Optimally controlled five-laser infrared multiphoton dissociation of HF

Abstract

Simulations of the quantum dynamics of the HF molecule immersed in a field of five overlapping, intense, linearly polarized, infrared laser pulses of subpicosecond duration are performed. The HF molecule, initially in its ground state, is modeled as a rotating oscillator interacting with a classical laser field via electric dipole interaction. Realistic potential and dipole functions are used. Optimal overlaps of the five laser pulses, as well as the optimal carrier frequencies of the laser pulses, are found which maximize the HF dissociation yield. A maximal yield of 45% in a single combined pulse is achieved using the best available potential and dipole moment functions. The optimal infrared multiphoton dissociation pathway for the HF molecule includes a series of the Δv=1 vibrational-rotational transitions followed by a series of Δv≥2 vibrational-rotational transitions. The latter is necessary as a consequence of the vanishing Δv=1 transition moment around v=12. In the Δv=1 regime, both P and R branch transitions are found to be important. The angular distribution of the dissociative flux is computed. Robustness of the results with respect to changes in the interatomic potentials, dipole functions and reduced mass, as well as to changes in laser pulse parameters (carrier frequencies, timings, phases, field amplitudes, and pulse durations) is investigated.

Exploring quantum control landscapes: Topology, features, and optimization scaling

“…The OCT literature collectively suggests that the required search effort to find an optimal control may be independent of the complexity (i.e., here captured by N ) of the target quantum system [2,3,9,13,15,17,. The results from this work systematically verify this behavior and identify the control conditions sufficient for the search effort scaling to be independent of N .…”

“…Despite the wide variation in absolute search effort with the choice of algorithm, the scaling of the search effort with respect to system complexity exhibited the same qualitative trends for all algorithms examined. Similarly, in OCT studies from the literature, gradient-based algorithms typically converge in ∼ 100 iterations [2, 9, 17, 39, 41-46, 49, 51, 53-59], while non-gradient simplex and evolutionary searches typically require several hundred iterations [3,13,40]. Importantly, these numbers do not appear strongly dependent on N .…”

“…Intuitively, the expectation is that finding a suitable control field would become more difficult as the size of the system increases, because additional control pathways involving a larger number of quantum states become accessible. The collective OCT literature, however, suggests that the required search effort to find an optimal control is generally on the order of ∼ 10 2 iterations, [2,3,9,13,15,17,, and systematic optimization of P i→f using kinematic control variables indicates that the search effort scales at most very slowly with N [61]. Successful OCE studies ranging from control of atoms [20,25] to complex protein molecules [28,30] further suggest a practical level of invariance of search effort to system complexity.…”

“…In this work, the complexity of the system is measured by the Hilbert space dimension N , i.e., the number of accessible energy levels of H 0 . A large body of results from the OCT literature [2,3,9,13,15,17, performed on systems where N ranges from 2 to ∼ 10 2 suggest that the search effort required for population transfer does not scale strongly with N . Although the required effort will depend on the convergence criteria, the number of reported algorithmic iterations to achieve convergence is observed to be typically no more than ∼ 10 3 , and often ∼ 100 or fewer, regardless of N or the particular search algorithm employed.…”

“…OCT simulations have successfully controlled a variety of objectives, including state preparation [2,6,7], molecular isomerization [8][9][10][11][12], dissociation [13][14][15][16], and orientation/alignment [17][18][19]. OCE using ultrafast tailored laser pulses have achieved control over many processes including state preparation [20,21], selective molecular dissociation [22][23][24], generation of high order optical harmonics [25][26][27], and energy transfer and isomerization in large biomolecules [28][29][30].…”