The free-electron laser has proven to be an invaluable source of radiation in the infrared spectral range. Its main qualities are the spectral range (from near-IR to mm waves), the tuning range (at least a decade for a given FEL) and tuning speeds (minutes), the high peak power (hundreds of MW) and the short pulse lengths (down to 100 fs RMS). In the last decade, several FELs have been designed and built as user dedicated facilities. Presently, seven infrared FELs are running more than 1000 hours/year for users performing experiments in various scientific fields.

The Stanford Picosecond Free Electron Laser Center has been extremely productive, and requests for beam time far exceed the amount available. In this paper we review the unique characteristics of Stanford's lasers that make them so popular. We then describe our efforts to increase the available beam time by interleaving pulse trains of different wavelengths and delivering these to different users, and present some of our results.

We report on the status of the first Italian Infrared Synchrotron Radiation Beamline SINBAD (Synchrotron INfrared Beamline At DA(Phi) NE), that has been designed to work at wavelengths greater than 10 micrometers . SINBAD is being installed on DA(Phi) NE, the new collider of the Laboratori Nazionali di Frascati designed to work at 0.51 GeV with a beam current of 2 to 5 A. The infrared radiation extracted from a bending magnet under an angle of 50 X 50 mrad will be two orders of magnitude more brilliant than that of a black body at 2000 K at a wavelength of 100 micrometers . The beamline layout, which consists of two planar mirrors, two toroidal mirrors and one aspherical mirror, has been designed by ray tracing simulation. In this layout one ellipsoid focuses the radiation on a wedged CVD diamond-film window, the beam is then re-focused again on the entrance of an interferometer. With a calculated transmittance of the optics between 60% and 80% at 50 micrometers , this beamline will allow experiments which require a very high brilliance in the far infrared.

Results of researches on spectroscopy characteristics of a pulsed electron accumulator synchrotron radiation source are given. Source's parameters are: electron energy E_e <EQ 20 MeV, orbit radius R equals 3 divided by 4 cm and pulsed current I_e equals 2000 A. The spectral radiation power in v < 400 cm^-1 Hz frequency range twice exceeds the known sources. A method of energetic gap measurement in HTSC films measuring the time dependence of detector signals registering radiation transmitted through the film has been developed. Measurement of energetic gap wideness in YBCO film on MgO support were made, giving 2(Delta) equals 20 meV at T equals 20 K.

The success of the infrared program at the National Synchrotron Light Source has motivated the establishment of an infrared beamline at the Synchrotron Ultraviolet RAdiation Facility (SURF II) of the National Institute of Standards and Technology. Here, we describe the design of the infrared beamline and its associated infrared microscope instrumentation and show preliminary Fourier-transform infrared spectra. In addition, we present measurements of the long wavelength (> 1 cm) synchrotron emission and the noise spectrum of the infrared synchrotron radiation. The microwave measurements were undertaken to help assess the utility of SURF II as a submillimeter and far-infrared source.

Infrared microspectroscopy with a high brightness synchrotron source can achieve a spatial resolution approaching the diffraction limit. However, in order to realize this intrinsic source brightness at the specimen location, some care must be taken in designing the optical system. Also, when operating in diffraction limited conditions, the effective spatial resolution is no longer controlled by the apertures typically used for a conventional (geometrically defined) measurement. Instead, the spatial resolution depends on the wavelength of light and the effective apertures of the microscope's Schwarzchild objectives. We have modeled the optical system from the synchrotron source up to the sample location and determined the diffraction-limited spatial distribution of light. Effects due to the dependence of the synchrotron source's numerical aperture on wavelength, as well as the difference between transmission and reflection measurement modes, are also addressed. Lastly, we examine the benefits (when using a high brightness source) of an extrinsic germanium photoconductive detector with cone optics as a replacement for the standard MCT detector.

