Anti-Stokes emission

Por IR-Raman experiments, a mid-IR pump pulse from an OPA and a visible Raman probe pulse are used. The Raman probe is generated either by frequency doubling a solid-state laser which pumps the OPA [16], or by a two-colour OPA [39]. Transient anti-Stokes emission is detected with a monocliromator and photomultiplier [39], or a spectrograph and optical multichannel analyser [40]. 

There are the further advantages that rotational lines can be studied and that fluorescent substances can be investigated by the inverse Raman effect. Benzene and other molecular liquids have been studied by this method by McQuillan and Stoicheff 232) jhe required continuum radiation was anti-Stokes emission produced by passing the laser beam in liquid toluene. 

Antimony trifluoride, 16 181 preparation of, 7 14-15 solubility of, 7 6-7 Antimony trifluoroacetates, 17 12, 13 Antiperspirants, 36 16 Anti-Stokes emission, 35 342-343 Antitumor agents DNA and RNA cleavers, 45 252 phosphazotrihalides as, 14 90, 91... 

Figure 3-42 Initial apparatus for measuring anti-Stokes emission using a frequency-doubled Nd YAG pumped dye laser. L is a short focal lens (3-4 cm) S is the sample I is an iris for spatially filtering the two exciting beams F is a wideband interference filter D is the detector (usually a PIN diode) M is a monochromator (not usually necessary). Not shown are the PAR-160 box car integrator, chart recorder, and dye laser scan drive used to record spectra. (Reproduced with permission from Ref. 104.)...

Examples of anti-Stokes data contaminated by an SFG artifact are shown in Fig. 11a and c, where the higher energy C-H stretching transition of neat methanol is pumped at u>ir = 3020 cm-1. The artifact will be centered at col + from methanol vibrational transitions at 3020 cm-1 (actually the higher energy tail of the C-H stretch transition at 2940 cm-1). The spectral and temporal properties of the artifact can be independently characterized by purposely generating SFG in a thin ( 50 pm) slab of KTP placed at the location of the sample. However, the amplitude of the SFG artifact in the spectrum is unknown. 

In the ordinary Raman effect, few molecules are found in their excited vibrational state. The strong pumping action of a laser beam changes this situation drastically, so that an appreciable fraction of all molecules in the laser beam are soon made available for anti-Stokes emission. Classically, the anti-Stokes radiation is generated by the interaction of the laser beam with molecular vibrations, but the phase of the latter is established by the still more intense Stokes radiation. As a consequence, an index-matching requirement... 

Similarly to the generation of coherent fundamental Stokes and anti-Stokes radiation, higher-order stimulated Stokes and anti-Stokes emission can be produced when high pump intensities are employed. 

Let us now turn to two-photon excitation via an intermediary level. In this chapter we restrict ourselves to processes without energy transfer, that is, typical one-ion processes. A recent and intensity-rich example is Eu " in LaOCl (41). Excitation of the Do level of Eu (cf. Fig. 6) does not only yield the usual emission transitions from the Dq level, but also yields anti-Stokes emission from the higher Di,2,3 levels. The intensities of these emissions were at least one order of magnitude smaller (for excitation with a continuous dye laser pumped with an argon ion laser). 

The excitation mechanism of the anti-Stokes emission is as follows. First the ion is excited into the Dq level. Although the Fo- Do transition is highly forbidden (J = 0- J = 0), it has a rather high absorption strength in LaOCl due to the strong linear crystal-field component at the La CEu ) site. The lifetime of the Do level is long (of the order of milliseconds). A second photon is now absorbed, which raises the system to the charge-transfer state. This is an allowed transition. More-... 

A similar process has been described by Boulon et al. 42) for Gd ". These authors observed, on pumping into the first excited level P7/2 at 311.5 nm, an anti-Stokes emission from the l7/2 level at 278.9 nm (cf. Fig. 13). 

Figure 1. Graph of measured energy of first-Stokes and up to 13th anti-Stokes emission in H2 gas (at 5 atm pressure) excited by laser radiation at 545 nm using the experimental arrangement shown in Figure 2 [8].

The solid phase lends itself to the preparation of lanthanide arrays. Indeed, lanthanide doped yttrium aluminium garnets are weU known as laser materials, while doped materials containing more than one kind of lanthanide have proved very effective at upconversion of energy - the sequential absorption of two photons giving rise to anti-Stokes emission [9]. In such systems, excited state absorption by the intermediate state gives rise to formation of a high energy... 

The SRS spectrum of a benzene droplet ( 35 um in radius) is shown in Fig. 2. In addition to the Ist-order Stokes SRS emission at V2 = 992 cm for benzene droplets, up to the 6th-order Stokes (with 6V2 shift) were simultaneously detected with an optical multichannel analyzer (OMA). Anti-Stokes emission from benzene droplets was not detectable. The Raman shifts of the... 

Photochemical upconverston increases the energy expioitabie new (anti-Stokes) emission... 

Figure 3 Folded BOXCARS geometry applied in several transient nonlinear optical spectroscopies. In pump-probe spectroscopy, one of the three beams is blocked and the intensity of one of the incoming beams is monitored as a function of the time delay between the remaining two beams (e.g., beam 3 is blocked and beam 2 is monitored as a function of its delay with respect to beam 1, phase-matching condition would be k2 = ki — ki -I- k2>. Beams 4 and 5 are photon echo signals generated from beams 1 and 2. Beams 6 and 7 can be stimulated photon echo or transient grating signals generated from beams 1,2, and 3. In transient grating two of the beams are time coincident. In coherent anti-Stokes Raman spectroscopy, beams 1 and 3 are time coincident and carry the same frequency the difference between this frequency and that of beam 2 (so-called Stokes beam) matches a vibrational frequency of the system and beam 6 will correspond to the anti-Stokes emission.

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