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The Journal of Chemical Physics

Electronic and vibrational population transfer in diatomic molecules as a function of chirp for different pulse bandwidths

Bo Y. Chang, Bongsoo Kim, Ignacio R. Solá

Published Mar 27, 2003• 17 citations
Publisher

Abstract

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.

DOI
10.1063/1.1559009

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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.
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.
Abstract We have theoretically investigated the population transfer and the selective excitation of vibrational states of the target state in a three-state Na 2 molecule using the chirped adiabatic passage. The population transfer among electronic states is studied. Compared with the single negative chirped pulse, complete population transfer can be achieved by using a positive chirped pulse. The effects of laser parameters on vibrational population distribution are discussed, and the selective excitation of vibrational states can be achieved by adjusting the central frequency. Finally, we give the method of selective excitation of specific vibrational states.
We discuss electronic population transfer in diatomic molecules. By analyzing time-dependent wave packets on electronic potential-energy surfaces, we investigate the efficiency and robustness of population transfer with intense, chirped laser pulses. Recently, the magnitude of the population transfer was observed to have a remarkable dependence on the sign of the chirp [J. Cao, C. J. Bardeen, and K. R. Wilson, Phys. Rev. Lett. 80, 1406 (1998)]. In this work we explain the reason for this asymmetry, and show how the frequency detuning, intensity, and chirp rate affect the results. In particular, we demonstrate efficient, robust population transfer for both positive and negative chirps. A Landau-Zener model of multiple avoided crossings is invoked to model the results.
We have numerically explored the feasibility and the mechanism of population transfer to the excited E (1)Σ(g) electronic state of Li(2) from the v=0 level of the ground electronic state X (1)Σ(g) using the A (1)Σ(u) state as an intermediate. In this system, the use of transform limited pulses with a frequency difference greater than the maximum Rabi frequency does not produce population transfer when all possible radiative couplings are taken into account. We have employed two synchronous pulses far detuned from the allowed transition frequencies, mainly with the lower frequency pulse positively chirped, and both pulses coupling the successive pair of states, X-A and A-E. The adiabaticity of the process has been investigated by a generalized Floquet calculation in the basis of 12 field dressed molecular states, and the results have been compared with those obtained from the full solution of time dependent Schrödinger equation. The conventional representation of the process in terms of three (or four) adiabatic potentials is not valid. It has been found that for cases of almost complete population transfer in full calculations with the conservation of the vibrational quantum number, adiabatic passage is attained with the 12 state Floquet model but not with the six state model. The agreement between the full calculations and the 12 state Floquet calculations is generally good when the transfer is adiabatic. Another characteristic feature of this work is the gaining of control over the vibrational state preparation in the final electronic state by careful tuning of the laser parameters as well as the chirp rate sign. This causes time dependent changes in the adiabatic potentials and nonadiabatic transfers can be made to occur between them.
A new scheme of controlling molecular processes by a sequence of linearly chirped pulses is proposed and is applied to selective excitation of an energy level among closely lying ones and to complete electronic excitation of a diatomic molecule. The basic idea is quite different from the conventional ones utilizing chirped pulses in the sense that the present one does not rely on the idea of adiabatic rapid passage at all, but tries to control basic nonadiabatic transitions explicitly. Control of molecular processes can be achieved by controlling nonadiabatic transitions among Floquet (or dressed) states with use of the interference effects. The scheme can be formulated with use of the analytical theories of nonadiabatic transitions, and the proper control parameters can be estimated theoretically. Numerical demonstrations are provided to confirm the robustness of the method in comparison with the other conventional ones. Namely, the present scheme is shown to be stable against the variation of pulse area, complete and selective in population transfer, and fast to accomplish the transition. It is expected that the method can be applied to general multilevel systems and various types of wave packet dynamics. Its experimental realizability can also be expected, since linear chirping can now be relatively easily realized and manipulated.
Laser-controlled ultrafast state-selective vibrational dynamics of diatomic molecules, which are coupled to an unobserved quasiresonant environment is investigated using the reduced density-matrix formalism beyond and within a Markov-type approximation. Dissipative quantum dynamics in a classical electric field of shaped infrared ultrashort laser pulses is simulated for a one-dimensional nondissociative Morse oscillator, representing the local OH bond in the H2{\mathrm{H}}_{2}H2​ O and HOD molecules in the electronic ground state. Localization of population at a prescribed vibrational target level of OH up to v=10 with probability of about 90% is demonstrated on a picosecond time scale, while the strength of the quasiresonant molecule-environment coupling results in subpicosecond lifetimes of the vibrational states. The laser-controlled stabilization of selective excitation in the restricted set of vibrational states against a background of strongly diminished relaxation to lower vibrational states is also shown. The laser-control scheme may include a superposition of several laser pulses. A Markov-type approximation results in slightly increased lifetimes of the vibrational states, along with decreased predicted probability of state-selective excitation of a molecule by about 20--30%. Several results obtained within the fourth-order perturbation theory in interaction of a molecule with an environment are in a good agreement with the non-Markov analysis.