7 Ultrafast Laser-Induced Processes Described by Ab Initio Molecular
159
7.4 Examples of Laser-Induced Dynamics
The simulation of laser interactions is not trivial within the SH scheme. Although
the first developments date back to the 1990s [66–68], few applications can be found
in the literature until very recently. New implementations [70–72, 74] now make it
possible to fulfill a long-time dream: Unraveling ultrafast laser-modulated molecular
processes including all internal degrees of freedom.
When considering laser interactions, two extreme cases can be defined: The impulsive regime and the adiabatic regime. An ultrafast photoexcitation is referred to
as impulsive if its time scale is short compared with a considered molecular event
e.g. a vibrational period (τ laser < τ vib ) [98]. A pictorial description is that the oscillator is kicked on a time scale shorter than its response time. In contrast, the
oscillator can accommodate itself slowly to the perturbation of the laser in the adiabatic limit. In this case, the time scale of the electromagnetic interaction is long
compared to the vibrational period (τ laser > τ vib ). In some cases, not the time scale
of the whole interaction but the time that is needed to switch on the laser—until the
maximum intensity is reached—is classified under the above criteria.
In the following, we present some examples of ab initio MD simulations employing SHARC which are attributed to the two categories just described.
7.4.1 Impulsive Regime
As a first scenario, we shall use SHARC to treat spin-orbit and strong laser-field
induced couplings at the same time. As in Sect. 7.3, we use the IBr molecule as a
model system and QD calculations for comparison. However, we now incorporate
all laser interactions explicitly in the simulations. As an extension to the model presented above, we shall also exert control on IBr via the non-resonant dynamic Stark
effect (NRDSE), as originally implemented experimentally by Sussman et al. [19].
The Stark effect is produced when an electric field energetically shifts the potentials of a molecule. If the field is time dependent, the process is called dynamic
Stark effect [99]. Especially interesting for control is the NRDSE case mentioned
above when the frequency of the time-dependent electric field does not induce a
one-photon transition but only induces potential shifts. Therefore, the effect is frequently described as a photonic catalysis [19, 42, 81–83, 86]. In order to influence
the potentials significantly, the field strength is chosen as intermediate, compared to
strong fields which would ionize the molecule, or weak fields which would introduce only a small perturbation.
In IBr, the control field is able to change the ratio Q (see Sect. 7.3) between the
possible dissociation products after photoexcitation. The latter is induced by a first
excitation pulse E e (t). The subsequent dynamics is then influenced by an infrared
control pulse E c (t). We extend the model employed in Sect. 7.3 by using realistic
polarizabilites α and dipole moments μ as given in Ref. [95].
The excitation pulse has a wavelength of 493.4 nm, a full width at half maximum of the Gaussian-shaped envelope of 50 fs and an intensity of 10 TW/cm 2 .
159
7.4 Examples of Laser-Induced Dynamics
The simulation of laser interactions is not trivial within the SH scheme. Although
the first developments date back to the 1990s [66–68], few applications can be found
in the literature until very recently. New implementations [70–72, 74] now make it
possible to fulfill a long-time dream: Unraveling ultrafast laser-modulated molecular
processes including all internal degrees of freedom.
When considering laser interactions, two extreme cases can be defined: The impulsive regime and the adiabatic regime. An ultrafast photoexcitation is referred to
as impulsive if its time scale is short compared with a considered molecular event
e.g. a vibrational period (τ laser < τ vib ) [98]. A pictorial description is that the oscillator is kicked on a time scale shorter than its response time. In contrast, the
oscillator can accommodate itself slowly to the perturbation of the laser in the adiabatic limit. In this case, the time scale of the electromagnetic interaction is long
compared to the vibrational period (τ laser > τ vib ). In some cases, not the time scale
of the whole interaction but the time that is needed to switch on the laser—until the
maximum intensity is reached—is classified under the above criteria.
In the following, we present some examples of ab initio MD simulations employing SHARC which are attributed to the two categories just described.
7.4.1 Impulsive Regime
As a first scenario, we shall use SHARC to treat spin-orbit and strong laser-field
induced couplings at the same time. As in Sect. 7.3, we use the IBr molecule as a
model system and QD calculations for comparison. However, we now incorporate
all laser interactions explicitly in the simulations. As an extension to the model presented above, we shall also exert control on IBr via the non-resonant dynamic Stark
effect (NRDSE), as originally implemented experimentally by Sussman et al. [19].
The Stark effect is produced when an electric field energetically shifts the potentials of a molecule. If the field is time dependent, the process is called dynamic
Stark effect [99]. Especially interesting for control is the NRDSE case mentioned
above when the frequency of the time-dependent electric field does not induce a
one-photon transition but only induces potential shifts. Therefore, the effect is frequently described as a photonic catalysis [19, 42, 81–83, 86]. In order to influence
the potentials significantly, the field strength is chosen as intermediate, compared to
strong fields which would ionize the molecule, or weak fields which would introduce only a small perturbation.
In IBr, the control field is able to change the ratio Q (see Sect. 7.3) between the
possible dissociation products after photoexcitation. The latter is induced by a first
excitation pulse E e (t). The subsequent dynamics is then influenced by an infrared
control pulse E c (t). We extend the model employed in Sect. 7.3 by using realistic
polarizabilites α and dipole moments μ as given in Ref. [95].
The excitation pulse has a wavelength of 493.4 nm, a full width at half maximum of the Gaussian-shaped envelope of 50 fs and an intensity of 10 TW/cm 2 .