Strong laser fields

Vibrational and Electronic Excitation of Molecules by Short-Pulse Strong Laser Fields... 

The interaction of even simple diatomic molecules with strong laser fields is considerably more complicated than the interaction with atoms. In atoms, nearly all of the observed phenomena can be explained with a simple three-step model [1], at least in the tunneling regime (1) The laser field releases the least bound electron through tunneling ionization (2) the free electron evolves in the laser field and (3) under certain conditions, the electron can return to the vicinity of the ion core, and either collisionally ionize a second electron [2], scatter off the core and gain additional kinetic energy [3], or recombine with the core and produce a harmonic photon [4]. 

The vibrational and electronic excitation of molecules has received less attention over the years, but the understanding of excitation processes is important for a number of reasons. Thus, in this paper, we will review the various mechanisms leading to vibrational and/or electronic excitation of molecules in strong laser fields. As we do this, it will become clear that exploring these mechanisms (1) reveals new features of the strong field interaction ... 

The simplest and first observation of the excitation of molecules by strong laser fields was the identification with TOF spectroscopy of the following dissociation channel —> I2+ + I°+ [31]. In this example, a short laser... 

Lastly, we mention one more excitation mechanism that has been observed in molecules. It is well-established that following strong field ionization in atoms and molecules, under certain conditions, the ionized electron can be driven back to the ion core where it can recombine to produce high-harmonic radiation, induce further ionization, or experience inelastic scattering. However, there is also the possibility of collisional excitation. Such excitation was observed in [43] in N2 and O2. In both molecules, one electron is tunnel ionized by the strong laser field. When the electron rescatters with the ion core, it can collisionally ionize and excite the molecular ion, creating either N + or Ol+ in an excited state. When the double ion dissociates, its initial state can... 

While excitation processes in strong laser fields have not received as much attention as ionization rates and high-harmonic generation, the study of excitation is an important aspect of our overall understanding of the behavior of molecules in strong laser fields. 

Important aspects of the interaction of strong laser fields with molecules can be missed in standard TOF experiments, most notably the population of electronically excited states. However, by studying vibrational excitation, the frequency and dephasing of the vibrational motion can be used to identify the electronic state undergoing the vibrational motion. In some cases, this turns out to be a ground state, and in others, an excited state. Once we have identified an excited state, we are left with the question of how and why the state was populated by the strong field. In one example above (the Ij A state discussed in Sect. 1.3.3), the excited state is formed by the removal of an inner orbital electron, in this case a iru electron. This correlates with the measured angular dependence for the ionization to this state. 

In this contribution recent results [13] on the control of the quantum mechanical phase of an atomic state in strong laser fields studied using the Autler-Townes (AT) effect [14] in the photoionization of the K (4p) state are discussed. We demonstrate quantum control beyond (i) population control and (ii) spectral interference, (i) We show, that for suitable combinations of the laser intensity of the first pulse and the time delay the second resonant intense laser pulse leaves the excited state population unchanged. However, the knowledge of the... 

One point which has not been addressed in the example of the time-independent harmonic oscillator is the non-perturbative treatment of the time dependence in the system Hamiltonians. Both the TL and the TNL non-Markovian theories employ auxiliary operators or density matrices, respectively, and can be applied in strongly driven systems [29,32]. This point will be shown to be very important in the examples for the molecular wires under the influence of strong laser fields. 

Figure 12.2 Effect of a strong laser field on the line shape for dissociation of an intermediate level at four hv2 IR frequencies and at two intensities of the IR laser. The spectrum on the hy2 axis (left-hand side) is computed IBr absorption spectrum in weak-field limit, starting from 960cm l above the intermediate level (which is 16,333.03 cm-1 above ground vibrational level). We see that broadening of the 16,333.03 cm-1 line occurs for I = 109 W/cift2.s whenever the hv2 photon is in near resonance with a strong predissocating line. (Taken from Fig. 2, Ref [388].) If...

The formalism developed so far is adequate whenever the motion of the atomic nuclei can be neglected. Then the electron-nucleus interaction only enters as a static contribution to the potential r(r, t) in Eq. (41). This is a good approximation for atoms in strong laser fields above the infrared frequency regime. When the nuclei are allowed to move, the nuclear motion couples dynamically to the electronic motion and the situation becomes more complicated. 

Theoretical study of the F + H2 reaction in the pres- 719 ence of a strong laser field... 

Multiphoton resonant processes with simplest fundamental quantum systems exposed to sufficiently strong laser fields attracted conspicuous attention over last years. Currently, this interest is being especially strongly stimulated by dramatic improvements in the precision of measurements presently attainable in spectroscopic experimental studies of hydrogenic and few-particle atoms. Using methods of ultra high precision Doppler-free spectroscopy, particularly impressive results have been recently obtained in studies of fundamental bounded systems such as hydrogen (H) and its natural isotopes deuterium (D) and tritium (T) [1,2,3,4,5,6,7], positronium [8,9], denoted Ps = (e+ — e ), muonium [10,11,9,12,13,14,15], denoted (M = — e ), and the helium atom (He) [16[... 

We remark that this effective Hamiltonian (190) constructed by the combination of a partitioning of the Floquet Hamiltonian, a two-photon RWT, and a final 0-averaging can be seen as a two-photon RWA, which extends the usual (one-photon) RWA [39,40], We have thus rederived a well-known result, using stationary techniques that allow us to estimate easily the order of the neglected terms. This method allows us also to calculate higher order corrections. We apply it in the next subsection to calculate effective Hamiltonians for molecules illuminated by strong laser fields. 

To determine an effective dressed Hamiltonian characterizing a molecule excited by strong laser fields, we have to apply the standard construction of the free effective Hamiltonian (such as the Born-Oppenheimer approximation), taking into account the interaction with the field nonperturbatively (if resonances occur). This leads to four different time scales in general (i) for the motion of the electrons, (ii) for the vibrations of the nuclei, (iii) for the rotation of the nuclei, and (iv) for the frequency of the interacting field. It is well known that it is a good strategy to take into account the time scales from the fastest to the slowest one. 

Consider the Menon-Agarwal approach to the Autler-Townes spectrum of a V-type three-level atom. The atom is composed of two excited states, 1) and 3), and the ground state 2) coupled by transition dipole moments with matrix elements p12 and p32, but with no dipole coupling between the excited states. The excited states are separated in frequency by A. The spontaneous emission rates from 1) and 3) to the ground state 2) are Tj and T2, respectively. The atom is driven by a strong laser field of the Rabi frequency il, coupled solely to the 1) —> 2) transition. This is a crucial assumption, which would be difficult to realize in practice since quantum interference requires almost parallel dipole moments. However, the difficulty can be overcome in atomic systems with specific selection rules for the transition dipole moments, or by applying fields with specific polarization properties [26]. 

An alternative method in which one could create a V-type system with parallel or antiparallel dipole moments is to apply a strong laser field to one of the two transitions in a A-type atom. The scheme is shown in Fig. 18. When the dipole moments of the 11) —> 3) and 2) —> 3) transitions are perpendicular, the laser exclusively couples to the 2) —> 3) transition and produces dressed states... 

Atoms and Molecules in Strong Laser Fields By F. Grossmann... 

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