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The vibrational dynamics of excited CO layers on Pt(111) were studied using infrared (IR) pump–probe methods. Resonant IR pulses of 0.7 ps duration strongly pumped the absorption line (ν≊2106 cm-1 ) of top-site CO. Weak probe pulses delayed a time tD after the pump were reflected from the CO-covered Pt(111) surface, and dispersed in a monochromator to determine the absorption spectrum of the vibrationally excited CO band, with time resolution <1 ps and monochromator resolution <1 cm-1. Transient spectra were obtained as a function of CO coverage, surface temperature, and laser fluence. Complex spectra for tD<0 show features characteristic of a perturbed free induction decay, which are expected based on multiple-level density-matrix models. For tD≥0, the CO/Pt absorption exhibits a shift to lower frequency and an asymmetric broadening which are strongly dependent on fluence (1.3–15 mJ/cm2 ). Spectra return to equilibrium (unexcited) values within a few picoseconds. These transient spectral shifts and the time scale for relaxation do not depend (within experimental error) on coverage for 0.1≤ΘCO≤0.5 ML or on temperature for 150≤Ts≤300 K. A model for coupled anharmonic oscillators qualitatively explains the tD>0 spectra in terms of a population-dependent decrease in frequency of the one-phonon band, as opposed to a transition involving a true CO(v=2) two-phonon bound state. The rapid relaxation time and its insensitivity to Ts and ΘCO are consistent with electron–hole pair generation as the dominant decay mechanism. [ABSTRACT FROM AUTHOR]

Tunable infrared picosecond pulses in the 5 μ region have been used for time-resolved pump–probe measurements of the population relaxation lifetime (T1) of CO(v=1) stretching vibrations in a series of metal–carbonyl cluster compounds in room temperature chloroform solution. T1 was the same for symmetric (ν=2084 cm-1, 90±10 ps) and antisymmetric (2014 cm-1, 87±10 ps) modes of the dicarbonyl Rh(CO)2(C5H7O2); T1 was the same for the B1 (2092 cm-1 , 710±130 ps) and B2 (2036 cm-1, 750±90 ps) modes of Rh2(CO)4Cl2. Similarly long T1 times were found for Rh4(CO)12 (2075 cm-1, 610±65 ps) and Rh6(CO)16 (2077 cm-1, 700±100 ps). The molecule Co4(CO)12 has also been compared to the corresponding rhodium analog and it exhibits an initially fast relaxation of 47±5 ps followed by a slower 396±70 ps decay. The transient response of the more complex systems to the single frequency experiment is found to be sensitive to frequency and can exhibit bleaching, absorption, and a combination of these effects. Such behavior is attributed to overlap of the IR pulse with v=1 to v=2 and higher transitions in the M4(CO)12 (M=Rh or Co) and Rh6(CO)16 molecules. The long CO(v=1) T1 values for the metal cluster molecules suggest relaxation via multiquantum transfer of vibrational energy to adjacent M–C stretch and M–C–O bend vibrations; energy transfer to vibrational or electronic states of the central metal core seems unimportant in determining T1 for these systems. [ABSTRACT FROM AUTHOR]

Distributions of internal states and kinetic energies are reported for the laser-induced desorption of NO from a platinum foil. Two desorption channels are observed, one with internal-state populations which are well fit by Boltzmann distribution functions that correspond to the peak surface temperature. The second desorption channel displays population distributions which are non-Boltzmann: kinetic energies exceed the peak surface temperature by a factor of approx. 5; kinetic energies depend on J; the spin–orbit states are inverted; and the vibrational population exceeds that associated with the maximum surface temperature by a factor of 50. The wavelength dependence of the non-Boltzmann desorption signal suggests that a substrate mediated desorption process is operative. [ABSTRACT FROM AUTHOR]

The response of the molecular stretch mode of CO/Cu(100) near 2086 cm-1 (ν1) to resonant infrared, and nonresonant visible and ultraviolet pumping is measured on a picosecond time scale. Fourier transform infrared measurements establish that ν1 is anharmonically coupled to the frustrated translation near 32 cm-1 (ν4), so that transient shifts in ν1 indicate population changes in ν4. The ν1 response to visible and ultraviolet pumping is characterized by a spectral shift near zero delay time, which decays with a ≊2 ps time constant to an intermediate value, which then decays on a ≊200 ps time scale. The data agree well with a model whereby ν4 couples to both the photogenerated hot electrons and to the heated phonons. The characteristic coupling times to these two heat baths are found to both be a few picoseconds. [ABSTRACT FROM AUTHOR]

