Tunability

Zare R N and Dagdigian P J 1974 Tunable laser fluorescence method for product state analysis Science 185 739-46... 

However, with the advent of lasers, the teclmique of laser-induced fluorescence (LIF) has probably become the single most popular means of detennining product-state distributions an early example is the work by Zare and co-workers on Ba + FLT (X= F, Cl, Br, I) reactions [25]. Here, a tunable laser excites an electronic transition of one of the products (the BaX product in this example), and the total fluorescence is detected as a... 

The foremost of the modem teclmiques is tlie use of lasers as spectroscopic tools. Lasers are extremely versatile light sources. They can be designed with many usetlil properties (not all in the same instmment) such as high intensity, narrow frequency bandwidth with high-frequency stability, tunability over reasonable frequency ranges, low-divergence beams which can be focused into very small spots, or pulsed beams with... 

Light sources can either be broadband, such as a Globar, a Nemst glower, an incandescent wire or mercury arc lamp or they can be tunable, such as a laser or optical parametric oscillator (OPO). In the fomier case, a monocln-omator is needed to achieve spectral resolution. In the case of a tunable light source, the spectral resolution is detemiined by the linewidth of the source itself In either case, the spectral coverage of the light source imposes limits on the vibrational frequencies that can be measured. Of course, limitations on the dispersing element and detector also affect the overall spectral response of the spectrometer. 

Seifert F, Petrov V and Woerner M 1994 Solid-state laser system for the generation of midinfrared femtosecond pulses tunable from 3.3-Mu-M to 10-Mu-M Opt. Lett. 19 2009-11... 

A RIKES experunent is essentially identical to that of CW CARS, except the probe laser need not be tunable. The probe beam is linearly polarized at 0° (—>), while the polarization of the tunable pump beam is controlled by a linear polarizer and a quarter waveplate. The pump and probe beams, whose frequency difference must match the Raman frequency, are overlapped in the sample (just as in CARS). The strong pump beam propagating tlirough a nonlinear medium induces an anisotropic change in the refractive mdices seen by tlie weaker probe wave, which alters the polarization of a probe beam [96]. The signal field is polarized orthogonally to the probe laser and any altered polarization may be detected as an increase in intensity transmitted tlirough a crossed polarizer. When the pump beam is Imearly polarized at 45° y), contributions... 

Kneipp K, Kneipp H and Seifert F 1994 Near-infrared excitation profile study of surface-enhanced hyper-Raman scattering and surface-enhanced Raman scattering by means of tunable mode-locked... 

A microwave pulse from a tunable oscillator is injected into the cavity by an anteima, and creates a coherent superposition of rotational states. In the absence of collisions, this superposition emits a free-mduction decay signal, which is detected with an anteima-coupled microwave mixer similar to those used in molecular astrophysics. The data are collected in the time domain and Fourier transfomied to yield the spectrum whose bandwidth is detemimed by the quality factor of the cavity. Hence, such instruments are called Fourier transfomi microwave (FTMW) spectrometers (or Flygare-Balle spectrometers, after the inventors). FTMW instruments are extraordinarily sensitive, and can be used to examine a wide range of stable molecules as well as highly transient or reactive species such as hydrogen-bonded or refractory clusters [29, 30]. 

Xu B, Flu Q and Mellooh M R 1997 Eleotrioally pumped tunable terahertz emitter based on inter-subband transitions App/. Rhys. Lett. 71 440-2... 

Matsuura S, Chen P, Blake G A, Pearson J and Pickett H M 1999 A tunable, cavity-locked diode laser system for terahertz photomixing IEEE Micro. Theory Technol. 48 380-7... 

In order to achieve a reasonable signal strength from the nonlinear response of approximately one atomic monolayer at an interface, a laser source with high peak power is generally required. Conuuon sources include Q-switched ( 10 ns pulsewidth) and mode-locked ( 100 ps) Nd YAG lasers, and mode-locked ( 10 fs-1 ps) Ti sapphire lasers. Broadly tunable sources have traditionally been based on dye lasers. More recently, optical parametric oscillator/amplifier (OPO/OPA) systems are coming into widespread use for tunable sources of both visible and infrared radiation. 

Figure Bl.5.12 SH and SF spectra (frill dots) for the CaF2/Si(l 11) interface (a) SH intensity as a fiinction of the photon energy of the tunable laser (b) SF intensity obtamed by mixmg the tunable laser with radiation at a fixed photon energy of 1.17 eV. For comparison, the open circles in (a) are signals obtained for a native-oxide covered Si(l 11). The fiill line is a fit to the theory as discussed in [79].

The development of ultrafast spectroscopy has paralleled progress in the teclmical aspects of pulse fomiation [Uj. Because mode-locked laser sources are tunable only with diflSculty, until recently the most heavily studied physical and chemical systems were those that had strong electronic absorption spectra in the neighbourhood of conveniently produced wavelengths. 

The OPA should not be confiised with an optical parametric oscillator (OPO), a resonant-cavity parametric device that is syncln-onously pumped by a femtosecond, mode-locked oscillator. 14 fs pulses, tunable over much of the visible regime, have been obtained by Hache and co-workers [49, with a BBO OPO pumped by a self-mode-locked Ti-sapphire oscillator. 

Wilhelm T, Piel J and Riedle E 1997 Sub-20 fs pulses tunable across the visible from a blue-pumped single-pass noncollinear parametric converter Opt. Lett. 22 1494-6... 

Shirakawa A, Sakane I and Kobayasi T 1998 Pulse-front-matched optical parametric amplification for sub-10 fs pulse generation tunable in the visible and near infrared Opt. Lett. 23 1292-4... 

Kane S, Squier J, Rudd J V and Mourou G 1994 Hybrid grating-prism stretcher-compressor system with cubic phase and wavelength tunability and decreased alignment sensitivity Opt. Lett. 19 1876-8... 

Yakovlev V V, Kohler B and Wilson K R 1994 Broadly tunable 30 fs pulses produced by optical parametric generation Opt. Lett. 19 2000-2... 

Reed M K, Steiner-Shepard M K and Negus D K 1994 Widely tunable femtosecond optical parametric amplifier at 250 kHz with a Ti sapphire regenerative amplifier Opt. Lett. 19 1855-7... 

Gale G M, Cavallari M, Driscoll T J and Hache F 1995 Sub-20 fs tunable pulses in the visible from an 82 MHz optical parametric oscillator Opt. Lett. 20 1562—4... 

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