SHG

Optical second-harmonic generation (SHG) has recently emerged as a powerful surface probe [95, 96]. Second harmonic generation has long been used to produce frequency doublers from noncentrosymmetric crystals. As a surface probe, SHG can be caused by the break in symmetry at the interface between two centrosymmetric media. A high-powered pulsed laser is focused at an angle of incidence from 30 to 70° onto the sample at a power density of 10 to 10 W/cm. The harmonic is observed in reflection or transmission at twice the incident frequency with a photomultiplier tube. 

SHG measurements have been made on a large number of interfacial sys-... 

SHG Optical second-harmonic generation [95, 96] A high-powered pulsed laser generates frequency-doubled response due to the asymmetry of the interface Adsorption and surface coverage rapid surface changes... 

Some recent advances in stimulated desorption were made with the use of femtosecond lasers. For example, it was shown by using a femtosecond laser to initiate the desorption of CO from Cu while probing the surface with SHG, that the entire process is completed in less than 325 fs [90]. The mechanism for this kind of laser-induced desorption has been temied desorption induced by multiple electronic transitions (DIMET) [91]. Note that the mechanism must involve a multiphoton process, as a single photon at the laser frequency has insufScient energy to directly induce desorption. DIMET is a modification of the MGR mechanism in which each photon excites the adsorbate to a higher vibrational level, until a suflBcient amount of vibrational energy has been amassed so that the particle can escape the surface. 

The second-order nonlinear optical processes of SHG and SFG are described correspondingly by second-order perturbation theory. In this case, two photons at the drivmg frequency or frequencies are destroyed and a photon at the SH or SF is created. This is accomplished tlnough a succession of tlnee real or virtual transitions, as shown in figure Bl.5.4. These transitions start from an occupied initial energy eigenstate g), pass tlnough intennediate states n ) and n) and return to the initial state g). A fiill calculation of the second-order response for the case of SFG yields [37]... 

The focus of the present chapter is the application of second-order nonlinear optics to probe surfaces and interfaces. In this section, we outline the phenomenological or macroscopic theory of SHG and SFG at the interface of centrosymmetric media. This situation corresponds, as discussed previously, to one in which the relevant nonlinear response is forbidden in the bulk media, but allowed at the interface. 

In tills section, we provide a brief overview of some experimental issues relevant m perfomiing surface SHG and SFG measurements. 

The main panel of Figure B1.5.6 portrays a typical setup for SHG. A laser source of frequency to is directed to the sample, with several optical stages typically being introduced for additional control and filtering. The combination of a... 

From this expression, we may estimate typical signals for a surface SHG measurement. We assume the following as representative parameters = 10 esu, = 1, and sec B = 4. For typical optical frequencies,... 

The basic physical quantities that define the material for SHG or SFG processes are the nonlinear susceptibility elements consider how one may detemiine these quantities experimentally. For... 

An alternative scheme for extracting all tliree isotropic nonlinear susceptibilities can be fomuilated by examining equation B 1.5.39. By choosing an appropriate configuration and the orientation of the polarization of die SH radiation e 2a) such that the SHG signal vanishes, one obtains, assuming only surface contribution with real elements, ... 

Given the interest and importance of chiral molecules, there has been considerable activity in investigating die corresponding chiral surfaces [, and 70]. From the point of view of perfomiing surface and interface spectroscopy with nonlinear optics, we must first examhie the nonlinear response of tlie bulk liquid. Clearly, a chiral liquid lacks inversion synnnetry. As such, it may be expected to have a strong (dipole-allowed) second-order nonlinear response. This is indeed true in the general case of SFG [71]. For SHG, however, the pemiutation synnnetry for the last two indices of the nonlinear susceptibility tensor combined with the... 

A schematic diagram of the surface of a liquid of non-chiral (a) and chiral molecules (b) is shown in figure Bl.5.8. Case (a) corresponds to oom-synnnetry (isotropic with a mirror plane) and case (b) to oo-symmetry (isotropic). For the crj/ -synnnetry, the SH signal for the polarization configurations of s-m/s-out and p-m/s-out vanish. From table Bl.5.1. we find, however, that for the co-synnnetry, an extra independent nonlinear susceptibility element, is present for SHG. Because of this extra element, the SH signal for... 

