The study of nanometer-thick molecular thin films deposited on a solid surface, due to recent technology applications, has become an important subject. Effective tools for unraveling the intrinsic structure within the molecular films and their growth mechanism, however, are still in the searching. Consequently, little is known about the structure and the most important factors controlling deposition of thin molecular films. This paper summarizes the demonstration showing that the nonlinear optical phenomenon- second harmonic generation- because of its symmetry properties can be
used effectively to characterize the structure within the molecular films. Experiments show that disorder-order phase transitions, glass transitions, crystallization kinetics and nucleation processes, and interfacial molecular structure within the thin molecular films can be characterized. The nonlinear optical studies have revealed the mechanisms and established the most important criteria for the deposition and growth of ultrathin molecular films.
We describe the design and performance of an all solid state uv laser source that produces several hundred milliwatts at 266 nm and up to one hundred milliwatts at 213 nm. Examples of using this source to ablate borosilicate glass and to expose (beta) -chloroethyl silsequioxane on silicon are discussed.
Reflective transient grating experiments were conducted using two different experimental configurations to study carrier dynamics. Using an 800 nm pump and 400 nm probe, a signal attributed to bleaching was observed, and the carrier energy relaxation time was measured to be approximately 600 fs. Experiments were also conducted with a 400 nm pump and 800 nm pump. For this configuration, the observed TG signal decay was attributed to carrier diffusion and recombination.
We demonstrate the se of an optical differential reflectivity technique to measure the rate of a surface reaction. The technique can be easily extended to a temporal resolution of nanoseconds and a spatial resolution of micrometers . The technique is used to study the reactions of acetylene on Cu(100). In the limit of low coverage acetylene undergoes two reactions: the first channel is desorption and the second channel is isomerization to vinylidene, which remains irreversibly bound to the surface. The rates of both the desorption and isomerization reactions have been measured. The desorption rates are in quantitative agreement with the predictions of transition state theory; the isomerization rates are in qualitative agreement.
Using mass spectrometry flight-time measurements, photodesorption of HFCO, H2CO, CH2CO and CH3Cl from Ag(111) under pulsed nanosecond laser irradiation has been investigated in the experimental photon energy range of 1.17 eV <EQ hv <EQ 4.67 eV. All these molecules are physisorbed on Ag(111). No threshold behavior has been established within this energy range. The translational energy distribution of the desorbing molecules is characterized by a Maxwell-Boltzmann temperature in the range 110 - 150 K. The low translational temperatures and low photon energy thresholds, though in striking contrast to the high average translational temperatures and threshold behavior reported for photodesorption of chemisorbed molecules from metal surfaces, can be understood in terms of the prevailing electron attachment model. However, it requires that the substrate electrons attach to the molecules with positive electron affinities. In an alternative model, electron scattering excites the high frequency, v equals 1 molecular vibrational levels, possibly through dipole interactions. In this case, desorption results from vibrational predissociation in the adsorbate- surface bond.
The reflectivity change induced by adsorbates on Cu(100) at (lambda) equals 632.8 nm is investigated. It is found that physisorbed molecules (water, methanol, acetone) induce no observable reflectivity change, while chemisorbed molecules (carbon monoxide, oxygen, acetylene) do. The magnitude of the reflectivity change is such that submonolayer sensitivity is obtained for chemisorbed species. Furthermore, the change is found to vary linearly with coverage. The results show that this technique is a versatile, inexpensive, and straightforward optical probe of the surface coverage, and as such is applicable to many different surface investigations.
The second harmonic generation from pyridine multilayers adsorbed on Ag(111) has been studied for the elucidating the structure of the adsorbed layers. An irreversible phase transition in the pyridine overlayers was observed when the pyridine film/Ag crystal is annealed above approximately 145 K. The phase transition is speculated to be from the formation of ordered domains in the pyridine multilayers.
The dynamics of photocarrier diffusion was studied by a two color transient grating technique in reflection geometry. A single exponential decay feature was observed immediately following the electron excitation pulse and was attributed to band edge carrier diffusion.
Transient IR reflection-absorption spectroscopy with diode lasers has been used to characterize the adsorption kinetics and the nature of the bonding of CO on Cu(100). Although the strong dynamic dipole coupling between the CO molecules adversely affect the absorption intensity and frequency, it can be used to reveal the interadsorbate interactions. It was found that the adsorption of CO on Cu is correlated, resulting from a repulsive interaction between the first and second nearest neighboring sites. A model based on static screening of the charge transferred from CO to Cu is proposed to account for all the observations. Furthermore, a nonresonant reflectivity change, which results only from chemisorption, but not physisorption, was identified in both the IR and visible regions.
The spectroscopy and dynamics of highly excited vibrational levels of the a1A1 and b1B1 states of CH2 were studied using time-resolved Fourier transform emission spectroscopy. The use of a Fourier transform spectrometer allows efficient acquisition of dispersed fluorescence spectra over several thousand cm-1 range in the visible, with better than 1 cm-1 resolution, from this short lived and low concentration species. Furthermore, the temporal evolution of the dispersed fluorescence spectra due to collisional relaxation can be monitored with 50 ns time-resolution. The results presented and discussed in this paper are: (1) the state-to-state rotational energy transfer and reactive cross- sections for b1B1 (0, 16O, 0) CH2; and (2) rotational analysis of several previously unobserved high vibrational levels of the CH2 a1A1 state.
The orientation dependence in the spin changing collisions C2H2O2(S1)+AryieldsC2H2O2(T1)+Ar and C2H2(S1)+AryieldsC2H2(T)+Ar has been examined by time-resolved laser induced fluorescence studies of the intersystem crossing rates in the C2H2O2-Ar and C2H2-Ar complexes with different isomeric structures. It was found that when Ar interacts primarily with the n(O) orbital the S1yieldsT1 transition rate is about two orders of magnitude faster than that induced by Ar interacting primarily with the (pi) *(CO) orbital. On the other hand, the studies in acetylene show that the Ar induced intersystem crossing rate is nearly identical for both the (pi) and (pi) * orbitals.
A nonlinear optical technique based on second harmonic generation (SHG) is introduced to probe the transient temperature jump induced by pulsed laser excitation of a metal surface. This technique is surface sensitive, nonintrusive and has time resolution limited only by the probe laser pulsewidth. For the Ag(1 10) surface, a strong temperature dependence of SHG enhanced by interband transitions is observed, and is used to detect a > 1010 K/sec heating rate induced by a nanosecond IR excitation pulse. A much stronger temperature dependence is also observed in the SHG resonantly enhanced by a surface state transition on Ag(110) and can in principle be used for surface temperature measurements. This kind of surface temperature measurement method can be generally applied to metals, semiconductors and solids that have temperature dependent electronic transitions.
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