1.   Diffusion of hydrogen adatoms on Cu(111) using linear optical diffraction methods and Xe density gratings as laser-erasable templates

Using linear optical diffraction from hydrogen density gratings on Cu(111), we studied diffusion kinetics of hydrogen atoms on Cu(111) from 140K to 210K.  We used a novel method to prepare periodic hydrogen density profiles on Cu(111).  We first formed a Xe density grating from a pre-adsorbed Xe monolayer on Cu(111) at 35K using dual-beam LITD.  Using the Xe grating as the template, we formed a hydrogen density grating complementary to the template.  We then removed the template using single-beam LITD.

To measure diffusion constants of hydrogen adatoms on Cu(111) from 140 K to 210 K, we detected the linear optical diffraction from the hydrogen density grating on Cu(111).  We used two set-ups to produce hydrogen gratings with periodicities of a = 10.3 μm and 2 μm.  It enabled us to measure diffusion constants from 2×10^-9 to 1×10^-14 cm²/sec (> 5 orders of magnitude).  Fig. 1 shows one of the set-ups.

The normalized 1^st-order linear optical diffraction signal is given by (Fick's law)

S₁(t) = exp(-α(T)t)                         (1)

The diffusion constant D(T) is related to the exponent by

D(T) = α(T) (a²/8π²)                     (2)

In Fig. 2, we display a set of real time traces of S₁(t).  Fig. 2a shows the signals obtained with a = 10.3 µm.  Fig. 2b shows the signals obtained with a = 2 µm.  By fitting S₁(t) to Eq. (1) we obtained α(T).  Through Eq. (2) we found the hydrogen diffusion constants D(T) from 140K to 210K.  In Fig. 3, we display the Arrhenius plot of D(T).

Over the temperature range of investigation, the diffusion rates are described well with an Arrhenius function with single activation energy,

D(T) = D₀exp(-E_diff/k_BT)               (3)

By fitting the experimental D(T) to Eq. (3) (solid line in Fig. 3), we found

D₀ = 48 cm²/sec                            (4)

E_diff = 0.44 eV (10 kcal/mol)         (5)

We are currently extending the measurement to below 140K using a new optical set-up that produces hydrogen density gratings with periodicity of a = 0.5 μm.  This will enable us to explore the on-set of quantum tunneling diffusion.

2.   Interlayer mass transport of Xe multi-layers studied from the decay of surface plasmon polariton waves excited with Xe multilayer gratings

A novel application of excitation of a surface plasmon polariton wave (SPPW) on a metal surface coupled by an adsorbate density pattern is the study of mass transport of the adsorbates.  A SPPW is an electromagnetic wave confined at the interface between a metal (ε_m) and a transparent material (ε_s).  It has a well-defined wave-vector along the interface q = (ω/c)(ε_mε_s/(ε_m+ε_s))^1/2.  One way to excite a surface-plasmon polariton wave is to form a dielectric grating (with spatial period of a) at the interface.  When a mono-chromatic light is incident from the transparent material onto the interface, the SPPW is excited.  At incidence angle θ_SPR where sinθ_SPR = (ε_mε_s/(ε_m+ε_s))^1/2 – λ/a, the reflectance is maximally attenuated.  The reflectance attenuation or dip depends on the depth of the grating.  In the limit that the thickness of the grating is much less than λ, the reflectance dip varies quadratically with the depth of the grating η₁.  If η₁ decays due to mass transport of molecular constituents of the grating, one can study the mass transport of molecular constituents of the grating by following the dip experimentally.

To demonstrate this concept, we excited the SPPW on Cu(111) using patterned Xe multi-layers as gratings.  We deposited a thin Xe layer of 6 monolayers.  Using LITD, we formed a Xe grating with mean thickness of 4 monolayers and modulation depth of η₁ = 2 monolayers.  The spatial periodicity was a = 5.45 μm.  To detect the reflectance dip due to the SPPW excitation, we used a converging He-Ne laser beam with the incidence angle spanning 8º and measured the angle-resolved reflectivity difference (OI-RD).  We observed a sharp dip in the OI-RD signal near 70.2º.

By following the decay of the reflectance dip, we determined the decay exponents α'(T) from 46K to 54K.  Assuming that α'(T) related to apparent diffusion constants D'(T) through Eq. (2), we obtained D'(T) shown in Fig. 4 as an Arrhenius plot.  We are exploring Xe interlayer mass transport kinetics potentially responsible for the behavior as exhibited in Fig. 4.