Open AccessVelocity‐resolved Laser‐induced Desorption for Kinetics on Surface Adsorbates
Author(s) -
Papendorf Kim,
Golibrzuch Kai,
Zhong Tianli,
Schwabe Sven,
Kitsopoulos Theofanis N.,
Wodtke Alec M.
Publication year - 2022
Publication title -
chemistry ‐ methods
Language(s) - English
Resource type - Journals
ISSN - 2628-9725
DOI - 10.1002/cmtd.202200017
Subject(s) - desorption , molecular beam , soft laser desorption , chemistry , thermal desorption , analytical chemistry (journal) , laser , kinetics , ion , femtosecond , ionization , mass spectrometry , molecule , chemical physics , adsorption , matrix assisted laser desorption/ionization , optics , organic chemistry , chromatography , physics , quantum mechanics
Most experimental methods for studying the kinetics of surface reactions – for example, temperature programmed desorption (TPD), molecular beam relaxation spectrometry (MBRS) and velocity‐resolved kinetics (VRK) – employ detection schemes that require thermal desorption. However, many adsorbates – for example reaction intermediates – never leave the surface under reaction conditions. In this paper, we present a new method to measure adsorbate concentrations on catalytic surfaces and demonstrate its utility for studying thermal desorption kinetics. After a short‐pulsed molecular beam deposits CO or NH 3 on Pt (111), the surface is irradiated with an ultrashort laser pulse that induces desorption. Another tightly focused ultrashort laser pulse ionizes the gas‐phase molecules by a non‐resonant multiphoton process and the ions are detected. This two‐laser signal is then recorded as a function of time after the dosing molecular beam pulse and decays exponentially. First‐order thermal desorption rate constants are obtained over a range of temperatures and found to be in good agreement with past reports. Ion detection is done mass selectively with ion‐imaging, dispersing the gas phase molecules by their velocities. Since laser‐induced desorption (LID) produces hyperthermal gas phase molecules, they can be detected with little or no background. This approach is highly surface‐specific and exhibits sensitivity below 10 −4 ML coverage. Because the signals are linearly proportional to adsorbate concentration, the method can be employed at lower temperatures than VRK, whose signal is proportional to reaction rate.