Nonadiabatic coupling between C-O stretching and Pt substrate electrons enhanced by frustrated mode excitations

We report the transient enhancement of coupling between adsorbate internal stretching vibration and substrate electrons induced by femtosecond laser excitation. A Pt(111) surface covered with carbon monoxide (CO) is irradiated with a 150 fs pump pulse at 400 nm. Transient changes in the imaginary part of nonlinear susceptibility in the C-O stretching region are probed by using femtosecond time-resolved infrared-visible sum-frequency generation with phase-sensitive detection. The line shape of C-O stretching shows remarkable asymmetry due to Fano interference with a continuum of electron-hole pair excitation. Excitations of CO-Pt frustrated modes play a key role in the enhancement of the nonadiabatic coupling.

Figure 1

(Color online) (a) A schematic of pulse timing and a setup for TR-IVSFG with heterodyne detection. LO: local oscillator, Q: quartz plate, S: sample, and F: filter. (b) SFG spectra of on-top CO on Pt(111) observed with heterodyne (black) and homodyne detection (red, light gray) in addition to a spectrum of LO (blue, dark gray).

Figure 2

(Color online) Typical $t_{\text{d}}$ dependence of real (dotted line) and imaginary (solid lines) part of $r\left(\omega \right)$. The real part is shown only for $t_{\text{d}}=-10\text{ps}$. The absorbed pump fluence was $13\text{mJ}/{\text{cm}}^2$. The horizontal axis is converted to ${\omega }_{\text{IR}}$. Open circles are fitting results by using Eq. (2).

Figure 3

(Color online) Parameters obtained by fitting $\text{Im}\left[r\left(\omega \right)\right]$ with Eq. (2): (a) nonresonant background $\delta {\chi }_{\text{NR}}$, (b) the asymmetric parameter $\tau$, (c) the dephasing rate $\gamma$, and (d) the resonance frequency shift $\delta {\omega }_0$. Absorbed fluences were 13 (red square), 8 (green filled circle), and $4\text{mJ}/{\text{cm}}^2$ (black open circle). Solid, dashed, and dotted curves in (a) are surface electronic temperature $T_{\text{e}}$ calculated with the two temperature model for absorbed fluences of $13\text{mJ}/{\text{cm}}^2$, $8\text{mJ}/{\text{cm}}^2$, and $4\text{mJ}/{\text{cm}}^2$, respectively. Experimental errors are estimated to be $\pm 1.8\text{fs}$ for $\tau$, $\pm 0.8{\text{cm}}^{-1}$ for $\gamma$, and $\pm 0.5{\text{cm}}^{-1}$ for ${\omega }_0$. Errors for $\delta {\chi }_{\text{NR}}$ are $\pm 5\mathrm{\%}$ of the maximum modulation magnitude at $I=13\text{mJ}/{\text{cm}}^2$.

Figure 4

(Color online) Plots of (a) vibrationally nonresonant background $\delta {\chi }_{\text{NR}}$ vs substrate electron temperature $T_{\text{e}}$, (b) $\tau$ vs $T_{\text{e}}$, and (c) $\tau$ vs adsorbate temperature $T_{\text{a}}$ estimated by using ${\eta }_{\text{e}}=1/0.4{\text{ps}}^{-1}$. The color code indicates different fluence ranges denoted in the figure. Data sets of $\delta {\chi }_{\text{NR}}$ with different fluence conditions are normalized at 2000 K.