Ultrafast Transient Dynamics of Adsorbates on Surfaces Deciphered: The Case of CO on Cu(100)

Time-resolved vibrational spectroscopy constitutes an invaluable experimental tool for monitoring hot-carrier-induced surface reactions. However, the absence of a full understanding of the precise microscopic mechanisms causing the transient spectral changes has limited its applicability. Here we introduce a robust first-principles theoretical framework that successfully explains both the nonthermal frequency and linewidth changes of the CO internal stretch mode on Cu(100) induced by femtosecond laser pulses. Two distinct processes engender the changes: electron-hole pair excitations underlie the nonthermal frequency shifts, while electron-mediated vibrational mode coupling gives rise to linewidth changes. Furthermore, the origin and precise sequence of coupling events are finally identified.

Figure 1

Schematic chronograph of the coupling mechanisms underlying the IS frequency shifts of CO on Cu(100). (Left) Example of the time evolution followed by the electron and phonon temperatures when an UV pump laser starting at $t=0.3\mathrm{ps}$ heats the surface. (Right) Time evolution of the hot-carrier-induced CO dynamics. First-order NC process is active between around 0.3 and 0.6 ps (blue frame and arrow). Electron-mediated vibrational mode couplings start at around 0.3 ps and operates until equilibration (green frame). The specific time intervals are indicated by corresponding green arrows.

Figure 2

Transient changes in the $\mathrm{CO}/\mathrm{Cu}(100)$ system induced by 400 nm pump-laser pulses with 150 fs duration. (a) Electron $T_e(t)$ and (b) lattice $T_l(t)$ temperatures as a function of time for an absorbed fluence $F=170\mathrm{J}/{\mathrm{m}}^2$. Initial temperature is 100 K. Time-dependent (c) frequency and (d) linewidth changes of the IS mode under nonthermal condition induced by the UV pump at $t=0.3\mathrm{ps}$. Blue, orange, and green represent the results for different $F$. (e),(f) Contribution-resolved analysis of frequency and linewidth changes when $F=170\mathrm{J}/{\mathrm{m}}^2$. Blue represents the contribution coming from the first-order NC term and orange shows the EMPPC contribution, while gray circles are the sums of the two. Light blue and magenta points show the corresponding results for $t=50$ and 100 ps, respectively. Green squares are experimental frequency shifts obtained from Refs. [11].