A pulsed uniform Laval expansion coupled with single photon ionization and mass spectrometric detection for the study of large molecular aggregates

Abstract

We report on a new instrument that allows for the investigation of weakly-bound molecular aggregates under equilibrium conditions (constant temperature and pressure). The aggregates are formed in a Laval nozzle and probed with time-of-flight mass spectrometry in the uniform postnozzle flow; i.e. in the equilibrium region of the flow. Aggregates over a very broad size range from the monomer to particle sizes of 10-20 nm can be generated and studied with this setup. Soft ionization of the aggregates is performed with single photons from a homemade vacuum ultraviolet laser. The mass spectrometric detection provides molecular-level information on the size and chemical composition of the aggregates. This new instrument is useful for a broad range of cluster studies that require well-defined conditions.

Overview: Homogeneous nucleation from the vapor phase—The experimental science

“…244,250,251,[265][266][267] For weakly bound clusters, significant progress is also being made using a sophisticated combination of techniques. [252][253][254][255] As instrumentation, including mass spectrometry, continues to develop it will become possible to test critical cluster characteristics derived via the nucleation theorems against the values determined by more of these direct experimental methods and from molecular modeling studies. The ability to make cluster resolved measurements on systems during nucleation events is an incredible step forward in nucleation research.…”

“…A bigger challenge is to use mass spectrometry to study weakly bound clusters under conditions where T and p are well defined. Most of the compounds investigated in standard nucleation experiments form weakly bound clusters, and, thus, it is encouraging that Signorell and co-workers [252][253][254][255] are developing mass spectrometric methods to interrogate these systems as well.…”

“…By applying high ion extraction voltages, they are able to observe cluster distributions [253][254][255] that include the monomer, dimer, trimer, etc., through to particles with diameters between 10 and 20 nm. As illustrated in Fig.…”

Can we Observe Gas Phase Nucleation at the Molecular Level?

“…The experimental setup combines a Laval nozzle for cluster formation with soft mass spectrometeric detection (Section 2). We use a pulsed Laval nozzle and match the background gas after the nozzle to ensure a uniform postnozzle flow, i. e. the temperature, the pressure (number density) and the gas velocity (Mach number) are constant in the postnozzle flow [19,20]. The design is similar to that described in refs.…”

“…The design is similar to that described in refs. [19][20][21][22]. Uniform Laval expansions have been successfully used before for gas kinetic reaction studies because they allow one to investigate processes under well-defined conditions (thermal equilibrium and constant pressure) (see refs [19,[21][22][23][24][25][26][27][28][29][30][31] and references therein).…”