Molecular vibrational energy relaxation at a metal surface: Methyl thiolate on Ag(111)

Abstract

The lifetime of the first excited level of the symmetric C-H stretching mode of CH3S chemisorbed on a Ag(lll) surface was measured by picosecond vibrational spectroscopy. A biexponential decay (--3and 63-ps lifetimes at 300 K) was observed, with a substantial temperature dependence of the slow component. Both decay processes are assigned to intramolecular vibrational relaxation. The decay rates are 2 orders of magnitude too fast to be explained by electron-hole-pair damping by the metal substrate.PACS numbers: 82.20. Rp, 34.30.+h, 78.47.+p Despite the fundamental importance of vibrational energy transfer to chemical processes at metal surfaces, there have been no time-domain measurements of vibrational lifetimes for adsorbates on single-crystal surfaces. ] This Letter reports such measurements for the first excited level of the symmetric C-H stretching mode of methyl thiolate, CH3S, chemisorbed on a Ag(l 11) surface. Previous indirect measurements based on linewidth have assigned the vibrational relaxation of this mode in similar adsorbates [CH 3 O/Cu(100) and CH 3 C/Pt(l 11)] to electron-hole-pair excitation in the metal. 2 The present results show that the vibrational linewidth is not determined in CH3S by vibrational energy relaxation. In addition, the relaxation itself is assigned to intramolecular energy transfer. The previous assignments must be reconsidered. The results demonstrate the importance of intramolecular energy transfer for small polyatomic molecules even where electron-hole-pair damping has previously been assigned dominance.To measure vibrational relaxation times, a resonant infrared pulse populates the first excited state of the symmetric C-H stretching mode at 2918 cm -1 . The subsequent population relaxation is monitored by transient sum-frequency-generation (SFG) spectroscopy. 3 In this method, an SFG signal is generated by mixing a visible probe pulse with a vibrationally resonant infrared probe pulse at the surface. The vibrationally resonant SFG signal is proportional to the square of the population difference between the lower and upper levels of the transition probed. 3 Its square root is therefore a measure of the population difference on the vibrational transition. If So is the SFG signal with no pump and S(r^) is the signal at delay r</ after the pump, 1 -[Sir^)] ,/2 /v^o is then analogous to a transient absorption probe at resonance; 4 this normalized difference of square roots is proportional to the pump-induced change in the population difference, which is dependent on the probe delay. This description assumes the excited-level absorption is shifted by anharmonicity, so that it does not interfere with the ground-state probe. In the present case, the shift of the excited-state absorption of the symmetric C-H stretching mode is 50-100 cm" 1 , well outside the probe-pulse bandwidth.Visible pulses with 4-ps duration (5-cm -1 bandwidth) and infrared pulses of 3-ps duration (6-cm -1 bandwidth) are generated in a picosecond laser system, as previously described. 3 The infrared pulses...

The interface structure of n-alkylthiolate self-assembled monolayers on coinage metal surfaces

“…An early spectroscopic investigation of methylthiolate on Ag(111) showed that it yields a LEED pattern corresponding to (77)R19.1º ordering [29]. The fact that atomic S was known to give an ordered phase on this surface (but that the spectroscopic data showed clearly that the thiolate was not dissociated in this study) with the same periodicity led to an assumption that the two structures have strong similarities.…”

pH dependent surface enhanced Raman study of Phe+Ag complex and DFT calculations for spectral analysis

“…In the SERS process, in addition to the enhancement in intensity of the vibrational modes, a significant frequency shift and change in Raman line-width compared to those in the free molecules are also observed [5]. To understand the time scale involved in the SERS process, time domain measurements of vibrational life-time for adsorbates on single crystal surface have been reported using picosecond vibrational spectroscopy [6]. In ref.…”

The role of reconstruction in self-assembly of alkylthiolate monolayers on coinage metal surfaces

“…On Ag(111) it is well established that methylthiolate forms an ordered (√7×√7)R19.1º ordered phase from the LEED pattern recorded in an early spectroscopic study of this system [16]. Atomic S also produces a surface phase with this same unit mesh, but in this case the structure has been attributed to multilayers of f-cubic Ag 2 S(111) which has a very close match in lattice parameter [17].…”