New generation UV Resonance Raman Spectrograph

Chinese Academician Can LI，Dalian Institute of Chemical Physics (DICP), Chinese Academy of Sciences (CAS)，and his study group developed the first UV Resonance Raman Spectrograph, received the CAS second prize and the second prize of national innovation. On April 8th,  established the Modern Instruments Joint Laboratory with ZOLIX, which facilitates a faster transition from research to actual production.

UV Resonance Raman Spectrograph Description
Major parts of the UV Resonance Raman system:
1. Laser: UV or Visible laser, UV-tunable narrow linewidth laser.
2. Spectrograph: Triple monochromator and the high sensitive science CCD.
3. Signal acquisition; High efficiency spectrograph acquisition component.
Features of the UV Resonance Raman system:
◆ Appropriate UV laser excitation can completely avoid the interference of fluorescence background.
◆ As the Raman signal intensity is proportional to the biquadrate of excitation laser frequency， UV laser can excite  Raman signal in more efficient( Equal-power 266nm laser could inspire 16 times higher than the 532nm laser Raman signal).
◆ Resonance Raman can provide a very high resonance enhancement factor，（The theoretical limit can be up to 106 times）which can greatly enhance the detection limit.
◆ Selective excitation can be achieved. When we tune to the excitation wavelength of the substance, the signal of only that wavelength resonance can be enhanced for several decades.  That can further enhance the signal to noise ratio, This is favorable for the catalytic and biological research.
◆ As a result of the triple monochromator filtered Rayleigh scattering, rather than the notch filter, equipment can test the Raman spectrum with several wave numbers.
Specification

1. Laser ◆ 325nm HeCd laser: 325nm, TEM00 mode, with the output power from 30W to 50W. ◆ 244nm frequency doubling tunable argon ion laser: 244nm, TEM00 mode, with the output power 24mW. Spectral line: 229, 238, 248, 250, 257, 264nm. ◆ 532nm green DPSS laser: ◆ narrow line width tunable Ti doped sapphire lasers.

 

	Tunable spectral region	Output average power	Tunable spectral region of single crystal
Basic Frequency	700-960nm	>1W	100nm
Double Frequency	350-480nm	90-500mW	50nm
Triple Frequency	233-320nm	20-250mW	33nm
Quadruple Frequency	193-240nm	5-100mW	25nm
Spectral line width	<0.1cm-1	Power stability	<3% rms

NOTE. If you want to cover the entire spectrum, you need to replace the crystal.
Tips: Resonance enhancing dose not begin in a particular wavelength suddenly, there is a wavelength range.
2、Spectrograph
A. monochramator
◆ Focus: 500mm, f/6.5
◆ Grating size: 68mm×68mm or 68mm×84mm
◆ Step size : 0.005nm
◆ Mirror reflectivity: In the UV and visible area can reach 90%.
B. Subtraction mode Raman spectral collection
◆ Resolution: 4.0cm-1(UV), 3.0 cm-1(Visible light).
◆ wave number range: 50-4000 cm-1(UV), 25-4000 cm-1(Visible light)
C. Spectrographic Detector

CCD or EMCCD	Spectrographic CCD	Spectrographic CCD	Spectrographic EMCCD
Pixels	1024×256	2048×512	1600×400
Pixels size um	26×26	13.5×13.5	16×16
Image area　mm	26.6×6.7	27.6×6.9	25.6×6.4
lowest refrigerating temperature oC	-100	-100	-100
Electronic Gain	NA	NA	1-1000

UV Raman Spectrum characterization of different types of molecular sieve.

UV Raman Spectrum in-situ research molecular sieve synthesis mechanism.

 UV Raman Spectrum testing the ultra-low content of skeleton
Ti in the molecular sieve skeleton.

