Velocity‐resolved Laser‐induced Desorption for Kinetics on Surface Adsorbates

Abstract

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 NH3 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.

Time-resolved surface reaction kinetics in the pressure gap

“…The NAP-VMI technique should be applicable to a range of other gas-condensed phase interfaces where high pressures are scientically interesting, e.g. combined with LID-VRK, 70 or unavoidable, such as scattering from liquid microjets. Further developments to the instrument are possible to allow operation at higher pressures and to move towards ambient pressure VMI.…”

Spiers Memorial Lecture: New directions in molecular scattering

“…386 More recently, a family of related techniques have become possible that take advantage of modern pulsed molecular beams and pulsed lasers as well as ion imaging. The velocity-resolved kinetics (VRK) methods [387][388][389] now make the experimental determination of reaction rates much more quantitative. Discovered serendipitously while performing state-to-state time-of-ight measurements involving molecular beams scattering from Pt, 390 VRK relies on laser methods that simultaneously obtain the densities and velocities of molecules, allowing accurate determinations of molecular ux.…”

“…This calls for new means for sensitive and specic, yet universal, detection of adsorbates. Laser-induced desorption followed by gas-phase ionization 389 has the potential to provide massspectrometric-like signatures of intermediates, if it can be combined with a universal means of detection (e.g., electron bombardment ionization or so VUV ionization). Multi-mass imaging 130 and covariance imaging 129 are two other excellent experimental methods that are likely to nd application in problems of reactive scattering from surfaces.…”

Electrochemical and Photoelectrochemical Water Splitting: Operando Raman and Fourier Transform Infrared Spectroscopy as Useful Probing Techniques

“…Ultrahigh vacuum conditions (<10 −9 hPa), ultrapure single crystal surfaces, and controlled adsorbate coverages down to the 0.1 monolayer range are required for atomic‐level surface studies. [ 26 ] In some techniques, the systems are often maintained at room temperature or below to allow for pristine surface conditions and controlled adsorbate coverages. Such studies are necessary to understand the kinetics and thermodynamics of reaction mechanisms in heterogeneous catalysis, but require a great deal of effort to obtain such carefully controlled conditions.…”