Bjarne Amstrup

Control of HOD photodissociation dynamics via bond‐selective infrared multiphoton excitation and a femtosecond ultraviolet laser pulse

A scheme for controlling the outcome of a photodissociation process is studied. It involves two lasers—one intense laser in the infrared region which is supposed to excite a particular bond in the electronic ground state, and a second short laser pulse in the ultraviolet region which, at the right moment, excites the molecule to a dissociative electronic state. We consider the HOD molecule which is ideal due to the local mode structure of the vibrational states. It is shown that selective and localized bond stretching can be created in simple laser fields. When such a nonstationary vibrating HOD molecule is photodissociated with a short laser pulse (∼5 fs) complete selectivity between the channels H+OD and D+OH is observed over the entire absorption band covering these channels.

Two‐pulse laser control of bond‐selective fragmentation

We elaborate on a two‐pulse (pump‐pump) laser control scheme for selective bond‐breaking in molecules [Amstrup and Henriksen, J. Chem. Phys. 97, 8285 (1992)]. We show, in particular, that with this scheme one can overcome the obstacle of intramolecular vibrational relaxation. As an example, we consider an ozone molecule with isotopic substitution, that is, 16O16O18O. It is shown that asymmetric bond stretching can be created in simple (intense) laser fields. We predict that an alternating high selectivity between the channels 16O+16O18O and 16O16O+18O can be obtained when such a non‐stationary vibrating ozone molecule is photodissociated with short laser pulses (∼10–15 fs) with a time delay corresponding to half a vibrational period (∼17 fs).

Coherent control of HOD photodissociation dynamics in the first absorption band

Abstract The coherent control scheme suggested by Brumer and Shapiro is recast into a time-dependent framework. We apply this scheme to the photodissociation dynamics of HOD in the first electronic absorption band where the branching ratio between H + OD and D + OH is considered. Electronic excitation is carried out with continuous wave lasers and it is shown that by varying the phase difference between two different excitation paths substantial control can be achieved at selected energies.

Chirped pulse control of CsI fragmentation: an experimental possibility

Abstract The strength of chirped pulse optimal control as an experimental possibility is demonstrated in this paper. The suggested experiment is a pump-dump experiment that first creates an unstable - dissociating - molecule on a repulsive level and then stops fragmentation. The experiment is envisioned by using a start pulse (a doubled green pulse) that inverts the population. The control pulse - that stops the fragmentation - is the chirp optimized green pulse itself. Optimization includes second and third order chirp expansion terms. Optimization is made for the weak and strong field limits with the help of the simulated annealing method. Weak field optimization shows the expected red chirp results. Strong field optimization, on the other hand, gives rise to blue chirp as optimal solution. It is argued, that the blue chirp is the consequence of the need for locking the upper and lower state wavefunctions for efficient population inversion.

Identification of born-oppenheimer potential energy surfaces of diatomic molecules from optimized chirped pulses

Abstract The optimal control of molecules has been widely studied in recent years. In this paper we explore the feasibility of applying adaptive feedback optimal control to determine Born-Oppenheimer (BO) surfaces. If the molecular wave packet is well localized, laser pulses optimized to transfer population between two surfaces contain information on the potential energy difference in the region scanned by the wave packet in the duration of the pulse. By iterating the experiment with the different values of the nuclear separation large regions of the unknown BO surface can be measured if one of the surfaces is known. The method is simulated with a model experiment on CsI. The results show that the BO surface can be determined with reasonable accuracy. Some limitations and possible extensions of the method are discussed.

On kinetic energy densities

Abstract The concept of kinetic energy densities has been discussed, with particular emphasis on the expression that may be derived from the Weyl-Wigner phase-space representation of quantum mechanics. The actual form of this expression, and of three other expressions for the kinetic energy density, has been calculated and plotted for the atoms hydrogen, neon and argon.

Laser-induced vibrational dynamics of ozone in solid argon

Abstract We consider the vibrational dynamics, induced by an intense infrared laser pulse, in an ozone molecule with isotopic substitution, that is, 16 O 16 O 18 O and compare the dynamics in the gas phase and in solid argon. The vibrational dynamics is not perturbed by argon on a time-scale of a few picoseconds and selective bond-breaking in the molecule should be possible following the same laser control scheme as suggested in the gas phase.