Concept of a Single Temperature for Highly Nonequilibrium Laser-Induced Hydrogen Desorption from a Ruthenium Surface

Laser-induced condensed phase reactions are often interpreted as nonequilibrium phenomena that go beyond conventional thermodynamics. Here, we show by Langevin dynamics and for the example of femtosecond-laser desorption of hydrogen from a ruthenium surface that light adsorbates thermalize rapidly due to ultrafast energy redistribution after laser excitation. Despite the complex reaction mechanism involving hot electrons in the surface region, all desorption product properties are characterized by equilibrium distributions associated with a single, unique temperature. This represents an example of ultrahot chemistry on the subpicosecond time scale.

Figure 1

Desorption of ${\mathrm{D}}_2$ from Ru after excitation with a 130 fs Gaussian laser pulse at 800 nm, $F=140\mathrm{J}/{\mathrm{m}}^2$. Left panel: time-dependent behavior of the electron, phonon, and adsorbate temperatures from a 2TM or 3TM, and the desorption rate from the 3TM according to Eq. (2). Right panel: the hot electron-mediated dynamics is mapped to an effective heating mechanism.

Figure 2

(a) Comparison of 6D $\mathrm{MDEF}(T_{\mathrm{el}})$ translational energies (blue, dark gray) to experimental results (green, light grey) for different fluences. (b) Comparison of the effective temperature model (open red squares and triangles) and $\mathrm{MDEF}(T_{\mathrm{el}})$ simulations (solid blue squares and triangles) as a function of $T_w$. The ideal thermal behavior $E_{\mathrm{trans}}=E_0+2k_B{T}_w$ is also shown (dashed line, $E_0=180\mathrm{meV}$). (c) The dependence of the reaction yield on $F$ obtained from our temperature model (red, symbols) and a nonlinear fit $Y\propto F^n$ (solid black lines) is shown for ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$. (d) Isotope ratios are compared to experiment [5].

Figure 3

Quantum mechanical DIET-Antoniewicz simulations for ${\mathrm{D}}_2$ in 3D. Shown is the population of rotational states $m$ for the vibrational states $\nu =0$ (black squares), $\nu =1$ (red triangles), and $\nu =2$ (blue dots). Boltzmann fits are depicted by dashed lines.