Active control of decoherence of excited resonance states by means of laser pulses

A. García-Vela

Instituto de Física Fundamental, Consejo Superior de Investigaciones Científicas, C/ Serrano 123, 28006 Madrid, Spain

As the pump pulse temporal width decreases, its spectral bandwidth (FWHM) increases from 0.15 cm-1 for 200 ps to 12 cm-1 for 2.5 ps. As a result, an increasingly broader wave packet is created with increasing an(t=0) weights for the v’-1 resonances overlapping with the v’ ground resonance, gradually enhancing the interference effects.

For v’=27 overlapping and interference of the v’ ground resonance occurs essentially with only one v’-1 resonance (the most intense one in the v’=27 spectrum, labeled by 9). When the bandwidth of the wave packet prepared is very narrow (as for long pulses) the corresponding lifetimes reflect the properties of the specific energies excited, namely about 24 ps for the resonance energy and decreasing lifetimes as the excitation energy becomes increasingly off resonance. However, when the bandwidth of the superposition created becomes broad enough as to populate the v’-1 resonances overlapping with the v’ resonance state, the onset of interference appears and the lifetime of the v’ ground resonance changes substantially, increasing or decreasing depending on the excitation energy.

The behavior of the v’ ground resonance lifetime for v’=35 is qualitatively similar to that of the v’=27 case. The main difference is that when the wave packet prepared is centered at the resonance energy, the decrease of the lifetime is proportionally more extensive (from 10.7 ps to 1.7 ps, see TABLE 1) than in the v’=27 case. The reason is that the v’ ground resonance overlaps and interfere with a larger number of v’-1 resonances for v’=35, producing more intense interference effects.

The lifetimes of TABLE 1 show that the excitation energy becomes an additional control parameter (in addition to the pulse width). Indeed, for v’=27 a small change of 0.42 cm-1 in the excitation energy allows one to vary the v’ resonance lifetime from practically 0 ps to about 24 ps. The range of excitation energies allowed for control depends on the v’ resonance width.

Introduction: This work explores how to exploit the mechanism of quantum interference occurring between overlapping zero-order resonances within a system [1-3] in order to change and control the lifetime of a specific excited resonance. The system chosen is the Br2(B,v’)-Ne van der Waals (vdW) cluster, since there has been previously shown [4-6] that it presents a range of initial v’ vibrational manifolds for which the ground vdW resonance overlaps with other resonances near in energy corresponding to the v’-1 vibrational manifold. Thus, modification and control of the predissociation lifetime of the ground vdW resonance of Br2(B,v’)-Ne is investigated through wave packet simulations by changing the width of the pulse that creates a coherent superposition of v’ and v’-1 overlapping resonances. Specifically, three different vibrational states v’ are studied corresponding to the isolated (nonoverlapping) resonance regime (v’=16), the sparse overlapping resonance regime (v’=27), and the dense overlapping resonance regime (v’=35). The effect of changing the excitation energy (i.e., the center of the wave packet prepared) along the excitation spectrum of the v’ ground resonance excited is also explored as an additional control parameter.

Conclusions: The mechanism of quantum interference that occurs between overlapping resonances in a system can be exploited in order to control the lifetime of a specific resonance by creating a coherent superposition of such resonance states. Varying the spectral bandwidth of the superposition prepared, by changing the temporal width of the pump pulse used to create the superposition, allows one to control the interference mechanism between the overlapping resonances and its effects on the lifetime of a given resonance of the superposition. It is found that extensive control can be achieved, both increasing and decreasing remarkably the resonance lifetime by changing the excitation energy on one hand, or keeping the lifetime nearly constant regardless of the excitation energy (either in resonance or off resonance) on the other hand. In addition to the pulse width, the excitation energy becomes also a control parameter due to the large dependence of the resonance properties on energy.

References [1] E. Frishman and M. Shapiro, Phys. Rev. Lett. 87, 253001 (2001). [2] P.S. Christopher, M. Shapiro, and P. Brumer, J. Chem. Phys. 123, 064313 (2005). [3] D. Gerbasi, A.S. Sanz, P.S. Christopher, M. Shapiro, and P. Brumer, J. Chem. Phys. 126, 124307 (2007). [4] A. García-Vela, and K.C. Janda, J.Chem. Phys. 124, 034305 (2006). [5] A. García-Vela, J. Chem. Phys. 126, 124306 (2007). [6] A. García-Vela, J. Chem. Phys. 129, 094307 (2008). Acknowledgements: Ministerio de Ciencia e Innovación, Spain, Grant No. FIS2010-18132, the Consolider program, Grant No. CSD2007-00013 and the Centro de Supercomputación de Galicia (CESGA).

Basic equations

Plot of the predissociation lifetimes of the v’ ground vdW resonance of Br2(B,v’)-Ne for v’=27 and 35 obtained by changing the width of the pump pulse from 200 ps to 2.5 ps, for three different excitation energies (in resonance and off resonance).

Excitation spectra for the v’ground vdW resonance of Br2(B,v’)-Ne for three different v’ vibrational states corresponding to the isolated (nonoverlapping, v’=16), sparse (v’=27), and dense (v’=35) resonance regime.

Survival probability of the Br2(B,v’)-Ne ground resonance for v’=16 (exciting the resonance energy) and v’=27 (exciting an energy -0.42 cm-1 off resonance) obtained with different pump pulse widths. In the case of v’=16 there is practically no change of the resonance lifetime (about 71 ps, see TABLE 1) with the pulse width, since interference is absent. By contrast, for v’=27 the lifetime increases and an interference pattern of unudulations appears as the spectral bandwidth of the pump pulse increases from 200 to 2.5 ps.

Predissociation lifetimes of the v’ ground vdW reonance of Br2(B,v’)-Ne for v’=16, 27, and 35 obtained by changing the width of the pump pulse from 200 ps to 2.5 ps, for three different excitation energies (in resonance and off resonance).