Hot carrier excitation of adlayers: Time-resolved measurement of adsorbate-lattice coupling

Abstract

Picosecond time-resolved infrared absorption measurements of CO/Cu(100) following visible and ultraviolet laser pumping allow a determination of the time scales for photoexcited carriers and lattice phonons to couple to the CO frustrated translation vibrational mode.PACS numbers: 68.35.Ja, 42.62.Fi, 63.20.Kr, 78.47.+p Since low frequency vibrational modes of adsorbates are the precursors to motion about and away from a surface, energy transfer through these modes plays important roles in a wide range of interface processes. For example, calculations of surface diffusion rely upon knowledge of the frictional damping rates of frustrated translations [1], Reactions between molecules on neighboring sites rely on activation of these modes to bring the molecules together. Recently, the field of femtosecond surface photochemistry has opened up a realm of novel interactions of molecules with nonequilibrium substrate degrees of freedom. Ultimately, an understanding of these processes requires knowledge of the various pathways by which the low frequency modes can couple to the substrate degrees of freedom [2][3][4][5]. However, the low cross sections for optical absorption and the lack of available time-resolved spectroscopies at these frequencies have prevented the direct measurement of the lifetimes of these modes.By observing the vibrational response of an adsorbate during ultrashort laser heating of a substrate, the time scales and mechanisms for surface-molecule energy transfer can be revealed. In this Letter, time-resolved measurements of the transient response of CO/Cu(100) to picosecond laser excitation are presented. Unlike previous measurements of substrate-adsorbate coupling strengths [6], these results unambiguously establish the contribution of transiently heated substrate electrons and phonons in activating the low frequency adsorbate vibrational modes and allow the coupling times of the frustrated translational mode to hot carriers and phonons to each be determined.The 0.5 monolayer (ML) (1 ML=7.67xl0 14 cm" 2 ) (V5xV2)/?45° overlayer of CO on Cu(100) at 100 K has been well characterized in terms of both adsorbate vibrational modes and low lying unoccupied electronic states. The electronic levels of the system have been characterized using inverse photoemission, indicating two unoccupied levels centered at 2.6 and 3.6 eV above the Fermi level, related to the CO 5<r and 2/r* levels [7,8]. Infrared reflection-absorption spectroscopy (IRAS) [9,10] and helium atom scattering [11] have identified the four fundamental vibrational modes: the CO stretch at vi =2086 cm" 1 , the Cu-CO stretch at v 2 =345 cm" 1 , the frustrated rotation at v>3=285 cm" 1 , and the frustrated translation at V4 = 32 cm" 1 . We have made independent steady state IRAS measurements [12] on the same crystal used for the time-resolved experiments, determining the temperature dependence of the CO stretch mode vi =2086-0.031 [T(K) -100] cm" 1 and width 8v x =3.9 + 0.005[r(K)-100] cm" 1 . This behavior is readily accounted for by dephasing of ...

Femtosecond Surface Vibrational Spectroscopy of CO Adsorbed on Ru(001) during Desorption

“…The observed redshift can be explained by a weakening of the C-O bond caused by the increasing amplitude of the (thermally excited) frustrated translation, that displaces the CO molecule from its on-top position towards a bridge position, where it has a lower vibrational frequency. The good agreement between the one-mode theory and our low-fluence experiment has been observed previously in transient band shifts experiments [4] and T -dependent IR data [13]. In contrast, for the high-fluence data, apparently a regime is entered where the assumption of coupling to a single mode is no longer adequate: the (maximal) observed change in width is an order of magnitude larger than predicted by theory, indicating that other modes of higher frequency determine the line shape.…”

“…Using picosecond (ps) techniques, information about the dynamics of energy exchange between adsorbed carbon monoxide (CO) and metal substrates has been obtained [4], by monitoring transient band shifts of the C-O vibration after optical excitation. The technique has further been used to observe vibrational energy relaxation [5] and reversible transient chemical transformations [6] at surfaces.…”

Substrate-Adsorbate Coupling in CO-Adsorbed Copper

“…Frustrated translational vibrations are considered particularly important in surface chemistry because they are typically very low in energy (a few meV) and are thus thermally activated. This mode has been found to be extremely short-lived for CO on copper, with a lifetime in the few picosecond range [4][5][6]. The mechanisms governing the relaxation of this mode are of considerable interest.…”

Lateral hopping and desorption of a single CO molecule on a Cu(110) surface induced by femtosecond laser pulses

“…Bartels et al calculated T el (t) and T ph (t) for Cu using the two-temperature model, 18 and T FT (t) usingh FT = 4 meV, η el = 1/(5.1 ps), and η ph = 1/(4.2 ps) at F = 30 J/m 2 and at the substrate temperature of 22 K. These coupling times to the electronic and phononic heat baths were deduced from the infrared response of the C-O stretch mode on Cu(100) to heating of the substrate. 19 In what follows, we present the full formula we use here in order to reproduce T el (t) and T ph (t) calculated by Bartels et al 1 The simulation done by Bartels et al is necessarily repeated in detail to share the same T el (t), T ph (t), and R al,ac (t) with them and to demonstrate an indispensable role of an excitation of the FR mode in CO hopping on Cu(110) surface as confirmed in real-time observation on a stepped Pt surface. 3 The well-established two-temperature model reads as…”