Thermal Desorption Spectroscopy (TDS / TPD) Lab

1. Fundamentals of Thermal Desorption & Polanyi-Wigner Kinetics

Thermal Desorption Spectroscopy (TDS)—alternatively designated Temperature Programmed Desorption (TPD)—is an ultra-high vacuum analytical methodology that establishes the thermodynamic binding energies and kinetic pathways of atomic and molecular species adsorbed on surfaces or trapped within crystal lattices. The sample is heated linearly at a constant rate $\beta = dT/dt$ while desorbing species are quantified in real time by a high-vacuum Quadrupole Mass Spectrometer (QMS). The desorption flux is governed by the Polanyi-Wigner relation:

where $\theta$ is the instantaneous adsorbate coverage, $\nu_n$ is the pre-exponential transition frequency factor ($\sim 10^{13}\,\text{s}^{-1}$ for first-order desorption), $n$ is the reaction order, and $E_d$ is the activation energy of desorption.

2. Kinetic Reaction Orders: Molecular vs. Recombinative

• First-Order Desorption ($n = 1$): Typical of intact molecular desorption (e.g., CO from Pt, $\text{H}_2\text{O}$ outgassing from photoresists). The peak temperature $T_p$ is invariant with initial coverage $\theta_0$. As coverage increases, peak amplitude grows while maintaining a stationary peak maximum.
• Second-Order Desorption ($n = 2$): Governs associative recombination of dissociated atomic species (e.g., $2\text{H}_{\text{lattice}} \to \text{H}_{2\text{(gas)}}$). The desorption rate is proportional to $\theta^2$. As initial coverage increases, the peak temperature $T_p$ systematically shifts toward lower temperatures because of higher collision probability at elevated concentrations.

3. Hydrogen Embrittlement Trapping in Advanced Alloys

In automotive gigapascal Advanced High-Strength Steels (AHSS) and aerospace titanium fasteners, hydrogen causes catastrophic delayed fracture. TDS is the sole method capable of distinguishing dangerous diffusible hydrogen from benign trapped hydrogen:

• Reversible Diffusible Hydrogen ($E_d \approx 20\text{--}35\,\text{kJ/mol}$): Trapped at edge dislocations, low-angle boundaries, and vacancy clusters. Releases between $100^\circ\text{C}$ and $250^\circ\text{C}$. This mobile hydrogen diffuses to crack tips under mechanical tensile load, initiating micro-void coalescence.
• Irreversible Deep Traps ($E_d \approx 70\text{--}120\,\text{kJ/mol}$): Coherent interfaces around transition-metal carbide/nitride nanoprecipitates (TiC, NbC, VC). Releases only above $500^\circ\text{C}$ to $800^\circ\text{C}$. Deep traps safely immobilize hydrogen, neutralizing embrittlement risk.

4. Redhead Peak Analysis & Arrhenius Inversion

By setting the second derivative of the Polanyi-Wigner equation to zero at the peak maximum ($d^2\theta / dT^2 = 0$), the famous Redhead equation relates peak temperature $T_p$ directly to activation energy:

This provides sub-0.5% accurate binding energy extraction from a single heating cycle, bypassing multi-rate heating campaigns.