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CRC Press eBooks

Applications of Attosecond Pulses

Zenghu Chang

Published Apr 19, 2016• 0 citations
Publisher

Abstract

If another linearly polarized attosecond pulse (the probe pulse) centered at 100 eV doubly ionizes the helium atom in the mixed state as illustrated in the right graph of Figure 9.1, the ionization probability depends on the time delay between the pump and the probe pulses. The probability is higher when the two electrons are located near each other, as depicted in Figure 9.2. Experimentally, one can measure the doubly ionized helium ion (He2þ) signal as function of time delay between the probe pulse and the pump pulse. The He2þ yield as a function of the time delay would inform us how the electron separation changes with time. Hu and Collins’s simulation shows that the separation changes periodically with a 2 fs cycle, which can be observed with the attosecond pulses. The cycle is much larger than the 200 as corresponding to the energy difference between the ground state and the first excited state (1s2p) because other excited states are involved.

DOI
10.1201/b10402-14

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If another linearly polarized attosecond pulse (the probe pulse) centered at 100 eV doubly ionizes the helium atom in the mixed state as illustrated in the right graph of Figure 9.1, the ionization probability depends on the time delay between the pump and the probe pulses. The probability is higher when the two electrons are located near each other, as depicted in Figure 9.2. Experimentally, one can measure the doubly ionized helium ion (He2þ) signal as function of time delay between the probe pulse and the pump pulse. The He2þ yield as a function of the time delay would inform us how the electron separation changes with time. Hu and Collins’s simulation shows that the separation changes periodically with a 2 fs cycle, which can be observed with the attosecond pulses. The cycle is much larger than the 200 as corresponding to the energy difference between the ground state and the first excited state (1s2p) because other excited states are involved.
Metastable states are important actors in the ionisation of atoms and molecules. Sub-femtosecond extreme ultraviolet pulses can coherently populate several transiently bound states at once, thus starting the attosecond clocks which are required to monitor and control ultrafast electronic evolution above the ionisation threshold. Here we illustrate, from a theoretical point of view, the effects coherent superpositions of 1 P o doubly excited states in the helium atom have on channel-resolved photoelectron spectra as well as on the transient absorption spectrum of the atom in the extreme ultraviolet region, when they are created by a single-attosecond pulse in the presence of a strong few-cycle near-infrared/visible pulse which acts as a probe. Interference fringes varying rapidly with the pump-probe time delay are visible in both photoelectron and transient absorption spectra. From such fringes, the wave packet itself can conceivably be reconstructed. Conversely, all observables are modulated by the characteristic beating periods of the wave packet, so that control of partial ionisation yields, branching ratios, and light absorption or amplification can be achieved.
Based on the numerical solutions of the time-dependent Schrödinger equation within the single-active-electron approximation, we propose a method for observing instantaneous atomic level shifts in an oscillating strong infrared (IR) field with sub-IR-cycle time resolution, by using a single tunable attosecond (SA) pulse to probe excited states of the perturbed atom. The ionization probability in the combined fields depends on both the frequency of the attosecond pulse and the time delay between both pulses, since the IR field periodically shifts SA-pulse-excited energy levels into and out of resonance.
The strong coupling between two autoionizing states in helium is studied theoretically with the pump-probe scheme. An isolated 100-as XUV pulse is used to excite helium near the 2s2p(1P)2s2p{(}^{1}\phantom{\rule{0.16em}{0ex}}P)2s2p(1P) resonance state in the presence of an intense infrared (IR) laser. The laser field introduces strong coupling between 2s2p(1P)2s2p{(}^{1}\phantom{\rule{0.16em}{0ex}}P)2s2p(1P) and 2p2(1S)2{p}^{2}{(}^{1}\phantom{\rule{0.16em}{0ex}}S)2p2(1S) states. The IR also can ionize helium from both autoionizing states. By changing the time delay between the XUV and the IR pulses, we investigated the photoelectron spectra near the two resonances. The results are used to explain the recent experiment by Gilbertson et al. [Phys. Rev. Lett. 105, 263003 (2010)]. Using the same isolated attosecond pulse and a 540-nm laser, we also investigate the strong coupling between 2s2p(1P)2s2p{(}^{1}\phantom{\rule{0.16em}{0ex}}P)2s2p(1P) and 2s2(1S)2{s}^{2}{(}^{1}\phantom{\rule{0.16em}{0ex}}S)2s2(1S) by examining how the photoelectron spectra are modified versus the time delay and the possibility of observing Autler-Townes doublet in such experiments.
We investigate the response of two three-body Coulomb systems when driven by attosecond half-cycle pulses: The electron transfer between two hydrogen atomic centers and the double ionization of the helium atom. Using very short half-cycle pulses (HCPs) which effectively deliver ``kicks'' to the electrons, we first study how a carefully chosen sequence of HCPs can be used to control to which of one of the two fixed atomic centers the electron gets reattached (HCP assisted electron transfer). Moving from one electron in two atomic centers to two electrons in one atomic center we then study the double ionization from the ground state of He by a sequence of attosecond time-scale HCPs, with each electron receiving effectively a ``kick'' from each HCP. We investigate how the net electric field of the sequence of HCPs affects the total and differential ionization probabilities.
The attosecond pump probe, in close analogy to the standard femtosecond probing technique, has been proposed and theoretically demonstrated with its application to explore ultrafast electron motions inside atoms. We have performed realistic modeling for the full dynamics of both the femtosecond pumping and the attosecond probing processes. Our simulations have illustrated that an ultrashort oscillation period of 2.0 fs can be mapped out for a wave packet in low-lying excited states of the helium atom. This opens the prospect of a wealth of similar pump probe experiments to examine ultrafast electronic or atomic motions.