Coherent Control

Manipulating vibrational motions might allow the laser control of chemical reactions or be used to improve 2D IR spectroscopy. It is well-known that simple chirped pulses can lead to high vibrational excitation. We demonstrated that sophisticated mid-IR pulses can manipulate the populations of individual vibrational levels. Shown in Fig. 1(a) is the pump-probe spectrum measured for a transform-limited pump pulse (55 fs duration).The spectrum consists of a negative feature at 1983 cm-1 followed by a series of positive and progressively weaker peaks at lower frequencies. This spectrum is typical of femtosecond pump-probe studies. The negative feature corresponds to a bleach of the ground state and ν=1–0 stimulated emission. The positive peaks are sequence bands arising from ν=1–2, 2–3, 3– 4, etc., absorptions. The diminishing peak intensities indicate that a transform-limited pulse becomes increasingly less efficient at populating progressively higher vibrational levels.

Fig. (4). Experimental pump-probe spectra for W(CO)6 in n-hexane performed with (a) a transform-limited pump pulse,(b) a pulse optimizing the ν=1-2 vs 0-1 features, (c) a pulse optimizing the ν=2 -3 vs 1-2 features, and (d) a pulse optimizing the ν=3-4 vs 2-3 feature.

In contrast, when we use an evolutionary algorithm to build pulses which optimize the population of specific peaks, we see very different pump-probe spectra (b-d). Optimizing the v=2-3 transition, for example, leads to strong enhancement of this transition relative to that in the conventional pump/probe spectrum, and we see a negative peak, indicative of stimulated emission, for the v=1-2 transition. Phase optimizations improved the ratio of the targeted peaks by 2.2–5.8 times the transform limited pulse.