A theoretical model has been developed to describe the generation of Sum Frequency (SF) light from a monolayer film adsorbed to the surface of a dielectric and metal composite substrate. This type of substrate provides enhanced SF signals from the monolayer without modifying the intrinsic (hydrophilic) properties of the dielectric surface. The fundamental equations of both resonant and nonresonant SF generation have been extended from a single interface to encompass the two displaced sources, the air/dielectric interface at which the monolayer film is located, and the dielectric/metal interface. The model describes the coherent addition of three separate SF signals which coherently interfere with each other, thereby affecting the line-shape observed in the net SF spectrum. Representative calculations have been made for a highly ordered monolayer of an aliphatic hydrocarbon molecule, adsorbed on a dielectric with the optical properties of mica, which is itself in contact with a metal modeled to resemble gold. The SF spectral line-shapes are predicted as a function of the thickness of the dielectric material for the methyl terminal groups of the hydrocarbon chain. The model can be applied to any dielectric/metal composite by modifying the optical properties appropriately.

IR visible sum-frequency spectroscopy (SFS) has been used to investigate the structural changes occurring in a dodecan-1-01 monolayer adsorbed on a model hydrophobic surface between 15 and 70 "C. Up to 40.5 "C, the SF spectrum of the monolayer shows predominantly terminal methyl resonances and very weak methylene chain resonances arising from a densely packed monolayer with a high degree of conformational order. Above 40.5"C, there is a sharp decrease in the intensity of the methyl resonances and an increase in the methylene resonances, indicative of a surface phase transition. Modelling the resonance profiles shows that both monolayer coverage and conformational order decrease above the phase-transition temperature.

A composite dielectric/metal substrate, consisting of a thin mica sheet backed with a gold film, has been employed to enhance the intensity of sum frequency vibrational spectroscopy (SFS) signals arising from model monolayers of octadecylsiloxane (ODS) adsorbed at the mica/air interface. In addition to enhanced intensities in comparison to SF spectra of ODS on mica sheets without gold backing, resonant line profiles were found to vary as a function of the thickness of the mica component of the composite substrate. This experimental result concurs with a recent theoretical study of the composite substrate which showed that interference between SF signals arising from the ODS monolayer and from the displaced gold surface results in variations in the resonant phase of the SF vibrational mode. The interference phase/thickness effect has been investigated for several composite samples, each containing a series of steps of well-defined mica thicknesses. A periodic relationship between the mica thickness and the measured interference phase was observed for both the symmetric (r + ) and antisymmetric (r -) methyl stretching modes of the ODS monolayers. The empirically determined periodicities were 3.3 and 3.1 µm for the r + and rresonances, respectively. These values agree within error with one of two theoretically predicted periodicities, namely, 3.01 µm. Scatter in the experimental data however is symptomatic of a more complicated interference phase/thickness behavior involving the second shorter theoretically predicted periodicity (162 nm). These experimental observations have been examined in the light of the theoretical model used to simulate the composite substrate. It is concluded that the shorter periodicity component (162 nm) is more susceptible to variations in experimental parameters such as distributions of incident beam angles, nonplanarity of mica, etc. and is partially "averaged out" to leave the less susceptible long periodicity component (3.01 µm) dominant.

Transient, time-delayed, four-wave mixing (TDFWM) experiments have been performed on the Na D doublet with use of a novel angled-beam geometry. The effects of petahertz superposition-state modulations have been observed in the integrated TDFWM signal as a function of the time delay. As the time delay is varied, the lowest-order mixing signal modulates with a period of 980 as-corresponding to the sum frequency of the two Na D lines. Higher-diffraction-order mixing signals contain modulation components at integral multiples of the doublet sum frequency. PACS numbers: 42.65.Ma, 32.90.+a, 42.50.Md, 42.65.Ft Research on ultrafast phenomena is characterized by a mixture of techniques and instrumentation of varied nature. 1,2 Several practical schemes provide light pulses in the middle femtosecond regime and recent developments now yield excitation pulses with durations as short as 6 fs. 3 Less direct methods using broad-band light sources have the potential of exploiting short autocorrelation times in order to study ultrafast phenomena by way of transient four-wave mixing. 4 " 10 These latter experiments have been shown to be capable of obtaining both spectroscopic and relaxation information in the picosecond and femtosecond regimes. The extension of this technique to a much shorter time regime is promising. In the ultrafast regime, the establishment of a well defined delay time poses a potential difficulty because of . E-Eo{e-is "[e /k, + e \+e 'fe i'ki-r I /kj-r- -rewhere the primed and unprimed k vectors correspond to the frequencies a and a', and r is a variable relative delay between the prompt and delayed fields which are indicated by the subscripts 1 and 2, respectively. The optics were adjusted so that ki -k2 = k{ -k2. u A mixing signal is generated with wave vector 2k2~ki which modulates at the difference frequency ft'-ftasa function of r. In the case of the Na /Mine excitations the difference frequency is 530 GHz resulting in a modulation with a period of 1.9 ps. Similar results have been seen with use of broad-bandwidth excitations in atomic vapors of both Na 10 and Rb. 7 In the case of the Rb D line the difference frequency is much larger yielding a beat with a period of 139 fs. To see such beats in the attosecond regime the difference frequency would have to be in the petahertz regime. Alternatively, attosecond beats can be realized by the adjustment of the beams so that ki-k2"" -(kj-ki), whereupon the modulation frequency is the sum rather than the difference of the individual excitation frequencies. The penalty is that mixing signals are no longer (see Ref. 11) produced when the angling of the excitation beams with respect to each other. This problem is relevant, however, only when we make relaxation measurements, not when we look at modulation effects.The observation of high-frequency modulation effects in time-delayed four-wave mixing (TDFWM) does not require the use of broad-band excitation pulses when relaxation times are long. This is best seen by our viewing the modulation process not as a conse...

Vibrational sum frequency spectroscopy (SFS) has been used to study monolayer films adsorbed from aqueous solutions of didodecyldimethylammonium bromide (DDAB) and dodecan-1-01 (C,,OH) onto a planar model hydrophobic surface. The DDAB film adsorbed from pure solution is conformationally disordered and becomes steadily more disordered as the solution concentration decreases or the temperature increases. In contrast, the C I 2 0 H film adsorbed from saturated solution produces a densely packed monolayer which exhibits a first-order phase transition near 313 K. The difference in conformational orders is thought to arise from the fact that DDAB has a large positively charged head group and two non-parallel alkyl chains which prevent close-packing in the monolayer. In solutions containing both DDAB and C,,OH coadsorption occurs when the DDAB concentration is at least 1/10 of its critical aggregation concentration (c.a.c.) and persists well above the phase-transition temperature of C, ,OH. For DDAB concentrations below 1/10 c.a.c. the adsorbed film contains only C, ,OH. For DDAB concentration equal to 5 c.a.c. the adsorbed film contains a fractional C, ,OH coverage of ca. 1/5. Experiments using deuteriated dodecan-1-01 (d-C,,OH) show that DDAB in coadsorbed films is more conformationally ordered than the pure DDAB monolayer.