Abstract
Light-induced infrared emission spectroscopy (LIRES) is a novel technique that permits to receive high-quality spectra in the mid-infrared region. Low-intensity visible light connected to a highly sensitive FTIR spectrometer is more advantageous for studying any samples, including biological samples without any damage. This technique permits obtaining unique information on the molecule structure via vibrational excitation fundamental frequencies, overtones, and combination modes. It also enables a direct observation of vibrational radiation transitions in vibrationally excited molecules as well as the channels of vibration energy redistribution, which is not allowed with any other method. In this work, the LIRES is being tested as a technique for studying of vibrationally-excited molecules of carbon tetrachloride and benzene in the liquid phase. On the other hand, using transparent liquids, we had tried to understand some of the physical phenomena that can drive emission in mid-IR. The characteristics of the infrared emission of both liquid species produced by different wavelength radiation from various types of light systems (100 - watt Xe-lamp and Nd:YAG laser; lambda = 1064 nm (8 mW) and lambda = 532 nm (4 mW)) are presented. We demonstrated that the IR-signal, as well as spectral properties of carbon tetrachloride and benzene, was dependent on the wavelength and power of excitation beam. Results obtained with different light sources show that the visible light produces a nonlinear IR-emission signal in transparent liquids. We believe that the visible light is the source of the nonlinear response and is producing the vibration excitation as well as photostimulated transformations of the molecules possessing the high activity for the nonlinear response.
Highlights
Great progress has been lately achieved in the development of spectroscopic feasibility for emission measurements in the mid-infrared region
Spectral Features of Liquid CCl4 and C6H6 in Mid-Infrared Region (2100 - 400 сm−1) Contrary to our expectations, in both investigated species a low-power laser like a Xe-lamp excitation produced IR-emission signal, which was high enough to allow the reliable registration by the Fourier-transform infrared (FTIR) technique
Mode labeling follows Herzberg’s notation. As it follows from the experimental data, the bands with symmetry class a2u, e2u, e1g, e2g and randomly b1u and b2u are enhanced in the light-induced infrared [17] [19] [20] emission (LIRE) spectra of benzene
Summary
Great progress has been lately achieved in the development of spectroscopic feasibility for emission measurements in the mid-infrared region (see, reviews of infrared emission spectroscopy in Refs. [1]-[6]). Most of the research works reported to date rely on the use of highly sensitive low-temperature detectors and heating elements in order to heat gases or solids above the detector temperature or the cooling the entire instrument [7] [8] While these devices are highly optimized in many respects, there are some serious limitations imposed by a damaging effect of high temperatures and hardly removable background radiation of the heaters Special attention should be paid to a methodological approach that involves the use of a continuum-wave-laser light to heat a sample surface and to record IR-emission in the mid-infrared region. Such FT-IR emission spectroscopic measurements require that the internal source must be replaced by the source of interest, i.e. by a sample itself. The underlying phenomenon is more complex process than that of laser heating or photoinduced fluorescence as it was previously described in [9] [10] and [11]-[14], respectively
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