A novel gas-vacuum interface for environmental molecular beam studies

Abstract

Molecular beam techniques are commonly used to obtain detailed information about reaction dynamics and kinetics of gas-surface interactions. These experiments are traditionally performed in vacuum and the dynamic state of surfaces under ambient conditions is thereby excluded from detailed studies. Herein we describe the development and demonstration of a new vacuum-gas interface that increases the accessible pressure range in environmental molecular beam (EMB) experiments. The interface consists of a grating close to a macroscopically flat surface, which allows for experiments at pressures above 1 Pa including angularly resolved measurements of the emitted flux. The technique is successfully demonstrated using key molecular beam experiments including elastic helium and inelastic water scattering from graphite, helium and light scattering from condensed adlayers, and water interactions with a liquid 1-butanol surface. The method is concluded to extend the pressure range and flexibility in EMB studies with implications for investigations of high pressure interface phenomena in diverse fields including catalysis, nanotechnology, environmental science, and life science. Potential further improvements of the technique are discussed.

Understanding water interactions with organic surfaces: environmental molecular beam and molecular dynamics studies of the water–butanol system

“…The EMB technique has been described in detail elsewhere, 20,21 and is only briefly presented here. The EMB apparatus consists of a multi-chamber high-vacuum system and the main components of the experimental setup are illustrated in Fig.…”

“…The gas is allowed to expand into vacuum and part of the molecular flow is selected in space and time using a skimmer and a rotating chopper that is frequency locked to the 60 Hz beam source. 20,21 This generates a pulsed directed low-density flow of D 2 O molecules with a kinetic energy of 0.29 AE 0.01 eV. The molecular beam flow is directed through a series of high-vacuum chambers and targeted on a temperaturecontrolled substrate surface situated inside an environmental chamber.…”

“…A 10 Â 10 mm 2 grating serves as an interface between the environmental chamber and the surrounding ultra-high vacuum system, and enables the molecular beam to enter and exit the environmental chamber. 21 A highly oriented pyrolytic graphite crystal (grade ZYB, 4 Â 4 mm 2 , Advanced Ceramics Corp.) is used as the surface substrate. It is positioned 1.6 mm behind the grating and the beam collides with the substrate at an incident angle of 431 with respect to the surface normal direction.…”

“…The experimental setup has been designed to reduce the path length of the molecular beam within the environmental chamber to 4.4 mm, in order to limit attenuating collisions between the beam molecules and the background gas within the chamber. 21 The formation and maintenance of a condensed n-butanol layer on the substrate is monitored by probing the surface with a laser (670 nm, 860 mW). The laser is directed at the surface with an incident angle of 31 and a photodiode detects the change in reflected intensity due to constructive and destructive interference caused by reflections from the graphite-butanol and butanol-gas interfaces as the condensed layer grows on the graphite surface.…”

Near-Ambient Pressure Velocity Map Imaging

“…65 The Environmental Molecular Beam (EMB) technique, which has been developed in Gothenburg to allow scattering experiments from volatile liquids, uses an aperture or a grating to separate the high pressure surface from the low pressure detector region. 66,67 This allows high pressure and very short flight distances of the molecules through the high pressure region. The use of electron ionization provides universal detection but prevents quantum state specific measurements.…”