Quantum Control

Rotational Alignment


Figure 1: A linearly polarized infrared field induces a dipole and creates an aligning potential well.
Since molecules are not generally spherically symmetric, the orientation of colliding molecules is a key factor in determining the ensuing chemistry. As well, in photoinitiated reactions molecular orientation influences the transition matrix elements when the light is polarized. The ability to align molecular axes offers the opportunity control these geometric influences, and to eliminate the intrinsic rotational averaging that occurs in typical measurements because it offers the ability to bring the molecular frame, where molecular processes occur, into the lab frame where measurements are made. This will advance emerging techniques for the study of molecular structure and dynamics where orientational averaging of the molecular frame leads to a considerable loss of information. Examples include time-resolved photoelectron spectroscopy, time-resolved x-ray diffraction, electron diffraction, and timed Coulomb explosion.

The NRDSE can be use to induce alignment in molecules: The application of a linearly polarized infrared field induces a dipole which subsequently interacts with the field. As a result a rotational potential is created along the laser polarization axis. However, if the alignment field is applied continuously, any control or observation technique will not access the field-free molecules.


Figure 2: Optical Cross correlation of Swtiched pulse demonstrating adiabatic rise and sudden fall

The Switched Wavepacket technique was developed to generate alignment of one molecular axis under field-free conditions. Through the adiabatic application and sudden truncation of a linearly polarized NRDSE interaction, a localized rotational Switched Wavepacket is created and its subsequent evolution exhibits revival structure and hence transient field-free axis alignment.

Figure 3: The optical Kerr effect as a measure of alignment. Wavepackets revive following the switch at t=0 producing transient alignment.
Figure 4: Fourier transform of the Kerr alignment signal (Fig 3). The peak structure can be used to determine rotational constants.

 

The advantage of the Switched Wavepacket technique is that very little information is required about the molecules in question: The control field needs only by adiabatic on the rise and sudden on the fall. There is no need to tune pulse durations or wavelengths. As a result, the approach has broad applicability[1,2]. Strategies have also been developed for field-free alignment of all molecular axes using NRDSE. Two orthogonally polarized, time-separated laser pulses can be used to effectively torque different molecular axes independently as a molecule rotates. By carefully timing the pulses, the revival time of the molecular axes can be made to coincide[3, 4].


Figure 5: Field-free 3D alignment of SO2 molecules using time-separated laser fields.

1: Phys. Rev.: 90, 223001 (2003)
2: Phys. Rev. A: 73, 053403 (2006)
3: Phys. Rev.: 94,143002 (2005)
4: Phys. Rev.: 97:173001 (2006)