Nuclear spin selective laser control of rotational and torsional dynamics

Abstract

We explore the possibility of controlling rotational-torsional dynamics of non-rigid molecules with strong, non-resonant laser pulses and demonstrate that transient, laser-induced torsional alignment depends on the nuclear spin of the molecule. Consequently, nuclear spin isomers can be manipulated selectively by a sequence of time-delayed laser pulses. We show that two pulses with different polarization directions can induce either overall rotation or internal torsion, depending on the nuclear spin. Nuclear spin selective control of the angular momentum distribution may open new ways to separate and explore nuclear spin isomers of polyatomic molecules.

Laser-Controlled Torsions: Four-Dimensional Theory and the Validity of Reduced Dimensionality Models

“…For a stronger adiabatic field (red line), the results are less intuitive: although the agreement between the two models is excellent for short time scales, the 2D model does not fully account for the dephasing that the 4D model exhibits as time evolves. Similar non-monotonous behavior has been observed earlier in the dynamics of rotational and torsional wavepackets [6,24,25].…”

“…Figure 2 shows the rotational alignment factor cos 2 θ (a) and the torsional alignment factor cos 2 2ρ (b) for B 2 F 4 after excitation with a short, x-polarized laser pulse within the 4D (colored lines) and 2D (dotted line) approaches. In all cases, we observe a considerable increase of the torsional alignment immediately after the interaction: the small torsional barrier and the initially twisted geometry of B 2 F 4 promote excitation of torsional motion [6]. In the absence of the nanosecond pulse, the molecular axis is not aligned, the angular distribution of the molecules remains isotropic, as indicated in oval (I) in Fig.…”

“…This approach was first proposed in 2007 [8] and later explored in more detail in several theoretical and experimental studies [7,[9][10][11][12]. Extending these ideas, many interesting topics have been studied, including the interconversion of enantiomeres of axially chiral molecules [4,13], nuclear spin selective control of molecular torsion [6] and torsional alignment in a dissipative environment [14].…”

“…The interaction with the femtosecond pulse then creates a rotationaltorsional wave packet that can be written as a superposition of pendular states. As the pulse is short compared to the rotational-torsional dynamics, we can employ the impulsive approximation [6,24] to obtain the expansion coefficients in that superposition. To quantify the degree of rotational and torsional control, we calculate the alignment factor for the torsion cos 2 2ρ and the alignment of the principal axis with respect to the polarization of the first laser pulse cos 2 θ .…”

“…Among the variety of applications of torsional control that have fueled this interest are the control of energy transfer [1], charge transport [2] and chemical reactions [3], the enhancement of transient absorption spectroscopy [4] and the design of molecular rotors [5][6][7]. A relatively recent approach to manipulating the conformation of molecules in the electronic ground state involves the application of a moderately intense, non-resonant laser field to generate a broad torsional wavepacket that is correspondingly localized in the conjugated angle space.…”