Frequency-network mechanism for alignment of diatomic molecules by multipulse excitation

There are several effective nonadiabatic alignment control schemes that use multilaser pulse excitation. To explain the control mechanisms in a unified manner through a case study, we apply nonresonant optimal control simulation to a rovibrational model of N₂. For experimental feasibility, we introduce a penalty and/or constraint to the simulation to impose restrictions on the number of optically accessible rotational states. When the control time is set to one rotational period, optimal pulses with linear polarization and a wavelength of around 800 nm are composed of three subpulses. From the power spectra of the intensities of the optimal pulses, we see that the optimal pulses construct a frequency network that connects the lowest three rotational Raman transitions. The frequency-network mechanism is confirmed by calculating the degree of alignment using sets of three Gaussian pulses. It shows several combinations of pulses that (approximately) satisfy the frequency-network condition, which can explain why there exist several control schemes that achieve a high degree of alignment.