Laser-induced thermal desorption of CO on Ni(111): Determination of pre-exponential factor and heat of desorption

Abstract

We show that by combining laser· induced thermal desorption with conventional thermal desorption spectroscopy, one can determine separately the activation energy E. and the pre-ilxponential factor /I for desorption with good accuracy. Results on CO on Ni( Ill) yield E.",,25.5 kcal/mol and 11"" 10 13 /s. They agree well with values obtained by other techniques and suggest that above room temperature, CO on Ni( 111) is in a highly mobile gas-like phase.

Site-Hopping of Adsorbed CO in c(4×2)-CO/Ni(111) by Laser-Induced Temperature Jump: Time-Resolved Sum-Frequency Generation Observation

“…Note the slight delay between the immediate drop in the hollow CO signal and the subsequent strengthening of the atop CO signal. To interpret this time profile, two different effects have to be taken into account; the change in the population itself, and the thermal broadening/weakening of the atop CO peak. , The recovery profiles of both the hollow and atop signals reflect that variation in surface temperature according to heat diffusion on a metal surface. − A quantitative analysis of this mechanism will be given elsewhere.

4 Transient changes in SFG signals at (a) 1904 cm -1 for hollow CO, and (b) at 2040 cm -1 for atop CO at 120 K.

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“…7,8 The recovery profiles of both the hollow and atop signals reflect that variation in surface temperature according to heat diffusion on a metal surface. [4][5][6][7][8][9][10] A quantitative analysis of this mechanism will be given elsewhere.…”

“…Adsorbed CO on metal surfaces, one of the simplest adsorbates, has attracted significant interest in the study of catalysis, and the temporal behaviors of these adsorbates have been investigated extensively by ultrashort laser spectroscopy. − The desorption of CO under pulsed laser irradiation has been monitored by mass analysis 4,5 or surface spectroscopy, and the spectral change of thermally or vibrationally excited CO on surfaces has been examined by surface vibrational spectroscopy. , Only a few attempts have been made, however, to make direct observations of chemical reactions at surfaces initiated by pulsed laser pumping. , Site-hopping of adsorbates among adsorption sites is also a topic of interest, and is expected to play an important role in surface phenomena such as diffusion of adsorbed species, island formation and adsorption-assisted desorption…”

“…Irradiation with near-infrared (NIR) pulses thermally excites the surface, giving rise to a change in surface vibrational spectra. − The CO stretching mode of adsorbed CO is unharmonically coupled with low-frequency frustrated modes, and thermal excitation causes a frequency shift and broadening of the CO stretching mode. ,,− Irradiation with intense pulses also results in desorption of CO from the surface, , with kinetics well explained by the rate equation of a thermal process.…”

“…[4][5][6][7][8][9][10] The CO stretching mode of adsorbed CO is unharmonically coupled with low-frequency frustrated modes, and thermal excitation causes a frequency shift and broadening of the CO stretching mode. 7,8,[12][13][14] Irradiation with intense pulses also results in desorption of CO from the surface, 4,5 with kinetics well explained by the rate equation of a thermal process.…”