There are a number of ray trace programs currently used for the design of synchrotron beamlines. While several of these programs have been written and used mostly within the programmer's institution, many have also been available to the general public. This paper discusses three such programs. One is a commercial product oriented for the general optical designer (not specifically for synchrotron beamlines). One is designed for synchrotron beamlines and is free with restricted availability. Finally, one is designed for synchrotron beamlines and is used primarily in one institution. The wealth of information from general optical materials and components catalogs is readily available in the commercial program for general optical designs. This makes the design of an infrared beamline easier from the standpoint of component selection. However, this program is not easily configured for synchrotron beamline designs, particularly for a bending magnet source. The synchrotron ray trace programs offer a variety of sources, but generally are not as easy to use from the standpoint of the user interface. This paper shows ray traces of the same beamline using Optikwerks, SHADOW, and RAY, and compares the results.

Far-IR Reflection Absorption Infrared Spectroscopy (RAIRS) has been used to probe submonolayers of adsorbates created under clean controlled conditions on small area single crystal surfaces, using the newly commissioned Daresbury 13.3 Far-IR synchrotron beamline. Adsorbed formate species on Cu(110) were studied as an example of an adsorbate for which a large structural data-base already exists in the literature from other surface science techniques. Our high resolution Far-IR data has allowed two distinct vCu-O vibrations to be monitored for 0.25 monolayer of formate adsorbed on Cu(110) at 300 K. We rule out a lower symmetry formate complex giving rise to these vibrations and, instead, attribute the two bands to at least two chemically distinct species at the surface, a possibility that has hitherto not been included in the analyses of this system using other techniques. In addition, we also report the first RAIRS spectrum of the vCu-O stretching vibration for adsorbed atomic O on the Cu(110) surface at 300 K. The dissociative adsorption of oxygen, at room temperature, on this surface is known to induce a massive reconstruction of the surface in which `added' rows of Cu-O-Cu strings form on the surface in the [001] direction to give rise to the (1 X 2) missing row structure. The vCu-O vibration frequency is found to be invariant as a function of coverage, suggesting that the chemical nature of the Cu-O-Cu entity remains essentially unaltered during the growth of the reconstructed phase.

Electron synchrotron storage rings, such as the VUV ring at the National Synchrotron Light Source, product short pulses of IR radiation suitable for investigating time-dependent phenomena in a variety of interesting experimental systems. In contrast to other pulsed sources of IR, the synchrotron produces a continuum spectral output over the entire IR (and beyond), though at power levels typically below those obtained from laser systems. The infrared synchrotron radiation source is therefore well-suited as a probe using standard FTIR spectroscopic techniques. Here we describe the pump-probe spectroscopy facility being established at the NSLS and demonstrate the technique by measuring the photocarrier decay in a semiconductor.

The Stanford Free Electron Laser (FEL) is a source of high peak-power, short-pulsed radiation tunable throughout the mid- and far-infrared. This light source is ideal for the study of nonlinear spectroscopic processes such as the characterization of the vibrational dynamics of molecules or the nonlinear optical response of engineered quantum well structures. We have developed a flexible experimental apparatus for conducting these nonlinear experiments. We present here three examples of published work done at the Stanford FEL Center which relied heavily on the unique and flexible characteristics of the Stanford FEL: pump-probe measurements of the SD stretch mode realization in amorphous As₂S₃, photon echo measurements of CO in three systems, and measurements of second harmonic generation in InGaAs/AlAs quantum wells. These examples are indicatives of the quality and variety of experiments performed at the Stanford FEL Center in collaboration with outside users.

We have explored near-infrared (NIR)--far-infrared (FIR) two-color optical experiments in quantum-confined semiconductor systems, using NIR radiation from a tunable cw Ti:Sapphire laser and intense and coherent FIR radiation from the UCSB Free-Electron Lasers. In this paper two recent experiments are discussed, both of which provide new insight into the internal structure and dynamics of confined excitons: (1) We have observed for the first time FIR internal transitions associated with the direct exciton in GaAs/AlGaAs quantum wells. The spectrum of excitations is enriched by the complexities of the valence band and differ significantly from simple reduced-mass, hydrogenic models. We provide a critical test of detailed calculations including the valence-band mixing of Bauer and Ando. (2) We have discovered resonant nonlinear optical mixing of NIR and FIR radiation, which results in strong near-bandgap emission lines, or optical sidebands. The sidebands appear when optically-created excitons are driven strongly by intense FIR fields. The frequencies of the sidebands are (omega) _NIR +/- 2n(omega) _FIR, where (omega) _NIR is the interband exciton-creation frequency, (omega) _FIR is the frequency of the driving field, and n is an integer. The intensity of the sidebands exhibits pronounced resonances as a function of applied magnetic field, which are well- explained in terms of virtual transitions between magnetically-tuned energy levels in the excitons.

