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Pulse shaping has long been used to tailor femtosecond laser pulses for studying and controlling the fragmentation of polyatomic molecules [1, 2]. We employed 80-bit binary spectral phase functions (BPs) to reduce the search space while maintaining the ability to generate essentially any arbitrary phase [3]. Analysis of the reduced search space allowed us to gain insight into which pulse parameters most influence the fragmentation of triethylamine $(N(C_2H_5)_3)$ [4]. We focused on influencing the relative yield of m/z 86, corresponding to the loss of a methyl group. Given that peak intensity is a confounding variable in strong-field control [5], we evaluated thousands of BPs that generated similar second harmonic intensity ($I_{SHG}$), and found some produced twice the product yield, a difference not attributable to noise. Analysis of these pulses, including their duration, instantaneous frequency, power spectra, pulse trains, and autocorrelation, revealed that pulses with features spaced by ~2 ps enhanced the desired product. This was further confirmed by pump-probe measurements.
We evaluated 200 additional 80-bit BPs with pulse structures spaced 1.25–2.5 ps and compared them to 200 randomly generated BPs, all with identical $I_{SHG}$ values to eliminate peak intensity as a variable. The selected BPs produced nearly twice the m/z 86 yield compared to the randomly shaped pulses, outperforming both transform-limited and pump-probe pulses. This enhancement is attributed to control via a dissociative Rydberg state with an estimated 2 ps lifetime in the neutral molecule [6]. The second feature in the shaped pulse likely ionizes the 3s Rydberg state, leading to m/z 86 formation. These findings suggest that open-loop control with BPs could uncover new control mechanisms and reveal key pulse parameters that influence laser-matter interactions.
[1] A. Assion, et al. Science 282, 919 (1998).
[2] M. Dantus, V. V. Lozovoy, Chem. Rev. 104, 1813 (2004).
[3] V. V. Lozovoy, M. Nairat, M. Dantus, J. Opt-UK 19, 105506 (2017).
[4] J. Stamm, S. Kwon, M. Dantus, Chem. Phys. Lett. 15, 12464 (2024).
[5] V. V. Lozovoy, et al., J. Phys. Chem. A. 112, 3789 (2008).
[6] T. I. Sølling, C. Kötting, and A. H. Zewail, J. Phys. Chem. A 107, 10872 (2003).
