It's hard to measure the surface temperature during the LITD process because the laser pulse width is 6 nano seconds. Therefore, we need to simulate the surface temperature to know the gas molecule desorption rate. To calculate the temperature evolution, we used Laplace thermal diffusion equation. If you click the left picture, you can see clearer result of the simulation. The temperature of the Nickel surface is heated up to 1066K within 10 nano seconds. Though the cooling process takes more time, we don't need to worry about it because the hydrogen molecules desorption process happens during the heating procedure. (FWHM of the desorption rate is ~3 nano seconds.) It gives us a good advantage to measure the TOF of desorption gas molecules.

Left picture showes the hydrogen desorption on the metal(Ni) surface. Because the laser beam is Gaussian, hydrogen isn't desorbed completely at the outside from the beam center while it's desorbed completely at the beam center. However, the desorption amount is changing more rapidly than Gaussian from 1 to 0 at x is around 20 (it depends on the laser power, and x= the distance from the laser center. unit = 1/20 sigma). That's because the desorption depends on the surface temperature exponentially. With this sharp desorption resolution and the focusing of laser beam, we can use LITD to get a good spatial relution on the metal surface experiment.

We measure the LITD signal using SX200 mass spectrometer. The desorption happens within 20 nano seconds, and the relaxation time to get the thermal equilibrium by the surface molecule is in the order of pico seconds. Therefore, the TOF of the gas molecule showes the thermal condition of the metal surface when the molecules are desorbed. The TOF data with different angles between the mass spectrometer and the metal sample also give us the informations of the angular dependence of the disorption.