Infrared absorption microspectroscopy is a useful technique to analyze biological tissues, as it can rapidly and non- destructively provide quantitative information about the molecular composition of tissue on a small spatial scale. At the Stanford Picosecond Free Electron Laser Center, a Scanning Near-field Infrared Microscope (SNIM) with the Free Electron Laser (FEL) as its illumination source has been used for in situ microspectroscopic characterization of constituents in human atherosclerotic tissue. The system consists of a Near-field Scanning Optical Microscope utilizing a tapered chalcogenide fiber as the scanning probe. The Stanford mid-infrared FEL provides high power infrared radiation that can be easily coupled into the chalcogenide fiber and whose wavelength is continuously tunable from 3 to 15 micrometers. With the FEL, the SNIM can acquire an image at a single wavelength of a 200 micrometer square region with 2 micrometer spatial resolution in under 30 minutes. It can also obtain infrared spectra at sub- wavelength resolution. The SNIM was used to examine unstained, frozen microtone sections of human atherosclerotic lesions. Spectra from localized regions in the sample were taken and analyzed to determine the distribution of various protein, lipid, and mineral constituents among the tissue microstructures. These findings were compared with results obtained by polarization microscopy and traditional histological staining techniques. The molecular information obtained in these studies can potentially lead to a greater understanding of atherosclerosis. Moreover, they demonstrate the usefulness of SNIM towards micrometer-scale vibrational microspectroscopy.

Infrared Microspectroscopy, using a globar source, is now widely employed in the industrial environment, for the analysis of various materials. Since synchrotron radiation is a much brighter source, an enhancement of an order of magnitude in lateral resolution can be achieved. Thus, the combination of IR microspectroscopy, and synchrotron radiation provides a powerful tool enabling sample regions only few microns size to be studied. This opens up the potential for analyzing small particles. Some examples for hair, bitumen and polymer are presented.

Chemical mapping of proteins and lipids inside a single living cell and at a resolution of a few microns, has been performed using synchrotron infrared microspectrometry. Modifications of the chemical distributions upon mitosis and necrosis has been investigated.

Infrared microspectroscopy combines microscopy and spectroscopy for the purpose of chemical microanalysis. Light microscopy provides a way to generate and record magnified images and visibly resolve microstructural detail. Infrared spectroscopy provides a means for analyzing the chemical makeup of materials. Combining light microscopy and infrared spectroscopy permits the correlation of microstructure with chemical composition. Inherently, the long wavelengths of infrared radiation limit the spatial resolution of the technique. However, synchrotron infrared radiation significantly improves both the spectral and spatial resolution of an infrared microspectrometer, such that data can be obtained with high signal-to-noise at the diffraction limit, which is 3 - 5 micrometers in the mid-infrared region.

The population lifetime of the amide I vibration (v₁₀ fundamental, ca. 1650 cm^-1) in the protein myoglobin in D₂O has been determined by picosecond infrared pump- probe spectroscopy using the Stanford mid-infrared free electron laser to be 1.3 +/- 0.2 ps. In a glass forming mixture of deuterated glycerol and D₂O, the vibrational lifetime was found to increase from 1.3 +/- 0.2 ps at 310 K to 1.8 +/- 0.2 ps at 10 K. In addition to determining the time-scale of vibrational relaxation, we also observed multi-level vibrational excitation which has implications regarding the anharmonicity and homogeneous linewidth of the mode.

The first IR beamline at the advanced light source, Beamline 1.4, is described. The design of the optical and mechanical systems are discussed, including choices and tradeoffs. The initial commissioning of the beamline is reported. The beamline, while designed primarily for IR microscopy and only initially instrumented for microscopy (with a Nicolet interferometer and microscope), will have the potential for surface science experiments at grazing incidence, and time- resolved visible spectroscopy.