A quantity of interest in many studies of surfaees and interfaees is tire eoneentration of adsorbed atomie or moleeular speeies. The SHG/SFG teelmique has been found to be a usefid probe of adsorbate density for a wide range of interfaees. The surfaee sensitivity afforded by the method is illustrated by the results of figure Bl.5.9 [72]. These data show the dramatie ehange in SH response from a elean surfaee of silieon upon adsorption of a fraetion of a monolayer of atomie hydrogen. 

We now consider how one extracts quantitative infonnation about die surface or interface adsorbate coverage from such SHG data. In many circumstances, it is possible to adopt a purely phenomenological approach one calibrates the nonlinear response as a fiinction of surface coverage in a preliminary set of experiments and then makes use of this calibration in subsequent investigations. Such an approach may, for example, be appropriate for studies of adsorption kinetics where the interest lies in die temporal evolution of the surface adsorbate density N. ... 

The second issue concerns molecular specificity. For a simple measurement of SHG at an arbitrary laser frequency, one caimot expect to extract infomiation of the behaviour of a system with several possible adsorbed species. To make the technique appropriate for such cases, one needs to rely on spectroscopic infomiation. In the simplest iiiiplementation, one chooses a frequency for which the nonlinear response of tlie species of interest is large or dominant. As will... 

The SHG/SFG technique is not restricted to interface spectroscopy of the delocalized electronic states of solids. It is also a powerful tool for spectroscopy of electronic transitions in molecules. Figure Bl.5.13 presents such an example for a monolayer of the R-enantiomer of the molecule 2,2 -dihydroxyl-l,l -binaphthyl, (R)-BN, at the air/water interface [ ]. The spectra reveal two-photon resonance features near wavelengths of 332 and 340 mu that are assigned to the two lowest exciton-split transitions in the naphtli-2-ol... 

Figure Bl.5.13 Spectra of the various non-chiral [p-in/p-oiit (filled circles) and s-in/p-oiit (filled diamonds)] and chiral [p-in/s-oiit (triangle)] SHG signals of (R)-BN molecules adsorbed at the air/water interface. (From [80].)...

The SHG and SFG teclmiques are also suitable for studying dynamical processes occurring on slower time scales. Indeed, many valuable studies of adsorption, desorption, difhision and other surface processes have been perfomied on time scales of milliseconds to seconds. 

In a typical time-resolved SHG (SFG) experiment using femtosecond to picosecond laser systems, two (tlnee) input laser beams are necessary. The pulse from one of the lasers, usually called the pump laser, induces the... 

The influence of an applied magnetic field, as introduced in section Bl.5.2.2. is quite different from that of an applied electric field. A magnetic field may perturb the interfacial nonlinear response (and that of the weak bulk tenns), but it does not lead to any dipole-allowed bulk nonlmear response. Thus, in the presence of magnetic fields and magnetization, SHG remains a probe that is highly specific to surfaces and interfaces. It... 

Experimental investigations of the model system of dye molecules adsorbed onto surfaces of polystyrene spheres have finuly established the sensitivity and surface specificity of the SHG method even for particles of micrometre size [117]. The surface sensitivity of die SHG process has been exploited for probing molecular transport across the bilayer in liposomes [118], for measurement of electrostatic potentials at the surface of small particles [119] and for imaging... 

Figure Bl.22.7. Left resonant seeond-hannonie generation (SHG) speetnimfrom rhodamine 6G. The inset displays the resonant eleetronie transition indueed by tire two-photon absorption proeess at a wavelength of approximately 350 mn. Right spatially resolved image of a laser-ablated hole in a rhodamine 6G dye monolayer on fiised quartz, mapped by reeording the SHG signal as a fiinetion of position in the film [55], SHG ean be used not only for the eharaeterization of eleetronie transitions within a given substanee, but also as a mieroseopy tool.

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