Photoionisation and structures of jet-formed toluene clusters Tonia M. Di Palma a, * , Attila Bende b , Antonio Borghese a a Istituto Motori – CNR, Via Marconi 8, 80125 Napoli, Italy b Molecular and Biomolecular Physics Department, National Institute for R&D of Isotopic and Molecular Technologies, Donath Street, No. 65-103, RO-400293 Cluj-Napoca, Romania article info Article history: Received 23 April 2010 In final form 19 June 2010 Available online 23 June 2010 abstract Here we report an experimental and computational study of toluene clusters that were formed in a He supersonic jet. A tunable vacuum ultraviolet radiation source was used to measure the ionisation poten- tials of dimers, trimers and tetramers from the onset of their photoionisation efficiency curves. DFT cal- culations were performed for different structures of stacked and non-stacked dimers and trimers as well as for a fully-stacked tetramer. Through comparison of the measured and calculated ionisation potentials, we show that under our experimental conditions, toluene nucleation starts from stacked dimers and proceeds through non-stacked trimers and tetramers. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Molecular aggregates or clusters can be considered physical ob- jects that bridge the transition from the gaseous to the condensed phase. They have been extensively studied in the last 30 years be- cause the knowledge of their structure and energetics helps in the exploration of all macroscopic systems at the molecular level. Molecular clusters are the seeds of the nucleation process and are involved in key issues concerning aerosol formation in the Earth’s atmosphere [1]. Moreover, cluster physics is the foundation of the increasingly important field of nanotechnology, with appli- cations ranging from material science to biology [2]. Clusters of aromatic molecules are important systems for mod- elling molecular interactions in biomolecules and their solvation. The interaction between aromatic residues, usually termed p–p interactions, is often involved in specific, energetically favourable rearrangements such as base stacking in DNA or stabilisation in protein structures [3]. Most reports have been made on clusters of benzene [4], the archetypal aromatic molecule, because it is the simplest prototype for p–p interactions. Investigations of substituted benzene clusters are more complex due to the pres- ence of substituents that introduce steric effects and dipole mo- ments and often participate in hydrogen-bond interactions. Moreover, it has been shown that the methyl substituents on the benzene ring can act as hydrogen-bond donors [5] and, thus, pro- vide a source of stabilisation in clusters containing aromatics. The dynamics of toluene (TOL) clustering in a supersonic beam were first studied by Bernstein in the 1980s [6,7]. Recently, the tol- uene dimer (TOL) 2 has been proposed as the most realistic proto- type for the p–p interaction in proteins. In particular, this model is well-suited for the study of the steric hindrances and dipole interactions in clusters [8]. Few studies have examined homoge- neous toluene clusters. One experiment has been performed using vacuum ultraviolet (VUV) synchrotron radiation [9], wherein the ionisation potential (IP) and the dissociation energy were provided for the smallest toluene cluster, i.e., the toluene dimer. Recent RE- MPI experiments [10] have shown a close similarity between the absorption spectrum of the dimer and the absorption spectra of larger clusters; authors have found that there is little change in the bonding around the absorption site as the clusters grow. It has been proposed that the (TOL) 2 dimer unit is a stable entity, and that a dimer core is likely present in larger clusters. In addi- tion, theoretical studies [11–13] have reported that stacked config- urations that are parallel (PA), anti-parallel (PA) or crossed (CR) are the most stable dimer structures. To assess the dynamics of cluster inception requires knowledge of the earliest structures that act as seeds for the later, larger ones. This Letter aims to compare experimental IP values of small (TOL) n clusters with those that were predicted by numerical modelling to shed light on the earliest seeding structures. To this end, we have used a tunable vacuum ultraviolet radiation from a laser produced plasma (LPP), we recently developed as soft single photon ionisa- tion (SPI) source and coupled to time of flight mass spectrometry (TOF-MS) [14–16]. First, we experimentally determined the photoionisation efficiency (PIE) curves of toluene clusters (TOL) n that were spectrally resolved in the 8–10 eV range of ionising pho- ton energy. We then inferred the ionisation potentials (IPs) of the first four toluene clusters (n 6 4). Next, we performed DFT theoret- ical calculations for a set of structures corresponding to (TOL) n clusters with n 6 4; the optimised geometric structures of toluene oligomers were obtained using the local version of second order Møller–Plesset perturbation theory (LMP2), whereas the IP values were calculated by considering the M06-2x DFT exchange-correla- tion functional. Finally, we compared these theoretical predictions to the experimental results and discussed our conclusions. 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.06.053 * Corresponding author. Fax: +39 0812396097. E-mail address: t.dipalma@im.cnr.it (T.M. Di Palma). Chemical Physics Letters 495 (2010) 17–23 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett