We measured the fully resolved 3-dimensional velocity distributions of nitric oxide photodesorbed from a gold single crystal. These experiments combine time-of-flight measurements and the velocity map imaging technique to yield velocity distributions resolved in three dimensions for a prototypical surface-adsorbate system. Nitric oxide adsorbed on Au(100) was photodesorbed using a 355 nm laser beam. The desorbed NO molecules were ionised in the gas-phase by resonance-enhanced multi-photon ionisation within a set of velocity map imaging optics. The NO molecules preferentially leave the surface along the surface normal with a very narrow angular distribution, indicating a non-thermal desorption process.
We report velocity and internal state distributions of nitric oxide photodesorbed from an Au(100) single crystal using 355 nm and 266 nm photons. The velocity distributions were measured in all three dimensions independently using our novel 3D-velocity map imaging setup. Combined with the internal energy distributions, we reveal two distinct desorption mechanisms for the photodesorption of NO from gold dependent on the photon wavelength. The 355 nm desorption is dominated by a non-thermal mechanism due to excitation of an electron from the gold substrate to the adsorbed NO; this leads to a super-thermal and noticeably narrow velocity distribution, and a rotational state distribution that positively correlates with the velocity distribution and can be described by a rotational temperature appreciably above the surface temperature. Desorption with 266 nm photons leads to a slower average speed and wider angular distribution, and rotational temperatures not too far off the surface temperature. We conclude that in the absence of occupied orbitals in the substrate and unoccupied orbitals on the adsorbed NO separated by 4.7 eV, corresponding to 266 nm, the shorter wavelength desorption is dominated by a thermally-activated mechanism.
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