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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities 5 VACUUM ULTRAVIOLET AND EXTENDED ULTRAVIOLET REGION: 200 to 10 nm PRESENT CAPABILITIES Conventional Lasers Four-wave sum and difference frequency generation in metal vapors and rare gases has advanced significantly in recent years to cover a substantial portion of the vacuum ultraviolet (VUV) region. It is now possible in the range from 190 to 80 nm to achieve pulse energies corresponding to 1012 photons per pulse at 10 Hz in bandwidths as narrow as 200 MHz and with pulse widths of 10 ns. Shorter pulses with lower pulse energies are also possible. Although different regions require different vapors, it is possible to obtain complete coverage from 190 to 80 nm. The tuning limits of conventional lasers are also being pushed toward shorter wavelengths by the use of high-order harmonics from Ti-sapphire-based terawatt lasers. These lasers have the particular advantage of high peak power (0.25 TW) and short pulses (around 100 fs), which make them ideal for nonlinear and pump-probe experiments. Currently, such experiments are performed with photon energies up to 50 eV (25 nm) for the probe, while tests on high-power lasers have shown that harmonics up to about 160-eV (8-nm) photon energy can be produced with useful efficiency. Beyond that energy the field ionization of the gases used for generating harmonics imposes a natural cutoff. These lasers are unlikely to excite deeper core levels. In general, high-harmonic generation sources are inefficient; terawatts are put in and kilowatts are obtained in any one harmonic. Synchrotron Radiation Current third-generation synchrotron light sources cover the range from 10 eV (124 nm) to about 20 keV (0.6 Å), a region including the binding energies for the sharpest core levels of all the elements (10 to 1000 eV), and in particular the “organic” elements carbon, nitrogen, and oxygen (290, 400, and 550 eV). Under normal operating conditions, peak powers from 100 mW at 100 nm (12.4 eV) to 5 W at 10 nm (124 eV) are available at the Advanced Light Source (ALS) at Lawrence Berkeley Laboratory, while peak powers of about 1 kW will be available at the Advanced Photon Source (APS) at Argonne National Laboratory. The pulse structure of these systems is designed for high average flux as opposed to high peak power. Typically, pulses are tens of picoseconds in duration at intervals of about 1 ns
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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities to 1 ms achieved in the single bunch mode. A flux of roughly 1015 to 1016 photons per second is available in a 0.1% bandwidth and in a beam of angular divergence of 5 mrad in an energy range extending from 10 eV to 10 keV. The spectral brightness of undulators makes it possible to obtain core-level photoelectron spectra limited only by the intrinsic lifetime broadening (typically 3 × 10−4 of the binding energy for the sharpest levels). These sources are also essential for element-resolved microscopy at a resolution of 0.1 mm or better. Free Electron Lasers There are no existing FEL sources in the wavelength range from 200 to 10 nm. The FELs at Laboratoire pour l'Utilisation du Rayonnement Electromagnetique (LURE) and Novosibirsk have produced light with wavelengths as short as 250 nm and 240 nm (about 5 eV), respectively. Various proposals would extend the wavelength range down to 75 nm. At very short wavelengths the problem of finding suitable mirrors becomes a limitation on the use of oscillators, and a transition to high-gain, single-pass amplifiers (using SASE as described in Chapter 2) may be needed. The wavelength range down to 100 nm overlaps with that of conventional lasers using harmonics, but FELs would have superior peak power and average power. WHAT IS ENVISIONED FOR A VACUUM ULTRAVIOLET FREE ELECTRON LASER For the sake of simplicity, the range of wavelengths from 200 nm to 10 nm is labeled as the VUV. One proposal suggests a system operating from 300 to 75 nm at a repetition rate of 360 Hz with pulse durations from 6 ps to 200 fs and pulse energies of roughly 1 mJ at 100 nm, corresponding to roughly 400-MW peak power. The key technical challenge of a VUV FEL is the achievement of high electron-beam quality along with high peak current. An FEL oscillator with normal incidence mirrors may be possible down to about 100 nm using clean Al mirrors in ultrahigh vacuum (Kortright, 1990). From 100 nm to about 60 nm, SiC mirrors would permit lasing using an FEL with a gain of 10. Down to 10 nm, the FEL oscillator would have to rely on hypothetical multilayered mirrors with reflectivity of about 50%, although such mirrors might restrict the tunability. Even if the reflectivity of a material is sufficient for laser operation, thermal distortion and damage may prevent or severely restrict the use of mirrors. This is an area that would benefit from further research and would be important for an FEL operating below 100 nm. It appears that a VUV FEL would require further research and development. The components for this wavelength range have been separately demonstrated in laboratory experiments, but no working device has been constructed. The proposals
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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities for VUV FELs estimate the cost to be $30M to $50M, depending on whether the system is a demonstration model or a full user facility. A few years of research and development in key areas could improve the reliability and possibly lower the cost of a VUV FEL. Areas for further study should include (1) the RF photocathode electron gun, to produce short pulses with high beam quality, (2) beam transport, in order to maintain the necessary beam quality through the accelerator and to the FEL undulator, (3) the undulator design, (4) mirror technology, particularly for wavelength extension down to 10 nm, and (5) system optimization. The use of the gain in higher-frequency harmonics, operating simultaneously on multiple colors, and developing two-stage FELs also deserve additional research. These design options present more risk now, but with research and development they have the potential to allow short-wavelength operation at reduced size and cost. SCIENTIFIC OPPORTUNITIES There are areas of chemistry, physics, biology, and materials science that would benefit from improved photon sources in the VUV region. Third-generation synchrotron sources are likely to be a proving ground for many of these ideas. The committee focuses here on four areas in which a VUV FEL would provide important advances. Photodissociation Dynamics Specific quantum-state analysis for chemical studies using pump-probe techniques has been made possible through the development of lasers. Applied to photochemistry, this technique uses an initial laser pulse to excite a molecule to a specific state; the molecule then dissociates, producing a set of fragments that a second laser probes to determine their internal state or velocity distribution. These studies require lasers because of the need for high pulse energy (especially in the pump), tunability (to allow one to pump and probe a well-defined state), and accurate timing (to allow for synchronization of the pump and probe pulses). Because of the current wavelength limitation of lasers, the initial photolysis step, which requires the highest intensity, accesses only the lowest valence levels of most molecules. The photochemistry that occurs after absorption to higher valence levels or to Rydberg levels is inaccessible to analysis, even for such basic molecules as CH4, H2O, and CO2. The availability of a tunable laser in the VUV with the requisite 1015 photons/pulse, in a bandwidth comparable to Rydberg absorption levels (ca. 1 cm−1), and with subpicosecond timing would open up this area of photochemistry for study.
