Demos.MolElectronic.HCl+
For the example of HCl cation, the effect of extremely short and intense pulses on a (sub-)femtosecond timescale is studied. Emphasis is on the validity of the (Floquet) picture of photon dressed states in the limit of pulses encompassing fewer and fewer optical cycles.
Model
All simulations are based on high level ab initio calculations published in of work by A. D. Pradhan, K. P. Kirby, and A.Dalgarno. In particular, we use tabulated data of the following functions:
Ultrafast dynamics of the electronic excitation of diatomic molecules under the control of a femtosecond shaped light pulses is demonstrated for the (non-rotating) HCl cation . The parameters of a sin2-shaped light pulse (ω=0.1295 Eh/ħ, perpendicular polarization) have been chosen such that only the A-state becomes populated. For further details of the laser parameters, see our work in Comp. Phys. Comm.. In the following, the effect of very intense pulses that are very short in comparison with the optical cycle (2π/ω=48.5 h/Eh=1.17 fs) are considered.
Pulse duration of 3 fs
In the following, results of simulations using bare states (upper panel) versus those using 7 dressed states (lower panel) shall be compared. Despite of the very short duration of the pulse (less than three optical cycles!), the overall population dynamics is captured correctly in the Floquet picture, in particular the populations at the end of the pulse are in good quantitaative agreement. However, the fast oscillations of the bare state simulation (which are in phase with the carrier oscillations) are not reproduced by the truncated Floquet approach.
Moreover, the picture of dressed (Floquet) states reveals details on the photoexcitation mechanism. While essentially being a one-photon process, there are some minor contributions of two- and three photon contributions.
Bare states
7 dressed (Floquet) states
Pulse duration of 1 fs
Even for a pulse that is slightly shorter than the optical cycle, the Floquet picture of dressed states can still be used! Again, the overall population dynamics is captured correctly in the Floquet picture, but the fast oscillations of the bare state simulation (which are in phase with the carrier oscillations) are not reproduced by the truncated Floquet approach.
Bare states
7 dressed (Floquet) states
Pulse duration of 0.5 fs
Finally, for a pulse that is slightly shorter than half an optical cycle, the Floquet picture of dressed states starts to break down! The overall population dynamics in the Floquet (7-state) picture deviates by more than 40 %.
