Vinod Kumarappan

Manipulating the torsion of molecules by strong laser pulses.

We demonstrate that strong laser pulses can induce torsional motion in a molecule consisting of a pair of phenyl rings. A nanosecond laser pulse spatially aligns the carbon-carbon bond axis, connecting the two phenyl rings, allowing a perpendicularly polarized, intense femtosecond pulse to initiate torsional motion accompanied by an overall rotation about the fixed axis. We monitor the induced motion by femtosecond time-resolved Coulomb explosion imaging. Our theoretical analysis accounts for and generalizes the experimental findings.

Aligning molecules with long or short laser pulses

We briefly review recent experimental developments of laser induced alignment of molecules with pulses that are either long (adiabatic) or short (nonadiabatic) compared to the molecular rotational periods. The main focus is on one-dimensional (1D) alignment obtained with linearly polarized laser pulses. Both linear and asymmetric top molecules are used to illustrate the alignment methods. Three-dimensional (3D) alignment, where all three molecular axes of a rigid asymmetric top molecule are orientationally confined, is also discussed.

Control of alignment dynamics of asymmetric top molecules

We introduce a new tool to control the nonadiabatic alignment dynamics in asymmetric top molecules. Alignment is induced using nonresonant, linearly polarized laser pulses with pulse durations that are short compared to the rotational periods of the molecule. Through a series of nonresonant Raman transitions a rotational wave packet is created. It is concluded with an interesting connection between the duration of the alignment pulse and the wave packet dynamics, providing a new tool to transform the non-periodic revival structure of an asymmetric top, into stable periodic rotations about the C-axis of the molecule. The experimentally observed features are also reproduced qualitatively in numerical simulations.

3D Alignment by Holding and Spinning Molecules

We introduce a new method for controlling the rotational motion of asymmetric top molecules. The method is capable of producing strong 3-dimensional (3D) alignment, i.e. confine all three principal molecular axes along three orthogonal space fixed axes. The method relies on combining two laser pulses that are short and long with respect to the natural molecular rotational periods. We conclude that strong 3D alignment can be obtained by combining a long and a short laser pulse, linearly polarized orthogonal to each other. Field-free 3D alignment should be experimentally obtainable by rapidly truncating the long laser pulse near the arrival of the short pulse. Our results also indicate 3D alignment revivals from spinning molecules, as the revival times correspond to the rotational constant A.