Atomic ionization, time-resolved

The second form of ionization is similar to autoionization.31 In a nonhydrogenic atom is not a good quantum number and bound states of high nt are coupled to Stark continua of low n j. This form of ionization applies to those states other than the reddest Stark states. The extreme red Stark states have nx = 0 and ionize, as do the red H states, at the classical ionization limit given by Eq. (6.35), modified by Eq. (6.37) for m 0 states. This point has been demonstrated explicitly by Littman et al,32 who measured the time resolved ionization of Na m = 2 states subsequent to pulsed laser excitation. Their measured rates are in good agreement with the rates obtained by extrapolation of the rates of Bailey et al.20 shown in Fig. 6.8. 

In the second part, we discuss possible applications of attosecond laser pulses to future studies of time-resolved electron dynamics in strongly driven systems. We discuss our current understanding of the time-dependent behaviour of non-perturbatively driven electrons in atoms, molecules and clusters. In Sect. 3.4 we discuss differences that arise when the generation of attosecond pulses is performed in different atomic media. This is followed in Sect. 3.5 by a description of the role of electron dynamics in dynamical alignment and enhanced ionization of molecules. Finally, in Sect. 3.6 the role of electron dynamics in laser heating of large clusters is discussed. 

In this chapter, we turn to problems of quantum chemistry and of many-electron atomic and molecular physics for which fhe desideratum is the quantitative knowledge and easy conceptual understanding of dynamical processes and phenomena thaf depend explicifly on time. We focus on a theoretical and computational approach which computes q>(q,t) by solving nonperturbatively the many-electron TDSE for unstable states of atoms and small molecules. The time evolution of fhese states is caused either by the time-independent Hamiltonian, Ham ( -g-/ case of time-resolved autoionization—see below) or by the time-dependent Hamiltonian, H t) = Ham + Vext(f), where Vext(f) is the sum of the identical one-electron operators that couple the field of a strong pulse of radiation to the electronic and nuclear moments of N-electron atomic or molecular states of inferest, thereby producing, during and at the end of the interaction, final stafes in the ionization or the dissociation continua. 

To illustrate atomic dynamics, which require subfemtosecond resolution. Fig. 6.58 shows the time-resolved field ionization of neon atoms in the optical field of a 5 fs laser pulse which consists of only three optical cycles within the pulse half width [750]. The whole process proceeds within about 6 fs, but one can clearly see peaks in the ionization probability at times of maxima in the optical field, which means that the time resolution is below 1 fs. 

The study of Rydberg spectra and ionization thresholds of ten lanthanides and actinides has been reported by PAISNER et al. [8.80]. In these experiments, high-lying states were accessed by time-resolved stepwise excitation using pulsed dye lasers tuned to resonant transitions. Atoms excited to levels within 1000 cm of the ionization limit were then photoionized by 10.6 ym radiation from a pulsed CO2 laser. The measurements allowed the accurate determination of ionization thresholds from Rydberg convergences to within 0.0005 eV. 

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