Spectral Steering

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Science  10 Jul 2009:
Vol. 325, Issue 5937, pp. 127
DOI: 10.1126/science.325_127d

When a molecule absorbs light, its electrons channel the energy into rearranging their relative positions. The process often entails a rather complex series of subtle movements, and researchers yearn to steer the trajectory with an ever increasing degree of external control. To this end, they have developed sophisticated techniques to modulate the spectral phases and amplitudes of the illuminating pulse, which in turn guide the excitation pathway. Roth et al. show that such coherent control methodology can induce distinct fluorescence behavior in two compounds that possess nearly identical electronic absorption spectra. The molecules in question were flavins that differed by the presence or absence of a phosphate group on a side chain appended to the central aromatic chromophore. The authors applied two sequential excitations, a leading ultraviolet pulse that induced fluorescence and a trailing infrared pulse that quenched it; they then used feedback to discover ultraviolet pulse shapes that either maximized or minimized the depletion ratio of the phosphate-bound to phosphate-free flavin emission. The optimal pulses achieved nearly 30% differentiation in either direction, an effect the authors attribute to the sensitive manipulation of low-frequency vibrational modes along the side chain.

Phys. Rev. Lett. 102, 253001 (2009).

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