direct absorption interferes only when near-infrared lasers (e.g., 1064 nm) are used. Therefore, water is an ideal solvent. However, since the laser is focused to a relatively small spot size, rapid heating of samples can occur. When resonance Raman spectra are recorded, however, sample heating and photo-bleaching can cause damage and a change to the Raman spectrum obtained. Furthermore, if the absorbance of the sample is high (> OD 2) over the wavelength range in which the Raman spectrum is recorded then inner-filter effects (reabsorption of the Raman scattering by the sample) can decrease signal intensity dramatically. Typically, the sample is placed into a tube, which can then be spun to decrease the sample's exposure to the laser light, and reduce the effects of photodegradation. Gaseous, liquid, and solid samples can all be analyzed using RR spectroscopy. Although scattered light leaves the sample in all directions the collection of the scattered light is achieved only over a relatively small solid angle by a lens and directed to the spectrograph and CCD detector. The laser beam can be at any angle with respect to the optical axis used to collect Raman scattering. In free space systems, the laser path is typically at an angle of 180° or 135° (a so-called back scattering arrangement). The 180° arrangement is typically used in microscopes and fiber optic based Raman probes. Other arrangements involve the laser passing at 90° with respect to the optical axis. Detection angles of 90° and 0° are less frequently used. The collected scattered radiation is focused into a spectrograph, in which the light is first collimated and then dispersed by a diffraction grating and refocused onto a CCD camera. The entire spectrum is recorded simultaneously and multiple scans can be acquired in a short period of time, which can increase the signal-to-noise ratio of the spectrum through averaging. Use of this (or equivalent) equipment and following an appropriate protocol can yield better than 10% repeatability in absolute measurements for the rate of Raman scattering. This can be useful with resonance Raman for accurately determining optical transitions in structures with strong Van Hove singularities. Resonance hyper-Raman spectroscopy Resonance hyper-Raman spectroscopy is a variation on resonance Raman spectroscopy in which the aim is to achieve an excitation to a particular energy level in the target molecule of the sample by a phenomenon known as two-photon absorption. In two-photon absorption, two photons are