Time-of-flight scattering and recoil spectrometry (TOF-SARS) applied to molecular liquid surfaces: A new approach to surface composition and orientation

In spite of their importance in many systems, liquid surfaces have been explored at the microscopic level to a much lesser extent than solids. Most surface analysis must take place in vacuum, a major drawback for liquids. The technique of time-of-flight scattering and recoil spectrometry (TOF-SARS) has been applied to molecular liquid surfaces for the first time. The apparatus borrows key elements from previous TOF-SARS experiments on solids and from molecular beam scattering (MBS) and features excellent surface specificity and the ability to detect all elements. A high-vacuum time-of-flight spectrometer was developed for the purpose of measuring the surface atomic concentration of atoms in low-vapor pressure liquid samples, and hence to infer preferred surface orientations.; The TOF-SARS experiment involves surface bombardment with inert gas ions in the 1–3 keV energy range. During the interaction surface atoms may either (a) induce scattering of primary ions or (b) recoil from the surface. A binary collision model describes the kinematics and dynamics of the interactions well, allowing prediction of velocities and probabilities of particles leaving the surface. Particles that reach a detector along a ∼1.1 in flight path are separated by velocity, and signals are collected as a histogram, revealing relative measured intensities that are converted to ratios of accessible surface atoms. Comparing the measured atomic ratios with computer-simulated accessible atomic ratios for various possible orientations gives insight into preferred surface orientation.; A number of systems were explored in this work: liquids including a complementary pair of molecules having distinct ‘head-tails’ structures; glycerol as a highly H-bonded system, and a room-temperature molten salt. Preliminary results reveal that surface molecules appear in most cases to adopt some preferred orientation at the interface. The TOF-SARS technique was able to distinguish ‘head’ from ‘tail’ in molecules exhibiting that structure, suggesting only part of the head was accessible. In glycerol, all but two possible orientations were ruled out but the symmetrical nature of the molecule prohibits definitive assignment. The ionic liquid was found to have the cation and anion sharing the surface population roughly equally, and a preferred orientation for the substituted aromatic anion was discovered.