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Abstract
Frequency-scanned techniques of ultrafast spectroscopy were applied to infrared four-wave mixing (IRFWM) in order to determine their effectiveness in the detection and quantification of vibrationally coupled modes. Frequency-scanned ultrafast techniques are a mixed version of frequency and time domain approaches with some advantages of each domain. The frequency domain advantages include the ability to select individual components from a mixture; time domain advantages include the ability to temporally discriminate many non-linear pathways that can congest spectra and interpretation.
These advantages have been experimentally verified with doubly vibrationally enhanced (DOVE) and triply vibrationally enhanced (TRIVE) FWM using dilute carbon disulfide as a model system. DOVE and TRIVE are multi-dimensional vibrational coupling techniques analogous to multi-dimensional NMR spin coupling techniques. They differ from NMR in that DOVE and TRIVE spectroscopy occur on the ultrafast (<1 ps) timescale and can provide new chemical dynamics information.
Quantification of DOVE signal has been performed computationally of dilute carbon disulfide and agrees with values extracted from linear infrared spectra. Additional parameters from computational models show population dephasing contributions of dilute carbon disulfide's major combination band and strongly absorbing fundamental to be values not far from expected ones. An advantage of TRIVE over the standard IR-IR pump-probe methods is in the possible reduction of the total number of interfering pathways. Pathways not typically explored in the pump-probe paths may be useful in their line narrowing of correlated vibrational modes.
The issue of broadening of spectral lines in mixed domain spectra due to the breadth of the electric fields has been addressed with the concept of gated line narrowing. Shaped input pulses can narrow homogeneously broadened lines when gated at a far time. In TRIVE, gating can take place in the form of up-conversion, and is demonstrated for nickel carbonyl compounds at small gate times. The up-conversion technique may provide higher sensitivity than other detection schemes and should be capable of detecting standard vibrational modes.
