Probing the Structure and Dynamics of Molecular Clusters Using Rotational Wave Packets

Abstract

Rotational wave packets of the weakly bound C 2 H 2 -He complex have been created using impulsive alignment. The coherent rotational dynamics were monitored for 600 ps enabling extraction of a frequency spectrum showing multiple rotational energy levels up to J ¼ 4. spectrum has been combined with ab initio calculations to show that the complex has a highly delocalized structure and is bound only by ca. 7 cm −1 . The experiments demonstrate how highly featured rotational spectra can be obtained from an extremely cold environment where only the lowest rotational energy states are initially populated. DOI: 10.1103/PhysRevLett.113.043004 PACS numbers: 33.20.-t, 33.80.-b, 36.40.Mr Weakly bound molecular complexes are important model systems for condensed matter held together by van der Waals forces [1]. The weak binding leads to large amplitude motion and delocalized structures that are often reflected in complex rotational spectra that cannot easily be categorized through rigid rotor models. Furthermore, obtaining these spectral features imposes practical problems associated with the very low temperatures that are required to form weakly bound complexes. The low temperatures imply the population of only the lowest quantum states. Consequently, the features in conventional rotational (microwave) spectroscopy comprise only a few lines-often not enough for a comprehensive analysis of the structure of the complex [2].A potential solution to this problem is to use impulsive alignment to generate rotational wave packets through the nonresonant interaction of an intense laser field with a molecule, aligning it in space [3]. Tuning the laser pulse duration and intensity controls the number of rotational eigenstates that contribute to the wave packets. Using this technique for isolated molecules it is possible to monitor the evolution of alignment in time, simultaneously obtaining information on the rotational dynamics and structure [3][4][5][6][7][8][9][10]. The extension of this technique to complex systems where weak interactions are important has been postulated theoretically [11], and demonstrated on the very simplest of systems, noble-gas dimers [12]. The extension of impulsive alignment to van der Waals complexes with internal and delocalized structures remains a challenge. This is due in part to the weak binding and floppy nature of the complex, but also due to the increased complexity of the resulting rotational spectrum, such that currently, to the best of our knowledge, no such measurement has been reported.This approach may also have potential for exploring liquids, where the wave-packet dynamics will be sensitive to dephasing. A particularly promising liquid to begin such studies is superfluid helium. Frequency-domain spectra of molecules embedded into large superfluid helium droplets show sharp rotational transitions, suggesting that wave packets will not dephase [2,13]. However, the recent attempt to impulsively align molecules in large helium droplets by Pentlehner et al. suggests that this may not be th...

The infrared spectrum of the He–C2D2 complex

“…We note that the present infrared results are about 10 times more accurate than the RWPS results 20,21 (of course, the RWPS technique may be capable of higher accuracy in the future). Furthermore, the assigned RWPS transitions all occur within a given j-stack, which means they depend strongly on the intermolecular distance, but are relatively insensitive to anisotropy in the potential surface.…”

“…The root mean square (rms) deviations between theory and experiment are about 0.035 cm −1 for IPESA and 0.018 cm −1 for IPESB, as compared to the estimated 20,21 experimental uncertainty of 0.03 cm −1 . However, the situation is different for the infrared results on He-C 2 D 2 : up to J = 4 in the j ≤ 1 stacks, the rms deviations are about 0.054 cm −1 for IPESA and 0.105 cm −1 for IPESB (see Table I).…”

“…They noted impressive agreement between experiment and theory, and also claimed 20,21 that their potential (IPESB) is superior to IPESA, based on the implication that the RWPS results are accurate enough to discriminate between slightly different predicted equilibrium structures. To investigate this comparison further, we have calculated rovibrational levels for He-C 2 D 2 (Table I) and He-C 2 H 2 (Table V) using both surfaces.…”

“…As noted, Galinis et al 20,21 recently reported spectra of He-C 2 H 2 obtained using rotational wave packet spectroscopy (RWPS) and a new intermolecular potential energy surface (which we label IPESB) which was used to help interpret the spectra. They noted impressive agreement between experiment and theory, and also claimed 20,21 that their potential (IPESB) is superior to IPESA, based on the implication that the RWPS results are accurate enough to discriminate between slightly different predicted equilibrium structures.…”

“…18,19 Very recently, pure rotational Raman-type transitions with ∆J = 2 have been measured in He-C 2 H 2 by observing rotational wave packets in the time domain following impulsive alignment by a femtosecond laser pulse. 20,21 In these papers, comparisons were also made between the observed transitions and rovibrational energies computed using a new ab initio potential energy surface. In the present paper, we report our experimental infrared spectra of He-C 2 D 2 in conjunction with calculations based on the helium-acetylene potential energy surface of Fernández et al 22 In addition, we make comparisons with the new surface of Galinis et al 21 and find that our He-C 2 D 2 spectra are accurate enough to discriminate between the two surfaces.…”