Gaining Mechanistic Insight with Control Pulse Slicing: Application to the Dissociative Ionization of CH₂BrI

Abstract

In quantum control experiments with shaped femtosecond laser pulses, adaptive feedback control is often used to identify pulse shapes that can optimally steer the quantum system toward the desired outcome. However, gaining mechanistic information can pose a challenge due to the varied structural features of the control pulses and/or the often complex nature of the associated simulations of the experiments. In this article, we introduce control pulse slicing (CPS) as an easy-to-implement experimental analysis tool that can be employed directly in the laboratory without the need for modeling, to gain mechanistic insights about control experiments, regardless of whether the pulse is optimal or chosen by other means. As an illustration, we apply CPS to dissociative ionization of CHBrI with mass spectral detection, where two pulses with similar intensities are investigated, with each capable of distinctively controlling the ratio of Br/CHBr. These two control pulses were, respectively, first identified with closed loop and open loop procedures, and then the multispecies experimental data was analyzed with CPS. By comparing the dynamical evolution of the observed multiple fragment ion yields upon slicing scans of the two distinct pulses, we were able to reveal insights about the control mechanism for manipulating the objective ratio. In addition, we also identified the relationship between the temporal structures of the control pulses and the associated key reaction pathways involved in ionic as well as neutral electronic states, in spite of the signals only directly being from the ionic species. The CPS technique is not limited to controlled fragmentation mass spectrometry, and it may be applied to gain mechanistic insights in any control experiment, reflected in the nature of the recorded signal.

Coherent Control of Molecular Dissociation by Selective Excitation of Nuclear Wave Packets

“…However, shaped pulse control fails to enhance specific dissociation pathways in certain molecules including p -nitrotoluene Lozovoy et al (2008) . Moreover, the “black box” nature of closed-loop control makes it difficult to fully understand the physical mechanisms by which an optimal pulse shape achieves product selectivity, even using additional specialized pulse shaping procedures Xing et al (2017) .…”

Ultrafast control of the LnF+/LnO+ ratio from Ln(hfac)3

“…So, the pulse effectively appears to be ''shorter''. 26 This explains why the negatively chirped pulse is more effective for increasing the PrF + /PrO + ratio.…”

“…25 We also saw that the yield of PrF + relative to PrO + could be controlled to some degree by simply chirping fs laser pulses (i.e., stretching the laser pulses tends to increase the PrF + /PrO + ratio). In this section, we apply control pulse slicing (CPS) 26 to provide insights into the control mechanism for this phenomenon. Two pulses (obtained through a GA optimization with a pixel basis as controls) that either maximize or minimize the PrF + /PrO + ratio were selected for CPS analysis.…”

“…Our findings are also supported by the results of timeresolved measurements taken under the control pulse slicing (CPS) mechanism analysis protocol. 26 CPS is a versatile technique that allows for extracting time-resolved information from laser-control experiments, including those involving an optimally determined laser field, E(t). The CPS data is attained through a series of experiments in which a pulse's time profile E(t) is ''sliced-off'' after a time t = t (i.e., E(t) is smoothly made equal to 0 for t 4 t).…”

Quantitative Analysis of Nitrotoluene Isomer Mixtures Using Femtosecond Time-Resolved Mass Spectrometry

“…Although pulse shaping is effective for isomer discrimination, it is difficult to gain physical insight into the origin of the fragmentation patterns for each isomer upon interaction with shaped pulses in the absence of specialized experiments, making it difficult to predict optimal pulse shapes a priori . Hence, broad parameter spaces of second and third order chirping or more than 1000 random spectral phases are typically sampled to identify one or more pulse shapes that effectively discriminate between isomers of any particular molecule.…”