Tunable continuum source

Excitation-resolved imaging Tunable continuum source... 

Fig. 4.6. (A) Schematic of femtosecond Ti Sapphire laser-pumped tunable continuum source (TCS) (B) fluorescence intensity image and (C) in situ...

Another fruit of modern technology is the advent of synchrotron radiation from electron storage rings This produces a pulsed, polarized tunable continuum source extending into the X-ray region, having a typical pulse duration 0.4 ns and a repetit ion (1—10) MHz, and is an ideal source for time-resolved spectroscopy in the vacuum ultraviolet. The expected intensity of the system at Daresbury, for example,... 

For 2PA or ESA spectral measurements, it is necessary to use tunable laser sources where optical parametric oscillators/amplifiers (OPOs/OPAs) are extensively used for nonlinear optical measurements. An alternative approach, which overcomes the need of expensive and misalignment prone OPO/OPA sources, is the use of an intense femtosecond white-light continuum (WLC) for Z-scan measurements [71,72]. Balu et al. have developed the WLC Z-scan technique by generating a strong WLC in krypton gas, allowing for a rapid characterization of the nonlinear absorption and refraction spectra in the range of 400-800 nm [72]. 

In addition to the continuum sources just discussed, line sources are also important for use in the UV/visible region. Low-pressure mercury arc lamps are very common sources that are used in liquid chromatography detectors. The dominant line emitted by these sources is the 253.7-nm Hg line. Hollow-cathode lamps are also common line sources that are specifically used for atomic absorption spectroscopy, as discussed in Chapter 28. Lasers (see Feature 25-1) have also been used in molecular and atomic spectroscopy, both for single-wavelength and for scanning applications. Tunable dye lasers can be scanned over wavelength ranges of several hundred nanometers when more than one dye is used. 

The experiments reported here were designed to demonstrate the feasibility of the measurement and to provide an initial test of the theory. As single beam experiments, the results are laser noise limited. Planned elaboration of the equipment to make double beam measurements should provide an increase in sensitivity. Other modifications which may improve detectability are cell design changes to reduce cell wall absorptions while maintaining minimal cell volume, laser output feed-back control, and signal averaging. With improved sensitivity the use of lower power tunable laser excitation will be feasible. Eventual improvement of sensitivity to the level required for use of continuum sources is at present doubtful. 

Depending on the desired excitation wavelength, different ways can be used to pump the sample. Part of the amplified pulses can be used to excite the sample directly (after intensity reduction with neutral density filters) as is done, for instance, by Woodbury et al One can also amplify a selected wavelength region of the generated continuum light in a one-or two-stage amplifier. In this way a broadly tunable excitation source is... 

There are two other sources worth noting, although they are currently used in a very small fraction of the instruments employed. Continuum sources can be used if their intensity is sufficient to minimize noise levels and if the spectrometer has sufficient dispersion to make the spectral bandpass comparable with the absorbing line width. While feasibility has been demonstrated in research laboratories, there currently is no commercial instrument available. At the other extreme, using a very bright, stable somce with a narrow line width has produced viable absorbance readings that are two to three orders of magnitude below those available with HCLs and EDLs. The somce that provides this detection enhancement is a tunable diode laser. 

In the mid-1960s, the availability of tunable ultraviolet radiation laboratory sources led to the replacement of electron beams with photon sources. Rare gas resonance lamps producing continuum sources of radiation that could then be passed through monochromators allowed the field of photoionization mass spectrometry to develop. The determination of photoion yield as a function of ionization energy, the photoionization efficiency curve, led to determination of ionization potentials with accuracies exceeding those of electron impact methods. Modern photoionization experiments often utilize laser or synchrotron light sources with narrow bandwidths and may employ collimated molecular beam sources that reduce the effects of... 

When lasers first emerged from the laboratory and became commercially available in the late 1960s, they have found application in diverse areas. Spectrometric analysis was among the technology s earliest applications. Unlike established continuum sources, the monochromatic, coherent beams from lasers delivered high energy density. The result was a quantum leap in spatial and temporal resolution, sensitivity and speed of acquisition. For more than 40 years, spectrometric techniques implemented sources with narrower wavebands, shorter pulses and tunable emission. 

