Quantum coherent control of the lifetime of excited resonance states with laser pulses

A. García-Vela

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

Conclusions: Two control schemes are proposed based on the interference effects occurring between overlapping resonances populated in a coherent superposition. The schemes are applied to a realistic model of the Br2(B,v’)-Ne predissociation dynamics in order to modify and control the lifetime of the v’=27 ground resonance, which overlaps with orbiting resonances of the v’-1vibrational manifold. In the first control scheme interference between resonances is controlled by changing the population of the different resonances in the superposition by varying the temporal width of the pump pulse preparing the superposition state. The second control scheme uses two pump pulses which excite two overlapping resonances (in the v’ and v’-1 manifolds, respectively). In this scheme two typical experimental parameters like the delay time between the two pump pulses and their intensities are used as control parameters. While the two schemes provide an extensive degree of control over the resonance lifetime, the scheme using two pulses exhibits a higher sensitivity of control and a stronger enhancement of the lifetime.

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] A. García-Vela and K.C. Janda, J. Chem. Phys. 124, 034305 (2006). [4] A. García-Vela, J. Chem. Phys. 129, 094307 (2008). [5] A. García-Vela, J. Chem. Phys. 136, 134304 (2012). [6] A. García-Vela, J. Phys. Chem. Lett. 3, 1941 (2012).

Acknowledgements: This work was supporteded by CICyT, Ministerio de Ciencia e innovación (MCINN), Spain, Grant No. FIS-2011-29596-C02-01, the Consolider program, MCINN, Spain, Grant No. CSD2007-00013, COST Action program, Grant No. CM1002, and the Centro de Supercomputación de Galicia (CESGA).

Single pulse control scheme. Plot of the lifetimes obtained for different pulse widths and the three excitation energies using the single pulse control scheme in the case of v’=27. These lifetimes are collected in the table along with the lifetimes corresponding to v’=16. In the case of v’=27 there is a substantial variation of the resonance lifetime due to interference between the v’ and v’-1 overlapping resonances.

Two pulse control scheme. (a) Survival probabilities calculated for A2=0 (i.e., excitation of resonance ψ1 only), and for the delay time Δt=160 ps and the three intensity ratios A2=0.2A1, A2=0.4A1, and A2=A1. (b) Typical survival probabilities calculated for several delay times and pulse intensity ratios along with their corresponding fits used to estimate the resonance lifetime. The curves have been rescaled for convenience. (c) Survival probabilities calculated for different delay times in the case of A2=0.4 A1.

Two pulse control scheme. The upper panel shows the Gaussian temporal profiles of the two pump pulses for different delay times Δt=t2-t1 between them. The center of one of the pulses is always fixed at t1=0 ps. The lower panel displays the resonance lifetimes obtained for the ground resonance of Br2(B,v’=27)-Ne for different delay times and A2/A1 intensity ratios between the pump pulses. An enhancement of the lifetime by a factor of three is found.

Introduction: This work explores how to exploit the mechanism of quantum interference occurring between overlapping resonances within a system [1,2] in order to modify 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 [3,4] 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 (Fig. 1). Thus, modification and control of the predissociation lifetime of the ground vdW resonance of Br2(B,v’)-Ne is investigated through exact wave packet simulations by creating coherent superpositions of v’ and v’-1 overlapping resonances, following two different control schemes. In one of them the system is excited with a single laser pulse of varying width in order to change the population of the different overlapping resonances in the superposition [5]. Two different vibrational states v’ are studied corresponding to the isolated (nonoverlapping) resonance regime, v’=16, and the sparse overlapping resonance regime, v’=27 (see Fig. 1). 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 (Fig. 2) is also explored as an additional control parameter. In the second control scheme [6] two laser pulses are used to excite two overlapping resonances, one being the ground resonance of Br2(B,v’=27)-Ne, and the other one being an orbiting resonance in the v=v’-1 manifold. The delay time and the ratio of intensities between the two excitation pulses are used as control parameters.

Single pulse control scheme. 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. For 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 undulations appears as the spectral bandwidth of the pump pulse increases from 200 to 2.5 ps.

Basic equations

Fig.2. Excitation spectra of the Br2(B,v’)-Ne ground vdW resonance for v’=16 (upper panel) and v’=27 (lower panels). The three excitation energies used in the single pulse scheme are indicated by the arrows in the middle panel. The pulses used in the two pulse scheme are shown in the lower panel.