Application directions:
◆ Catalysis ◆ Biochemistry ,Life Sciences ◆ Materials science, polymer science ◆ nano science ◆Optoelectronic Materials

 
Appendix 1: UV Raman and resonance Raman Principle and Application 
The problems of fluorescence interference and the low sensitivity block the  extensive application of conventional Raman spectroscopy. But the above problems have been solved with the development of  UV Raman spectroscopy in recent years. The advent and development of UV Raman spectroscopy technology extends the scope of UV Raman spectroscopy application. On the right is the schematic diagram of avoiding the UV-fluorescence interference in Raman spectroscopy. Fluorescence often appears in the 300nm to 700nm, or in a longer wavelength region. Fluorescent seldom appears in the UV region( less than one wavelength). Therefore, for the

substances having a strong fluorescent interference in the visible Raman spectrum, such as oxides and so on, can avoid the fluorescence interference and get a high ratio of signal to noise Raman spectrogram successfully using the UV Raman spectroscopy. From the above AlPO-5 molecular sieve spectrogram, we find that UV Raman resonance spectrum technology can avoid the fluorescence interference, and can be used for the  micro-mesoporous materials  characterization.

Another prominent feature of UV Raman spectrum technology is that Raman signal can be enhanced by resonance Raman signals. Resonance Raman effect can be explained by the Raman scattering cross-section formula: according to the Kramers-Heisenberg-Dirac scattering formula:         ωri is the frequency of the energy difference between the ground state and the excited state. ωL is the  incident laser frequency. When the excitation light frequency near the electronic absorption band, the first denominator tends to zero, so its scattering cross-section increases abnormally, leading to some specific Raman scattering intensities increase up to 104 ~ 106 times. With the different excitation lines, spectral peak intensity of resonance Raman spectrum shows different changes in the ordinary Raman.

Raman scattering & Resonance Raman scattering energy level diagram and the corresponding Raman spectrogram.

        When the UV Resonance Raman used for characterization of multi-component system, you can selectively stimulate certain components, so these components Raman signal are greatly enhanced, and resonance Raman spectra are obtained.
This resonance-enhanced or resonance Raman effect is a very useful technology, which not only greatly reduce the detection limit of the Raman measurements, but also can be introduced into the e-option. Thus, if we use resonance Raman spectrum to study the samples, can not only see its structural characteristics, but also can get its electronic structure information. Metal porphyrins, carotenoids, as well as a range of other important biological molecules, their transition energy difference between the electronic energy levels are in the visible range, which makes them into ideal research materials for resonance Raman spectra.
There is a very practical application for resonance selection techniques. That is half of the chromophores spectrum will be enhanced because of this resonance effect, and its surroundings will not. This means that for the chromosomes, using visible light can detect the active absorption center, and the array of proteins around them will not be impacted (this is because these proteins need ultraviolet light to produce resonance enhanced effect).
Resonance Raman spectroscopy is also an important technology on the detection of metal center compounds, fullerene molecule, diacetyl, and other rare elements, because these materials all have strong absorption for visible light.
Much more molecules absorption spectra are in the UV region, so they need UV laser for the resonance excitation, we call it UlraViolet Resonance Raman Spectroscopy. UV resonance Raman spectroscopy is an important tool to study catalytic and complex biological systems for molecular analysis. Most of the biological systems can absorb ultraviolet radiation, so all of them can provide enhanced UV resonance Raman. Such a high Raman resonance selection effects makes Raman spectra of the important biological targets such as proteins and DNA to be greatly enhanced, while other substances are not, very easy for target recognition and analysis. For example, 200nm excitation light can enhance the vibration peaks of amide; and 220nm excitation light can enhance the specific vibration peaks of aromatic residues. Water Raman scattering signal is very weak, this technology makes it possible to analyse water-related weak system Raman signal.

Appendix 2: experiment examples

◆Ultra-low content of isolated transition metal ion( Ti-MCM-41) in the micro-mesoporous materials skeleton can be identified by UV resonance Raman spectroscopy reliably and accurately.

◆Using the characteristics of UV Raman which can avoid the fluorescence and increase detection sensitivity, we can study the precursors, intermediate and the evolution of molecular sieve crystals  in synthesized process.

◆UV Raman spectroscopy can selectively get the surface information of the materials (for example, TiO2 and ZrO2) which have strong absorption in the UV Region.