Quantum coherent control (QCC) has been successfully demonstrated experimentally and theoretically for two-and threephoton optical excitation of atoms and molecules. Here, we explore QCC using spectral phase functions with a single spectral phase step for controlling the yield of H 3 + from methanol under strong laser field excitation. We observe a significant and systematic enhanced production of H 3 + when a negative 3 /4 π phase step is applied near the low energy region of the laser spectrum and when a positive 3 /4 π phase step is applied near the high energy region of the laser spectrum. In some cases, most notably the HCO + fragment, we found the enhancement exceeded the yield measured for transform limited pulses. The observation of enhanced yield is surprising and far from the QCC prediction of yield suppression. The observed QCC enhancement implies an underlying strong field process responsible for polyatomic fragmentation controllable by easy to reproduce shaped pulses.Published under license by AIP Publishing. https://doi.
Strong-field ionization, involving tunnel ionization and electron rescattering, enables femtosecond time-resolved dynamics measurements of chemical reactions involving radical cations. Here, we compare the formation of CH3S+following the strong-field ionization of the isomers CH3SCN and CH3NCS. The former involves the release of neutral CN, while the latter involves an intramolecular rearrangement. We find the intramolecular rearrangement takes place on the single picosecond timescale and exhibits vibrational coherence. Density functional theory and coupled-cluster calculations on the neutral and singly ionized species help us determine the driving force responsible for intramolecular rearrangement in CH3NCS. Our findings illustrate the complexity that accompanies radical cation chemistry following electron ionization and demonstrate a useful tool for understanding the cation dynamics after ionization.
We report on the theoretical and experimental evaluation of state‐selective bond activation for coherent Raman spectroscopy and control of chemical reactivity. We compare six different strategies for state‐selective activation using shaped broadband femtosecond laser pulses. Results for a pair of long pulses with different wavelengths are also considered. For each of these approaches, we report on their theoretical and experimental ability to excite a desired vibrational mode while minimizing excitation of other modes including harmonics. We find that the double parabola spectral phase function gives the most intense Raman excitation with 10 cm−1 selectivity with the lowest background. The sinusoidal phase function produces intense Raman excitation with high selectivity but gives a series of harmonic excitations which could hinder selectivity. Implications of this study on several fields ranging from sensing, to controlling chemical reactions, and selective excited state excitation by X‐ray pulses are discussed.
Identifying and quantifying mixtures of compounds with very similar fragmentation patterns in their mass spectra presents a unique and challenging problem. In particular, the mass spectra of most per-and poly-fluoroalkyl substances (PFAS) lack a molecular ion. This complicates their identification, especially when using the absence of chromatographic separation. Here, we focus on linear, nonpolar, short-chain PFAS, which have received less attention than amphipathic PFAS despite their longer environmental lifetimes and greater global warming potentials. We identify and quantify n-C 5 F 12 and n-C 6 F 14 in binary mixtures by analyzing small changes in abundances of the main fragment ions following femtosecond tunnel laser ionization, without the need of chromatographic separation. Time-resolved femtosecond ionization mass spectrometry reveals that the metastable cation of both compounds undergoes predissociation within 1−2 ps of ion formation, with yields of C 3 F 7 + showing evidence of coherent vibrational dynamics. These coherent oscillations are compared to low-level ion-state calculations and supported the idea that the oscillations in the C 3 F 7 + ion yield are due to vibrations in the C 5 F 12 +• and C 6 F 14 +• radical cations and are associated with the predissociation dynamics of the metastable molecular ion. Surprisingly, we find that the fragment ions used for quantifying the mixtures have similar fragmentation dynamics. Conversely, the odd-electron C 2 F 4 +• fragment shows different time dependence between the two compounds, yet has negligible difference in the relative ion yield between the two compounds. Our findings indicate that femtosecond laser ionization may be a useful tool for identifying and quantifying mixtures of PFAS without the need of chromatography or high-resolution mass spectrometry.
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