The transient infrared response of CO/Pt(111) following picosecond visible excitation is reported. A spectrally broad decrease in reflectivity correlates with heating of the Pt lattice, and an observed shift in the CO(v=0→1) transition is interpreted as heating of the 60 cm-1 in-plane frustrated translational mode. A phenomenological three temperature model that assumes the adsorbate vibrational temperature Tads exclusively couples to either the electronic temperature Te (with a time constant τe) or to the lattice temperature Tlat (with a time constant τlat) describes the temporal response of the adsorbate vibrations. The lattice phonon temperature Tlat(z,t) and measured temperature dependence of the optical constants predict the observed spectrally broad reflectivity change. Density matrix methods model the infrared response of the transiently heated molecule. Limits of τe=2±1 ps or τlat<1 ps are established by comparison of predicted spectra and the data. [ABSTRACT FROM AUTHOR]

State-specific diagnostics are used to characterize the laser-induced desorption of NO from Pt(111). Two desorption channels are observed; one is consistent with thermal activation, while the other is driven by adsorbate interactions with hot carriers. For this latter channel, the observed dependence of the desorption yield on the wavelength of the incident laser pulse (1907, 1064, 532, and 355 nm) and the wavelength dependence of the kinetic energy distributions establish the nonthermal nature of the excitation process. The inverted spin–orbit population, the non-Boltzmann rotational state distributions, and the vibrational state population are interpreted in terms of a desorption mechanism involving a temporary ion resonance. [ABSTRACT FROM AUTHOR]

Picosecond infrared pump–probe experiments determined the vibrational population lifetimes (T1) of the hydroxyl fundamental stretching mode OH(v=1) in 12 alcohols (R3COH) and 8 silanols (R3SiOH) in dilute room temperature CCl4 solutions. T1 for the silanols is in the range 185

Picosecond infrared spectroscopy was used to measure the vibrational energy relaxation time T1 of OH(v=1) and OD(v=1) groups chemisorbed on silica surfaces over the temperature range 100≤T≤800 K. The observed T1 times and their temperature dependencies are discussed in terms of a multiphonon relaxation mechanism. Limiting low temperature lifetimes are T1=220±20 ps (1σ) for OH(v=1) and T1=149±10 ps for OD(v=1). [ABSTRACT FROM AUTHOR]

Picosecond infrared transmission spectroscopy was used to directly measure the vibrational energy relaxation time T1 of hydroxyl groups chemisorbed on the surface of colloidal silica (SiO2). T1 was obtained for OH(νstretch=1) in the strongly bound ‘‘isolated sites’’ of fumed silica particles in vacuum and dispersed in several liquids at T=293 K. At the SiO2/vacuum interface, T1=204±20 ps. When the SiO2 particles are surrounded by solvents, the relaxation time of the surface OH(v=1) groups decreases: for the liquids CCl4, CF2Br2, CH2Cl2, and C6H6, T1(ps)=159±16, 140±30, 102±20, and 87±30, respectively. T1 does not depend on the size of the SiO2 particles for the range 70 Å≤ diameter ≤150 Å, or on the surface OH coverage up to an average density of 4 OH/100 Å2. Significant amounts of physisorbed water (5 H2O/100 Å2) decreased T1 for the isolated OH(v=1) to T1=56±10 ps. For comparison to the surface hydroxyls, the vibrational deactivation time for OH(v=1) groups in the bulk of fused silica (OH/SiO2≊130 ppm by weight) was determined to be T1=109±11 ps. These observations are discussed in terms of the possible mechanisms of vibrational energy flow in these systems. The observed T1 values demonstrate that the spectral linewidths (e.g., IR and Raman) observed for these surface vibrations are too large (by factors of 200–2000) to be caused solely by T1 uncertainty broadening. The slow transfer of vibrational energy between surface and lattice vibrations may have important implications for surface chemistry. [ABSTRACT FROM AUTHOR]

The first results of a study of the vibrational spectrum of hydrogen, chemisorbed on the surface of platinum by isotope dilution neutron spectroscopy are presented. (AIP) [ABSTRACT FROM AUTHOR]

Internal state distributions were obtained by laser-excited fluorescence for NO desorbing from a saturation coverage on Pt(111). Two major peaks are observed in the thermal desorption spectrum at crystal temperatures Ts=200 and 340 K. The rotational state distributions for each desorption feature were Boltzmann, described by the rotational temperatures TR=195±10 K and 305±10 K, respectively. The two spin-orbit states were occupied to a degree consistent with equilibration between spin and rotational degrees of freedom, both Lambda doublet species were statistically populated and no molecular alignment was observed. [ABSTRACT FROM AUTHOR]