Attosecond probing

In this section, by applying the heterodyne interferometry to a mixed gas of H2 and D2 molecules, we probe attosecond dynamics of nuclear wavepackets in the molecules. We find that not only the single molecule responses but also the propagation effects of harmonics differ between the two isotopes and that to discuss dynamics of molecules in the single molecule responses, the propagation effects need to be excluded from the raw harmonic signals. The measured relative phase as well as intensity ratio are found to be monotonic functions of the harmonic order and are successfully reproduced by applying... 

We illustrate the use of multichannel single-ionization scattering states for the interpretation of time-resolved experiments in the case of the attosecond-XUV-pump IR-probe attosecond interferometric spectroscopy of the doubly... 

So far we have considered the absorption of an XUV pump pulse with attosecond extension. The next step in the simulation is to apply a probe pulse. During the time between the pump and the probe, the system undergoes two different kinds of temporal evolution. One is the semiperiodic... 

K. Kliinder, J.M. Dahlstrom, M. Gisselbrecht, T. Fordell, M. Swoboda, D. Guenot, et al., Probing single-photon ionization on the attosecond time scale, Phys. Rev. Lett. 106 (14) 143002. 

J. Mauritsson, T. Remetter, M. Swoboda, K. Kliinder, A. L HuiUier, K.J. Schafer, et al., Attosecond electron spectroscopy using a novel interferometric pump-probe technique, Phys. Rev. Lett. 105 (2010) 053001. 

N. Choi, T. Jiang, T. Morishita, M.-H. Lee, C.D. Lin, Theory of probing attosecond electron wave packets via two-path interference of angle-resolved photoelectrons, Phys. Rev. A 82 (2010) 013409. 

Nordlund D, Ogasawara H, Bluhm H, Takahashi O, OdeHus M, Nagasono M, Pettersson LGM, Nilsson A. (2007) Probing the electron delocalization in liquid water and ice at attosecond time scales. Phys Rev iMt 99 217406. 

The pump-probe pulses are obtained by splitting a femtosecond pulse into two equal pulses for one-color experiments, or by frequency converting a part of the output to the ultraviolet region for bichromatic measurements. The relative time delay of the two pulses is adjusted by a computer-controlled stepping motor. Petek and coworkers have developed interferometric time-resolved 2PPE spectroscopy in which the delay time of the pulses is controlled by a piezo stage with a resolution of 50 attoseconds [14]. This set-up made it possible to probe decoherence times of electronic excitations at solid surfaces. 

Probing ofthe electron in water at attosecond time scale D. Nordlund etal. [34]... 

Femtochemistry has enabled chemical kinetics to progress from descriptions in terms of rather vague concepts such as activation and transition state to a much more detailed picture of molecules in the act of crossing energy barriers. Nevertheless, it does not yet mark the end of the road on which Wilhelmy, Arrhenius and others set out in the nineteenth century. Developments in laser technology will eventually lead to shorter pulses lasting not femtoseconds, but attoseconds (1 as = 1 X 10 s). Thus even more powerful techniques will become available to probe what actually happens in the act of chemical transformation as chemical bonds are broken and new ones are formed. 

The validity of the physics that adopts the point of view of decaying states depends on the characteristics of the process of excitation-preparation. Specifically, one must assume that the duration of the pulse of excitation energy is much shorter than the lifetime of the unstable state. This implies that indeed the system is prepared in a nonstationary state at f = 0, i.e., in the localized state (T o/ Eo)/ while losing memory of the excitation step. For long-lived unstable states, this is expected to be achievable easily. For shortlived unstable atomic or molecular states, say of the order of 10 s, this is also achievable, in principle, via modern pump-probe techniques with time-delays in the range of a few femtoseconds or of a couple of hundreds of attoseconds. 

For sub-femtosecond pulses a new techniques has been developed which is called CRAB complete reconstruction of attosecond bursts). Instead of using a nonlinear interaction for generating the signal, a weak femtosecond pulse is used to probe the attosecond pulse by measuring the energy spectrum of photo-electrons produced by photo-ionization of atoms by the attosecond pulse (see Fig. 6.59 and Sect. 6.2.5). 

Using such short attosecond XUV pulses, the temporal evolution of the Auger process after inner shell excitation can be followed with the pump-and-probe technique. 

The time of excitation to the Franck-Condon state is approximately 1 fs (10 s). Thus, these techniques will freeze out any equilibrium that involves the actual movement of nuclei, such as a cis-trans interconversion. Electron exchange can, however, be faster than a vibration. These very fast electron transfer processes can be probed by exposing a sample to laser radiation with a pulse shorter than 10 s, i.e., the realms of femtosecond (10 s) and attosecond (10 s) spectroscopy. ... 

As a fundamental study on field induced chemical reactions, Neidel and Vrakking et al. observed attosecond d3mamics of electrons in a series of small- and medium sized neutral molecules by monitoring time-dependent variation of the parent molecular ion 3delds [296]. The information on electron dynamics was extracted from experimental data on the basis of the relation between the time dependent dipole and ionization. This was performed in the two-color femtosecond near infrared (NIR) pump-attosecond extreme ultraviolet (XUV) probe experiment. They claim that the time-dependent dipole induced by the moderately strong NIR pulse field is monitored with attosecond time resolution. The oscillations are interpreted in terms of a time dependent screening induced by the polarization of the molecule, which alters the photoionization yield of the neutral molecule. This scheme can be considered as the first example of molecular attosecond Stark spectroscopy. 

While the electron wavefunction can be used to obtain the energy and other properties of the electron, the question arises, in quantum mechanics generally, as to what the wavefunction itself means . This has been, and still is, the subject of much debate and there is currently intense research activity into using attosecond spectroscopy to probe atomic wavefimctions [16]. The most useful interpretation of the wavefunction for chemistry is that due to Born, who, by analogy to a light wave, where the intensity is proportional to the square of the amplimde, suggested... 

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