chapter we describe advances in the femtosecond time-resolved multiphoton photoemission spectroscopy (TR-MPP) as a method for probing electronic structure and ultrafast interfacial charge transfer dynamics of adsorbate-covered solid surfaces. The focus is on surface science-based approaches that combine ultrafast optical pump probe excitation to induce nonlinear multi-photon photoemission (MPP) from clean or adsorbate covered single crystal surfaces. The photoemitted electrons transmit spectroscopic and dynamical information, which is captured by their energy analysis in real or reciprocal space. We examine how photoelectron spectroscopy and microscopy yield information on the unoccupied molecular structure, electron transfer and relaxation processes, light induced chemical and physical transformations and the evolution of coherent single particle and collective excitations at solid surfaces. 

To investigate possible detection of the history of sequential transitions between electronic states by a pulse train with time-resolved femtosecond photoelectron spectroscopy, we compute the quantum wavepacket dynamics of the LiH system. The LiH system is suited for our purpose of studying time evolution of electronic interaction in that (1) the lowest excited state... 

Fig. 6.16. Probing the transient electronic structure of TbTes in the course of its ultrafast insulator-to-metal transition induced by femtosecond-laser excitation (a) Time- and angle-resolved photoemission spectroscopy. A TbTcs sample was excited by an IR pulse (hi Pump = I.SeV, about 50 fs duration) and probed after a time delay At with a UV pulse (hi/pump = 6 eV, about 90 fs duration). The photoelectron intensity and kinetic energy E ,i were measured as a function of the emission angles (a, 9). (b) Insulator-to-metal transition Above the critical temperature Tc (or 100fsafterlaserexcitation)the band gap of the CDW phase closes, (c) "Snapshots" of the electronic band structure E(k)in TbTej fordifferenttimedelaysAt.Afterlaserexcitation, the gap has closed and the band dispersion near the Eermi level, Ep, changed after a time delay of 100 fs. Such a delayed collapse of the band gap is characteristic of the "Peierls type" mechanism (see text).

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