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Physical Review A

Inducing changes in the bond length of diatomic molecules by time-symmetric chirped pulses

Bo Y. Chang, Seokmin Shin, Ignacio R. Solá

Published Dec 27, 2010• 21 citations
Publisher

Abstract

We show numerically that it is possible to change the structure of a simple molecule, that is, a diatomic molecule, where the bond length is modified at a precise timing with symmetrically chirped laser pulses. In the adiabatic regime, the process is fully time reversible, making it possible to design slow vibrations with large bond elongations. The scheme relies on the preparation of a separable state of both nuclear and electronic degrees of freedom with predominant amplitude on the dissociative (antibonding) electronic wave function. Shorter laser pulses can be used to dynamically induce larger bond elongations, preparing a highly excited vibrational wave packet in the ground potential as the laser is switched off.

DOI
10.1103/physreva.82.063414

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We show numerically that it is possible to change the structure of a simple molecule, that is, a diatomic molecule, where the bond length is modified at a precise timing with symmetrically chirped laser pulses. In the adiabatic regime, the process is fully time reversible, making it possible to design slow vibrations with large bond elongations. The scheme relies on the preparation of a separable state of both nuclear and electronic degrees of freedom with predominant amplitude on the dissociative (antibonding) electronic wave function. Shorter laser pulses can be used to dynamically induce larger bond elongations, preparing a highly excited vibrational wave packet in the ground potential as the laser is switched off.
In this paper we propose to control the bond length of a diatomic molecule and its vibrational coherent motion over arbitrary distances. The method requires (a) mixing a dissociative potential with bound electronic configurations in order to stabilize the continuum, and (b) manipulating the wave-packet transfer below the threshold of adiabatic passage. Using asymmetric pulses it is possible to implement sudden-adiabatic transitions to stop or induce the motion in the light-induced potential, controlling the nonadiabatic dynamics in a most efficient way.
Using the concept of light-induced potentials introduced by Garraway and Suominen [Phys. Rev. Lett. 80, 932 (1998)], we study alternative schemes of adiabatic population transfer in a diatomic molecule from the lowest vibrational level of the ground electronic state to the lowest vibrational level of a third electronic state via allowed optical transitions from the ground and third electronic states to a second electronic state. Our chirped-pulse rapid adiabatic passage scheme is implemented with short-duration optical pulses, and can be understood in terms of ``road design engineering'' of the light-induced time-dependent adiabatic potential upon which the dynamics occurs. We explicitly show that complete adiabatic vibronic population transfer occurs using chirped picosecond-duration pulses in the sodium dimer example.
We study the dynamics of two-photon nonresonant electronic excitation of diatomic molecules driven by chirped pulses. While the majority of the experimental results address the role of the chirp for fixed pulse bandwidth, we analyze the possibility of selective excitation for fixed time, as a function of the pulse bandwidth, depending on the sign of the chirp. With strong picosecond pulses and positive chirp it is shown that the dynamics always prepare the molecule in the ground vibrational level of the excited electronic state. The robustness of the dynamics inherits the properties of an effective Landau–Zener crossing. For negative chirp the final state is very sensitive to the specific pulse bandwidth. The dynamics of the system follow a complex convoluted behavior, and the final state alternates between low vibrational levels of the excited electronic state and excited vibrational levels of the ground potential, which become increasingly more excited with increasing bandwidth. The final electronic populations follow a double-period oscillatory behavior. We present a model based on sequential independent crossings which correlates the long-oscillation period with changes in the final vibrational state selected. We show that the short-oscillation period is related with nonadiabatic effects that give rise to fast dynamic Rabi flipping between the electronic states, providing only information of the field–molecule effective coupling. Although the short-oscillation period partially masks the expected results of the final populations, we show that it is still possible to retrieve information from the long-oscillation period regarding the frequencies of the electronic potentials. In order to do so, or in order to control the outcome of the dynamics, it is necessary to perform experiments scanning very different pulse bandwidths, and we propose a possible experimental implementation. All the numerical results of the paper are calculated for a model of the Na2 dimer.
We propose a scheme for squeezing vibrational wave packets in the ground (or excited) states of diatomic molecules that uses pulse sequences of strong nonresonant short pulses. The physical mechanism of the scheme is a two step process involving adiabatic and diabatic transformations of the wave packet. The first step is the adiabatic stretching of the wave packet on a light-induced potential, and the second step is dynamic squeezing by free evolution after ultrashort (i.e., diabatic) population transfer. We show that this sequence of transformations can be performed very efficiently with a simple two pulse sequence with slow switch on and fast switch off pulse shapes, and we analyze the required physical resources that are needed. We test the scheme on a model of harmonic oscillators and on the Rb2{\mathrm{Rb}}_{2}Rb2​ dimer. Finally, different possible generalizations are proposed.
Recent calculations [1, see also 2-5] indicate that suitably timed sequences of electronically preresonant femtosecond laser pulses can induce large amplitude coherent molecular vibrations of dilute chromophores in low temperature hosts, without undue electronic absorption or other loss processes. Here we demonstrate that frequency modulation, in the forms of variable pulse center frequency and variable chirp rate, can significantly further enhance the efficiency of impulsive stimulated Raman preparation of coherent vibrations in the electronic ground state.