Overview of the molecular-beam/surface-science UHV apparatus with FTIR spectrometer and external infrared detector for RAIRS detection.
Schematic of the molecular beam path including the supersonic expansion nozzle, molecular beam skimmer, chopper disk, pyroelectric detector, beam flag for King & Wells measurements, and the single crystal target surface mounted on a surface science manipulator.
Relevant dimensions (mm) in the molecular beam path.
(a) Photographs of the sample holder: (A) Pt(111) single crystal; (B) tungsten heating wires; (C) K-type thermocouples monitor the sample and the holder temperature; (D) copper arm are attached with sapphire spacers (E) to the central dewar assembly (F) filled with liquid nitrogen; (G) heating wire; (H) stainless steel liquid nitrogen cryotube; (b) Pt (111) sample heated to Ts = 1000 K.
Detection of vibrationally excited molecules in the molecular beam prepared by IR pumping and detected by a pyroelectric detector. The figure shows the laser power dependence of pyroelectric signal for excitation of CH4(ν3) originating from the J = 0 and J = 1 rotational states via the R(0) and R(1) transition.
RAIRS analysis of the CH3(ads) uptake on Pt(111) at Ts = 150 K for an incident molecular beam of 3% CH4 in (Et = 47.7 kJ/mol, Tn = 700 K): (a) Time evolution RAIRS signals during 80 min molecular beam deposition and (b) final RAIR spectrum after 80 min deposition.
Calibration of the RAIRS absorption signal CH3(ads) at 2281 cm−1 in terms of C coverage on Pt(111) as determined by AES detection. The calibration data indicate a linear relationship between RAIRS absorption signal and CH3(ads) coverage with a conversion of θ(CH3) [ML] = 211 [ML/abs]. RAIRS + 0.025.
(a) RAIRS measurement of methyl uptake from a ∼3% CH4/He (Tn = 900 K) molecular beam incident on Pt(111) at a surface temperature of 150 K; the solid line shows the fit of a site-blocking Langmuir uptake model (see text) to the data points to extract the coverage dependent sticking probability S(θ) from the slope of the uptake curve; (b) simultaneous measurement of S(θ) by the King & Wells (K&W) method (a); and (c) comparison of S(θ) measured by RAIRS (a) and K&W (b).
RAIR spectra of the nascent chemisorption products for the five different methane isotopologues dissociating on Pt (111) at Ts = 150 K, activated by ∼50 kJ/mol incident translational energy and 5–10 kJ/mol of thermal vibrational excitation from nozzle heating to 700 K. Both C–H and C–D cleavage products are observed for all partially deuterated methane isotopologues. Signals near 2060 cm−1 (defect sites) and 2080 cm−1 (terrace site) are due to a small coverage (≤0.3% ML) of a CO impurity in the molecular beam.
Initial reactivity S 0 for (laser-off •) and (ν3 state-specific ■) as a function of normal incident translational energy Et for dissociative chemisorption of CH4 on Pt(111) at 150 K. The vibrational efficacy η(υ3) ≈ 0.7 is given by the offset between the state-resolved sticking curves (dashed lines) and the vibrational energy of the υ3 state. Error bars (∼23%) are estimated from the relative errors in the coverage measurements (10%) and in the molecular beam flux measurements (13%).
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