Femtosecond time-resolved measurement of desorption

Abstract

Desorption of NO molecules from a Pd(l 11) surface induced by femtosecond visible laser pulses has been investigated in real time. The measurements, accomplished by means of a two-pulse correlation scheme, reveal a subpicosecond response time. The findings indicate that the desorption arises from coupling to the nonequilibrium electronic excitation in the substrate.PACS numbers: 68.45. Da, 42.65.Re, 78.90.+t, The nature of coupling and the rate of energy flow between an adsorbate and a solid surface constitute key issues in surface dynamics. l " 7 Despite considerable experimental and theoretical effort, a complete understanding of these fundamental issues concerning the interaction between the localized excitation of the adsorbate and delocalized electronic and lattice excitation in the substrate remains elusive. For metal surfaces, strong coupling is generally present and typical relaxation rates lie in the picosecond to femtosecond range. 6,7 While these processes cannot be examined in real time using conventional surface science probes, laser-based techniques provide the possibility of time-domain studies. 8 Recently, real-time studies of relaxation of electronic 9 excitation at surfaces and of vibrational 7,10,11 excitation in adsorbates have been performed using pulsed laser excitation. In this Letter we report the results of the first direct timeresolved measurements of desorption from a surface under femtosecond excitation. 12 The system of nitric oxide (NO) on Pd(lll) was chosen as a model for desorption of a nondissociating chemisorbed molecule from a surface. In our laboratory we have previously characterized the yield and final-state energy distributions for NO molecules desorbed from Pd (111) by femtosecond laser pulses. 13 In contrast to the results obtained for desorption by nanosecond pulses, 14 the findings for desorption by femtosecond pulses could not be understood on the basis of equilibrium behavior, nor could the results be explained by a simple photochemical mechanism. 4,5 In the present work, we investigate the dynamics of desorption in the time domain. The time-resolved experiments are performed using a two-pulse correlation scheme in which the total desorption yield is measured as a function of temporal separation between a pair of excitation pulses. In this manner, we obtain femtosecond time resolution despite the time delay required before the desorbed molecules can be detected. 15 Examining correlation signals for both equal and unequal excitation pulses, we demonstrate that the excitation responsible for desorption has a finite lifetime not exceeding 1 ps. This result implies that nonequilibrium electronic excitation in the substrate is responsible for the desorption process, since mechanisms associated with coupling to substrate phonons would reflect the longer time scale required for cooling of the phonons to occur. We further show that the principal features of the time-resolved data can be understood through consideration of the substrate electronic temperature, which in the su...

Hot-electron-mediated desorption rates calculated from excited-state potential energy surfaces

“…Photoinduced desorption had already been observed for a few adsorbate systems 1,2 using lowintensity nanosecond laser pulses, but high-intensity femtosecond laser pulses have been shown to induce desorption in a large class of adsorbate systems [3][4][5][6][7][8][9][10] and induce chemical reactions which cannot proceed by thermal heating. 11 The mechanism attributed to these reactions is excitation of substrate electrons by the laser pulse.…”

Electronic Excitation and Dynamic Promotion of a Surface Reaction

“…Closer insight into the dynamics of this reaction pathway is obtained by two-pulse correlation (2PC) experiments [10]. Two equally intense fs-laser pulses are sent onto the Ru sample with variable time delay t. In Fig.…”

Dynamics of surface melting. Final report for period 1 September, 1993 - 30 April, 1996

“…Nonetheless, it provides an initial basis for comparison of the experimental results. Using the approach taken by Quinn,n we can estimate the lifetime of the excited electrons by calculating the imaginary component of their energy, since the probability decays exponentially in time according to e- 'l',with~= /i/(2El …”