# Real-Time Measurement of Photodissociation with a Static Modulated Fourier Transform Spectrometer
## 1 Introduction  
Recently, environmental monitoring of greenhouse gases has become an important part of addressing global warming. Most monitoring technologies can provide spectroscopic information acquired under limited conditions. However, sometimes, the real-time remote sensing spectroscopic technology with short measurement times and mechanical robustness is needed but are not easily obtained because there has not been any appropriate spectrometer applied in the field.  
When spectral information need to be obtained in a wide measurement band, from visible to far infrared, Fourier transform spectroscopy can provide a potent solution. Although a dispersion optical system using a diffraction grating can obtain spectral information in a relatively wide band, it has the drawback of smaller diffraction angles in longer wavelength regions. In addition, without any additional optical system, it is difficult to obtain an intense incident light compared with that of a Fourier transform spectrometer [1]. On the other hand, the Fourier transform spectrometer can have a high spectral resolution in the fingerprint region because the spectral resolution improves as the wavelength increases [2], and it is possible to obtain spectral information from a much wider wavelength band ina relatively short time than that of a grating optical system [3]. However, since a moving mirror is supposed to be operated for measurement, conventional Fourier transform spectroscopy should be performed in a strict laboratory environment without vibrations [2]. There have been reports of static modulated Fourier transform spectrometers designed to not have moving parts in the structure [1; 2; 3; 4; 5; 6; 7; 8; 9]. Though most reports have suggested various structures with the conveniences of mechanical stability and short measurement times, it has also been pointed out that they are not practical because of their very low spectral resolution compared with that of conventional dynamic modulated Fourier transform spectrometers [1; 2; 3; 4; 5; 6; 7; 8; 9]. The optical path difference in a static modulated Fourier transform spectrometer usually depends on structural design factors such as wedge angle, thickness, and a refraction index of optical components. However, recently, various studies that can greatly improve spectral resolutions have been reported [10; 11; 12; 13; 14; 15]. They are based on increases in the maximum optical path difference so that a sensor-shifting technique and additional optical components, such as stepped mirrors and a phase retarder, are employed. Since these are based on hardware changes, the spectrometers become more complex. In addition, some research is much interesting because it shortens the measurement time dramatically. In those reports, including our previous publication [13], it is possible to obtain spectral information very quickly, which leads to the fast changing spectral information of a target material.  
In this study, we report the photodissociation process of solid dye samples that were fabricated in our laboratory with a static modulated Fourier transform spectrometer that can remotely and rapidly measure changing spectral signals in real time, which cannot be achieved with existing spectroscopic systems.