Assume that an excited level Ej has been selectively populated by optical pumping with a laser (Fig.8.28). If the sample is irradiated with the spectral continuum of a broad-band source, the total absorption spectrum of molecules in all levels is obtained by measuring the transmitted intensity dispersed by a spectrometer (see Sect.8.1). When the intensity I of the pump laser is chopped, the specific absorption of molecules in level Ej can be selected by a lock-in detector tuned to the chopping frequency. The spectrometer may be spared if the continuum source is replaced by a tunable laser 12 The difference in the absorption 612(0 2) = a(o)2)Axl2 with and without the pump laser gives the absorption spectrum of molecules in the excited level E directly. More sensitive is the excitation spectroscopy (Sect.8.2) where the fluorescence intensity ifi (0)2) induced by L2 is monitored as a function of the frequency 0 2 the tunable laser L2 (Fig.8.28b). 

Grant, D. M., Elson, D. S., Schimpf, D., Dunsby, C., Requejo-Isidro, J., Auksorius, E., Munro, I., Neil, M. A. A., French, P. M. W. Nye, E., Stamp, G. and Courtney, P. (2005). Optically sectioned fluorescence lifetime imaging using a Nipkow disk microscope and a tunable ultrafast continuum excitation source. Opt. Lett. 30, 3353-5. 

The femtosecond transient absorption studies were performed with 387 nm laser pulses (1 khz, 150 fs pulse width) from an amplified Ti Sapphire laser system (Model CPA 2101, Clark-MXR Inc). A NOPA optical parametric converter was used to generate ultrashort tunable visible pulses from the pump pulses. The apparatus is referred to as a two-beam setup, where the pump pulse is used as excitation source for transient species and the delay of the probe pulse is exactly controlled by an optical delay rail. As probe (white light continuum), a small fraction of pulses stemming from the CPA laser system was focused by a 50 mm lens into a 2-mm thick sapphire disc. A schematic representation of the setup is given below in Fig. 7.2. 2.0 mm quartz cuvettes were used for all measurements. 

Essentially, a small part of the laser pulse train that is ultimately used to trigger the photocathode is split off to create a synchronized laser probe pulse train. The probe line is equipped with different nonlinear optical devices that permit the tunability of the probe beam from the near UV to the NIR. Available probe sources include the laser fundamental (790 nm) and second harmonic (395 nm), a white-light continuum (470-750 nm) generated in a sapphire plate, and a continuously tunable Optical Parametric Amplifier (470-750 nm, 1000-1600 nm, and 240-375 nm by SHG), able to deliver light pulses shorter than 30 fs after compression. 

A suprananosecond kinetic spectrometer is schematically shown in Figure 2. The excitation source is a Q-switched Nd YAG laser (Continuum Surelite 1) which is capable of a iO-Hz repetition rate but is typically used in the replicate-one-shot mode. The nominal 6-ns pulse can contain up to 450 mJ at 1064 nm, which decreases after harmonic conversions to 532 nm, 355 nm, or 266 nm. In addition to these harmonic lines, the 355-nm line can be used to pump an OPO (Opotek Magieprism), which provides tunable radiation in the range 420 nm to ca. 900 nm. The selected photolysis beam is incident on one face of a 10 mm x 10 mm quartz cuvette containing the sample. 

The instrument uses an excitation source of one of the harmonics (266 nm/355 nm/532 nm, 7 ns FWHM) of a Nd YAG laser (Continuum Surelite I) or tunable visible light from an optical parametric oscillator (OPO) (Opotek). Other pulsed lasers can be employed, the criteria being that a short pulse of light (<20 ns) be produced, with sufficient per pulse energy to generate enough absorbance to rise above the noise inherent in the spectrometer. During the experiment, the laser beam excites the sam-... 

Figure ID. If a tunable laser is used for secondary excitation of molecular ions, resonance dissociation spectroscopy of molecular cations may be performed. Here excited cationic levels are subject to laser spectroscopy. They serve as intermediate states for the process of resonance enhanced multiphoton dissociation. This is quite similar to resonance ionization spectroscopy of neutrals. The difference is that a dissociation instead of an ionization continuum is finally reached by multiphoton excitation. The advantage of this technique is that it is independent of high ion numbers (as necessary for absorption spectroscopy), fluorescence [necessary for laser-induced fluorescence (LIF)] or predissociation and therefore is fairly general. In addition, mass selectivity is intrinsic and one may benefit from state selective ion formation if resonance multiphoton ionization is used as an ion source.

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