A quantum cascade laser cw cavity ringdown spectrometer coupled to a supersonic expansion source.

Area of Science:

• Molecular Spectroscopy
• Physical Chemistry
• Instrument Development

Background:

• Cavity ringdown spectroscopy (CRDS) is a powerful technique for high-sensitivity absorption measurements.
• Quantum cascade lasers (QCLs) offer tunable mid-infrared light sources crucial for vibrational spectroscopy.
• Supersonic expansion provides a method for cooling molecules to low rotational temperatures.

Purpose of the Study:

• To develop and test a novel instrument coupling a supersonic expansion source to a continuous wave QCL-based CRDS.
• To acquire cold, rotationally resolved gas-phase spectra of molecules, with an initial focus on buckminsterfullerene (C60).
• To demonstrate the capability of the new system for high-resolution vibrational spectroscopy.

Main Methods:

• Construction of a new instrument integrating a supersonic expansion source with a CW Fabry-Perot QCL-CRDS.
• Acquisition of high-resolution spectra of the nu(8) vibrational band of CH(2)Br(2) around 1197 cm(-1).
• Assignment and fitting of spectral transitions to effective Hamiltonians.

Main Results:

• Successfully recorded high-resolution spectra of the CH(2)Br(2) nu(8) vibrational band, a band not previously rotationally resolved.
• Assigned 62 transitions for three isotopologues of CH(2)Br(2), fitting them to effective Hamiltonians with a small standard deviation (14 MHz).
• Achieved a noise-equivalent absorption coefficient of 1.4 x 10(-8) cm(-1) and confirmed rotational cooling to approximately 7 K via spectral simulations.

Conclusions:

• The developed instrument effectively couples supersonic expansion cooling with QCL-CRDS for high-resolution molecular spectroscopy.
• The system is capable of obtaining rotationally resolved spectra of cold molecules, paving the way for studying complex species like C60.
• This represents a significant advancement in high-resolution vibrational spectroscopy techniques.