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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities Photoelectron Spectroscopy of Dilute Species Photoelectron spectroscopy (PES) is a valuable tool for elucidation of the structure and the dynamics of molecules. The fact that ionization potentials generally lie in the VUV part of the spectrum, where few sources exist with bandwidths narrow enough to resolve rotational splitting, has severely limited the resolution attainable for these experiments. Multiphoton absorption techniques and the generation of VUV photons through harmonic processes have overcome this limitation to some extent. The combination of harmonic generation with threshold photoelectron analysis permits resolution in PES measurements at the level of individual rotations (on the order of a cm−1). In general, intensities from harmonic generation sources are low, and the tunability is restricted. The first limitation essentially removes from analysis a broad range of extremely important molecules, particularly radicals and weakly bound complexes that can be produced only in small quantities in molecular beams. Radicals are involved in some of the most important chemical processes, including in combustion and atmospheric chemistry. Studies of weakly bound complexes at the rovibronic level provide information not only about structure but also about chemical dynamics, including energy transfer and reaction processes. A Universal Detector A new application for a high-power vacuum ultraviolet source is as a universal detector. In many current studies of reactive and photodissociative chemical events, the products are detected by electron bombardment ionization. Not only is this process extremely inefficient, but it also offers only limited selectivity. A tunable, high-power VUV source should ionize products with nearly 10% efficiency. In addition, by tuning the source to the threshold energy for the species of interest, one might avoid ionizing background species of higher ionization potential. Such a detector would have many applications, but two are to molecular reaction dynamics and surface chemistry. Pump-Probe Photoemission Past pump-probe experiments have used an intense pump beam to put a significant density of electrons into a long-lived excited state (e.g., a band-gap state in semiconductors or an image state in metals). The probe beam then ionizes these electrons, and the resulting photoelectrons are detected, providing information about the energy, momentum, and lifetime of the excited state. The photon energies of the probe beam have ranged from 2 to 20 eV, but recent developments in high-order harmonic generation have pushed the energy to 50 eV, with an upper limit envisioned beyond 100 eV. This development makes it possible to detect the shift of core levels
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FREE ELECTRON LASERS AND OTHER ADVANCED SOURCES OF LIGHT: Scientific Research Opportunities induced by the pump beam, which is related to the charge transfer caused by the excitation of a specific atom. The technique promises information about the atomic identity of laser-excited intrinsic and defect states that is difficult if not impossible to obtain by any other technique. Current efforts are directed toward testing whether or not this concept is feasible. FINDINGS Modern third-generation synchrotrons will cover the vacuum ultraviolet and extended ultraviolet regions well and will provide opportunities to explore much new scientific research in this wavelength region. There are currently no free electron lasers that operate in this region. The construction of a device should be possible with some additional research and development. It may be possible to construct a large VUV free electron laser facility now, since most of the component pieces have been demonstrated, but construction of a VUV free electron laser using current technology would cost on the order of tens of millions of dollars and involve some risk. A user community for a VUV free electron laser is not well developed at this time but could grow with the exploition of third-generation synchrotron sources, demonstrating proof-of-principle experiments. RECOMMENDATIONS Scientific opportunities in the vacuum ultraviolet and extended ultraviolet wavelength range should be explored by the use of existing synchrotron sources. The development of technology for a vacuum ultraviolet free electron laser should be supported. Among the goals of this development should be lowering the cost of these devices and increasing their reliability. A free electron laser user facility in the vacuum ultraviolet should not be constructed at the present time. The scientific case for a VUV free electron laser facility does not justify the current cost, and the technology is not sufficiently well developed. REFERENCE Kortright, J.B. 1990. Chapter 13 in Free Electron Laser Handbook. W.B.Colson, C. Pellegrini, and A. Renieri, eds. The Netherlands: North-Holland Physics, Elsevier Science Publishing Co.
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