Spectroscopy pumping, optical

The large population density Nj which can be achieved in selectively excited levels E by optical pumping with lasers allows one to perform high-resolution spectroscopy of excited molecules with sufficient sensitivity. Many of the techniques applicable to molecules in their ground states (e.g., absorption spectroscopy, optical pumping, LIF, microwave spectroscopy) can be now transferred to transitions between excited states. In the following sections we briefly discuss some of these methods. 

Excimer lasers are of great importance for UV and vacuum UV (VUV) spectroscopy and photochemistry. They are also found in a wide range of applications. For example, they are used in micromachine medical devices, including refractive surgery, in photo-lithography for the microelectronics industry, for material processing, as optical pump sources for other type of lasers (dyes), and so on. More details about excimer lasers can be found in Rodhes (1979). 

Decomps, B., Dumont, M. and Duclqy, M. (1976). Linear and nonlinear phenomena in laser optical pumping. In Laser Spectroscopy of Atoms and Molecules. Topics in Applied Physics, vol. 2, ed. H. Walther, pp. 283-347 (Springer-Verlag, Berlin, Heidelberg, New York). 

Excited target atoms can be prepared in well-defined states by optical pumping with a tunable laser. Specific magnetic substates are excited by polarised light. Momentum distributions are observed for these states by electron momentum spectroscopy. 

Transient terahertz spectroscopy Time-resolved terahertz (THz) spectroscopy (TRTS) has been used to measure the transient photoconductivity of injected electrons in dye-sensitised titanium oxide with subpicosecond time resolution (Beard et al, 2002 Turner et al, 2002). Terahertz probes cover the far-infrared (10-600 cm or 0.3-20 THz) region of the spectrum and measure frequency-dependent photoconductivity. The sample is excited by an ultrafast optical pulse to initiate electron injection and subsequently probed with a THz pulse. In many THz detection schemes, the time-dependent electric field 6 f) of the THz probe pulse is measured by free-space electro-optic sampling (Beard et al, 2002). Both the amplitude and the phase of the electric field can be determined, from which the complex conductivity of the injected electrons can be obtained. Fitting the complex conductivity allows the determination of carrier concentration and mobility. The time evolution of these quantities can be determined by varying the delay time between the optical pump and THz probe pulses. The advantage of this technique is that it provides detailed information on the dynamics of the injected electrons in the semiconductor and complements the time-resolved fluorescence and transient absorption techniques, which often focus on the dynamics of the adsorbates. A similar technique, time-resolved microwave conductivity, has been used to study injection kinetics in dye-sensitised nanocrystalline thin films (Fessenden and Kamat, 1995). However, its time resolution is limited to longer than 1 ns. 

Time-resolved resonance Raman spectroscopy of 25 in 50% aqueous CH3CN proved that the final product 26 appears with a rate constant of 2.1 x 109 s 1 following pulsed excitation of 25.207 The appearance of 26 was slightly delayed with respect to the decay of (25), A = 3.0 x 109s, that was determined independently by optical pump probe spectroscopy in the same solvent. The intermediate that is responsible for the delayed appearance of 26, t 0.5 ns, is attributed to the triplet biradical 327.462 It shows weak, but characteristic, absorption bands at 445 and 420 nm, similar to those of the phenoxy radical. ISC is presumably rate limiting for the decay of 327, which cyclizes to the spiro-dienone 28. The intermediate 28 is not detectable its decay must be faster than its rate of formation under the reaction conditions. Decarbonylation of 28 to form p-quinone methide (29) competes with hydrolysis to 26 at low water concentrations. Hydrolysis of 29 then yields p-hydroxybenzyl alcohol (30) as the final product. 

The combination of collinear fast-beam laser spectroscopy and P-RADOP (radiation-detected optical pumping) has been used to measure nuclear spins and moments of neutron-rich isotopes of the light alkali elements jLi [72-74] and Na [75]. Here, the optically pumped fast atomic beam is implanted into a single crystal placed in a static magnetic field. The NMR signal is destroying the nuclear polarization detected by measuring the p-decay asymmetry. 

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