Complementary vibrational spectroscopy

Abstract

Vibrational spectroscopy, comprised of infrared absorption and Raman scattering spectroscopy, is widely used for label-free optical sensing and imaging in various scientific and industrial fields. The two molecular spectroscopy methods are sensitive to different types of vibrations and provide complementary vibrational spectra, but obtaining complete vibrational information with a single spectroscopic device is challenging due to the large wavelength discrepancy between the two methods. Here, we demonstrate simultaneous infrared absorption and Raman scattering spectroscopy that allows us to measure the complete broadband vibrational spectra in the molecular fingerprint region with a single instrument based on an ultrashort pulsed laser. The system is based on dual-modal Fourier-transform spectroscopy enabled by efficient use of nonlinear optical effects. Our proof-of-concept experiment demonstrates rapid, broadband and high spectral resolution measurements of complementary spectra of organic liquids for precise and accurate molecular analysis.

Fig. 1

Schematic and concept of CVS. a Schematic of CVS. The insets show the autocorrelation trace of the NIR pulses and the spectra of NIR and MIR pulses. BS: Beamsplitter, LPF: Long-pass filter, DM: Dichroic mirror, SPF: Short-pass filter, MCT: HgCdTe, APD: Avalanche photodetector. b Conceptual description of CVS. The figure shows a linear triatomic molecule as an example of molecular vibrations. The left panel displays CVS-IR process given by MIR pulses, while the right panel CVS-Raman process by NIR pulses. OPD: Optical path length difference

Fig. 2

CVS interferograms of toluene. a The upper and lower panels show the sequential interferograms measured by CVS-IR and CVS-Raman spectroscopy, respectively. b Magnified 15-averaged CVS-IR and CVS-Raman interferograms plotted as a function of the OPD

Fig. 3

Complementary vibrational spectra of toluene in the fingerprint region. The upper panel shows the comparison of the CVS-IR spectrum and the reference IR absorption spectrum measured by a standard FT-IR. The lower panel shows the comparison of the CVS-Raman spectrum and the reference Raman scattering spectrum measured by a spontaneous Raman spectrometer

Fig. 4

Complementary vibrational spectra of organic molecules. a benzene, b chloroform, c 4:1 mixture of benzene and DMSO. Red and blue curves represent CVS-IR and CVS-Raman spectra, respectively. The inset shows the zoomed spectrum of the Raman peaks of DMSO and benzene around 3000 cm⁻¹. The vibrational lines are found from 800 to 1700 cm⁻¹ in the CVS-IR spectrum and from 600 to 3100 cm⁻¹ in the CVS-Raman spectrum. The small spikes at 500–550, 750, 1250, and 2500 cm⁻¹ in the CVS-Raman spectra are attributed to instrumental noise, which can be removed by careful instrumentation