Site Specificity in Femtosecond Laser Desorption of Neutral H Atoms from Graphite(0001)

Femtosecond laser excitation and density functional theory reveal site and vibrational state specificity in neutral atomic hydrogen desorption from graphite induced by multiple electronic transitions. Multimodal velocity distributions witness the participation of ortho and para pair states of chemisorbed hydrogen in the desorption process. Very slow velocities of 700 and $400{\mathrm{ms}}^{-1}$ for H and D atoms are associated with the desorption out of the highest vibrational state of a barrierless potential.

Figure 1

Kinetic energy distribution for desorption out of five bound states for the para adsorption site, calculated from Eq. (2). The parameters are given in the text, and ${ϵ}_i={ϵ}_0$. The probabilities have been normalized to fit the figure, in fact the maximum value of $P_n(v)$ increases by an order of magnitude for each $n$. The $n=8$ state thus has a much larger desorption probability than the rest.

Figure 2

Observed and calculated velocity distribution of H atoms from graphite. Triangles and dotted line denote the experimental data, the solid and dashed lines represent the theoretical results.

Figure 3

Same as Fig. 2, but for D atoms. Note the distinct peak at very low velocities in both the experimental data and the theoretical results.