Step-scan FTIR (Absorption)


     An infared spectrum provides molecular information about the vibrational modes, the types of transitions, and the transition probability. Using Fourier-transform infrared spectrometer, we can readily obtain simultaneously those pieces of information about stable molecules. The main concept of absorption is population difference: if the population difference is large, strong absorption might occur, but a small population difference causes a weak absorption. When molecules are populated mainly in the ground state, the absorption method functions whereas the emission approach fails. Hence, absorption detection serves as an effective method to purvey information about the lower states. Moreover, FTIR equipped with a step-scan mode can elicit excellently both spectral and temporal information in monitoring a rapid chemical phenomenon. The spectral resolution can attain 0.13 cm-1 and temporal resolution up to 50 ns. Much research has been done in the condensed phase through step-scan IR absorption. We extended the method to a gaseous-phase system to investigate both the products upon photolysis and the reaction intermediates.




Pronciple of Step-scan FTIR

     The step-scan means that the moving mirror of the interferometer pauses at certain points, which are called zero-crossing points(ZCP), determined by the wavelength of He-Ne laser. When the moving mirror is stopped at the first zero-crossing point and stabilized, a chemical event initiated (by stimulation of some kind). During this event, the concentration of chemical substances might alter resulting in a modification of transmitted IR light. The detector receives the modulated light, and the signal (voltage or current) becomes recorded with an analog-to-digital converter. Hence, we obtain a temporal profile at this zero-crossing point. The moving mirror steps to the next ZCP and undergoes the same chemical event. The process finishes at the final required points and all temporal profiles are recorded. In an absorption experiment, we use AC/DC method to collect the data. The AC-coupled signals provide the time-resolved interferograms and the DC-coupled signal produces a spectral background and phase correction. Hence, the absorption difference ΔAt(v) can be deduced in the following form, ΔAt (v)= -log(1+ΔSt(v)/S0(v)).


      The setup can be separated to three components: FTIR sprectometer mounted with a multi-pass White cell, a laser system, and signal processor.
We use an instrument (NEXUS 870 FTIR) equipped of a step-scan module for absorption experiments. A multi- pass White cell is mounted in the path of the IR beam. The numbers of passes ordinarily is 32 with a base path length of 20 cm, i.e., the total absorption length is increased to 6.4 m. In addition, the spectrometer can be operated under both slave and normal trigger modes. The laser system was chosen for other purposes; light at wavelengths 193, 248, 355, 532 and 1064 nm are available in our lab. The signal processor includes one low-noise voltage amplifier (SR560) and one external ADC board (Gage 14100).


  1. Detection of ClSO with time-resolved Fourier-transform infrared absorption spectroscopy, L.-K. Chu, Y.-P. Lee, and Eric Y. Jiang, J. Chem. Phys. 120, 3179 (2004).
  2. Observation of CH4 (v2=1 or v4=1) in the reaction Cl+CH4 with time-resolved Fourier-transform infrared absorption spectroscopy, Y.-J. Chen, L.-K. Chu, S.-R. Lin, and Y.-P. Lee, J. Chem. Phys. 115, 6513 (2001).
  3. Detection of ClCO with time-resolved Fourier-transform infrared absorption spectroscopy, S.-H. Chen, L.-K. Chu, Y.-J. Chen, I-C. Chen, and Y.-P. Lee, Chem. Phys. Lett. 333, 265 (2000).