Pulsed lasers CARS spectroscopy

Byer RL, Duncan M, Gustafson E, Oesterlin P, Konig F (1981) Pulsed and CW Molecular Beam CARS Spectroscopy. In McKellar ARW, Oka T, Stoicheff BP (eds) Laser Spectroscopy V, Springer, Berlin, p 233... 

Nonlinear vibrational spectroscopy provides accessibility to a range of vibrational information that is hardly obtainable from conventional linear spectroscopy. Recent progress in the pulsed laser technology has made the nonlinear Raman effect a widely applicable analytical method. In this chapter, two types of nonlinear Raman techniques, hyper-Raman scattering (HRS) spectroscopy and time-frequency two-dimensional broadband coherent anti-Stokes Raman scattering (2D-CARS) spectroscopy, are applied for characterizing carbon nanomaterials. The former is used as an alternative for IR spectroscopy. The latter is useful for studying dynamics of nanomaterials. 

Conventional CARS spectroscopy is carried out using the two-color CARS process induced by continuous-wave or narrowband pulsed laser sources. On the basis of the progress in the broadband pulsed laser technology, broadband CARS methods are now drawing much attention [29-31]. One of most remarkable examples is... 

Ab initio calculations gave vibrational wavenumbers for 9 isomers of CNNS.524 A CARS study of the effects of 266 nm. pulsed laser photodissociation of NCNCS showed that the vapour contained both NCNCS and CNCN.525 IR and Raman spectra, with factor group analysis, gave vibrational assignments for crystalline ammonium dicyanamide, NH4[N(CN)2].526 Variable-temperature Raman spectroscopy was used to follow the solid state transformation of NI I4[N(CN)2] into NCN=C(NH2)2.527 Ab initio and/or DFT calculations gave vibrational wavenumbers for CH2=CH-N=C=X (X = O, Se) 528 NN-C(CN)2 529 nitroso-azide, NNN-N=0, and nitro-azide, NNN-N02.530... 

Fig. 16. Schematic of experiment to study shock-induced nanopore collapse in real time, (a) One element of a shock target array. A near-IR laser pulse generates shock waves by ablation of an absorbing surface layer. The shock front steepens up to <25 ps in the buffer layer. CARS spectroscopy is used to probe dye molecules in the nanoporous layer, which monitor strain and temperature, and a thin anthracene downstream gauge which monitors changes in the risetime of the shock front caused by pore collapse, (b) Scanning electron micrograph of the surface of the nanoporous layer. The pore size distribution is 100 nm 10%. The distribution appears broader in the image, since it sees only the pore cross-section in the surface plane. Reproduced from ref. [120].

CARS has been successfully used for the spectroscopy of chemical reactions (Sect. 8.4). The BOX CARS technique with pulsed lasers offers spectral, spatial, and time-resolved investigations of collision processes and reactions, not only in laboratory experiments but also in the tougher surroundings of factories, in the reaction zone of car engines, and in atmospheric research (Sect. 10.2 and [380, 381]). 

RL Byer, M Duncan, E Gustafson, P Oesterlin, F Konig. Pulsed and CW molecular beam CARS spectroscopy. In ARW McKellar, T Oka, BP Stoicheff, eds. Laser Spectroscopy V. Berlin Springer-Verlag, 1981, pp 233-241. 

CARS has been successfully used for the spectroscopy of chemical reactions (Sect. 13.4). The BOX CARS technique with pulsed lasers offers spectral, spatial and time-resolved investigations of collision processes and... 

Early picosecond studies were carried out by Schneider et al, [63] on the parent spiro-oxazine (NOSH in Scheme 8) and similar derivatives. In a back-to-back work, they also described a complimentary CARS (coherent anti-Stokes Raman spectroscopy) investigation [69], Simply put, these authors found that the closed spiro-oxazine ring opened in 2-12 psec after laser excitation. The reaction was slower in more viscous solvents. An intermediate state formed within the excitation pulse and preceded the formation of merocyanine forms. This transient was named X in deference to the X transient named by Heiligman-Rim et al. for the spiropyran primary photoproduct [8], (See also the previous section.) The name X has since been adopted by other workers for the spiro-oxazines [26,65],... 

Common to all narrow-bandwidth excitation schemes is sequential scanning of an experimental parameter in order to adjust the Raman shift in CRS detection. In order to obtain an entire CRS spectrum, this is not only time consuming but also prone to sources of noise induced by fluctuations in laser pulse parameters. As a consequence, dynamical changes in a CRS spectrum are difficult to follow. This problem can be circumvented by use of multiplex CRS spectroscopies [48, 49], which will be discussed in combination with CARS and SRS microscopy in Sects. 6.3 and 6.4, respectively. 

Here, E is the strength of the applied electric field (laser beam), a the polarizability and / and y the first and second hyper-polarizabilities, respectively. In the case of conventional Raman spectroscopy with CW lasers (E, 104 V cm-1), the contributions of the / and y terms to P are insignificant since a fi y. Their contributions become significant, however, when the sample is irradiated with extremely strong laser pulses ( 109 V cm-1) created by Q-switched ruby or Nd-YAG lasers (10-100 MW peak power). These giant pulses lead to novel spectroscopic phenomena such as the hyper-Raman effect, stimulated Raman effect, inverse Raman effect, coherent anti-Stokes Raman scattering (CARS), and photoacoustic Raman spectroscopy (PARS). Figure 3-40 shows transition schemes involved in each type of nonlinear Raman spectroscopy. (See Refs. 104-110.)... 

Several studies to determine the ablation mechanisms for picosecond laser ablation were focused on spectroscopy (coherent anti-Stokes Raman scattering (CARS), absorption, and ultrafast imaging) [108-113]. It has been shown that pulses in the picosecond range produce fast temperature jumps and solid-state shockwaves that are... 

Ultrashort pulses may be also used for vibrational spectroscopy with high frequency resolution. As a first example we have demonstrated FT-CARS of a supersonic expansion. Several advantages of the technique should be noted. The effect of transit time broadening can be eliminated. Artifacts via the nonresonant part of the third order susceptibility are negligible. A possible dynamic Stark effect during the excitation process does not influence the ns signal transient. Precise spectroscopic information is provided without narrow-band laser sources. 

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