Formation of coherent rotational wavepackets in small molecule-helium clusters using impulsive alignment.

Gediminas Galinis,Mirjana Mladenović,I C Edmond Turcu,Mark J Watkins,Slawomir Skruszewicz,Marius Lewerenz,Arnaud Rouzée,Jonathan G Underwood,Russell S Minns,Robert Irsig,Andrew M Ellis,Richard T Chapman,J Tiggesbäumker ,Lev Kazak ,M Siano ,E Springate ,S Göde ,Karl‐Heinz Meiwes‐Broer ,Céphise Cacho ,Luis Guillermo Mendoza‐Luna ,K Von Haeften

doi:10.1039/c4fd00099d

Abstract

We show that rotational line spectra of molecular clusters with near zero permanent dipole moments can be observed using impulsive alignment. Aligned rotational wavepackets were generated by non-resonant interaction with intense femtosecond laser pump pulses and then probed using Coulomb explosion by a second, time-delayed femtosecond laser pulse. By means of a Fourier transform a rich spectrum of rotational eigenstates was derived. For the smallest cluster, C(2)H(2)-He, we were able to establish essentially all rotational eigenstates up to the dissociation threshold on the basis of theoretical level predictions. The C(2)H(2)-He complex is found to exhibit distinct features of large amplitude motion and very early onset of free internal rotor energy level structure.

Highlights

We show that rotational line spectra of molecular clusters with near zero permanent dipole moments can be observed using impulsive alignment
The impulsive alignment technique is derived from the well-known rotational coherence spectroscopy method that has been intensively applied to the investigation of the rotational structure of free molecules 10–14, clusters 15–17 and liquids 18
In impulsive alignment an intense laser pulse interacts with molecules nonresonantly and populates rotational eigenstates via virtual states corresponding to the respective laser wavelength

Summary

Introduction

We show that rotational line spectra of molecular clusters with near zero permanent dipole moments can be observed using impulsive alignment. The impulsive alignment technique is derived from the well-known rotational coherence spectroscopy method that has been intensively applied to the investigation of the rotational structure of free molecules 10–14, clusters 15–17 and liquids 18. In impulsive alignment an intense laser pulse interacts with molecules nonresonantly and populates rotational eigenstates via virtual states corresponding to the respective laser wavelength. Following the propagation of the wavepacket in time provides dynamical information straightforwardly, such as the determination of coherence times This is important for the investigation of time-dependent interactions, such as molecular collisions 33. If the amplitude of the excitation laser pulse decreases much faster than the rotational period of the molecule, the rotational states remain populated and the wavepacket propagates in time and space even after the laser field has vanished 7. A particular benefit of using molecular beams is that the cluster samples are continuously renewed, so that 2|

Results

Discussion

Conclusion