Measurement of field-free alignment of jet-cooled molecules by nonresonant femtosecond degenerate four-wave mixing

Abstract

A femtosecond degenerate four-wave mixing (fs-DFWM) technique for probing the alignment of molecules is described. The rotational wave packet generated by a strong pump beam modulates the nonresonant third-order susceptibility of the gas and this modulation is observed using a noncollinear background-free setup. As a demonstration of the technique, we measure the alignment of a linear molecule (nitrogen), a symmetric top (benzene), and an asymmetric top (iodobenzene) in a supersonic jet. The measured signal is shown to agree well with calculations of the nonresonant third-order susceptibility from the solution of the time-dependent Schrödinger equation. The pump-DFWM measurement scheme can be used with dilute jet-cooled targets, allows arbitrary pump pulses to be used, and is rapid enough to be incorporated into a feedback optimization loop. The scheme can easily be made polarization-sensitive, indicating the possibility of measuring and optimizing three-dimensional alignment of asymmetric-top molecules.

N2 HOMO-1 orbital cross section revealed through high-order-harmonic generation

“…Impulsive alignment is achieved by creating a superposition of rotational quantum states which produces a time-dependent observable, revealed as a revival structure. Measurements like ionization [22,23], nonlinear optical methods [24,25], or HHG [11,26,27] are sensitive to this rotational wave packet and have been extensively used in the past [28,29]. Furthermore, recent studies have shown that changes in the revival dynamics can also occur when orbitals of different symmetry are involved [30][31][32].…”

Strongly aligned and oriented molecular samples at a kHz repetition rate

“…Recently, some relevant molecular beam setups with high repetition rates have been developed and some individual ingredients of the necessary control and detection details have been demonstrated [19][20][21]. A benchmark experiment demonstrating long-pulse alignment of molecules in a lowpressure continuous beam at 1 kHz repetition rate, probed with short ps x-ray pulses from a synchrotron source, has demonstrated weak alignment of cos 2 θ ≈ 0.4 [22].…”

Reconstruction of three-dimensional molecular structure from diffraction of laser-aligned molecules

“…If the angular distribution is not known, an estimated distribution can be used as a starting point, and then it is optimized by the genetic algorithm until the distribution that best matches the data is found. The result with lowest final error gives the best retrieval pattern 19 . Since most published reconstruction algorithms assume a pattern corresponding to perfect alignment to begin with, their functions are comparable to our second step, the iterative phase retrieval.…”